Ty Sok
email: sokty@itc.edu.kh

Chantha Oeurng

Vinhteang Kaing

Sabine Sauvage

Mathias G Kondolf

José Miguel Sanchez Perez

Assessment of Suspended Sediment Load Variability in the Tonle Sap and Lower Mekong Rivers, Cambodia

Keywords: Temporal variability, Sediment load, Mekong River, Tonle Sap Lake

The Mekong River in Southeast Asia, one of the world's great rivers, has been facing disruption of its sediment balance and resultant impacts on nutrient fluxes, aquatic ecology, floodplains and the delta. Using monitoring data from 1993-2018, we estimated the temporal variability of sediment loads in Tonle Sap and Lower Mekong Rivers in Cambodia, assessing the sediment linkage between the Tonle Sap Lake and the Mekong mainstem, which are connected by a seasonally reversing flow through the Tonle Sap River. We estimated the annual sediment in the Mekong mainstem of 72±38 Mt/year at Kratie (upstream) and 78±22 Mt/year at Chroy Changvar from 1993-2018 (just upstream of the Tonle Sap confluence). Our sediment load estimation of the Mekong River is consistent with other recent estimates of sediment load on the Lower Mekong.

However, the result is lower than reported in some older studies (prior to the 2000s), which is consistent with sediment trapping by dams on Upper Mekong mainstem and major tributaries. Our analysis indicates that Tonle Sap Lake provided 0.65±0.6 Mt/year of sediment annually to the Lower Mekong River from 1995 to 2000. However, since 2001, Tonle Sap Lake has become a sink for sediment, accumulating an average of 1.35±0.7 Mt annually. Net storage of sediment in Tonle Sap Lake reduces the annual sediment transport to the delta, further compounding the effects of reduced sediment delivery to the delta resulting from upstream dam development and instream sand mining.

Introduction

In many river basins, changes in sediment supply have affected the geomorphology of river channels, floodplains, and deltas [START_REF] Peng | Temporal variation of sediment load in the Yellow River basin, China, and its impacts on the lower reaches and the river delta[END_REF]. Sediment transport has important influences on river morphology, water quality and persistence of geomorphic features such as deltas [START_REF] Walling | The impact of global change on erosion and sediment transport by rivers: current progress and future challenges[END_REF]. In recent decades, river processes have been greatly influenced by anthropogenic activities such as hydropower dam development, which alters downstream river discharge and sediment load [START_REF] Vörösmarty | Anthropogenic sediment retention: major global impact from registered river impoundments[END_REF]. Recent studies have shown substantial changes in water and sediment fluxes for rivers globally from prior decades [START_REF] Li | Global trends in water and sediment fluxes of the world's large rivers[END_REF]. The largest rivers of the North and East Asia in Russia and China, such as Amur, Yellow, Yangtze, Pearl, evinced sediment load changes due to human impacts [START_REF] Chalov | Environmental and human impacts on sediment transport of the largest Asian rivers of Russia and China[END_REF]. Rivers with high relief and erodible lithologies like the Ganges, Danube and Amazon Rivers are likely to respond to higher than average precipitation with increased sediment yields [START_REF] Cohen | Global suspended sediment and water discharge dynamics between 1960 and 2010: Continental trends and intra-basin sensitivity[END_REF]. An increasing trend is also visible in long-term records of water and sediment fluxes of the Blue Nile in Africa from 1980-2010 [START_REF] Li | Global trends in water and sediment fluxes of the world's large rivers[END_REF]. However, at global scale, the mean annual water and sediment fluxes to seas from the world's large rivers have decreased [START_REF] Li | Global trends in water and sediment fluxes of the world's large rivers[END_REF]Walling et al., 2003;[START_REF] Walling | Human impact on land-ocean sediment transfer by the world[END_REF]. Within the same context, the estimated suspended sediment to the Arctic Ocean delivered from all Arctic rivers has declined (including Yenisei River) [START_REF] Bobrovitskaya | Regional patterns in recent trends in sediment yields of Eurasian and Siberian rivers[END_REF][START_REF] Fabre | Assessment of sediment and organic carbon exports into the Arctic ocean: The case of the Yenisei River basin[END_REF] Suspended sediment discharge in the Mississippi River declined by at least 50% due to impoundments and other engineering alterations since the early 1950s [START_REF] Meade | Causes for the decline of suspended-sediment discharge in the Mississippi River system[END_REF].

Declines in water discharge and suspended sediment flux of the Para Paraná River, one of the majors in South America, has been observed [START_REF] Stevaux | Changes in a large regulated tropical river: The Paraná River downstream from the Porto Primavera Dam[END_REF]. The sediment flux of all the major river of Mainland China have declined below their long-term averages [START_REF] Cheng | Sediment load reduction in Chinese rivers[END_REF], including about a 60-80% reduction from Qiantang and Yangtze the Pearl River Basin [START_REF] Liu | Analysis on variation of seagoing water and sediment load in main rivers of China[END_REF].

From its headwaters on the Tibet-Qinghai plateau in Asia and sharing the same water source Yangtze and Yellow River, the Mekong River flows 4,800 km with the average annual discharge of 470 km 3 . The river's uppermost 2,000 km (195,000 km 2 basin area), flows through China (including small part in Myanmar) and is known as the Lancang. The total area of the lower Mekong Basin is approximately 600,000 km 2 including Thailand, Laos, Cambodia, and Vietnam. This important basin is facing extensive landuse/cover disturbance, water diversion and reservoir construction, and mining of sediment from the river bed for construction aggregate [START_REF] Lu | Water discharge and sediment flux changes in the Lower Mekong River[END_REF].

Delivery of water and nutrients underpins a vibrant fishery and highly productive agriculture supporting over 60 M people, notably the highly productive Tonle Sap Lake, floodplains of Cambodia, and the Delta of Vietnam [START_REF] Arias | Impacts of hydropower and climate change on drivers of ecological productivity of Southeast Asia's most important wetland[END_REF]Kummu et al., 2008). Changes in the Lancang-Mekong River's sediment load have attracted wide research interest because such changes can affect river channel form, nutrient fluxes, river ecology, and the sustainability of the delta. Hydropower development is altering sediment loads in the Mekong, first from dams in upper basin [START_REF] Kummu | Sediment-related impacts due to upstream reservoir trapping[END_REF]. Analysis of sediment flux from Upper Mekong at Gaju to Khong Chiam at Lower Mekong at seven stations documented by [START_REF] Liu | Changes in the sediment load of the Lancang-Mekong River over the period 1965-2003[END_REF], at five stations in the Lower Mekong (from Chiang Saen to Khong Chiam located in Laos) from 1985-2000 by [START_REF] Wang | Sediment load estimates and variations in the Lower Mekong River[END_REF], and nutrient loads for these stations from 1985-2011 by [START_REF] Li | Rising flux of nutrients (C, N, P and Si) in the lower Mekong River[END_REF] all show decreases. However, these studies covered the river only down to Pakse in Laos. Sediment loads in the lowermost reach of the Mekong, through Cambodia and into the delta, including 3S river system (Sekong, Sesan, and Srepok), the biggest sub-basins of the Mekong Basin, and the Tonle Sap Lake system have received more limited study. Tonle Sap Lake, connected to the Mekong River via Tonle Sap River, has a unique hydrological environment that plays an important role in the active exchange between Tonle Sap Lake and the Mekong River through the seasonal reversal in flow [START_REF] Siev | Seasonal changes in the inundation area and water volume of the Tonle Sap River and its floodplain[END_REF]. A better understanding of the sediment exchanging between the Tonle Sap Lake system and Mekong mainstem is needed because sediment load provided by the Mekong River is pivotal for the productivity of Tonle Sap [START_REF] Arias | Impacts of hydropower and climate change on drivers of ecological productivity of Southeast Asia's most important wetland[END_REF]. Kummu et al. (2008) analysed sediment exchange between Tonle Sap Lake and Mekong River for the period 1997-2003 and found a net sediment inflow from the Mekong into the lake. In contrast, Lu et al. (2014a) studied the period 2008-2010 and concluded that the net sediment transfer was from the lake to the river.

