Led by a Franco-Brazilian team of scholars in the humanities, social sciences, arts and literatures, this joint research project is developing a digital platform for Transatlantic Cultural History to be published in four languages. In a series of essays exploring cultural relations between Europe, Africa, and the Americas, it presents a connected history of the Atlantic space since the 18 th century, highlighting the cultural dynamics of the Atlantic region and its crucial role in the contemporary process of globalization.

Modernist photography in Mexico: Modotti's Letter

Didier Aubert -Université Sorbonne Nouvelle Europe -North America

The Atlantic Space Within Globalization -The Consolidation of Mass Cultures Photography reinvented itself in Mexico between the two World Wars, through numerous exchanges between foreign and national artists and writers. This "Mexican moment" of photographic modernism was shaped by left-wing politics, ideas of Mexicanidad spreading at home and abroad, and the emergence of documentary forms.

On July 9 th, 1931, Italian-born photographer Tina Modotti, then in Moscow, wrote a letter to her Mexican friend and one-time student Manuel Álvarez Bravo. After inquiring about various people both of them knew, she moved on to "asuntos de fotografía." The short paragraph she devoted to these "photographic topics" clearly signals, in retrospect, the end of Modotti's career as a photographer, more than a decade before her death. Yet it also provides a privileged viewpoint to survey Mexico as one of the most vital and influential centers of photographic modernism in the Atlantic region. For two decades, from the moment Modotti and US photographer Edward Weston reached Mexico (August 1923) to the exhibit entitled Mexico: 8 Photographers at the New York Museum of Modern Art (December 1943), a small group of foreign and national photographers made post-revolutionary Mexico an international capital of modernist photography.

Letter from Tina Modotti to Manuel Alvarez Bravo, July 9th 1931. Alquimia 3 (May.-Ago. 1998), p. 40. Source : https://revistas.inah.gob.mx A few months after Modotti and Weston settled in Mexico City, the muralist David Alfaro Siqueiros already celebrated them as "world-famous photographers"-a remarkable premonition rather than an accurate assessment at the time, in spite of Weston's early career as a portrait photographer in California. [START_REF] Siqueiros | Una trascendental labor fotográfica : la exposición Weston-Modotti[END_REF] Twenty years later, in her monograph on Weston, historian of photography Nancy Newhall quoted another glowing praise by French painter Jean Charlot: "It was the good fortune of Mexico to be visited, at the time when the plastic vocabulary of the Renaissance was still tender and amenable to suggestions, by Edward Weston, one of the authentic masters that the United States has bred." [START_REF] Newhall | The Photographs of Edward Weston[END_REF] The couple's influence on Álvarez Bravo and thus indirectly on his wife Lola as well as Agustin Jimenez, Bravo's friendship with Henri Cartier-Bresson (France), and the latter's connections with such influential colleagues as Paul Strand (United States) and Pierre Verger (France), now constitute one of the standard and most remarkable chapters in standard histories of photography. Conversely, pictures published in Mexico, France and the United States by these men and women played a substantive role in establishing Mexican art and culture-both modern and pre-Colombian-as legitimate topics of art history beyond national frontiers. As is well-known, Mexican visual and political cultures were also influential in shaping modernism in the United States. This contribution came, to a large extent, through the publication and exhibition of photographs.

What, then, does Modotti's "asuntos de fotografía" tell us about the circulation, transmission and evolution of modernist photography in the Atlantic region in the 1920's and 1930's? A closer look at her letter and career can help explain the way photography reinvented itself in Mexico between the two World Wars, and to understand how the young nation embraced the medium as a way to promote its "renaissance" in Europe and the United States. The following essay will focus on four key aspects of photography's "Mexican moment": its connection to left-wing politics, the role played by this group in spreading ideas of Mexicanidad in Europe and the United States, the emergence of documentary as a category of modernism, and some of the ramifications of the original groups, as illustrated by the "8 photographers" chosen by the MoMA in 1943 to represent "Mexican" photography.

Politics: "no es posible hacer dos cosas sobre todo cuando las dos son tan importantes"

The first thing Modotti tells Álvarez Bravo in her 1931 letter is that she hasn't found time to take a single picture, because of her involvement in politics (she was an organizer for the International Red Aid in various European countries). An Italian teenager from Udine, Modotti had joined her father in San Francisco, where he ran a short-lived photo studio, in 1913. Like countless other Italian immigrants, she had first tried to make a living as a seamstress and had lived in a community with a vigorous working-class political culture.

Yet it was her experience in Mexico, between 1923 to 1930, which made her both a photographer and a political activist. Two years before becoming a member of the Mexican Communist Party in 1927, she had started working as a translator for El Machete-the organization's official organ, while photographing Soviet dignitaries, such as ambassador Stanislav Pestovsky, who were supporting the magazine. By 1928, Modotti was producing some of her most openly propagandistic pictures, which were soon visible in international left-wing publications such as The New Masses in the U.S. (four covers between October 1928 and September 1929) and the Arbeiter Illustrierte Zeitung in 1928. In 1929 her first solo show was organized at the newly-reorganized Universidad Nacional Autónoma, and advertised as "the first revolutionary photographic exhibition held in Mexico." Her work was shown at the Berkeley Museum of Art, and the Mexican government included some of her pictures as part of the photography exhibit at Sevilla's Ibero-American Fair, along with photographs by Álvarez Bravo and Frida Kahlo's German-born father Guillermo. Tina Modotti. Woman with Flag. A.I.Z. Vol. 10, n. 17, 1931. Private Collection. Source : Photo by Fine Art Images/Heritage Images/Getty Images By that time, however, Mexico's government had considerably restricted the Communist Party's activities. By the summer of 1929, El Machete, which had been the main protagonist of several of Modotti's pictures, had been closed down. A few months later, she was suspected of being involved in the death of Cuban communist leader Juan Antonio Mella, and in a conspiracy to assassinate President Ortiz Rubio. Modotti was forced to leave the country which she had adopted as her home. She travelled to Berlin, where she worked briefly with the agency Unionfoto GmbH Berlin. In October 1930, she reached Moscow and, for all intent and purposes, her photographic career was over.

Although not to the extent of Modotti's unparalleled story, the interplay between leftwing politics and photographic modernism was a key feature in the experience of most of her foreign colleagues who chose to work in Mexico in the 1920's and 1930's. When she first travelled by herself, in 1922, she carried prints made by her lover Edward Weston, himself a successful studio photographer with a well-established reputation in pictorialism. Their friend Ricardo Gomez Robelo had returned to his native land to participate in José Vasconcelos' ambitious cultural policy in the service of the postrevolutionary government. Robelo helped Modotti arrange an exhibit of Weston's work at the Academia de Bellas Artes.

Weston's decision to follow Modotti in 1923 was largely motivated by their affair and the success of this first exhibit, but also by a sense of frustration with cultural and political conformity in California, as well as a growing urge to reorient his work, both thematically and aesthetically. "My disgust for that impossible village of Los Angeles grows daily. Give me Mexico, revolution, smallpox, poverty, anything but the plague spot of America -Los Angeles" (cited in Maddow, 83). Although no one has ever claimed that Weston's work was primarily inspired by Mexico's socialist politics, he often commented favorably on the cultural activism he experienced there, as opposed to the complacency of his native land's celebrations of mass consumption. When he met Diego Rivera, he was not only impressed by his stature or eager to hear anecdotes about Matisse and Picasso in Paris: it was the vitality of Mexico's state programs of artistic and cultural education that made him reflect ironically that his country's own "government 'of the people, by the people and for the people' [didn't] foster great art." [START_REF][END_REF] Mexico City was a cultural capital compared to California's provincialism.

A similar curiosity attracted the French photographer Henri Cartier-Bresson and his U.S. counterpart Paul Strand one decade later. As had been the case with Weston, Strand was invited in 1932 by one of the artists associated to the Mexican government's Secretariat of Public Education, the composer Carlos Chávez. He brought with him fifty prints that were exhibited at the Palacio de Bellas Artes in February, and Strand was visibly touched by the "democratic character of the people who came." [START_REF] Krippner | Trace[END_REF] The popular appreciation of his work contrasted strongly with his experience as a modernist pioneer in Stieglitz's New York gallery, and his involvement in Mexican public education and cultural policy continued with a contract as an elementary school teacher, and then as Director of Photography and Filmmaking for the Mexican Department of Fine Arts of the Secretariat of Public Education. This appointment resulted in Redes (1937), a collaborative fiction acted by real-life fishermen in Alvarado. Although this project was ultimately completed without Strand, it contributed to strengthen his political commitment as a photographer and filmmaker, as his later involvement with Frontier Film (1934) made clear: more dramatically even than Weston, he felt increasingly foreign to the U.S.'s political and cultural climate, and ended up choosing exile in Europe in the early 1950's, after the Photo League, which he had helped create, was listed as a subversive organization by the U.S. Attorney General. Strand's social and political commitment was thus largely influenced by his Mexican years.

In France, Henri Cartier-Bresson was already close to left-wing artists and organizations such as the Association des Écrivains et Artistes Révolutionnaires before he reached Mexico in July 1934. Hired for his first real job as a photographer to document an expedition exploring potential itineraries for the Panamerican highway, Cartier-Bresson immediately felt at ease in the social and political effervescence that he encountered there. Although the survey mission never took off, he stayed for almost a year and began to develop his idiosyncratic style while getting to know major figures of Mexican arts and politics such as the painter Ignacio Aguirre, Rivera's former wife (and Weston model) Guadalupe Marin, or African-American writer and activist Langston Hughes.

