Yijian Zeng

Anne Verhoef

Dani Or

Matthias Cuntz

Lukas Gudmundsson

Lutz Weihermueller

Stefan Kollet

Jan Vanderborght

Harry Vereecken

Julia Guimond

GEWEX-ISMC SoilWat Project: Taking Stock and Looking Ahead

On January 28 th , the American Geophysical Union (AGU) Hydrology Section Student Subcommittee (H3S) welcomed 15 new members to the committee, ranging in career stage from undergraduate student to postdoctoral researcher. Since the rst meeting, H3S members and subcommittees have been busy brainstorming new initiatives, resources, and opportunities for students and early career researchers. On Friday, March 26 th , H3S hosted the rst virtual networking event of the year. is event was in the form of a speed networking social with approximately 30 one-minute blocks for early career researchers to meet and share interests. Similar events will be held virtually every month this spring with the hope of strengthening the early career community and building students' and early career researchers' networks. Events with similar goals are also in the planning stage, to be held this summer as well as at the 2021 AGU Fall Meeting.

One year ago, H3S launched its website (agu-h3s.org) to provide resources and highlight the research of early career scientists.

is spring, H3S members are working hard to expand the website, including absorbing the WaterPOC database, created by students from the University of Waterloo. is database is a tool to support e orts of diversity, equity, and inclusion in the eld of water research, including:

• Increasing the visibility of these researchers to their peers (both minoritized and not) up and down the career ladder

• Increasing recognition of the excellence of research conducted by these individuals

• Facilitating nominations and recruitment of these individuals for awards, leadership positions, and paid positions is database also highlights the intersections of identities within water researchers, including race, gender, and career stage. ere are currently 180+ entries in the database and it continues to grow. e WaterPOC database will soon be accessible through the H3S website for long-term access and maintenance.

Stay tuned for more event announcements in the coming months! Interested in learning more about H3S activities? Follow us on Twitter (@AGU_H3S) or Instagram (agu_hydrology).

e Soil and Water (SoilWat) initiative is a joint activity between the Global Energy and Water Exchanges (GEWEX) project and the International Soil Modelling Consortium (ISMC). SoilWat aims to bring together two research communities to improve the representation of soil and subsurface processes in climate models. e soil modeling community (represented by ISMC) and the climate modeling community (represented by GEWEX) are working together to identify the most pressing challenges and topics related to this e ort, in terms of understanding the role of soil properties and parameters, soil physical processes (e.g., in ltration, surface evaporation, water and heat ow), root hydraulics, groundwater dynamics, and their interactions with the vegetation and biogeochemical cycles, deploying a combination of Earth observations (in situ and remote sensing), modeling, and data assimilation.

Soil Properties and Pedotransfer Functions

ere is a longstanding tradition in soil science of developing and applying pedotransfer functions (PTFs) for the calculation of hydraulic properties [START_REF] Clapp | Empirical equations for some soil hydraulic properties[END_REF][START_REF] Cosby | A Statistical Exploration of the Relationships of Soil Moisture Characteristics to the Physical Properties of Soils[END_REF][START_REF] Pachepsky | Development of pedotransfer functions in soil hydrology[END_REF][START_REF] Vereecken | Using Pedotransfer Functions to Estimate the van Genuchten-Mualem Soil Hydraulic Properties: A Review[END_REF][START_REF] Van Looy | Pedotransfer Functions in Earth System Science: Challenges and Perspectives[END_REF]Dai et al., 2019a). ese functions use basic soil properties that are widely available, such as texture, organic matter, and bulk density, to estimate hydraulic, solute transport, thermal, and biogeochemical parameters [START_REF] Van Looy | Pedotransfer Functions in Earth System Science: Challenges and Perspectives[END_REF]. e soil hydraulic properties (water retention characteristic and the hydraulic conductivity curve) [START_REF] Brooks | Hydraulic properties of porous media[END_REF][START_REF] Van Genuchten | A Closed-form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils[END_REF][START_REF] Montzka | A global data set of soil hydraulic properties and sub-grid variability of soil water retention and hydraulic conductivity curves[END_REF]Gupta et al., 2020;[START_REF] Gupta | Global prediction of soil saturated hydraulic conductivity using random forest in a Covariate-based GeoTransfer Function (CoGTF) framework[END_REF] are pivotal in describing the ow and storage of water in soil and its availability for vegetation via root water uptake [START_REF] Verhoef | Modeling plant transpiration under limited soil water: Comparison of di erent plant and soil hydraulic parameterizations and preliminary implications for their use in land surface models[END_REF].