Given the critical role of the sediment and nutrient transport linkage between the Mekong mainstem and Tonle Sap River for the highly productive Tonle Sap Lake system and for sediment delivery to the Mekong delta, the objective of our study was to comprehensively analyse the sediment exchange between Tonle Sap Lake and the mainstem Mekong River over 25 consecutive hydrological years from 1993-2018.

Materials and Methods

Study area

Located in Southeast Asia, the Mekong River is the world's 12 th longest river (4,800 km), has the 8 th largest average annual runoff (470 km 3 ) and the 21 st largest basin area (795,000 km 2 ). The basin of Mekong River is commonly subdivided into the Upper Mekong basin (or UMB) covering 24% of total basin (21% in China, 3% in Myanmar), and the Lower Mekong basin (or LMB), with 76% of the total basin area (Laos 25%, Thailand 23%, Cambodia 20% and Vietnam 8%) (Figure 1).

The Mekong Basin is characterized by distinctly wet and dry seasons, driven by the Southwest Monsoon. The wet season commonly extends from May to late September (or early October). The average rainfall of Mekong floodplain of Cambodia and the delta of Vietnam annually are up to 1,500 mm, while the maximum intensity occurs in central Laos. At elevations above 500 m above mean sea-level, dry season temperatures are lower, though not by much. The average rainy season temperatures greatly decline through south to north, from 26-27°C at Phnom Penh Cambodia to 21-23°C at northern part of Thailand. The discharge of Mekong river reaching to the sea averages 15 000 m 3 /s [START_REF] Adamson | The hydrology of the Mekong River, The Mekong[END_REF][START_REF] Gupta | The Mekong from satellite imagery: A quick look at a large river[END_REF].

In Cambodia, the Mekong River connects with Tonle Sap Lake (TSL) (the largest permanent freshwater lake in the Southeast Asia) via Tonle Sap River at Chaktomuk confluence at Phnom Penh (Figure 1). Tonle Sap Lake was seasonally influenced by Mekong River flood pulse. The lake is approximately 120 km long and 35 km wide and covers 2,500 km 2 in the dry season, but it expands up to 250 km long and 100 km wide, and covers 17,500 km 2 in the wet season because high stages in mainstem Mekong drive flow into TSL through the Tonle Sap River [START_REF] Campbell | Tonle Sap Lake, the heart of the lower Mekong[END_REF]. From October to April, flow in the mainstem Mekong recedes, and flows reverse direction, draining from Tonle Sap Lake to the Mekong mainstream via Tonle Sap River [START_REF] Fujii | Hydrological roles of the Cambodian floodplain of the Mekong River[END_REF][START_REF] Masumoto | Modelling of multi-functional hydrologic roles of tonle sap lake and its vicinities[END_REF]. The majority of water in the Lake in the wet-season is from the Mekong mainstem (Lu et al., 2014b), and the delayed release of this water provides important fresh water to the Mekong delta in Cambodia and Vietnam in low flow period (dry season) and prevents saltwater intrusion from the sea into fertile agricultural lands [START_REF] Hai | Development of a two-dimensional finite element model for inundation processes in the Tonle Sap and its environs[END_REF].

We selected three stations with continuous records of discharge and total suspended solids: The Mekong River at Kratie, the Mekong at Chroy Changvar (just upstream of the Tonle Sap confluence) reflecting discharge and sediment load above interactions with the Tonle Sap, and the Prek Kdam station on the Tonle Sap River. Background information on these three stations of this study is detailed in [START_REF] Vinhteang | Temporal Variation of Sediment and Nutrients (Nitrate and Phosphorus) Dynamics in the Mekong River in Cambodia, The 4th International Symposium On Conservation and Management of Tropical Lakes "Featuring Environmental Health and Lake Basin Management[END_REF]. We analyzed flow and sediment by hydrological year from May 1 to April 30 following year (Kummu and Sarkkula, 2008;Lu et al., 2014b).

Water Discharge and Total Suspended Solids Data

Daily water discharges were generated at the three sites using rating curves and water level data provided by the Cambodian Ministry of Water Resources and Meteorology (MOWRAM) during period 1993 to 2018. Also, from MOWRAM, we obtained total suspended solids (TSS, hereafter referred as suspended sediment) concentration data collected under the Mekong River Commission framework: Water Quality Monitoring Network (WQMN). The WQMN dataset has been used to estimate suspended sediment loads in previous studies such as [START_REF] Kummu | Sediment-related impacts due to upstream reservoir trapping[END_REF], Kummu et al. (2008) and [START_REF] Wang | Sediment load estimates and variations in the Lower Mekong River[END_REF]. TSS concentrations were obtained from water quality sampling conducted on a monthly basis by MOWRAM from 1995 to 2018 on the Mekong at Kratie, from 1993 to 2017 on the Mekong at Chroy Changvar, and from 1995 to 2018 on the Tonle Sap River at Prek Kdam. Table 1 summarizes the data (TSS and water discharge) coverage period and number of samples used in this study. In the WQMN sampling program, individual samples were collected 0.3 m below the surface in the middle of the river cross section. The samples were analyzed at laboratories designated by MRC, following recommended analytical methods for TSS analysis (2540-D-TSS-SM) [START_REF] Kongmeng | Lower Mekong Regional water quality monitoring report[END_REF]. The reader should bear in mind that these were not depth-integrated samples, but single, near-surface "grab" samples from the approximate mid-point of the river. To the extent that the suspended sediments in the river are well-mixed, the samples may be representative of true loads, but if concentrations are heterogeneous in the vertical columns or across the channel, the measured TSS may not be representative of the average sediment concentration channel-wide, and may under-represent suspended sand especially, as sand concentrations would tend to greater near the bed.

Moreover, total suspended solids (TSS) may not be directly comparable to suspended sediment concentration (SSC) determinations, because TSS samples are collected from only a single point (0.3 m below the water surface mid-river) in the cross section, while SSC samples are collected from the full water column and at multiple points on the cross section. If the sediments are not well-mixed through the water column but stratified, the two methods can yield different suspended sediment concentrations. Moreover, there are differences in analytical methods that can yield different particle-size distributions [START_REF] Gray | Comparability of suspended-sediment concentration and total suspended solids data[END_REF][START_REF] Murphy | Changing suspended sediment in United States rivers and streams: linking sediment trends to changes in land use/cover[END_REF]. SSC is determined by measuring the dry weight of all sediment from a water sample of a known volume, normally large samples (1 L) which to represent the entire water column. Several techniques are used to determine TSS, and most techniques similarly measure the dry weight of all sediment from a water sample of a known volume. However, as defined by the TSS protocol (APHA, 1995) this technique weighs the sediment in only a 100-250 mL sub-sample from the original water sample.