When he returned to France, he started collaborating with such publications as the Communist daily Ce Soir. Two of his closest friends and colleagues in Paris, David Seymour (born in Poland) and Robert Capa (who had emigrated from Hungary) also ended up publishing important photo essays from Mexico at the end of the decade, as they made their way from Civil War Spain to the United States. Seymour crossed the Atlantic from Sète (France) to Veracruz aboard the S.S. Sinaia with Republican exiles (1939) and his pictures were just as unmistakably "modernist" in style as they were political and newsworthy in content. On assignment for Life, Capa documented Mexican Communism and the 1940 presidential elections. Some sources suggest that the two men, who had met Modotti in Spain, also tried in vain to convince her to pick up the camera again. Yet, as she had written Álvarez Bravo from Moscow, she no longer believed she could practice politics and photography at the same time.

"Mexicanidad" at home and abroad: "Vi en los periódicos de México algunas reproducciones de fotos de [Agustín] Jiménez"

The circulation of photographs, whether in the form of prints to be shown and exhibited, or published in books and the illustrated press, contributed decisively to what has been called in the case of the United States "the enormous vogue of things Mexican" (Delpar). Even in Europe, Modotti's letter informed Álvarez Bravo, some of their colleague Agustin Jimenez's pictures could be found. Although Mexico's cultural influence in the 1920s and 1930s was largely based on monumental art-whether pre-Colombian pyramids or muralism, photography played a decisive role in making these works, as well as Mexico's popular culture, available for publics in Europe and in the United States. Here again, Weston and Modotti must be considered pioneers.

In 1926, Mexican-American writer Anita Brenner embarked Weston, his son Brett and his "assistant" Tina Modotti on a research trip commissioned by the Secretariat of Education. Their mission would result in Idols behind Altars, first published in 1929 in the United States, and which celebrated native Mexican culture and its resistance to colonial power. Although Modotti's actual contribution to the iconography remains disputed, this project quickly lead to others. Mexican Folkways, edited by anthropologist Frances Toor, was published in Mexico, yet it was bilingual and directed at an international readership: it featured reproductions of Rivera and Siqueiros' murals taken by Modotti. Anita Brenner's « acknowledgement », the author of Idols Behind Altars (1929) calls Edward Weston and Tina Modotti "masters of their craft" and writes that they "shared the commission" of providing photographs for the book. Source : Archive.org Remarkably enough, it seems that it was the publication of Modotti's more personal work in the same magazine which prompted a young Manuel Álvarez Bravo to get in touch with her. What is certain is that he took over her position as photographer for Mexican Folkways after she left. A spectacular example of his role as a cultural link between the U.S. and Mexico can be found in an article on muralist painting published by The American Magazine of Art in July 1934. Its author, the classically-trained painter George Biddle, had visited Mexico, worked with Rivera, and This type of commissioned or illustrative work was only one aspect of the photographers' contribution to the cultural renaissance of Mexico. The celebration of native culture that was the heart of a new mexicanidad was also decisive for their own aesthetic pursuits. Both Weston and Modotti were looking for ways to include vernacular and popular Mexican culture in their visual expression of modernity, in contrast with the romanticized vision of Hugo Brehme's pictorialist photobook Mexico Pintoresco, which was so successful in 1923 that it was published in several languages.

Weston, for example, called his work "a fight to avoid [Mexico's] natural picturesqueness." [START_REF]Edward Weston in Mexico[END_REF] Modotti, planning in a 1929 letter to Weston what would become her UNAM exhibit, echoed similar feelings, combined with a sense of duty, not only to her art but also to her adopted nation: "...casi que le debería al país una exposición, no tanto por lo que yo he hecho aquí, sino en especial por lo que aquí se puede hacer, sin recurrir a las iglesias coloniales y a los charros y a las chinas poblanas y a la basura similar que practica la mayoría de los fotógrafos." [START_REF] Modotti | Una mujer sin país. Las cartas a Edward Weston y otros papeles personales[END_REF] Later discussions of the photographic forms of mexicanidad given shape by Álvarez Bravo (and later his ex-wife Lola), Paul Strand, and Henri Cartier-Bresson, have constantly grappled with the issue of authenticity and exoticism, identity and otherness. At the time, Rivera recognized Weston and Modotti as transnational artists. Weston's "extreme modernity" combined "the plasticity of the North and the living tradition of the land of the South," which made him a thoroughly "American" modernist. As for Modotti's "abstract," "intellectual," and "Italian temperament," it "flower[ed] perfectly in Mexico." [START_REF] Rivera | Edward Weston and Tina Moddoti [sic][END_REF] In foreign contexts, visual tropes of revolution and history, religion and folklore, race and class, were conveyed through various prisms. Strand's work was reserved to a small audience in the exclusive fine art photogravure portfolio published in 1940. In the May 10, 1937 issue of Life magazine, an article entitled "An American Photographer Does Propaganda Movie for Mexico" claimed that Strand's images "constitute[d] some of the loveliest photographs that ever came out" of that country, adding that "the virtue of the film as propaganda lies in its simple demonstration that workers must organize." In the dual context of Roosevelt's New Deal and Good Neighbor Policy, Strand's celebration of a strengthened Mexican democracy could thus gain a four-page layout in the United States' most influential magazine.

In France, it was Álvarez Bravo's work that was given remarkable exposure, not so much because he knew Henri Cartier-Bresson as because he met André Breton in 1938. The French writer dubbed Mexico the surrealist country par excellence and presented some of Bravo's photographs in an exhibition entitled Mexique in March 1939. His essay "Souvenir du Mexique" was published with a selection of ten Álvarez Bravo pictures, which were then reintroduced and recontextualized for the Mexican public in a show curated by Breton the following year.

In spite of a few exceptions in Modotti's or Álvarez Bravo's work (his first break came when he won an industrial photography contest in 1925), it was mostly pictures focusing on Indianness, pre-Colombian history and Mexico City's popular culture which appealed to foreign publics. As had been the case with Cartier-Bresson originally, it was the Musée ethnographique du Trocadero which sent Pierre Verger to Mexico in 1936. Both men also knew of each other through the Alliance Photo agency. Two years later, Verger published simultaneously in Paris and Mexico a book of 170 photographs with a preface by ethnographer Jacques Soustelle (Au Mexique, 1938). It was a significant example of the way the Museum had come to consider ethnographic photography both in documentary and aesthetic terms, a visual culture serving both science and a larger public.

Other related examples confirm that history, exoticism and aesthetics were often associated in photographic exploration of Mexico. Laura Gilpin's trip, 1932 in Mexico, produced a series of slides of Yucatan that were shown at American Museum of Natural History, and were later bought by the Library of Congress. As a landscape photographer, she also contributed to the recognition of a Southwestern visual culture, in the shadow of such artists as Weston, photographer Ansel Adams or painter Georgia O'Keefe. An architect from the University of California, Esther Born also spent several months in Mexico taking pictures and collecting postcards of mexicanidad. But her travels and research resulted in a much more original photographic project, in the form of a book on entitled The New Architecture in Mexico (Wm. Morrow & Co., 1937). The striking duality of her work, which looked both at the colonial churches photographed by Strand as well as Juan O'Gorman's functionalist buildings, is indicative of the diversity of Mexico's visual modernism.

"Quiero una Leica": Modernism, "anti-graphic" and documentary photography

Cartier-Bresson's Mexican photographs were first shown through the Department of Public Education in March 1935, in an exhibition shared with Manuel Álvarez Bravo, who became a close friend. The same show was then adapted for the Julien Levy Gallery in New York. Moving across the border, this exposition moved from a public to a private institution, and added photographs by Walker Evans (April-May 1935). With this seminal event, Levy attempted to define a certain photographic style stretching from Mexico to New York and Paris, and called it "documentary and anti-graphic photographs." The term "anti-graphic" was mostly relevant in terms of what these photographers were not doing, and contributed to a vital discussion about the nature of the photographic image which also signaled a transition in modernist photography.

All of the "Mexican" photographers of the time responded to this evolution in their own way. Levy's definition of "anti-graphic" was in fact largely influenced by Cartier-Bresson's approach: he had first used the term in a 1933 solo show of the French photographer's work. He wrote at the time that this quality in was "unanalyzable," except as a "residuum." A photograph could be called "anti-graphic" when of all the usual formal criteria of "good" photography (classic composition, darkroom techniques) had been left aside. Levy offered that the new photographic modernity relied on ambiguity and accident, discarding "consideration and polish" which seemed precisely to define what photographers like Weston, Siqueiros and Strand had long defended as the very definition of modernism (Brooklyn Daily Eagle, 8 Oct. 1933).

It is hard not to link this definition to the technological revolution of the Leica and the 35 mm format which had been adopted by Cartier-Bresson and was becoming the basis for "instantaneous" photography and street reportage. Both Cartier-Bresson and Álvarez Bravo produced a body of work in the street of Mexico City which seemed to be "more dynamic, startling and inimitable," to quote Levy, than Weston and Strand's "straight photography," which implied pre-visualization, large format negatives and high quality contact prints. Cartier-Bresson had already published photographs taken in Spain in the illustrated weekly Vu. His mastery of the small-format Leica camera made his work magazine-ready, whereas the most spectacular outcome of Strand's work was the luxurious portfolio Photographs of Mexico (New York, Virginia Stevens, 1940) which was anything but "anti-graphic."