Besides applications in soil science, ecology, engineering, and hydrology, soil hydraulic functions and their PTFs are used extensively to derive hydraulic parameters for land surface models (LSMs). LSMs are embedded in global climate and Quarter 2 2021 numerical weather prediction (NWP) models to enable regional to global coverage and calculation of soil-and land surfacerelated processes, pertaining to the water, energy, and carbon balances. Consequently, uncertainty in basic soil properties propagates into the derived soil parameters and can a ect uxes and state variables simulated by LSMs [START_REF] Weihermüller | Choice of Pedotransfer Functions matters when simulating soil water balance uxes[END_REF]Dai et al., 2019b). For example, the evaluation of European Centre for Medium-Range Weather Forecasts (ECMWF) soil moisture and temperature analyses against in situ data collected on the Tibetan Plateau (TP) revealed that the persistent systematic model bias is mainly caused by nonrepresentative values of saturated hydraulic conductivity used in the model and simpli cations of certain soil physical processes (e.g., freeze-thaw) [START_REF] Su | Evaluation of ECMWF's soil moisture analyses using observations on the Tibetan Plateau[END_REF][START_REF] Yu | Liquid-Vapor-Air Flow in the Frozen Soil[END_REF]. A further investigation into the ve existing soil texture databases over the TP demonstrated a wide uncertainty range across di erent climate regions of the TP (i.e., arid, semi-arid, and subhumid) [START_REF] Zhao | Analysis of soil hydraulic and thermal properties for land surface modeling over the Tibetan Plateau[END_REF]. e results of these kinds of studies show that the choice of soil information and soil databases matters for LSMs (Dai et al., 2019a;[START_REF] Montzka | A global data set of soil hydraulic properties and sub-grid variability of soil water retention and hydraulic conductivity curves[END_REF][START_REF] Fatichi | Soil structure is an important omission in Earth System Models[END_REF].

Information on basic soil properties such as soil texture and porosity is also used to derive the parameters required in the equations used to calculate thermal soil properties (thermal conductivity and heat capacity) in LSMs (Dai et al., 2019a;[START_REF] Johansen | ermal conductivity of soils[END_REF][START_REF] Lu | An Improved Model for Predicting Soil ermal Conductivity from Water Content at Room Temperature[END_REF][START_REF] He | A new model for predicting soil thermal conductivity from matric potential[END_REF]Ghanbarian and Daigle, 2016;Dai et al., 2019c). Hence, the term PTF can also be used in this thermal context. Furthermore, other than considering thermal property equations and their PTFs, SoilWat ultimately aims to develop a uni ed underlying theory to yield both thermal and hydraulic parameters in a physically-consistent way [START_REF] Lu | Closed-Form Equation for ermal Conductivity of Unsaturated Soils at Room Temperature[END_REF]. is will help facilitate the harmonization of soil hydro-thermal properties and their PTFs as used in LSMs, to avoid artifacts potentially originating from the choice of soil property maps and PTFs [START_REF] Weihermüller | Choice of Pedotransfer Functions matters when simulating soil water balance uxes[END_REF], and confusion with uncertainties stemming from di erent model structures and physics. ermal Properties" and "Pedotransfer functions and land surface parameterization" (https://soil-modeling.org/science-panels/ working-groups/pedotransfer-functions). Analysis of the SP-MIP data is currently ongoing. e recent paper by [START_REF] Weihermüller | Choice of Pedotransfer Functions matters when simulating soil water balance uxes[END_REF] explored the e ect of using di erent PTFs [START_REF] Clapp | Empirical equations for some soil hydraulic properties[END_REF][START_REF] Cosby | A Statistical Exploration of the Relationships of Soil Moisture Characteristics to the Physical Properties of Soils[END_REF][START_REF] Pachepsky | Development of pedotransfer functions in soil hydrology[END_REF]) (see Fig. 1) on key components of the water balance for an area in Germany (using a 30 year set of meteorological driving data). It employed the widely used and tested HYDRUS model (Šimůnek et al., 2016) to avoid confounding e ects of choice of PTF and model structure. e e ects on the evapotranspiration were considerable, and they varied depending on the choice of soil type, land cover (bare, grass, or crop), and whether groundwater was present. Follow-on studies are currently being conducted to test the e ects with other independent soil physical models, as well as the e ects on the thermal regime.