As described by [START_REF] Gray | Comparability of suspended-sediment concentration and total suspended solids data[END_REF], generally, only finer suspended particle sizes are captured by TSS, while SSC characterizes the entire suspended particle-size distribution of the water column. The bias of TSS compared to SSC is most important at sites with larger proportions of sand-sized sediment. In the Lower Mekong River, the suspended sediments are generally dominated by silt and clay [START_REF] Walling | Evaluation and analysis of sediment data from the Lower Mekong River[END_REF]. [START_REF] Koehnken | IKMP discharge and sediment monitoring program review, recommendations and data analysis[END_REF] found that from downsteam of Pakse, suspended sediments are mainly composed of silt-and clay-size particles, and typically at Kratie all suspended sediments size are less than 63 µm. Generally, suspended particles smaller than 60 µm are uniformly vertically distributed in the water column [START_REF] Partheniades | Unified view of wash load and bed material load[END_REF]. Given that the suspended sediments are well-mixed in the water column, the load estimation using TSS should be comparable to the load estimation using SSC in the Lower Mekong River. [START_REF] Allison | Accumulation rates and stratigraphic character of the modern Atchafalaya River prodelta, Louisiana[END_REF]-20181993[START_REF] Chalov | A toolbox for sediment budget research in small catchments[END_REF][START_REF] Allison | Accumulation rates and stratigraphic character of the modern Atchafalaya River prodelta, Louisiana[END_REF]-2018[START_REF] Allison | Accumulation rates and stratigraphic character of the modern Atchafalaya River prodelta, Louisiana[END_REF]-2018-Discharge data 1995-20181993[START_REF] Chalov | A toolbox for sediment budget research in small catchments[END_REF][START_REF] Allison | Accumulation rates and stratigraphic character of the modern Atchafalaya River prodelta, Louisiana[END_REF]-2018[START_REF] Allison | Accumulation rates and stratigraphic character of the modern Atchafalaya River prodelta, Louisiana[END_REF]-2018 Daily

Sediment Load Estimation and Trend analysis of annual sediment load

LOADEST: Sediment Loads were estimated at Kratie, Chroy Changvar, and Prek Kdam Station using the LOAD ESTimator (LOADEST) [START_REF] Runkel | Load Estimator (LOADEST): A FORTRAN program for estimating constituent loads in streams and rivers[END_REF]. LOADEST provides three methods for load estimation, such as Maximum Likelihood Estimation (MLE), Adjusted Maximum Likelihood Estimation (AMLE), and Least Absolute Deviation (LAD) (Table 2) [START_REF] Vinhteang | Temporal Variation of Sediment and Nutrients (Nitrate and Phosphorus) Dynamics in the Mekong River in Cambodia, The 4th International Symposium On Conservation and Management of Tropical Lakes "Featuring Environmental Health and Lake Basin Management[END_REF].

M o i i m j L a a X m s V = = + ∑ AMLE Normal distribution  2 1 exp( ).H( , b, , , ) M o i i j L a a X m s α κ = = + ∑
Where -m is the number of degree of freedom, 2 s is the residual variance, and V is a function of the

explanatory variables. - 2 [ ( , , )] m g m s V
is the bias correction factor -of the maximum likelihood method.

-a and b are functions of the explanatory variables.

-α and κ are parameters of the gamma distribution.

-2 [ ( , , , , )] H a b s α κ is the bias correction factor.

-e is the residual error, and n is the number of uncensored observation in the calibration dataset.

Trend analysis of annual sediment load

To detect if trends can be identified in annual or seasonal series, we applied the Mann-Kendall, a non-parametric test originally proposed by [START_REF] Mann | Nonparametric tests against trend[END_REF] and updated later by [START_REF] Kendall | Rank correlation measures[END_REF]. The test has 3 alternative hypotheses in the series evolution: negative (i.e., decreasing trend), null (no trends in the series), and positive (increasing trend) [START_REF] Howden | Statistical analysis of nitrate concentrations from the Rivers Frome and Piddle (Dorset, UK) for the period 1965-2007[END_REF]. Pre-whitening (PW) is used to eliminate serial correlation [START_REF] Yue | Applicability of prewhitening to eliminate the influence of serial correlation on the Mann-Kendall test[END_REF].

Results

Water

LAD

Non-normal distribution

 1 1 exp( ) exp( ). n k M k o i i j e L a a X n = = = + ∑ ∑
We calculated descriptive statistics such as minimum, maximum, mean, and standard deviation to illustrate the temporal distribution of important water quality parameters and to highlight specific characteristics. Additionally, we calculated skewness and kurtosis to determine if the data were skewed with positive or negative tails, and to determine the peakness of the distributions, respectively. The summary statistics of TSS concentrations from 1993-2017 at Kratie and Chroy Changvar are summarized in Table 3. Between June and October, water in the Mekong River was opaque, with TSS concentrations from 90 to 200 mg/l as it carried sediments derived from erosion across the basin. On the falling limb beginning in December, the TSS concentration drop to less than 20 mg/l on average. Figure 3 illustrates the rating curve of observing daily water discharge and TSS at Kratie and Chroy Changvar station (1993 to 2017). Discharge and TSS concentrations showed significant statistical relations, with R 2 =0.53 for Kratie station and R 2 =0.65 for Chroy Changvar.

The TSS and Q relations in rivers, especially for a large river like the Mekong, are typically governed by multiple and relatively complex processes, which can produce hysteresis loops. [START_REF] Higgins | Suspended sediment transport in the Magdalena River (Colombia, South America): Hydrologic regime, rating parameters and effective discharge variability[END_REF] outlined four factors affecting hysteresis loops: a sudden increase in rainfall, the availability of sedimentary material in riverbeds, the interaction of deep soil layers, and aggressive flooding that causes the damming of sediment in flood plains upstream. [START_REF] Williams | Sediment concentration versus water discharge during single hydrologic events in rivers[END_REF] identified five common types of hysteresis loops: a single-valued line, a clockwise loop, a counter-clockwise loop, a single-valued line plus a loop, and a figure-8 loop. However, our TSS data set consists of monthly samples, and hysteresis loops would be evident only in data collected on a shorter time step, such as daily. To identify possible hysteresis loops and their possible causes would require in-depth studies such those undertaken by [START_REF] Pietroń | Model analyses of the contribution of in-channel processes to sediment concentration hysteresis loops[END_REF] and [START_REF] Chalov | A toolbox for sediment budget research in small catchments[END_REF]. 4). TSS concentrations for outflow from lake to the Mekong (October to March) averaged 41 mg/l, while TSS concentrations during reverse flows from the Mekong River towards TSL (May to September) averaged 74 mg/l. Thus, TSS concentrations of the inflow were nearly twice the concentrations of the outflow from TSL, implying a net transfer of sediment into the Lake

Temporal Variability of Water Discharge and Sediment Load in the Mekong River at Kratie and Chroy Changvar

Annual water discharge and computed sediment loads on the mainstem show higher flows and sediment loads at Kratie than at Chroy Changvar (Figure 6). Annually, 80% of annual flow occurred during rainy season (from May to October), while 20% of the annual flow occurred during dry season (from November to April). On average, the annual water discharge in Kratie was 404,000 Million cubic meters (Mm 3 /yr), 36,000 Mm 3 /yr higher than the annual water discharge at Chroy Changvar (368,000 Mm 3 /yr). This pattern is similar to the pattern of the flow in Kratie and Stung Treng (Cambodia-Laos border, about 150 km upstream of Kratie). MRC This overland flow was reported to average 2,500 Mm 3 /yr by Lu et al. (2014b). On the left bank of the main river at Kampong Cham, part of the Mekong flow is partly diverted into the Tonle Toch River, which then discharges back into the Mekong further downstream of Chroy Changvar in the Mekong delta. The bypass discharge by this river has not been previously reported in the literature. The trend analysis confirmed a decreasing trend in annual water discharge at Chroy Changvar (statistically significant (p<0.05)), but we found no significant trend at Kratie upstream (Table 5).

The annual sediment load over the 24-year period averaged 72±26 Mt/yr at Kratie, 78±22 Mt/yr at Chroy Changar (Figure 6 The monthly discharge and sediment load distribution curves of the entire recorded periods The seasonal outflow of TSL averaged 68,000 Mm 3 , which was equivalent to 18% of annual discharge of Mekong River at Chroy Changvar. The seasonal reverse flow into the lake from the Mekong River averaged 36,000 Mm 3 , about 10% of annual discharge of Lower Mekong River at Chroy Changvar. Thus, the outflow from the lake was almost twice the reverse flow from the Mekong River. Some of this difference can be attributed to runoff from the 760,000 km 2 drainage lake basin, some to overbank flow from the Mekong mainstem across floodplains into the lake.

Mean annual overland runoff into the lake was estimated as 2,600 Mm 3 , while runoff from the drainage basin of the lake was nearly the same, about 2,700 Mm 3 (Lu et al., 2014b). Looking at temporal trends, the reverse flows to the lake showed decreases over the 24-year period, but these were not statistically significant (p, 0.05); outflows also showed decreases, which were significant (p<0.05) (Table 6).