Levy's label is useful not as a reliable aesthetic category, but rather as an entry point in a discussion that had originated long before him. In 1928, already, a review by Catalan writer Marti Casanovas had defended Modotti's work claiming that her ability to create "pure" artistic photography was in no way detrimental to its social content and propaganda value ("Las fotos de Tina Modotti: el anecdotismo revolucionario," ¡30-30!, 1 July 1928, p. 4). Her work was hailed as both graphic and documentary, making it in effect "transcendental" and political. For the 1935 exhibit, Levy reintroduced "documentary" as a companion term to "anti-graphic," thus signaling the continuation of a debate over what could be broadly-termed photographic "realism" as an artistic and political category. A few years later, in La querelle du réalisme (Editions sociales internationales, 1936), French poet Louis Aragon-himself influenced by the official Soviet doctrine of socialist realism-singled out Cartier-Bresson's Mexican work as a model of politically significant work grounded in authenticity. Increasingly, the modernity of Álvarez Bravo and Cartier-Bresson's "street" photography would be legitimized by expositions in institutions such as Mexico's Sociedad de Arte Moderno (Álvarez-Bravo, 1945) and the Museum of Modern Art in New York (Cartier-Bresson, 1947).

Around 1930, Weston gave his version of the modernist ethos which had earned him a cover of the Irradiador published by the Estridentista group: his photograph of an Ohio factory was textbook industrial modernism, the kind of photograph Charles Sheeler made in Detroit and Álvarez Bravo for the Totelca competition that had launched his career. Siqueiros had celebrated Modotti and Weston's achievement as the "purest photographic expression" of modernity. In 1931, as he was about to publish his aptly titled The Art of Edward Weston, the US photographer still defended the role of the artist's all-powerful vision as the creative force which made the most of the medium's specificity. Realism, to Weston, was not social and even less socialist although he admired his Mexican friends' political commitment. In a short declaration for Experimental Cinema, he conceded that lightweight cameras were able to "stop a bullet's flight." Yet the "objective, the physical facts of things" (a remarkably redundant phrase) were merely means to express "the primal, subjective motive" or the representation of an "abstract idea." [START_REF]Statement[END_REF] His role as an artist was to convey his vision through the camera in purely photographic terms, never to reproduce an elusive "reality." Edward Weston, « Statement, » Experimental Cinema (3 February 1931): 14-15. Source : Archive.org Weston's defense of modernist photography as a celebration of photographic "purity" is a well-known chapter in the history of the United States' "straight photography" movement, stretching from Paul Strand's portfolio in Camera Work's last issue (June 1917) to the F:64 Manifesto signed by a group of California photographers in November 1932 (a group Weston belonged to). Evidently, it must also be understood as part of a transnational modernist movement which compelled most artists of the time to define their own aesthetic and political positions (Álvarez Bravo submitted a couple of prints to Weston, who was in charge of the American selection for Stuttgart's 1926 Film und Foto exhibit). This wider context does not change the fact that Weston's Mexican experience constituted a turning point for him. In his Daybooks, he recorded how his visit at the Museo Nacional helped him refine his approach, midway between the "purely scientific and commercial uses" of photography and the "conventionalized nature, superb forms [and] decorative motives" of painting.

"That the approach to photography must be through another avenue, that the camera should be used for recording life, for rendering the very substance and quintessence of the thing itself, whether it be polished steel or palpitating flesh [...] I feel definite in my belief that the approach to photography is through realism -and its most difficult approach." [START_REF] Weston | The daybooks of Edward Weston[END_REF] By and large, in spite of Experimental Cinema's insistence that Weston's work sent "a permanent message to future proletarian technicians ... against the bourgeois 'technique.'" (Experimental, p. 13), Weston's definition of "realism" failed to include a social and political dimension that Modotti already found crucial. In her only published statement about photography, in 1929, she did celebrate "honest photographs, without distortions or manipulations" in terms reminiscent of Weston's, but she also used the word "documentary," which her former partner was always reluctant to use: "Photography, precisely because it can only be produced in the present and because it is based on what exists objectively before the camera, takes its place as the most satisfactory medium for registering objective life in all its aspects, and from this comes its documental value. If to this is added sensibility and understanding and, above all, a clear orientation as to the place it should have in the field of historical development, I believe that the result is something worthy of a place in social production, to which we should all contribute." 10 Documents play a role in "social production" and "historical development," and the transformation of a socially-determined visual culture emerges in Modotti's letter to Álvarez Bravo as affected by technological changes, various media markets and national cultures, but also the way photographic practices evolve. As a cultural and political worker, she tells her friend that she must change her tool (her "work companion") to keep her relevance: "Ya no quiero ni tengo ganas de usar más la Graflex. Quiero una Leica... Tan pronto se venda la Graflex compro una Leica y entonces me será más fácil hacer algo. De todos modos me sorprendió a mí misma la indiferencia con la cual vi alejarse mi pobre y fiel compañera de trabajo."

Once again, these lines are easy to understand in retrospect as telling signs of a growing disenchantment with photography. Yet her decision to sell her Graflex is mostly a response to the evolution of photographic culture that she experienced first-hand in Germany. When she first arrived in Berlin in May 1930, she was initially surprised to find that she couldn't find film and paper for the camera format of her Graflex. She also quickly realized that her "propaganda pictures" would not be in demand since most people had cameras and "worker-photographers" (a movement initiated by the Arbeiter Illustrierte Zeitung in 1926) were already doing this kind of work more efficiently than she ever could, since they documented their own lives. What's more, she considered that photojournalism was "a man's job" and that she was not "aggressive" enough to pursue this new form of photographic work (Letter to Weston, 23 may 1930).

All of these circumstances certainly contributed to her "indifference" to the separation with her "faithful workmate," but also to the realization that the new visual world brought about by the availability of small format cameras and the illustrated press had profoundly transformed the way photography could do social and political work. In Levy's gallery as well as the radical press, documentary and photojournalism seemed to be the new forms of committed photographic expression, and Modotti sensed it much more acutely than her teacher Weston. Photojournalism and documentary were increasingly shaping "the field of historical development" to which she had so eagerly contributed.

"yo también pienso lo que usted llama 'arbol genealógico'": photography in Mexico after Weston and Modotti

Modotti had already left one of her cameras to Álvarez Bravo when she left Mexico, a symbolic transmission which was also an invitation to pursue her work. This is the last "photographic topic" she mentions in her 1931 letter: "yo también pienso lo que usted llama 'arbol genealógico'. Me causo mucho gusto oir que usted sigue usando la cámara." These remarks point to the artistic and technical legacy she left to the Mexican photographers who were to follow. Although it is fair to say that photography had been a medium of choice for discourses of Mexican identity since the Porfiriato, and even more decisively during the revolution, Modotti and Weston's work marked the beginning of an influential chapter in the history of the medium, which affected both the national culture and definitions of modernism abroad-a modernity which combined a purely "photographic" vision with narrative and politics, imagination and folk culture, rural communities and the "elemental expression" of the "primitive race". [START_REF] Daybooks | Bibliography See on Zotero Agostinis, Valentina. Vita, arte, e rivoluzione: lettere a Edward Weston[END_REF] In Mexico, Weston became both a modernist and a Californian photographer, far removed from the New York scene. Tina Modotti did work she could never replicate elsewhere. Strand redefined his political vision in a fishermen's community, away from the urban monuments of his earlier film, Manhatta (1921). Cartier-Bresson invented a peculiar style of street photography as a brand of social realism tinted with a surrealist's wit.

Mexico's "errant modernism" (to use Esther Gabara's term) reconfigured the colonial discourse of modernity, both in its European and nationalist forms, to question perception and truth, by mixing categories of "avant-garde" and popular, realism and fiction, and by constantly revisiting history and myth. It is often said that Álvarez Bravo "reanimated" Weston's abstractions, reintroducing narratives and fiction in documentary fragments. Agustin Jimenez, placed by Modotti's letter on the "family tree" of Mexican modernism, exhibited his work at the Galeria Moderna in 1931, and contributed photo essays to various Mexican publications, including Mexican Folkways. After having presumably worked with Sergueï Eisenstein (himself an admirer of Weston's work) on the filming of ¡Que viva México! (1930), he became one of Luis Buñuel's collaborators in the 1950's. At least one other name should be mentioned as belonging to Modotti's family tree: Álvarez Bravo's wife, Lola. Originally her husband's assistant, she started working independently after their divorce. She became chief photographer for El Maestro Rural (Department of Public Education) and then at the Instituto Nacional de Bellas Artes y Literatura. In 1943, her photographs of murals by Orozco and Alfredo Zalce were included in the Mexican Art Today exhibit at Philadelphia's Museum of Art. She would also photograph Pierre Verger in the 1950's.

It was also in 1943 that the MoMA opened Mexico: 8 Photographers. Predictably, pictures by Edward Weston, Paul Strand and Manuel Álvarez Bravo were shown. A representative of the younger generation, Antonio Reynoso (who seems to have been one of Álvarez Bravo's students), was also included. The other four photographers in the show had less sustained connections with Mexico, except perhaps Anton Bruehl, who already had established a solid reputation with his studio and magazine work: his 1932 Mexican journey was financed by Orozco's U.S. publicist Alma Reed, who published the visually stunning Photographs of Mexico the following year. Born in Germany, Fritz Henle was a successful photojournalist who still worked with largeformat cameras and published numerous photo essays and books on various countries, including Mexico.