Soil Infiltration Processes

In ltration is a key soil process that partitions precipitation Quarter 2 2021 into surface runo and water that enters the soil pro le. As part of the SoilWat initiative, Vereecken et al. ( 2019) reviewed the basic principles and current approaches to describe in ltration in LSMs, its numerical implementation, and its sensitivity to model parameters. ey found a large variation in in ltration approaches adopted by LSMs, also in relation to upscaling from the local to the grid-scale, in addition to the large variety of approaches used to estimate soil hydraulic properties as mentioned above.

ey identi ed several processes not yet considered in LSMs that in uence in ltration into soils such as soil structure [START_REF] Fatichi | Soil structure is an important omission in Earth System Models[END_REF], water repellency, crusting, freeze-thaw processes, hysteresis, ow instability, and swelling and shrinking. At present, we lack information on how, e.g., vegetation and land use may impact soil structure and thus soil hydraulic properties and the process of in ltration, although e orts by Gupta et al. (2020[START_REF] Gupta | Global prediction of soil saturated hydraulic conductivity using random forest in a Covariate-based GeoTransfer Function (CoGTF) framework[END_REF] have tried to take this into account.

Soil Surface Evaporation

Soil evaporation is an important component of the hydrological cycle that a ects plant available water and the surface energy balance. Knowledge of surface evaporation (de ned as evaporation from soil pores and canopy interception, but not transpiration) is important for separation of evapotranspiration to its components (transpiration and evaporation) to better link the water and carbon cycles and for various aspects of water resource management (e.g., irrigation scheduling). e dynamics of soil evaporation and internal drainage following rainfall are sensitive to soil properties; in other words, soil type (as de ned by its textural composition) a ects the rates by which soil water percolates to deeper layers, and the surface resistance to evaporation. It also a ects a characteristic depth below which water cannot be easily extracted by capillarity and related physical processes (of course, plant roots can access and extract soil water down to several meters). ese common but often overlooked soil e ects on soil evaporation have been formulated in a conceptual model by [START_REF] Or | Surface Evaporative Capacitance: How Soil Type and Rainfall Characteristics A ect Global-Scale Surface Evaporation[END_REF] termed the Surface Evaporation Capacitor (SEC), and the model has been applied globally to improve climatic surface evaporation predictions (highlighting shortcomings of some of the common approaches presently used to deduce this important land surface ux).

e SEC implements a simple and easy-to-use formulation of evaporation surface resistance [START_REF] Lehmann | Soil Texture E ects on Surface Resistance to Bare-Soil Evaporation[END_REF] that has been tested using FLUXNET data and decadal lysimeter records from arid regions [START_REF] Lehmann | Surface Evaporation in Arid Regions: Insights From Lysimeter Decadal Record and Global Application of a Surface Evaporation Capacitor (SEC) Model[END_REF]. More recently, the SEC model has been extended to include the competing e ects of drainage in an analytical framework (Lehmann and Or, under review).

is line of research not only ts into the SoilWat theme of linking soil and climate processes, but it also o ers readily useable improvements for global models as has been shown recently [START_REF] Or | Surface Evaporative Capacitance: How Soil Type and Rainfall Characteristics A ect Global-Scale Surface Evaporation[END_REF][START_REF] Decker | New turbulent resistance parameterization for soil evaporation based on a pore-scale model: Impact on surface uxes in CABLE[END_REF][START_REF] Mu | Evaluating a land surface model at a water-limited site: implications for land surface contributions to droughts and heatwaves[END_REF].