Net annual sediment loads averaged 3.7 Mt in reverse flows into Tonle Sap Lake, equivalent to 4.8% of annual average sediment load at Chroy Changvar of the Mekong mainstem. (Figure 9).

The sediment load from the lake to Mekong River was 2.9 Mt/yr, which is equal to 3.7% of average annual sediment load in Chroy Changvar. Thus, we can estimate that Tonle Sap Lake gained an average of 0.8 Mt of sediment annually from the Mekong River. However, as this term is calculated as a residual, it must be treated with caution, as such residual terms incorporate and hide errors in other terms [START_REF] Kondolf | Unmeasured residuals in sediment budgets: a cautionary note[END_REF]. Sediment loads in reverse flow increased over the observed period (statistically significant at p<0.05), while sediment load of the lake outflow evinced no trend. Sediment Load Linkage between Tonle Sap Lake and Mekong River and Towards the

Mekong Delta

The

Discussion

Sediment load in the Lower Mekong River

Estimation of the total flux of Mekong to the delta is complicated by interactions with the Tonle Sap system [START_REF] Kummu | Sediment-related impacts due to upstream reservoir trapping[END_REF] However, the more recent estimates cover relatively short periods after major dam construction in the upper Mekong mainstem. Looking over longer time scales reflected in stratigraphic analysis of Holocene sediment cores, [START_REF] Ta | Holocene delta evolution and sediment discharge of the Mekong River, southern Vietnam[END_REF] proposed 144±34 Mt/year of long-term mean sediment load to the sea, which is consistent with pre-dam estimate of 145 Mt/year by [START_REF] Liu | Changes in the sediment load of the Lancang-Mekong River over the period 1965-2003[END_REF].

As noted above, our study used TSS data from single-depth, mid-channel samples collected as part of a larger water quality dataset (WQMN). It would be preferable to use depth-integrated samples, but these data are extremely limited in time and space [START_REF] Koehnken | IKMP discharge and sediment monitoring program review, recommendations and data analysis[END_REF]. Prior studies also relied on TSS from single-depth mid-channel samples to estimate sediment load in the Lower Mekong

River [START_REF] Kummu | Sediment-related impacts due to upstream reservoir trapping[END_REF], (Kummu et al., 2008) and [START_REF] Wang | Sediment load estimates and variations in the Lower Mekong River[END_REF]. [START_REF] Wang | Sediment load estimates and variations in the Lower Mekong River[END_REF] compared the sediment loads calculated at Chiang Saen in Laos (the most upstream of Lower Mekong) using both the TSS data from the WQMN database and the suspended sediment concentrations (SSC) derived from depth-integrated sampling from the HYMOS dataset (another MRC dataset). This comparison revealed no underestimation using the TSS data at Luang Prabang and Khong Chiam (Laos). [START_REF] Walling | The changing sediment load of the Mekong River[END_REF]-2010, Lu et al. (2014a) collected suspended sediment samples at 3 evenly-spaced points across the Mekong and Tonle Sap rivers, at 3 depths each (0.5m, 0.2 and 0.8 depth), and found "no major differences in suspended sediment concentrations within the profile."

The results of our trend analysis of sediment load are broadly consistent with patterns observed in major rivers in the region and the world. With the exception of the Amazon, whose suspended sediment discharge increased about 20% 1995-2007 (while water discharge held steady) [START_REF] Martinez | Increase in suspended sediment discharge of the Amazon River assessed by monitoring network and satellite data[END_REF], most rivers have displayed decreases in sediment load. [START_REF] Vörösmarty | Anthropogenic sediment retention: major global impact from registered river impoundments[END_REF] suggested that large reservoirs controlled over 40% of runoff of global rivers and trap more than half of the pre-dam sediment load. Walling and Fang (2003) assessed sediment load trends for 145 rivers globally and found that most showed decreasing sediment loads. [START_REF] Li | Global trends in water and sediment fluxes of the world's large rivers[END_REF] assessed water and sediment fluxes measured at 138 rivers and found that finding 50 rivers had stable water flows but decreasing sediment flux, while 41 rivers decreased in both water and sediment fluxes.

Mean annual water and sediment fluxes to sea from the world's large rivers decreased by 58-98 km 3 /yr (0.25%) and 2617-2715 Mt/yr (20.8%) respectively over the recent five to ten year timescale, mainly due to the reduction in water and sediment fluxes that occurred in Asia (by 1.6%-2.0% in water discharge, and by 13.1%-13.2% in sediment fluxes [START_REF] Li | Global trends in water and sediment fluxes of the world's large rivers[END_REF].

Displaying a different trend, the largest rivers of North and East Asia (Russia and China), such as Yenisei, Amur, Yellow, Yangtze, Pearl Rivers have experienced suspended sediment load reduction attributed to sediment trapping by dams [START_REF] Chalov | Environmental and human impacts on sediment transport of the largest Asian rivers of Russia and China[END_REF]). An earlier study by [START_REF] Liu | Analysis on variation of seagoing water and sediment load in main rivers of China[END_REF] found that major rivers in Southern China (e.g., Yangtze, Qiantang, and Pearl rivers)

were transporting only about 60% to 80% of their pre-dam loads, while their annual runoff was reported relatively stable. [START_REF] Gupta | The role of mega dams in reducing sediment fluxes: A case study of large Asian rivers[END_REF] estimated that the combined annual sediment flux of the large Chinese rivers decreased from 1800 Mt to about 370 Mt over the previous five decades.

On the Mekong itself, a number of authors have converged on significant impacts to the river's sediment load. The first mainstem dam was the Manwan Dam on the Upper Mekong (Lancang) in China in 1993, which reduced sediment loads by about 60% [START_REF] Fu | Sedimentation in the Manwan reservoir in the Upper Mekong and its downstream impacts[END_REF]. Projecting effects of completing 8 large mainstem dams on the Lancang, Kummu et al. ( 2010) estimated a cumulative reduction by half of the river's annual load of 140 million tons. On the Lower Mekong, 11 mainstream dams are planned, of which three (Xayaburi, Pak Beng and DonSahong) are built or under construction [START_REF] Fox | Political borders, epistemological boundaries, and contested knowledges: Constructing dams and narratives in the Mekong River basin[END_REF]. The future of the proposed Sambor dam in the Kratie Province of Cambodia is uncertain, but if built as originally proposed, it would likely trap 38Mt/y of an estimated 77Mt/y that would be delivered to the dam under a scenario of 38 dams built on the mainstem and tributaries [START_REF] Wild | Mitigating dam conflicts in the Mekong River Basin, Conflict resolution in water resources and environmental management[END_REF].

The concern stems not only from dam development on Mekong mainstem but also on its main tributaries. For example, the Sekong, Sesan, and Srepok basin (so-called '3S basin', the largest tributary of the Mekong) formerly contributed a large portion of the sediment load reaching TSL and the Delta [START_REF] Wild | Managing flow, sediment, and hydropower regimes in the Sre Pok, Se San, and Se Kong Rivers of the Mekong basin[END_REF], but this has been mostly cut off by the recently completed Lower Sesan 2 dam, which blocks the Srepok and Sesan. Overall, construction of the approximately 140 dams planned for the Mekong and its tributaries would trap 96% of the sediment that was naturally delivered to the Mekong Delta [START_REF] Kondolf | Dams on the Mekong: Cumulative sediment starvation[END_REF]. Reduced sediment supply threatens the long-term sustainability of the Mekong Delta [START_REF] Campbell | Perceptions, data, and river management: Lessons from the Mekong River[END_REF][START_REF] Saito | Shrinking megadeltas in Asia: Sea-level rise and sediment reduction impacts from case study of the Chao Phraya Delta[END_REF]. Beside the dam development in Mekong River Basin, climate change and land-use change are also the crucial factors affecting sediment transport in the basin. Deforestation alters basin erosion, increases sediment delivery to reservoirs and can decrease productivity of hydropower [START_REF] Kaura | Benefits of forest conservation on riverine sediment and hydropower in the Tonle Sap Basin, Cambodia[END_REF]. [START_REF] Shrestha | Impact of climate change on sediment yield in the Mekong River basin: a case study of the Nam Ou basin, Lao PDR[END_REF] point to uncertainties in the direction and magnitude of variability of flow and sediment yields due to climate change, and [START_REF] Darby | Fluvial sediment supply to a mega-delta reduced by shifting tropicalcyclone activity[END_REF] have documented reductions in sediment supply from the basin as runoff from tropical cyclones, delivering about 32% of the suspended sediment load reaching the delta, has been halved since 1981 as cyclone tracks shifted north.