The last two photographers were more directly "political" in their approach, albeit in profoundly different ways: Bradley Smith, like Henle, published his work in masscirculation magazines. In its May 1944 "Picture of the Month" section, Popular Photography titled one of his photographs "Pan-American Youngsters": it showed a crowd receiving U.S. healthcare supplies in Mexico City. This photograph was actually part of the MoMA's exhibit, underlining the role of the Museum in the nation's cultural diplomacy. As for Helen Levitt, she was a left-wing street photographer connected to the Photo League, who acknowledged Cartier-Bresson's influence. Yet her work in Mexico was somewhat of an anomaly: it was the only foreign country she ever photographed.

Popular Photography, May 1944, p. 29. Source : Google Books Mexico: 8 Photographers did not leave an outstanding trace in the history of photography. Yet it testified to the MoMA's continued interest in visual culture from and about Mexico, at a time when the museum was making sustained efforts to legitimize photography as a fine art, while serving the United States' cultural and diplomatic relations with its Southern neighbor. Ironically, of course, Modotti's fame had faded by that time and she was not included in the show. She had died a year earlier in Mexico City.

  was influential in creating Franklin Roosevelt's public art programs. His essay celebrated the possibility of a new American art, and included two reproductions of murals by Pablo O'Higgins and Julio Castellanos photographed by Álvarez Bravo. This book-form edition of Mexican Folkways (1947), published in New York, included 25 photographs credited to Lola Alvarez Bravo, 2 to Fritz Henle, and 1 to Manuel Alvarez Bravo Source : Archive.org Along with Brenner and Toor, a third woman played an important role in the cultural exchanges between Mexico and the United States: journalist Alma Reed, whose articles in the New York Times had forced Harvard's Peabody Museum to return stolen Mayan artefacts to Mexico, put up an exhibit of Weston's photograph in her newly created Delphic Studio in October 1930. Her 1932 book on Jose Clemente Orozco, published in New York was the first ever published on the muralist: it was also illustrated by both Weston and Modotti, who had helped the painter advertise his mural work in the United States by sending prints as early as 1929.

. His essays on photography and photojournalism across the Americas have been published in various international journals including Visual Studies, and Etudes Photographiques. He co-edited Refaire l'Amérique in 2011 and L'Amérique des Images in 2013. He is a member of Transatlantic Cultures' editorial team.


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Yann Kerr

Passive microwave soil moisture downscaling using evaporative fraction

Abstract-This paper develops two different approaches for downscaling (disaggregation) passive microwave derived soil moisture from coarse resolution L-band observations. Ground and airborne data collected over Walnut Gulch experimental watershed during the Monsoon '90 experiment were used to test these approaches. These data consisted of eight micrometeorological stations (METFLUX) and six flights of the L-band Push Broom Microwave Radiometer (PBMR). For each PBMR flight, the 180m resolution L-band pixels covering the eight METFLUX sites were first aggregated to generate a 500m "coarse-scale" passive microwave pixel. The coarse-scale derived soil moisture is then downscaled to 180m resolution using two different surface soil moisture indices (SMIs): i) the evaporative fraction (EF), which is the ratio of the evapotranspiration to the total energy available at the surface; and ii) the actual evaporative fraction (AEF), defined as the ratio of the actual to potential evapotranspiration. It is well known that both SMIs depend on surface soil moisture. However, they are also influenced by other factors such as vegetation cover, soil type, and atmospheric conditions. In order to decouple the influence of soil moisture from the other factors, a land surface model was used in conjunction with a projection technique to account for the heterogeneity of vegetation cover, soil type, and atmospheric conditions. The projection increased the coefficient correlation between SMIs and surface soil moisture from 0.66 to 0.79 for EF and from 0.71 to 0.81 for AEF. When comparing the downscaled soil moisture obtained with EF and AEF, it is apparent that AEF is more directly linked to surface soil moisture, especially for high soil moisture values. Those results illustrate the potential use of high-resolution satellite-based estimates of instantaneous evapotranspiration acquired on clear-sky days for downscaling coarse-resolution passive microwave soil moisture. Index Terms-Downscaling, passive microwave, surface soil moisture, evapotranspiration, evaporative fraction.

I. INTRODUCTION

Soil moisture is one state variable that controls several earth surface related processes, including hydrology, meteorology, climate modelling, and agricultural management. It controls the portioning of rainfall into infiltration and runoff, and thus influences strongly the response of stream discharge to rainfall events, and playing a key role in the prediction of erosion and sediment loads in watershed streams and ponds [START_REF] Western | The Tarrawarra project: high resolution spatial measurement, modelling and analysis of soil moisture and hydrologic response[END_REF]. Soil moisture also controls the partitioning of available energy at the surface into sensible and latent heat fluxes, which triggers development of the atmospheric boundary layer. Soil moisture is also a key variable for sustainable management of irrigation water, which consumes about 85% of total available surface water in arid and semi-arid regions [START_REF] Chehbouni | The use of remotely sensed data for integrated hydrological modeling in arid and semi-arid regions: the SUDMED program[END_REF]. In moisture limited regions, soil moisture content has also been used as an indicator of the spatial distribution of precipitation and general plant health [START_REF] Perry | Analysis and estimation of soil moisture at the catchment scale using EOFs[END_REF].

The spatial and temporal dynamics of soil moisture is very complex since it depends on several factors. Beside the weather conditions, it also depends on a variety of surface features such as land cover/land use, topography and geology. One feasible way of monitoring this variability is through a dense network of continuous observation of soil moisture. This can be done for a confined experimental area during short periods but the establishment of a continuous in situ soil moisture monitoring program worldwide is not practical and economically feasible.

Consequently, the only possibility for deriving the spatially distributed soil moisture data required for the applications mentioned above, is through use of satellite observations. Satellite-based soil moisture can be obtained from passive microwave or active microwave sensors through to the large contrast between the dielectric properties of liquid water (≈ 80) and those of dry soil (≈ 4), and the resulting variability on dielectric properties of soil-water mixtures as they go from dry to wet (≈ 4 -30).

Active microwave sensors such as the European satellites ERS-1/2 C-band Synthetic Aperture Radar (SAR), ENVISAT C-band Advanced SAR, and the Canadian C-band RADARSAT-1/2 can provide resolutions from 10 to 100 m over a swath width of 50-500 km, which meet the spatial requirement for most basin-scale hydrological applications [START_REF] Moran | Estimating soil moisture at the watershed scale with satellite-based radar and land surface models[END_REF]. However, despite the multitude of models developed during the last decade to infer soil moisture from the backscatter coefficient, the issue of decoupling the effect of surface roughness and vegetation biomass from that of soil moisture is still unanswered. To date, no operational algorithm exists to invert soil moisture from SAR data -at least-with the existing space borne sensors [START_REF] Wagner | Operational readiness of microwave remote sensing of soil moisture for hydrologic applications[END_REF].

Passive microwave sensors represent an interesting alternative for monitoring soil moisture [START_REF] Njoku | Soil moisture retrieval from AMSR-E[END_REF]. The ability of passive microwave space-borne sensors such as the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave Imager (SSMI) and the Advanced Microwave Scanning Radiometer (AMSR-E) to provide a daily global coverage makes them very useful for several earth science disciplines. Airborne sensors which are operating at low frequencies (L-Band), such as the Push-Broom Microwave Radiometer (PBMR) and the Electronically Scanned Thinned Array Radiometer (ESTAR) have been found very effective for inferring surface soil moisture [START_REF] Schmugge | Applications of passive microwave observations of surface soil moisture[END_REF]. In this context, the Soil Moisture and Ocean Salinity (SMOS) mission [START_REF] Kerr | Soil moisture retrieval from space: The Soil Moisture and Ocean Salinity (SMOS) mission[END_REF] operating at L-band is scheduled for launch by European Space Agency (ESA) in mid 2008. This instrument is based on an innovative two-dimensional aperture synthesis concept, which will bring new and significant capabilities in terms of multi-angular viewing configurations and will allow for simultaneously retrieving soil moisture and vegetation biomass [START_REF] Wigneron | Retrieving near surface soil moisture from microwave radiometric observations: current status and future plans[END_REF][START_REF] Saleh | Estimates of surface soil moisture under grass covers using L-band radiometry[END_REF] with a revisit time ranging from 1 to 3 days and a mean ground resolution (pixel size) of about 40km. This instrument will provide a valuable global data set of soil moisture content to be implemented in general circulation and climate models. However, the use of coarse spatial resolution of instrument such as SMOS in the field of hydrology is not straightforward [START_REF] Entekhabi | An agenda for land surface hydrology research and a call for the Second International Hydrological Decade[END_REF]. Indeed, the scale at which most hydrological processes are better observed and modelled is less than 1km [START_REF] Merlin | Assimilation of disaggregated microwave soil moisture into a hydrologic model using coarse-scale meteorological data[END_REF]. It is thus of crucial importance to develop simple and robust procedures to disaggregate passive microwave-based soil moisture from its nominal scale to that needed for hydrologic application and/or watershed management.