Soil Water and Heat Flow

Most of the current operational LSMs describe water ow via Richards' equation and heat ow via Fourier's law, and the soil water and heat ow are "softly" coupled via soil heat capacity and thermal conductivity, both of which are dependent on soil water content. Although such model treatments of soil physics can broadly capture the soil water-heat dynamics over humid and subhumid regions, their performance over arid and semiarid lands (ASALs) (that make up >40% of the globe) are always compromised. Such model de ciency over ASALs is mainly caused by the missing mechanism of vapor transfer, which is key for actively and dynamically coupling soil water and heat ow, since it carries simultaneously mass and energy (Garcia [START_REF] Gonzalez | Incorporation of water vapor transfer in the JULES land surface model: Implications for key soil variables and land surface uxes[END_REF]Zeng et al., 2011a;Zeng et al., 2011b;[START_REF] Shahraeeni | Pore-scale analysis of evaporation and condensation dynamics in porous media[END_REF][START_REF] Shahraeeni | Pore scale mechanisms for enhanced vapor transport through partially saturated porous media[END_REF][START_REF] Shokri | Critical evaluation of enhancement factors for vapor transport through unsaturated porous media[END_REF].

A study with water vapor transfer incorporated into the JULES model found that the water vapor ux contributes signi cantly to the water and heat transfer in the upper soil layers over three semi-arid and temperate arid sites [START_REF] Gonzalez | Incorporation of water vapor transfer in the JULES land surface model: Implications for key soil variables and land surface uxes[END_REF]. e inclusion of vapor transfer has a direct and positive impact on the diurnal evolution of evaporation, soil moisture content, and surface temperature, which will contribute to the improved understanding and prediction of landair interactions. Furthermore, the consideration of soil vapor ow can further our wider process understanding, including land-atmosphere feedbacks at larger scales. For instance, we might be better able to grasp why afternoon rain is more likely over drier soils [START_REF] Taylor | A. Afternoon rain more likely over drier soils[END_REF], or why the semi-arid ecosystem plays a dominant role in the trend and variability of the land CO 2 sink [START_REF] Ahlström | e dominant role of semi-arid ecosystems in the trend and variability of the land CO 2 sink[END_REF].

Afternoon rain falls preferentially over dry soils, particularly over semi-arid regions, where surface uxes are sensitive to soil moisture and convective events are frequent [START_REF] Taylor | A. Afternoon rain more likely over drier soils[END_REF]. Semi-arid ecosystems were found to account for 39% of interannual variability of global Net Biome Production (NBP), which is mainly driven by the local compensatory e ects of soil water availability on NBP [START_REF] Ahlström | e dominant role of semi-arid ecosystems in the trend and variability of the land CO 2 sink[END_REF][START_REF] Jung | Compensatory water e ects link yearly global land CO 2 sink changes to temperature[END_REF]. To facilitate understanding the role of soil physical processes within the above context, SoilWat is conducting an in-depth survey on how key soil physical processes (water and heat ow) are represented in Earth System Quarter 2 2021 and Land Surface Models.

Soil-Root Hydraulics

Since the root system is the plant organ that is responsible for the acquisition of water and nutrients, its hydraulic properties play a key role in the functioning of the vegetation and in the water, carbon, and nutrient cycles of terrestrial ecosystems. Root hydraulics have been represented in LSMs in di erent ways: assuming a vertical "big root" [START_REF] Amenu | A model for hydraulic redistribution incorporating coupled soil-root moisture transport[END_REF][START_REF] Tang | Incorporating root hydraulic redistribution in CLM4.5: E ects on predicted site and global evapotranspiration, soil moisture, and water storage[END_REF] or assuming a network of parallel roots that take up water near the root tips and that are connected at the root collar [START_REF] Sulis | Incorporating a root water uptake model based on the hydraulic architecture approach in terrestrial systems simulations[END_REF][START_REF] Kennedy | Implementing Plant Hydraulics in the Community Land Model, Version 5[END_REF]. ese hydraulic root models are parameterized in a "top-down" approach by rst postulating a simpli ed root hydraulic structure and then de ning its parameters from root distributions and root segment hydraulic properties. Another approach is to start with a 3-D root hydraulic architecture model and solve the corresponding ow equations. is system of equations was scaled up to a 1-D model and general properties of the root system that describe total root water uptake and water uptake distributions as a function of the soil water potential distribution can be derived [START_REF] Vanderborght | From hydraulic root architecture models to macroscopic representations of root hydraulics in soil water ow and land surface models[END_REF].