Our study did not explicitly include bed-load due to a lack of data. Suspended sediment load data are more common globally because suspended sediment load is the dominant component of total load and is easier to measure [START_REF] Milliman | River discharge to the coastal ocean: a global synthesis[END_REF][START_REF] Walling | The impact of global change on erosion and sediment transport by rivers: current progress and future challenges[END_REF]. In the absence of direct measurements of bed load transport, it is frequently assumed that the bed load is up to 10% of the total load [START_REF] Gregory | Drainage basin form and process[END_REF][START_REF] Milliman | World-wide delivery of river sediment to the oceans[END_REF]. For most larger rivers, the bed load much less, e.g. <2% for the Yukon River of North America [START_REF] Brabets | Environmental and hydrologic overview of the Yukon River Basin[END_REF]; < 5% for the lowermost Mississippi River [START_REF] Nittrouer | Bedform transport rates for the lowermost Mississippi River[END_REF]. On the Mekong River, [START_REF] Koehnken | IKMP discharge and sediment monitoring program review, recommendations and data analysis[END_REF] reported bed-load in Kratie measured in 2011 was only 1.6 Mt/year, approximately 1.4% of suspended sediment load.

The Mekong River and major regional rivers in term of sediment

Of the total natural global sediment flux to the oceans of about 12.6 to 18 Gt/year, Asia exported the most sediments (~4.8 Gt/year) among continents [START_REF] Gordeev | Fluvial sediment flux to the Arctic Ocean[END_REF][START_REF] Syvitski | Sediment flux and the Anthropocene[END_REF][START_REF] Syvitski | Impact of humans on the flux of terrestrial sediment to the global coastal ocean[END_REF]. High sediment loads are a common feature of many Asian rivers, especially those originating from the Himalayan-Tibetan Plateau, such as the Yellow Rivers, the Yangtze, the Red and the Mekong, and due to the pronounced topographic relief of the region [START_REF] Evans | Water quality: assessment of the current situation in[END_REF][START_REF] Ludwig | River sediment discharge to the oceans; present-day controls and global budgets[END_REF][START_REF] Milliman | Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers[END_REF]. However, Africa and Asia showed the largest reduction in sediment flux to the coast in rivers (such as the Nile, Orange, Niger, and Zambezi in Africa and the Yangtze, Indus, and Yellow in Asia), and 31% of the total sediment load retained in reservoirs were indicated in Asia and 25% in Africa [START_REF] Syvitski | Impact of humans on the flux of terrestrial sediment to the global coastal ocean[END_REF]. The world's largest river, the Amazon exports around ~550-1500 Mt/yr of sediment to the Atlantic [START_REF] Dunne | Exchanges of sediment between the flood plain and channel of the Amazon River in Brazil[END_REF][START_REF] Gaillardet | Chemical and physical denudation in the Amazon River Basin[END_REF][START_REF] Guyot | Régime et bilan du flux sédimentaire de l'Amazone à Óbidos (Pará, Brésil) de 1995 à[END_REF][START_REF] Martinez | Increase in suspended sediment discharge of the Amazon River assessed by monitoring network and satellite data[END_REF][START_REF] Meade | Sediment loads in the Amazon River[END_REF]. The Mekong's annual sediment load is comparable to loads reported for other major rivers in Asia and elsewhere (Table 7).

Sediment exchange between Tonle Sap River and Mekong mainstream

As reported above, our analysis shows that from 1995 to 2000, the Tonle Sap contributed more sediment load to Mekong River than was deposited in the lake, on the average 0.65 Mt annually, but the rate decreased, and then since 2001, an average net 1.35±0.7 Mt of sediment has been deposited in the lake annually. Our results are in the same range but differ from prior studies, which covered shorter time periods, such as Kummu et al. (2008) average sediment load inflow of 6.3 Mt/year into the lake from Mekong mainstem and 7 Mt/year in the outflow, for a net deposition within Tonle Sap Lake, inconsistent with our results. Our different method for load estimation, Lu et al. (2014a) estimated sediment load base on the developed sediment rating curve and discharge at a given station. By using this, the estimated load can be represented a natural chrematistic flow and concentration with a specific time, however, it cannot be represented or cannot cover the range of characteristic of the water and sediment dynamic in the past and near future. While our estimation base on regression model that developed base on wide range of dataset that would be able to capture characteristic of the water and sediment dynamic for long period. Even there are some inconsistency, our study can provide anther confirmation to the conflict of previous study and better understand the sediment dynamic between

Mekong River and Tonle Sap Lake with longer study periods and extend the discuss within basin scale and reginal scale.

An assessment of water discharge and sediment loads variability of Mekong River and Tonle Sap system presented in this study helps clarify the exchange annual discharge and sediment load toward the Mekong delta. Tonle Sap Lake provided sediment load to the Mekong system and delta annually 0.65±0.6 Mt from 1995 to 2000, but since 2001 Tonle Sap Lake has become a sediment sink for about 1.35±0.7 Mt annually, thereby reducing the annual sediment transport to the Mekong delta. This reduction in sediment supply compounds the threat to the delta from accelerated subsidence and sea level rise [START_REF] Pokhrel | A review of the integrated effects of changing climate, land use, and dams on Mekong river hydrology[END_REF][START_REF] Syvitski | Going under: The world's sinking deltas[END_REF]. Decreased sediment loads to the delta and altered sediment transport processes will impact numerous livelihoods which depend on ecosystem services services [START_REF] Kondolf | Changing sediment budget of the Mekong: Cumulative threats and management strategies for a large river basin[END_REF]. The sudden change appears to be due to increased TSS concentrations from the Mekong to Tonle Sap Lake.

The concentration of TSS in Kratie appears to have been largely unchanged, but the river could have picked up sediment as it overflowed the wide floodplain, used for agriculture and thus exposed to erosion without the protection of native vegetation [START_REF] Chea | Evidence of water quality degradation in lower Mekong basin revealed by self-organizing map[END_REF]. The instream TSS levels in the lower part of the Mekong River are likely influenced by the interaction between land use/land cover, rainfall-runoff, and anthropogenic activities within the basin [START_REF] Ly | Linking Changes in Land Cover and Land Use of the Lower Mekong Basin to Instream Nitrate and Total Suspended Solids Variations[END_REF].

The Tonle Sap Compared with Other Lake-Channel Systems

The Tonle Sap is an example of a lake-channel system, a lake (usually on a floodplain) that connects with a main river (via defined channels as well as overbank flow), and that absorbs flood peaks and releases waters gradually back into the main river as flood stage recedes. Retaining floodwaters in the lake for extended periods of time can result in substantial deposition of sediment from suspension, with potentially significant influences on the riverine sediment budget. Thus, we can ask how the Tonle Sap system compares with other major lake-channel systems, such as Dongting Lake on the Yangtze River floodplain, the second largest freshwater lake in China, and the channel-floodplain systems of the Amazon River.