In this context, several disaggregation approaches with different degrees of complexity have been developed during the last decade. Without going into a comprehensive review of all existing methods which is beyond the scope of this paper, they can be categorised into three groups: i) methods based on the use of topography and soil depth information [START_REF] Pellenq | A disaggregation scheme for soil moisture based on topography and soil depth[END_REF];

ii) methods based on the combination of passive microwave data with high spatial resolution active microwave data [START_REF] Bindlish | Subpixel variability of remotely sensed soil moisture: An inter-comparison study of SAR and ESTAR[END_REF] or optical data such as surface temperature and vegetation index [START_REF] Chauhan | Spaceborne soil moisture estimation at high resolution: A microwave-optical/IR synergistic approach[END_REF]; and iii) methods based on the combination of coarse resolution passive microwave data, with fine-scale optical data and a surface process model [START_REF] Merlin | A combined modeling and multi-spectral/multi-resolution remote sensing approach for disaggregation of surface soil moisture: Application to SMOS configuration[END_REF][START_REF] Merlin | Assimilation of disaggregated microwave soil moisture into a hydrologic model using coarse-scale meteorological data[END_REF]. This paper follows from that of [START_REF] Merlin | A combined modeling and multi-spectral/multi-resolution remote sensing approach for disaggregation of surface soil moisture: Application to SMOS configuration[END_REF], but with two fundamental differences. The two differences are: i) there is no need to have dual angle observations of surface temperature; and ii) a simple energy balance model can be used in place of a complex surface process model. Two different energy balance approaches are developed and tested for downscaling (disaggregating) the coarse resolution passive microwave derived soil moisture that can be retrieved from L-band radiometry. These two approaches are based on two different soil moisture indices (SMIs): i) the evaporative fraction (EF), which is the ratio of the evapotranspiration to the total energy available at the surface; and ii) the actual evaporative fraction (AEF), which is computed as the ratio of the actual to potential evapotranspiration. The hypothesis is that these indices, which can be computed at fine spatial resolution, can provide information on the fine scale distribution of surface soil moisture. The projection technique developed in [START_REF] Merlin | A combined modeling and multi-spectral/multi-resolution remote sensing approach for disaggregation of surface soil moisture: Application to SMOS configuration[END_REF] is then implemented with a surface energy balance model to decouple the effects of external factors (i.e. land cover, soil properties, meteorological forcing) on the relationships between SMIs and surface soil moisture.

Ground and airborne data collected over the Walnut Gulch experimental watershed during the Monsoon'90 experiment were used to test the performance of these two approaches. These data consisted of eight micro-meteorological stations (METFLUX) and six flights of the L-band PBMR. For each PBMR flight, the 180m resolution L-band pixels covering the eight METFLUX sites were first aggregated to generate a 500m resolution "coarse-scale" passive microwave pixel. The coarse resolution derived soil moisture is then downscaled using the two approaches outlined above, and evaluated against the ground-based data. The applicability of such downscaling methods to SMOS scale is finally discussed in a separate section.

II. MONSOON'90 DATA

The Monsoon '90 experiment was conducted during the summer of 1990 over the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona, USA [START_REF] Kustas | An interdisciplinary field study of the energy and water fluxes in the atmosphere-biosphere system over semiarid rangelands: Description of some preliminary results[END_REF][START_REF] Kustas | Monsoon ' 90 multidisciplinary experiment[END_REF].

The purpose of the experiment was to remotely sense moisture fluxes in a semiarid climate during a dry-down. A network of eight meteorological surface energy flux (METFLUX) stations covering the main study area (about 150 km 2 ) were situated in grass-dominated and shrubdominated ecosystems and in the transition zones containing both vegetation types. The data collected at each METFLUX site from Julian day (Jd) 204 to Jd 222 consist of 20 minute estimates of: 0-5cm soil moisture, meteorological conditions at screen height including air temperature, relative humidity, wind speed and solar radiation, surface fluxes composed of net radiation, soil heat flux measured at -5 cm, sensible heat flux and latent heat flux.

As part of the Monsoon'90 campaign, the NASA PBMR was flown on six flights of the C-130 aircraft during a 10-day period in July and August of 1990 [START_REF] Schmugge | Push broom microwave radiometer observations of surface soil moisture in Monsoon ' 90[END_REF]. The objective was to map the surface brightness temperature at a wavelength of 21-cm (L band) and to infer surface soil moisture from these data. The 4 beams of PBMR point at ± 8º and ± 24º incidence angle with a 3dB beam width of about 30% of the altitude. For Monsoon '90 the PBMR flights were at an altitude of 600 m, which yielded an IFOV of 180m. Available PBMR data of the Monsoon '90 experiment are nadir H-polarized brightness temperatures. To create the images of the brightness temperature at nadir, the outer beams were corrected for incidence angle effects during each PBMR flight by multiplying them by the ratio of the average of the inner beam to the outer beam on each side [START_REF] Schmugge | Push broom microwave radiometer observations of surface soil moisture in Monsoon ' 90[END_REF].

In this study, a time series of six 500m resolution microwave pixels was generated by aggregating the eight closest 180m resolution PBMR pixels covering each of the eight METFLUX stations on each day of PBMR observations. The low resolution soil moisture is retrieved by using the linear regression of PBMR brightness temperature versus ground-based 0-5cm soil moisture derived by [START_REF] Schmugge | Push broom microwave radiometer observations of surface soil moisture in Monsoon ' 90[END_REF]. These coarse resolution soil moisture values are subsequently downscaled, using ground measurements of evapotranspiration. A discussion about the operational application of such methods to SMOS and optical sensors is provided.

III. METHOD

The surface soil moisture retrieved from generated coarse-scale microwave pixels is downscaled by using two fine-scale SMIs at the eight METFLUX sites: i) the evaporative fraction (EF) and ii) the actual evaporative fraction (AEF). The downscaling approaches are based on a linear relationship between surface soil moisture and the SMI. To decouple the effect of other factors on this relationship, the projection technique of [START_REF] Merlin | A combined modeling and multi-spectral/multi-resolution remote sensing approach for disaggregation of surface soil moisture: Application to SMOS configuration[END_REF] is used in conjunction with a surface energy balance model and surface properties at high resolution. The diagram of Figure 1 illustrates the different steps and parameters involved in the downscaling procedure.

The general idea behind the approach is to assume a linear relationship between a SMI and surface soil moisture, and scale this relationship at high resolution with the observed low resolution soil moisture, and the slope of the correlation between the SMI and surface soil moisture. The slope (noted 1 f ) of the correlation between the SMI and surface soil moisture is therefore assumed to be invariant across scales i.e. 1 f is constant in space within the microwave pixel. In the paper, parameter 1 f is also assumed to be constant in time due to the constancy of EF/AEF during most daytime. This allows calibrating it during a training period, independently from the application data set.

A. General approach

Assuming a linear relationship between surface soil moisture and the SMI, the downscaled soil moisture values H W (subscript H for high resolution = 180m) can be expressed as: the slope of the correlation between surface soil moisture and the SMI.

( ) > < - + = obs H obs H L obs L H SMI SMI f W W , , , 1 , (1)
In [START_REF] Merlin | A combined modeling and multi-spectral/multi-resolution remote sensing approach for disaggregation of surface soil moisture: Application to SMOS configuration[END_REF], the SMI was the soil temperature inverted from dual angle radiative surface temperature and the slope 1 f was retrieved from SMOS observation by using the multi-angular, bi-polarized information of surface soil emission. As this information may be difficult to extract due to vegetation effects on SMOS observation, this papers tests other SMIs for which the 1 f parameter does not vary much in time and/or can be estimated indirectly from a different source of data. This is the rationale for choosing EF and AEF as SMIs. Both ratios are in general nearly constant during the daytime [START_REF] Shutlleworth | FIFE: The variation in energy partition at surface flux sites[END_REF][START_REF] Sugita | Daily evaporation over a region from lower boundarylayer profiles measured with radiosondes[END_REF][START_REF] Crago | Conservation and variability of the evaporative fraction during the daytime[END_REF][START_REF] Crago | Daytime evaporation and the self-preservation of the evaporative fraction and the Bowen ratio[END_REF][START_REF] Gentine | Analysis of diurnal evaporative fraction behavior[END_REF]. Moreover, they are more directly related to surface moisture condition [START_REF] Kustas | Relationships between evaporative fraction and remotely sensed vegetation index and microwave brightness temperature for semiarid rangelands[END_REF] and less dependent on incoming radiation than evapotranspiration or surface temperature [START_REF] Nishida | An operational remote sensing algorithm of land surface evaporation[END_REF]. In the paper, parameter 1 f is therefore assumed to be constant in time. As it is also constant in space within the microwave pixel (hypothesis of linearity of the correlation between SMI and surface soil moisture), the slope 1 f is assumed to be a constant. In the paper, it is calibrated using ground-based data during a training period.

B. Soil moisture indices

In the paper, EF and AEF are both calculated from the surface fluxes and meteorological data monitored at the eight METFLUX sites.

The observed EF is calculated as: ( ) As an illustration of the dependence of EF to surface soil moisture, Figure 2 shows the time series of surface soil moisture and evaporative fraction measured at the METFLUX stations.

        + + ∆         - + - ∆ =
Both the maximum and the minimum values measured at the eight METFLUX stations are plotted to illustrate the range of spatial variability observed within the coarse-scale microwave pixel. The difference between the maximum and minimum surface soil moisture varies from approximately 10 to 20 % vol. from Jd 212 to Jd 221. It is apparent that dynamics of EF plotted between 10am and 2pm is link to the dynamics of surface soil moisture observed in the period Jd 212 and Jd 221. The difference between the maximum and minimum values of EF varies between 0.2 and 0.5. Note that the values of EF greater than 1 are due to the presence of clouds, which make the energy available suddenly decrease while the surface is still evaporating.