It was found [START_REF] Vanderborght | From hydraulic root architecture models to macroscopic representations of root hydraulics in soil water ow and land surface models[END_REF] that the upscaled equations can be approximated by a form that is equivalent to the parallel root model. is implies that parameters of the parallel root model can be derived in a bottom-up approach from 3-D root hydraulic architectures, whereas this was not possible for the big-root model, which contains twice the number of parameters as the parallel root model (see Fig. 3, on cover). A parallel root model that was parameterized in a bottom-up approach predicted the root water uptake pro les better than parallel root or big root models that were parameterized in a top-down approach. ese results show that simple 1-D root hydraulic models used in LSMs can represent uptake by complex 3-D root hydraulic architectures. Root density pro les could be used to parameterize root architecture models that are subsequently used to parameterize the upscaled root water uptake model. e 3-D root hydraulic architecture is currently being coupled with a soil-root transfer model that describes the water ow from the bulk soil to the root surface and represents non-linearities of soil water ow equations. First results show that upscaling the linear equations of the 3-D root hydraulic architecture, and then coupling the upscaled equations to the non-linear soil equations, results in good predictions of the root water uptake pro les compared to predictions by the full 3-D model coupled with the nonlinear soil equations. e upscaling reduces the number of non-linear equations that need to be solved so drastically that this approach may be feasible for use in operational LSMs.

Soil-Groundwater Interactions

e groundwater system is an essential component of the global hydrological cycle, and is directly connected with the soil pro le via the deep drainage at the bottom of the soil pro le (groundwater recharge), and root water uptake and capillary rise from the groundwater for relatively shallow groundwater levels. Hence, groundwater can moderate soil water content, and therefore the root water uptake and related transpiration and evaporation processes. It also controls root distribution patterns, together with local climate conditions (i.e., precipitation-in ltration depth) [START_REF] Fan | Incorporating water table dynamics in climate modeling: 1. Water table observations and equilibrium water table simulations[END_REF]. e rst generation LSMs did not include simulation of groundwater and simply applied a free drainage boundary condition to the bottom of a xed soil column. More recently, however, groundwater simulation has been included in a number of LSMs (Yeh andEltahir, 2005a, 2005b;[START_REF] Maxwell | Development of a coupled land surface and groundwater model[END_REF], and tested at a range of scales from single basins to the globe, with approaches ranging from lumped to distributed models. Note that lateral groundwater ow is not generally included in the majority of global hydrological models and LSMs, with some exceptions, e.g., [START_REF] Felfelani | Representing Intercell Lateral Groundwater Flow and Aquifer Pumping in the Community Land Model[END_REF]. Including water table dynamics has been shown to improve river discharge simulations (Yeh and Eltahir, 2005b;[START_REF] Koirala | Globalscale land surface hydrologic modeling with the representation of water Quarter 2 2021 table dynamics[END_REF] and including capillary ux from groundwater increases evapotranspiration, with the global mean simulated to rise by up to 16% [START_REF] Koirala | Globalscale land surface hydrologic modeling with the representation of water Quarter 2 2021 table dynamics[END_REF][START_REF] Niu | Development of a simple groundwater model for use in climate models and evaluation with Gravity Recovery and Climate Experiment data[END_REF][START_REF] Yeh | Regional groundwater evapotranspiration in Illinois[END_REF][START_REF] Anyah | Incorporating water table dynamics in climate modeling: 3. Simulated groundwater in uence on coupled land-atmosphere variability[END_REF]. Key SoilWat members, together with members of the wider GEWEX hydrological community, are currently in the process of resubmitting a paper on "Global groundwater modeling and monitoring: Opportunities and challenges" (Condon et al., under review) that looks at the complexities of implementing realistic groundwater modeling on the continental to global scale.