The Dongting Lake-Yangtze River system is similar to the floodplain lake-channel exchanges of TSL and Mekong River, in that Dongting Lake plays an important role in regulating flood stage and is an important sink of sediments. In flood season water pours into Dongting Lake through three natural distributary channels from the Yangtze (just downstream of its exit from Three Gorges onto the broad alluvial plain) to Dongting Lake. Unlike the Tonle Sap system, when Dongting Lake drains, it does not flow back through the same channel to the river, but drains through different channels back to the river when river stage declines. (Thus, the lake and its connecting channels function as a flood bypass and backwater.) The Yangtze River and Dongting Lake are strongly influenced by operation of The Three Gorges Project upstream. In addition to an important flood-flow-regulating function, Datong Lake functions as sediment sink, receiving 110 Mt/yr from the Yangtze River, while losing only 39 Mt/yr via return flow back to the Yangtze [START_REF] Dai | Impact of the Three Gorges Project operation on the water exchange between Dongting Lake and the Yangtze River[END_REF]. The area of Dongting Lake has been reduced over the centuries by construction of dykes to protect farmland encroaching onto the fertile lake bottom during dry years between big floods, a problem that was documented at least back to the Ming Dynasty and continued through the 20 th century [START_REF] Perdue | Official goals and local interests: Water control in the Dongting Lake region during the Ming and Qing periods[END_REF].

The Amazon is characterized by strong exchanges of water, sediment, nutrients, and biota between channel and its extensive floodplains. The Lago Grand de Curuaı´ floodplain is a complex system of more than 30 interconnected lakes, linked to the Amazon River by several channels, two of which (both with permanent flow) carry return flow from the floodplain into the mainstream [START_REF] Bourgoin | Temporal dynamics of water and sediment exchanges between the Curuaí floodplain and the Amazon River[END_REF]. Lago Grand de Curuaı´ floodplain is a sediment sink, with more than 80% of the suspended solids entering the floodplains deposited [START_REF] Mertes | Channel-floodplain geomorphology along the Solimões-Amazon river[END_REF], but the water storage in the floodplain is more transient than that of the Tonle Sap Lake system, which contributes baseflow back to Mekong River and delta through most of the dry season, as demonstrated in our analysis. On the Amazon floodplain, sediment accumulation (simulated deposition rate is of about 0.3 Mt km -1 yr -1 ) occurs during the five months of the flood rise, and export of sediments to the mainstream occurs during the recession limb of the seasonal hydrograph [START_REF] Bourgoin | Temporal dynamics of water and sediment exchanges between the Curuaí floodplain and the Amazon River[END_REF][START_REF] Mangiarotti | Discharge and suspended sediment flux estimated along the mainstream of the Amazon and the Madeira Rivers (from in situ and MODIS Satellite Data[END_REF].

Thus, the channel-floodplain systems of the Amazon River can act as important sinks of sediments, not only via channelized flow to the Lago Grand de Curuaı´ complex, but also via diffuse overbank flow. However, unlike the Amazon, most of the Mekong's course follows a narrow bedrockcontrolled path, so there is very limited exchange of sediment between channel and floodplain, until the reach downstream of Kratie, where the floodplains of the Cambodian lowlands and the Mekong delta are inundated, allowing important fluxes of material and energy between the floodplain and mainstem river channel [START_REF] Gupta | The Mekong from satellite imagery: A quick look at a large river[END_REF]. With its channel sized reverse flow pattern, combined with broad, shallow lateral inundation of floodplains during the wet season, the Tonle-Sap-Mekong exchange represents a uniquely developed and important channelfloodplain exchange. In fact, the Mekong-Tonle-Sap exchange is arguably among the best developed such river-floodplain-lake exchange systems in the world, and it supports a fishery that is globally exceptional in many respects [START_REF] Campbell | Tonle Sap Lake, the heart of the lower Mekong[END_REF].

Conclusion

The study assessed temporal variability of sediment loads in Lower Mekong River in Cambodia

Figure 1 .

 1 Figure 1. Maps of the study area. (a): The Mekong river Baisin, (b): three sampling sites located at Kratie of the Mekong River, Chroy Changvar at Phnom Penh, and at Prek Kdam of Tonle Sap River. (c). the Chatumuk confluence located near Phnom Penh City.

Figure 2

 2 Figure 2 shows the hydrographs of the daily water discharge and observed TSS at Kratie from 1995 to 2018 and Chroy Changvar from 1993 to 2017. The daily water discharge clearly reflects the annual dry and rainy seasons, with low flows in March and April (down to approximately 2,000 m 3 /s), followed by onset of the rainy season and increased discharges in May, and high flows in August and September (up to approximately 30,000 m 3 /s). At Kratie, the maximum discharge reached 58,205 m 3 /s in the rainy season of 1996 while the minimum discharge was 1,073 m 3 /s in the dry season of 1995. At Chroy Changvar station, maximum discharge reached 40,556 m 3 /s in the rainy season of 2014, while minimum lowest discharge was 405 m 3 /s in the dry season of 1995.

Figure 2 :

 2 Figure 2: Variation in the water discharge at the daily time step and total suspended solids concentration (monthly time step) at (a) Kratie (1995 to 2018) and (b) Chroy Changvar (1993 to 2017). Data from Cambodia MOWRAM.

Figure 3 :

 3 Figure 3: Relationship of TSS concentration and daily water discharge in the Lower Mekong River at (a) Kratie (from 1995 to 2018) and (b) Chroy Changvar (1993 to 2017). Data from Cambodia MOWRAM

  . The sediment rating curves at Prek Kdam show distinct relations when plotted separately for outflow and reverse-flow periods (Figure 5), with TSS concentrations lower in outflows from TSL to the Mekong (Figure 5.a) than in reverse flows to TSL (Figure 5.b), with all parameters having only a weak relationship in linearity with discharge (R 2 < 0.1) in both flow periods.

Figure 4 .

 4 Figure 4. Variation in the water discharge at the daily time step and total suspended solids (TSS) at Prek Kdam (monthly time step). The negative values of concentration and discharge are (reverse) inflow into the TSL from Mekong River, while positive values are the outflow from the lake towards Mekong River. Data from Cambodia MOWRAM.

Figure 5 :

 5 Figure 5: Relationship of TSS concentration and daily discharge of Tonle Sap River at Prek Kdam for the period 1995-2018. (a) Outflow from TSL to Mekong River (in low water stage) and (b) Reverse flow direction from Mekong River to TSL (in high water stage). Data from Cambodia MOWRAM.

(

  2019) reported in their observed flows for the Mekong mainstream stations over the period 2000-2017, Stung Treng discharge is found higher than the downstream at Kratie. After Kratie the Mekong enters extensive floodplains and then delta. Water flow in this reach is very complex (due to downstream backwater effects, overbank flows and temporary water storage on the floodplain) especially during the flood season when hydraulic conditions define the flow distribution between different river branches. The downstream reduction in gauged flow at Kratie and Chroy Changvar occurred mainly at higher flows and can be attributed to overbank flow from the Mekong River traversing the floodplain to TSL and by flow into major distributaries between Kratie and Chroy Changvar. During the flood season, water starts to spillover both banks of the Mekong River between upstream of Kompong Cham (150 km upstream of Chroy Changvar) and Chroy Changvar station. Part of the water spilling over the right bank reaches the Tonle Sap Lake as overland flow.

  ). A pronounced temporal trend was visible at Kratie, where annual sediment loads decreased from 123 Mt/yr in 1995 to 40 Mt/yr in recent years, with the MK test documenting a statistically significant (p<0.05) decreasing trend. At Chroy Changar, sediment loads fluctuated over the study period (from 32 Mt in the dry year 1998 to 128 Mt in 2011), but no temporal trend was detected (Table6).

Figure 6 :

 6 Figure 6: Annual sediment load dynamics and water discharge in the Mekong river at (a) Kratie (hydrological year 1995-2017) and (b) Chroy Changvar stations (hydrological year 1993-2016).

(

  hydrological year 1995-2017 at Kratie, hydrological year 1993-2016 at Chroy Changvar) (Figure7) show similar monthly patterns in high and low sediment load (i.e., equal or exceeding 5% and 95%). However, monthly highest discharge differed significantly. The flow distribution at Kratie and Chroy Changvar confirmed the downstream reduction in gaged flow at Chroy Changvar occurring at higher flows. The peaks of discharge and sediment loads were 133 km 3 /month and 40Mt/month at Kratie, 95 km 3 /month and 33 Mt/month at Chroy Changvar.