The results of a synthetic study are presented in Figure 3. The surface energy balance model of [START_REF] Norman | Source approach for estimating soil and vegetation energy uxes in observations of directional radiometric surface temperature[END_REF] is used to simulate the variation of both ratios EF ann AEF in response to surface soil moisture ranging from 0 to 35% vol. and for different LAI values from 0.5 to 4. Atmospheric forcing is fixed (air temperature Ta = 20 ºC; incoming radiation Rg = 900 Wm -2 ; relative humidity qa = 50%; wind speed u = 3 ms -1 ) and surface parameters are set as in [START_REF] Norman | Source approach for estimating soil and vegetation energy uxes in observations of directional radiometric surface temperature[END_REF]. The correlation between SMI and surface soil moisture appears to be non-linear above 20% vol., above which SMI tends to saturate. The use of EF and AEF as soil moisture index is therefore limited to soil moisture values below this threshold. The synthetic study also shows that the sensitivity of the SMI to surface soil moisture decreases with increasing LAI values.

C. Projection

The general idea behind the projection is to improve the correlation between the SMI and surface soil moisture at high resolution before applying the downscaling procedure of equation [START_REF] Western | The Tarrawarra project: high resolution spatial measurement, modelling and analysis of soil moisture and hydrologic response[END_REF]. The methodology used in the paper is the projection technique developed in [START_REF] Merlin | A combined modeling and multi-spectral/multi-resolution remote sensing approach for disaggregation of surface soil moisture: Application to SMOS configuration[END_REF]. It consists of using a land surface model to simulate the impact at high resolution of surface parameters such as vegetation cover, soil type and atmospheric conditions on the correlation between the SMI and surface soil moisture. In this paper, a simple surface energy balance model is used to simulate EF/AEF at high-resolution given: i) surface parameters available at high resolution and, ii) the same set of surface parameters estimated at low resolution. The projected EF/AEF noted obs H SMI , is written as the observed SMI subtracted by the difference of the two simulated SMIs:

( ) ( ) [ ] H H sim H H H sim H obs H obs H p W SMI p W SMI SMI SMI > < - - = , , , , , , (5) with
( )

H H sim H p W SMI , ,
(and subscript sim for simulated) being the SMI simulated using the surface parameters available at high resolution and

( )

H H sim H p W SMI > < , ,
the SMI simulated using the surface parameters averaged at the microwave resolution. The projected SMI is therefore a combination of the observed SMI and the SMI simulated by the surface energy model using fine-and integrated-scale surface parameters. Note that the projection does not require all the surface parameters involved in the surface energy budget (input of the model) to be available at high-resolution. If one parameter is available at high-resolution (vegetation cover for instance), the projection can be applied with respect to this parameter only.

By replacing the observed SMI by the projected SMI, the expression of the downscaled soil moisture of equation ( 1) is modified as follows:

( ) > < - + = obs H obs H L obs L H SMI SMI f W W , , , 1 , (6)
Note that the application of equation ( 5) requires iterating on soil moisture values, as H W is not known at the beginning of the procedure. In fact, the algorithm runs a loop on integer k with:

obs L k H W W , , = (7)
for k = 0 (initialization) and:

( ) > < - + = - - ) ( ) ( 1 , 1 , , 1 , , k H k H L obs L k H W SMI W SMI f W W (8)
for k > 0. Convergence of H W is typically reached after two or three iterations on k. Note that H W is the only parameter to vary in equation (8). In this paper, parameter 1 f is assumed to be constant in time and space (within the microwave pixel).

D. Land surface energy balance model

The energy balance model used for the application to the Monsoon '90 data is the N95 model developed by [START_REF] Norman | Source approach for estimating soil and vegetation energy uxes in observations of directional radiometric surface temperature[END_REF], revised by [START_REF] Kustas | Combining optical and microwave remote sensing for mapping energy fluxes in a semiarid watershed[END_REF] and further improved by [START_REF] Kustas | Evaluation of soil and vegetation heat flux predictions using a simple two-source model with radiometric temperatures for partial canopy cover[END_REF]. Briefly, it is a dual-source model treating the energy balance of the soil/substrate and vegetation using surface skin temperature observations at the zenith view angle [START_REF] Norman | Source approach for estimating soil and vegetation energy uxes in observations of directional radiometric surface temperature[END_REF] and remotely sensed images of nearsurface soil moisture [START_REF] Kustas | Combining optical and microwave remote sensing for mapping energy fluxes in a semiarid watershed[END_REF] for estimating the soil energy balance over the watershed of the Monsoon '90 experiment. In this study, the model revised by [START_REF] Kustas | Combining optical and microwave remote sensing for mapping energy fluxes in a semiarid watershed[END_REF] is preferred because the heterogeneity of the 0-5 cm soil moisture is accounted for in the estimation of surface fluxes. The model formulation explicitly computes the soil evaporation as a function of the resistance of the top soil layer to water vapor transfer. The resistance of surface soil layer r ss is parameterized with near-surface soil moisture [START_REF] Sellers | Relations between surface conductance and spectral vegetation indices at intermediate (100 m2 to 15 km2) length scales[END_REF]:

) exp( sat ss W W B A r - = (9)
with A and B being two calibration parameters and sat W the soil moisture at saturation (35% vol. for the Walnuch Gulch site). Total net radiation Rn is partitioned into Rns and Rnc as in [START_REF] Norman | Source approach for estimating soil and vegetation energy uxes in observations of directional radiometric surface temperature[END_REF]:

[ ]    - - = - = ) exp( 1 ) exp( LAI Rn Rnc LAI Rn Rns κ κ ( 10
)
with κ is estimated to be 0.45 for high solar zenith angles.

The simulated SMI involved in equation ( 5) can be expressed as: I. The space varying surface parameters are composed of A, B, LAI, canopy height, air temperature Ta, relative humidity qa, solar radiation Rg and wind speed u. The other input parameters in p are fixed to uniform values as in [START_REF] Kustas | Evaluation of soil and vegetation heat flux predictions using a simple two-source model with radiometric temperatures for partial canopy cover[END_REF].

( ) ( ) ( ) ( ) p W G p W Rn p W LE p W EF

IV. APPLICATION

Application of the downscaling approaches presented here include successively i) the projection of EF and AEF, ii) the estimation of the slope 1 f and iii) downscaling the coarseresolution soil moisture retrieved from the six generated microwave pixels. The approach is demonstrated using Monsoon'90 data and the downscaled results compared with ground-based measurements.

The projection technique is applied to the data set between Jd 212 and Jd 221. The projected EF and projected AEF are calculated using equation [START_REF] Njoku | Soil moisture retrieval from AMSR-E[END_REF], averaged between 10am and 2pm and compared to the 10am-2pm average of the measured EF and AEF from METFLUX stations.

Results are presented in Figure 4, where it is apparent that the correlation between SMI and W is improved by the projection in both cases; the correlation coefficient is increased from 0.66 to 0.79 and from 0.71 to 0.81 for EF and AEF respectively. The projection is therefore a useful tool to decouple the effect of other variables on the correlation between SMI and W, given that a surface energy model can be properly calibrated over heterogeneous areas, notably for different vegetation covers and soil properties.

The parameter 1 f in equation ( 5) is then calibrated by minimizing the root mean square difference between the soil moisture simulated with equation ( 5) and observations from Jd 206 to 211. The mean and standard deviation of 1 f is evaluated to 56 and 10, and 47 and 7 for EF and AEF respectively ( 1 f is in % vol.). In our analysis, parameter 1 f is assumed to be a constant in time. The same mean value obtained between Jd 206 and 211 is therefore used in the application during PBMR flights. Note that the time period for the calibration of 1 f (Jd 206 to Jd 211) is different from the time period of the application (Jd 212 to 221). This allows testing of the assumption of that 1 f can be held constant, using an independent data set.

The soil moisture retrieved at coarse resolution from the six generated microwave pixels is then disaggregated with equation (5) using the projected SMIs and the value of the slope 1 f estimated previously. Downscaled soil moisture is plotted against observations in Figure 5 for both the EF and AEF approaches. Table II reports the mean root mean square error (RMSE)

between the downscaled and measured soil moisture values for each of the six days of data.

When using all parameters at high-resolution (meteorological data, soil and vegetation parameters), the average of the RMSE is about 2% (vol. per vol.) and 3% for AEF and EF respectively. On Jd 214 however, the disaggregation error is greater than 5% vol. in both cases.

The poor correlation with ground observations on Jd 214 can be explained by (i) the great variations of f_1 with the presence of clouds (see time series of EF in Fig. 2) and (ii) the saturation of SMI for soil moisture values above 20% vol (see synthetic study in Fig. 3).

The comparison of downscaling results with EF and AEF shows that the AEF-based approach is more accurate in most cases. In Table II, the error on downscaled values is in general lower (except on Jd 216) with the use of AEF. As the N95 model was calibrated against EF observations, and not against AEF observations, results with the EF approach were expected to be superior. While the projection technique improves the correlation between EF and W, as compared to AEF and W, the correlation between the measured AEF and surface soil moisture is simply higher. The stronger link between AEF and surface soil moisture can be explained by several factors.