Summary and Outlook

ere are a number of publications addressing relevant issues linked to SoilWat aims. Ongoing studies include the importance and in uence of soil hydraulic and thermal properties, pedotransfer functions accounting for soil structure, soil-plant interactions (via root hydraulics), soil evaporation, coupled soil water and heat ow, as well as soil-groundwater interactions on land surface processes.

Outstanding issues are:

• Critically, how to systematically evaluate added value of improved soil process representation in LSMs (see [START_REF] Gupta | Global prediction of soil saturated hydraulic conductivity using random forest in a Covariate-based GeoTransfer Function (CoGTF) framework[END_REF] for some of the challenges of showing improvement) is plasticity has been addressed in LSMs by considering "optimal" distributions. However, in managed systems and when environmental changes occur faster than the Quarter 2 2021 process that optimizes these distributions, optimality cannot be assumed. Describing how vegetation will respond to changing environmental conditions (either by management or due to climate change) will therefore be critical to predict land-surface processes.

• How to better represent preferential (aka bypass) ows in LSMs (Gharari et al., 2019), for example, as caused by macropore ows (Rahman and Rosolem, 2017), ngered ows, and funnel ows (Pales et al., 2018;Demand et al., 2019). is would also require concerted e orts in collaboration with the soil biological and related modeling communities.

• In ASALs, soil water and heat transfer are strongly coupled, and the soil vapor ow dominates (i.e., carrying both mass and energy). Nevertheless, the soil vapor ow is largely ignored in LSMs, and the water ow is only weakly coupled with heat transfer through the heat capacity and thermal conductivity functions (i.e., via soil water content). On the other hand, land surface uxes and convective events are very sensitive to soil moisture over dry lands, and so is the interannual variability of NBP. erefore, it is essential to revisit the importance of representing soil vapor ow in LSMs for understanding land-air interactions over ASALs. Related to this issue is the topic of vapor adsorption and how to represent this in LSMs for ASALs (Saaltink et al., 2020;[START_REF] Verhoef | Adsorption of water vapor by bare soil in an olive grove in southern Spain[END_REF]. Saaltink et al. show that, on a yearly basis, inward vapor ux (adsorption) into soil can be around a third of the outward vapor ux (evaporation). Detailed soil physical models can reproduce these diurnal evolutions of soil vapor ow, but only if the driest part of the soil water retention curve is represented properly, which brings us back to the importance of choice of hydraulic and thermal properties, and their PTFs.

SoilWat will organize workshops and seminars to facilitate discussions on the above issues, and report back to the community on the outcomes.

  For this purpose, SoilWat is conducting a systematic assessment of the quality of resolved soil maps used in climate modeling, and the e ect of di erent soil maps on the prediction of land surface uxes and state variables. With this endeavor, we seek to improve the quality and resolution of soil maps used in LSMs. Current ongoing e orts include a "Soil Parameter Model Intercomparison Project" (SP-MIP, see Fig. 2); this involves eight LSMs [i.e., the Community Land Model (CLM), the Interaction Sol-Biosphère-Atmosphère (ISBA) model, the Joint UK Land Environment Simulator (JULES), the Jena Scheme for Biosphere Atmosphere Coupling in Hamburg (JSBACH) model, the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) model, the Minimal Advanced Treatments of Surface Interaction and Run O (MATSIRO) model, MATSIRO with Groundwater (MATSIRO-GW), and the Noah-Multiparameterization (Noah-MP) model] and the ISMC working groups of "Soil

Figure 1 .

 1 Figure 1. Summary of the SP-MIP effort. Comparison between experiments 1 and 2 will show the model diversity caused by the PTFs. Comparison between experiments 2 and 3 will show the model diversity that comes from different soil maps and aggregation schemes. Experiment 4 serves: (i) to quantify the effect of spatial variability of soil parameters on model diversity, (ii) to assess the sensitivity of each model to soil hydraulic parameters, and (iii) to investigate to which degree the spatial variability of key water and energy balance outputs is controlled by soil properties.

Figure 2 .

 2 Figure 2. Soil water retention curve (left) and hydraulic conductivity curves (right) for the United States Department of Agriculture (USDA) sand class for 13 PTFs (colored curves).