Figure 7 :

 7 Figure 7: Monthly water discharge and loads sediment load distribution curves at Kratie and Chroy Changvar station.

Figure 8 :Figure 9 :

 89 Figure 8: Seasonal water discharge exchange between TSL and Mekong River at Prek Kdam station for the hydrological year 1995 to 2017. The negative values of discharge are (reverse)

  overall balance of flow and sediment load between TSL and Mekong River over the observation hydrological year 1995-2017 shows a net contribution of water from Tonle Sap Basin, as would be expected from such a large drainage area (Figure 10.a). 2004 was an exception, when the Mekong River at Chroy Changvar reached its maximum discharge 60,000 Mm 3 . The annual water balance indicates that TSL contributes an average of 33,000 Mm 3 during the low-flow period to the Mekong delta. However, the net sediment transfers changed over the observation period.From hydrological year 1995 to 2000, Tonle Sap River contributed more sediment load to Mekong River than it received via reverse flows, but from hydrological year 2001-2017 the lake received more sediment from the Mekong River than it contributed (Figure 10.b), averaging 1.35±0.7 Mt per year.

Figure 10 :

 10 Figure 10: Water discharge and sediment loads balance between TSL and Mekong River through Tonle Sap River at Prek Kdam station for hydrological years 1995 to 2017. (a) Net water discharge, (b) Net sediment load. Annual water discharge and sediment load obtained as residuals by subtracting seasonal water discharge and load in reserve flow from Mekong River and outflow from TSL. Negative values reflect net inflow to the TSL from Mekong River, while positive values are the net outflow from the lake to the Mekong River.

  since large volumes of flood water enter the Tonle Sap floodplain, where a part of the sediment loads deposited. Our estimated loads at Kratie (72± 38 Mt from 1993-2017) and Chroy Changvar (78±22 Mt from 1995-2018) are similar to those proposed by prior authors, such as Dang et al. (2016), who reported suspended sediment flux of 87±28 Mt/year (1981-2005), Lu et al. (2014a), who reported 50-91 Mt/year of suspended sediment load from 2008 to 2010, and Manh et al. (2014), who estimated 106 Mt/year at Kratie (2010-2011).

  who estimated for the period 1997-2003 that the TSL received a net transfer of sediment from the Mekong of 3.7 Mt/y, from an average of 5.1 Mt/year to TSL from the Mekong by reverse flow, and an average of 1.4 Mt/year from TSL to the Mekong via outflow. For the period 2008-2010, Lu et al. (2014a) estimated

  and the sediment linkage between Tonle Sap Lake and Mekong River from 1993 to 2017. Sediment load in the main Mekong River averaged 72± 38 Mt at Kratie and 78±22 Mt at Chroy Changvar from 1993-2018, showing a significant decrease in sediment load, consistent with trends documented in other major rivers in the region and globally. The seasonal and annual sediment load linkage between the Mekong mainstem and Tonle Sap Lake are controlled principally by suspended sediment concentrations and water discharge, in both the reverse flows into the lake and outflows from the lake, both via the Tonle Sap River. The annual water inflow to the lake via reverse flow was 36 km3, while outflow to the Mekong was 68 km3. The great water outflow can be attributed to runoff from the Tonle Sap Lake basin and also flows across the floodplain lying to the west of the Mekong River. TSS concentrations of the reverse flows to TSL from Mekong river were nearly twice the concentrations of the outflow from TSL, resulting in a net transfer of sediment into the Lake. The net sediment loads averaged 3.7 Mt in reverse flows into Tonle Sap Lake, 2.9 Mt/yr in flows from the lake to Mekong River. We found Tonle Sap Lake provided 0.65±0.6 Mt of net sediment to the Mekong annually from 1995 to 2000, but was a sediment sink for an average of 1.35±0.7 Mt annually from 2001 onwards. The change pattern of sediment load linkage can be attributed to a trend of increased TSS concentrations from the Mekong to Tonle Sap Lake. This change in sediment concentration constitutes the most important variable in explaining sediment linkages between the Mekong River and Tonle Sap Lake. This study helps clarify the sediment exchange between Mekong River and Tonle Sap Lake and reveals an important change since 2001, with implications for the sustainability of the delta.

Table 1 :

 1 Data coverage period and the number of total suspended solids sampling and water

	discharge
					Prek Kdam
			Kratie	Chroy Changvar	TSL	Mekong	Time step
					towards Mekong	towards TSL
	TSS (number of samples)	246	259	170	72	Monthly
	TSS	Data
	coverage period

Table 2 .

 2 Load estimation methods in LOADEST

	Loads Estimation method	Model Assumption	residuals	Function
	MLE	Normal distribution		exp(	2 ).g ( , , )
					1

Table 3 :

 3 Descriptive statistics of sediment concentration at Kratie and Chroy Changvar Note: Min, max, mean, and SD are concentrations in mg/l; Skewness and Kurtosis are dimensionless.

		Month	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Jan	Feb	Mar	Apr
		Min	3	8	26	29	31	6	31	2	6	2	3	2
	Kratie	Max Mean	167 35	232 96	402 159	355 157	363 175	303 118	271 108	112 37	51 17	70 14	50 13	44 12

Table 4 :

 4 Descriptive statistics include minimum values (min), maximum values (max), mean values, standard deviation (SD), skewness coefficients and Kurtosis of sediment concentration at Kratie from 1995-2018.

		From Tonle Sap Lake	From Mekong River
		towards Mekong River	towards Tonle Sap Lake
	Min	1.0	3.0
	Max	265	335
	Mean	41.7	74.0
	SD	39.6	58.1
	Skewness 2.55	1.54
	Kurtosis	8.27	4.46
	Note: Min, max, mean, and SD are concentrations in mg/l;
	Skewness and Kurtosis are dimensionless.

Table 5 :

 5 Summary of annual water Discharge and sediment load in the Mekong River at Kratie

	and Chroy Changvar
	Location	Average Maximum	Minimum Trend direction Significant level
	Water Discharge at Kratie (Mm 3 )	404,000 532,000	256,000	No trend

Table 6 :

 6 The percentage share of water discharge and sediment load between TSL and Mekong River at Chroy Changvar, averaged over hydrological year1995-2017.

	Flow stations/Direction	Water discharge Percentage sharing with Mekong at Chroy Changvar (%) (Million m 3 )	Annual Trend Trend direction	Significant level
	TSL to Mekong River 67 600	18	Decrease	Significant at α= 0.05
	Mekong River to TSL 3 600	10	No trend
	Mekong mainstem at Chroy Changvar	368 000
	Sediment stations/Direction	Sediment Load (Mt)	Percentage sharing with Mekong at Chroy Changvar (%)	Annual Trend Trend direction	Significant level
	TSL to Mekong River 2.9	3.7	No trend
	Mekong River to TSL 3.7	4.8	Increase	Significant at α= 0.05

Table 7 :

 7 The annual mean of sediment is comparable with other major rivers in Asia and continents.

	River	Region	Basin area

Acknowledgement:

Ty SOK would like to acknowledge Les Bourses du Gouvernement Français (BGF) and Laboratoire Ecologie Fonctionnelle et Environnement (UMR CNRS/Université de Toulouse) for hosting during his PhD study.

estimate of mean annual sediment load in reverse flow from the Mekong River mainstem to Tonle Sap lake of 4.2 Mt/year is lower than the estimation of Kummu et al. (2008) (5.1 Mt) and Lu et al. (2014a) (6.3 Mt). The mean annual sediment outflow from the Tonle Sap lake estimated in our study (3.1 Mt /year) is higher than mean value (1.4 Mt/year) estimated by Kummu et al. (2008) but lower than the mean value of 7 Mt/year calculated by Lu et al. (2014a).