First, AEF is intrinsically more directly linked to surface moisture status than EF, as AEF is defined relatively to a "wet-surface", whereas EF is defined relative to a surface at the "thermal equilibrium". In the case of bare soil in particular, various analyses have shown that AEF can be expressed as a function of solely near-surface soil moisture alone [START_REF] Saxton | Estimating generalized soil-water characteristics from texture[END_REF][START_REF] Komatsu | Toward a robust phenomenological expression of evaporative efficiency for unsaturated soil surfaces[END_REF]. For vegetated surfaces, AEF is also dependent on vegetation characteristics and water potential in the root zone. However, one can argue that the normalization of AEF at saturation (AEF=1) makes this SMI more applicable to different vegetation covers in this range of soil moisture. Figure 2 illustrates how AEF scales with surface soil moisture, which is less dependent on LAI for high soil moisture values than EF.

The second explanation for a stronger link with AEF and soil moisture is that the diurnal variability of EF may limit the validity of the " 1 f constant" hypothesis in this case. The diurnal behaviour of EF depends on both surface conditions and atmospheric loading. Notably atmospheric demand for evapotranspiration is governed by solar radiation, relative humidity and, to a lesser extent, air temperature and wind speed; while surface control is exerted by soil moisture and vegetation condition. This issue has been heavily investigated from both experimental and theoretical perspectives [START_REF] Crago | Conservation and variability of the evaporative fraction during the daytime[END_REF][START_REF] Crago | Daytime evaporation and the self-preservation of the evaporative fraction and the Bowen ratio[END_REF][START_REF] Lhomme | Examination of evaporative fraction diurnal behaviour using a soil-vegetation model coupled with a mixed-layer model[END_REF]. Most of these studies have reported a typical concave-up shape for EF which can induce errors when assuming a daytime constant EF equal to the noon value, since the latter is always lower than the daily average [START_REF] Gentine | Analysis of diurnal evaporative fraction behavior[END_REF]. The assumption of the self conservation of EF during day time hour is only valid under relatively dry surface and clear sky conditions [START_REF] Sugita | Daily evaporation over a region from lower boundarylayer profiles measured with radiosondes[END_REF][START_REF] Lhomme | Examination of evaporative fraction diurnal behaviour using a soil-vegetation model coupled with a mixed-layer model[END_REF].

To check the stability of both SMIs with the Monsoon'90 data, the relative variability of EF and AEF is estimated by successively calculating the standard deviation of measurements at the eight METFLUX sites between 10am and 2pm, averaging the eight standard deviations, and normalizing the diurnal variability with the sensitivity of SMIs to surface soil moisture. The sensitivity of AE and AEF to surface soil moisture is evaluated as the difference between the SMIs calculated at 20% vol. (maximum value) and 0 (minimum value) using the linear regression of the correlations EF-W and AEF-W respectively. Table III lists the values of the relative diurnal variability (in %) of EF and AEF evaluated for each day between Jd 212 and Jd 222. The average diurnal cycle represents about 20% and 15% of the sensitivity to surface soil moisture for EF and AEF respectively. It is suggested that the superiority of the AEF-based approach is partly due to the relative stability of AEF for changing atmospheric conditions. In the case of EF, the assumption " 1 f constant" is less valid.

Finally, the comparison between the error on downscaled soil moisture in Table II and the diurnal variation of EF and AEF in Table III indicates that the performance of the disaggregation is well correlated with the stability of SMIs. For example, the disaggregation results are significantly improved on Jd 212, 217 and 221, for which the diurnal variability of AEF is approximately half that of EF. The temporal variability of atmospheric conditions is also presented in Table III for comparison with the diurnal variability of SMIs. The temporal variations of solar radiation, relative humidity, air temperature and wind speed are calculated as the spatial average at the eight METFLUX sites of the standard deviation between 10am and 2pm. It is apparent that variations in EF and AEF can be attributed to changes in solar radiation and relative humidity; the impact of air temperature and wind speed being less visible with these data.

V. LIMITATIONS AND APPLICABILITY TO SMOS Application to the Monsoon'90 data demonstrated the performance of the approach when using ground-based measurements over a limited range of surface conditions (8 locations distributed within a 150km 2 semi-arid area) and during a short period of time (20 days). This section aims to assess the applicability of the suggested method to SMOS like data at larger space-time scales. The assumptions underlying the development of equations ( 1) to ( 6) are listed and discussed. A sensitivity analysis of the algorithm is also conducted to evaluate the impact of uncertainties in ancillary data, high-resolution SMI observations and low-resolution surface soil moisture observations on the disaggregation results.

A. Assumptions

The methodology is based on five assumptions. Each assumption is followed by a discussion regarding its applicability to SMOS like data:

1) Cloud free conditions: evaporative fraction observations can be derived at large spatial scales using optical data [START_REF] Jiang | Estimation of surface evaporation map over southern Great Plains using remote sensing data[END_REF][START_REF] Nishida | Development of an evapotranspiration index from Aqua/MODIS for monitoring surface moisture status[END_REF][START_REF] Wang | Estimation of evaporative fraction from a combination of day and night land surface temperatures and NDVI: a new method to determine the Priestley-Taylor parameter[END_REF], but optical data will be available for clear sky conditions only. Note that clear sky conditions are also needed, as shown in the application to Monsoon'90, to meet the " 1 f constant" hypothesis.

2) Soil moisture index observations are available at approximately the same time as passive microwave observations, so as the low resolution surface soil moisture does not vary much between the two types of observations. With daily optical data such as the MODerate resolution Imaging Spectroradiometer (MODIS), this requirement is generally met: one can assume that the 40km soil moisture does not change significantly between 6am (SMOS overpass time) and 10am (MODIS overpass time).

3) The soil moisture index is assumed to be linearly correlated to surface soil moisture, which allows writing equations ( 1) and ( 6). The synthetic study of Fig. 3 showed that the relationship between EF/AEF and surface soil moisture is non-linear for high soil moisture values. The saturation of the SMI does not allow estimating high-resolution surface soil moisture above the saturation threshold (20% vol. in the case of the Walnut Gulch watershed). This represents a limitation of the downscaling method for applications where the studied process does not saturate above this soil moisture threshold.

4) Some parameters involved in the surface energy budget such as vegetation cover, soil properties, and meteorological forcing are assumed to be available at high resolution so as the projection technique of equation ( 5) can be used. It is however reminded that one parameter available at high resolution (e.g. LAI) is sufficient to apply the projection, as long as the other parameters can be estimated at low resolution.

5) The slope of the correlation 1 f between SMI and surface soil moisture is assumed to be relatively independent of meteorological forcing data. This allows :

B. Sensitivity analysis

To assess the impact of uncertainties in input data of the downscaling procedure, realistic measurement errors were added to high-resolution SMI and low-resolution surface soil moisture observations. Three cases are investigated: i) surface parameters such as vegetation characteristics (LAI, canopy height), soil properties (A,B), meteorological data (Rg,Ta,ua,qa) are available at low-resolution only (not at high-resolution as in the previous section);

ii) a bias ranging from -5 to +5% vol. on the low-resolution soil moisture observation;

iii) 10% and 20% error on SMI observations with LAI ranging from 0 to 4.

For each of the three cases, the impact on the downscaled soil moisture is evaluated and discussed.

1) Surface parameters available at low-resolution only

To evaluate the impact of resolution of ancillary data, the downscaling procedure is run with the same data set as in the previous section, but with the surface parameters -composed of LAI, canopy height, soil resistance parameters, meteorological forcing data-averaged at the scale of the microwave pixel. Table II lists the mean RMSE on the downscaled soil moisture obtained for each type of ancillary information separately. Results are to be compared to the case where all surface parameters are available at high-resolution. It is apparent that the spatial resolution of soil resistance parameters and canopy height (ranging from 0.1 to 0.6m for Monsoon'90) has no significant impact on the disaggregation results. However, the impact of the resolution of LAI is more significant: the mean RMSE increased from 2.7% vol. to 3.7% vol. for EF and from 2.2% vol. to 2.8% vol. for AEF. The impact of resolution of meteorological data is very low for both EF and AEF. Surprisingly, downscaling results are slightly better in general with low-resolution than with high-resolution atmospheric forcing. It is suggested that the projection is not able to correct efficiently the impact of meteorological conditions to the correlation between SMI and surface soil moisture, as this impact is rather small with EF and AEF.

As a summary, the critical ancillary data to be used at high resolution seem to be the LAI and soil parameters A and B. The resolution of meteorological data does not seem to have a significant impact on the downscaling results. Note however, that the spatial extent of the data set used in this paper is rather small (150km 2 ) compared to the SMOS pixel size (1600km 2 ), which means that higher heterogeneities are expected at the SMOS scale, with probably higher impacts on the downscaled soil moisture.

2) Bias on the low-resolution surface soil moisture observation

To evaluate the impact of a given bias on the low-resolution surface soil moisture observation to the downscaled soil moisture, a bias of -5, -4, -3, -2, -1, +1, +2, +3, +4 and +5% vol. was successively added to the low-resolution soil moisture observation. The resulting bias on the downscaled soil moisture H W is plotted as function of the input bias on L W in Figure 6 for EF and AEF separately. It is shown that the mean output bias is practically equal to the input bias, which confirms the linear link between high and low resolution surface soil moisture of equation ( 1) and [START_REF] Njoku | Soil moisture retrieval from AMSR-E[END_REF]. Note that the slight divergence of the high-resolution bias with respect to the 1:1 line for negative biases is due to the negative values of surface soil moisture that were forced to 0 in the simulations.

3) Uncertainty in remotely sensed evaporative fraction Space-based estimates of evaporative fraction are probably more uncertain than the groundbased estimates acquired during a field experiment such as Monsoon'90. The objective is to simulate realistic uncertainties on space-based evaporative fraction observations and evaluate its impact on the disaggregation results. A synthetic study was undertaken to quantify the error on the downscaled soil moisture associated with an error of 10% and 20% on SMI estimates, and with a LAI ranging from 0 to 4. Figure 6 illustrates the results obtained for EF and AEF separately. They indicate that an error of 5% vol. is achievable with 10% error on SMI.

However, an error of 20% on SMI impacts significantly the downscaled soil moisture with an error estimated to 10% vol.

VI. CONCLUSIONS

Two simple approaches for downscaling (disaggregating) L-band coarse resolution passive microwave soil moisture are developed, and tested using ground and airborne data collected over the Walnut Gulch experimental watershed during the Monsoon'90 experiment. These data consisted of eight micro-meteorological stations and six flights of the L-band PBMR. For each PBMR flight, the L-band pixels covering the eight stations were first aggregated to generate a 500m coarse-scale passive microwave pixel. The soil moisture retrieved from the low resolution microwave observations was then downscaled to 180m, using two different SMIs to describe the sub-pixel variability of surface soil moisture: i) the ratio of the evapotranspiration to the available energy (EF); and ii) the ratio of the actual to potential evapotranspiration (AEF). It is well known that both SMIs depend on surface soil moisture. However, they are also influenced by other factors such as vegetation cover, soil type, and atmospheric conditions. In order to decouple the influence of soil moisture from the other factors, a surface energy balance surface model is used in conjunction with the projection technique developed in [START_REF] Merlin | A combined modeling and multi-spectral/multi-resolution remote sensing approach for disaggregation of surface soil moisture: Application to SMOS configuration[END_REF] to account for the heterogeneity of vegetation cover, soil type and atmospheric conditions.

The projection was able to increase the correlation coefficient between SMI and surface soil moisture from 0.66 to 0.79 and from 0.71 to 0.81 for EF and AEF respectively. The main hypothesis of the downscaling approaches is the constancy of the correlation between SMI and surface soil moisture (slope 1 f ) during day time. This assumption was considered valid in most cases given the overall accuracy on downscaled values of about 3% vol. for EF and 2% vol. for in testing downscaling methods before SMOS data become routinely available, and validating downscaled soil moisture products once SMOS is on orbit.

  for low resolution = 500m and subscript obs for observed) being the coarse-scale soil moisture retrieved from the aggregated PBMR data,

  Rn and obsG being the latent heat flux, the net radiation and the ground flux measured by the METFLUX stations. The observed AEF is calculated as: evapotranspiration computed with the Penman-Monteith formula:

  pressure deficit of the air, ρ being the mean air density at constant pressure, Cp the specific heat of the air, ∆ the slope of the saturation vapor pressure temperature relationship, γ the psychrometric constant, obs a r , the aerodynamic resistance and min r the minimum surface resistance (fixed to 20 for the eight METFLUX sites). The canopy height used to calculate the aerodynamic resistance is from[START_REF] Weltz | Comparison of laser and field measurements of vegetation height and canopy cover[END_REF].

  latent heat flux, net radiation flux, ground heat flux, and potential latent heat flux simulated with the N95 model. The N95 model is calibrated against EF observations during a training period between Jd 206 and 211. The measured and simulated EF is averaged between 10am and 2pm and the root mean square difference between the average of the measured and simulated EF is minimized by varying the parameters A, B and LAI. Results of the site-specific calibration are presented in Table

o

  using the projection technique with low-resolution meteorological data only; and o calibrating 1 f during a training period that is independent from the application data set. Note that the value of 1 f still varies with any changes in vegetation cover. The calibration of 1 f should therefore be undertaken for each microwave pixel, and as often as the surface conditions (seasons, land use) change within the microwave pixel.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Time series of the (a) minimum and maximum surface soil moisture observed at the eight METFLUX sites and the (b) minimum and maximum evaporative fraction measured at the eight METFLUX sites between 10am and 2pm, from Jd 212 to Jd 221.

Figure 3 .

 3 Figure 3. Sensitivity of the (a) simulated evaporative fraction (EF) and the (b) simulated actual evaporative fraction (AEF) to surface soil moisture for increasing LAI values.

Figure 4 .

 4 Figure 4. Measured and projected soil moisture indices (SMIs) versus surface soil moisture observations between Jd 212 and Jd 221. The projected SMIs are also plotted versus measured SMIs for comparison.

Figure 5 .

 5 Figure 5. Downscaled versus measured surface soil moisture for the six microwave pixels generated on Julian days 212, 214, 216, 217, 220 and 221. The results obtained with the (a) EF and (b) AEF are compared.

Figure 6 .

 6 Figure 6. Estimated error on the downscaled soil moisture associated with a given uncertainty in SMI observations and a given bias on low-resolution soil moisture observations.

TABLE I

 I Calibration parameters at the eight METFLUX sites.TABLE IIMean root mean square error (RMSE) on the downscaled surface soil moisture for the six generated microwave pixels. Results obtained with EF and AEF, and with ancillary data available at high or low-resolution are successively presented.

	Site LAI	A B
	1	0.4	9	9
	2	0.3	8	5
	3	0.4	9	8
	4	0.4	8	4
	5	0.2	8	5
	6	0.2	8	5
	7	0.7	8	4
	8	0.6	8	4

TABLE III

 III Average at the eight METFLUX sites of the relative diurnal variability of EF and AEF, and standard deviation of solar radiation, relative humidity, air temperature and wind speed evaluated between 10am and 2pm for each day between Jd 212 and Jd 222. The six days of PBMR flight are underlined.

	Jd	Relative variability (%) EF AEF	g (Wm -2 ) R σ	a (ºC) q σ	a T σ (ºC)	a (ms -1 ) u σ
	212	26	12	110	3.5	1.0	0.6
	213	13	5	74	5.3	1.8	1.0
	214	21	28	260	5.7	1.9	0.8
	215	20	18	169	4.0	1.7	0.9
	216	10	11	101	3.9	1.6	0.6
	217	23	12	198	2.8	1.5	1.2
	218	31	32	84	7.4	1.7	0.8
	219	14	16	133	3.1	1.5	0.6
	220	17	11	114	3.9	1.6	0.7
	221	13	7	57	5.5	1.8	0.8
	222	13	3	54	3.0	1.8	0.8

ACKNOWLEDGMENT This research was funded by discovery grant DP0557543 from the Australian Research Council. The authors thank William Kustas, David Goodrich, Susan Moran, and Tom Schmugge for making the Monsoon '90 data available. In addition, the past and current support of the USDA-ARSSWRC staff in Tombstone and Tucson are gratefully acknowledged. The necessary financial support by NASA and the USDA to conduct the Monsoon '90 field experiment is also acknowledged. This included funds from NASA Interdisciplinary Research Program in Earth Sciences (NASA Reference IDP-88-086) and funds from the USDA-ARS Beltsville Area Office.

AEF, except under cloudy conditions. The comparison of EF and AEF indicates that AEF is more directly linked to surface soil moisture, especially for high soil moisture values. The diurnal variability of EF due to temporal changes in incoming radiation and relative humidity seems to explain the superiority of the AEF-based approach.

The assumptions underlying the equations of the downscaling method were discussed in terms of their applicability to SMOS. When applied to the scale of about 40km (SMOS pixel size), the main limitations are: (i) optical derived SMI estimates limited to clear sky conditions, (ii) the saturation of SMI for high soil moisture values, which does not allow retrieving downscaled values above the saturation threshold (about 20% vol. with the Monsoon'90 data), and (iii) the availability of ancillary data such as meteorological data and soil/vegetation characteristics at high-resolution. Note however, that the availability of all the ancillary data at high resolution (notably meteorological data) is not a requirement to apply the projection.

A sensitivity analysis of the method was undertaken to assess the impact of uncertainties in input data to the downscaled soil moisture. An error of 10% on SMI observations has an effect of about 2 to 5% vol. on the downscaled soil moisture, depending on soil moisture and LAI values. A given bias on the low-resolution soil moisture observation impacts the downscaled soil moisture with the same bias.

Those results illustrate the potential use of high resolution satellite-based estimates of instantaneous evapotranspiration obtained on clear-sky days for downscaling coarse-resolution passive microwave soil moisture. Recent studies have investigated the use of 1km-resolution optical data such as NOAA/AVHRR and MODIS to develop operational schemes for monitoring EF at regional and global scales [START_REF] Jiang | Estimation of surface evaporation map over southern Great Plains using remote sensing data[END_REF][START_REF] Nishida | Development of an evapotranspiration index from Aqua/MODIS for monitoring surface moisture status[END_REF][START_REF] Wang | Estimation of evaporative fraction from a combination of day and night land surface temperatures and NDVI: a new method to determine the Priestley-Taylor parameter[END_REF]. High-resolution microwave data collected during field experiments such as the National Airborne Field Experiment (NAFE) [START_REF] Panciera | The NAFE'05 data set: Towards SMOS calibration, downscaling and assimilation[END_REF], will be essential