By consider the same period (1997)(1998)(1999)(2000)(2001)(2002)(2003) with Kummu et al. (2008), we found that Tonle Sap Lake obtained 2.7 Mt from Mekong mainstem and returned 2.65 Mt annually while Kummu et al. (2008) suggested Tonle Sap Lake obtained 5.1 Mt from Mekong mainstem and returned 1.4 Mt. Kummu et al. (2008) used single-point surface water sample measuring TSS at Prek Kdam with monthly basis from MRC, which is the same data source as our study. This should be the reason, we come up with consistency. Whereas, the different in value might be from the different of method as we applied load estimator, LOADEST base on Least Absolute Deviation (LAD) method, while Kummu et al. (2008) applied sediment transport module of 3D EIA model. With the same study period (2008)(2009)(2010), we estimated the lake system received 3.9 Mt and shared 2.66 Mt of sediment annual from and to the Mekong mainstem. These values are lower than those suggested by Lu et al. (2014a) which was 6.3 Mt and 7 Mt. The inconsistency of the lake sink or source of sediment and the value between our study and Lu et al. (2014a) might come from various reasons. Firstly, the sampling points are different. We estimated the load from Prek Kdam stations in the Tonle Sap River, while Lu et al. (2014a) from Phom Penh port, which is only located few kilometers toward the Mekong mainstem. Secondly, the different in data source, we used water and sediment data collected by MOWRAM under the MRC water quality program. As noted above, these were not depth-integrated samples, so the accuracy of suspended load estimates from these depends on how well mixed are sediment concentrations vertically and horizontally.

Prior studies also relied on TSS from single-depth mid-channel samples to estimate sediment load in the Lower Mekong River e.g. [START_REF] Kummu | Sediment-related impacts due to upstream reservoir trapping[END_REF], Kummu et al. (2008) and [START_REF] Wang | Sediment load estimates and variations in the Lower Mekong River[END_REF]. While, sampling in 2008-2010 at multiple depths across the cross section of the Mekong at Chroy Changvar and the Tonle Sap River at Phnom Penh Port by Lu et al. (2014a) indicated that suspended sediment was well-mixed. However, within the MRC depth-integrated measurement campaign for year 2011, the inflow (6.4 t/yr) and outflow (1.5 Mt/yr) at Prek Kdam [START_REF] Koehnken | IKMP discharge and sediment monitoring program review, recommendations and data analysis[END_REF] were in line with our finding and Kummu et al. (2008). Thirdly, we used


Arnaud Taute
email: arnaud.taute@ec-lyon.fr

Régis Orobtchouk

Eva Kempf

Stéphane Monfray

Lotfi Berguiga

Xavier Letar- Tre

Sébastien Cueff

Emerging Low-Loss Phase Change Material Enabling Non-Volatile Phase Modulation

Sb 2 S 3 is an emerging phase change material featuring a promising transparency in the near infrared region. In this work we take advantage of the refractive index change of this material to control the propagation of light inside an optical waveguide to achieve phase modulation. This opens the door to active optical functions with very limited absorption.

Introduction

Phase change materials (PCMs) such as Ge 2 Sb 2 Te 5 have recently become a popular class of materials to actively tune integrated photonic devices. These PCMs are exploited for their unique physical properties, and most notably their ability to rapidly and reversibly switch between a crystalline and an amorphous phase exhibiting widely different optical and electrical properties. For example the tunability of their electrical behaviour, especially in resistivity, is industrially developed to create phase change memories [START_REF] Ahn | Highly manufacturable high density phase change memory of 64Mb and beyond[END_REF]. On the other hand, the dynamic modulation of their optical properties, i.e. refractive index and extinction coefficient, are exciting opportunities to create integrated amplitude modulators and switches [START_REF] Yu | Ultracompact electro-optical modulator-based Ge 2 Sb 2 Te 5 on silicon[END_REF][START_REF] Leonardis | Broadband electro-optical crossbar switches using low-loss Ge 2 Sb 2 Se 4 Te 1 phase change material[END_REF], to develop active optical function with tunable metasurfaces (working in reflection or transmission), or even to control the reflectivity of a sample without patterning [START_REF] Cueff | Reconfigurable Flat Optics with Programmable Reflection Amplitude Using Lithography-Free Phase-Change Material Ultra-Thin Films[END_REF]. The current roadblock that limits the applicability of conventional PCMs is their high optical absorption in the near-infrared region. Even though some lower-loss alternatives to Ge 2 Sb 2 Te 5 , such as GeSbSeTe have been reported [START_REF] Zhang | Reshaping light: reconfigurable photonics enabled by broadband low-loss optical phase change materials[END_REF], the level of optical absorption may still be too high for applications such as phase modulation in large scale photonic integrated circuits requiring a large refractive index change and transparency in the near-infrared region.

In this context, Sb 2 S 3 is a very promising emerging PCM since it has been demonstrated to present a large refractive index change upon crystallization and an ultra-low extinction coefficient down to the visible range [START_REF] Dong | Wide bandgap phase change material tuned visible photonics[END_REF]. The non-volatile nature of this index change is an advantage over conventional phase modulators relying on the plasma dispersion effect or the thermo-optic coefficient since continuous power is not required to maintain the phase modulation. Using the crystalline state of Sb 2 S 3 to control the effective index of a guided mode has been reported using a resonant microring [START_REF] Fang | Non-volatile reconfigurable integrated photonics enabled by broadband low-loss phase change material[END_REF]. Here we demonstrate the use of Sb 2 S 3 as a low-loss phase modulation material in the near-infrared region using a Mach-Zehnder interferometer.

Experiments

The base sample was fabricated by STMicroelectronics using SiN deposited by PVD as the waveguide material. A first 40 nm thick film of SiO 2 is deposited by electron beam evaporation on top of the base sample. We then patterned a thin film of 50 nm of Sb 2 S 3 deposited by electron beam evaporation on top of one of the arms of the MZI. The sample was subsequently capped using a 50 nm film of SiO 2 to avoid degradation of the film at high temperature.

A lensed optical fibre was used to inject light from a broadband light source into the MZI. This allows for the measurement of the transmission spectrum of the MZI between 1200 nm and 1700 nm. The MZI spectrum was first measured at room temperature, the Sb 2 S 3 film being in its as-deposited state. The sample was subsequently gradually crystallized by placing it on a temperature-controlled stage at the selected temperature. The sample was then allowed to cool down to room temperature before acquiring the MZI spectrum. This ensures that no crystallization occurs during the measurement and lifts the need to take thermo-optic effects into account.

Results

Optical microscope observations of the sample before and after the measurements confirm the crystallization of the film (Fig. 1). While the as-deposited film appears uniform, the crystalline structure of the Sb 2 S 3 is clearly visible after annealing. From the micrographs, the length of the portion of the waveguide covered by the Sb 2 S 3 patch is measured to be 185 µm. As the temperature increases the Sb 2 S 3 patch crystallizes causing a large increase of its refractive index (Fig. 2). This induces a change of the effective refractive index of the guided mode of the covered arm of the MZI. The Sb 2 S 3 acts as a phase shifter and is the cause of the shift of the interference pattern seen on the MZI spectra as the temperature gets close to 300°C (Fig. 3). After the 290°C annealing, only partial crystallization is achieved as denoted by the small interference pattern shift. By careful control of the temperature of the PCM it is then possible to reach intermediary states between fully amorphous and fully crystallized. These intermediary states are required to achieve not only binary but multi-level phase modulation. It should also be noted that the amplitude of the transmitted signal stays roughly constant as the Sb 2 S 3 crystallizes confirming the low absorption nature of this PCM. From the MZI transmission spectrum the phase shift is determined to be 3.02 rad around 1.55 µm which corresponds to a required length of 385 µm to achieve a 2π phase shift.

The setup chosen in this work only allows for the crystallization of the as-deposited PCM film. The amorphization of the PCM requires heating it up to its melting temperature and then rapidly quenching it which could be achieved using either an integrated micro-heater or a pulsed laser [START_REF] Delaney | A New Family of Ultralow Loss Reversible Phase-Change Materials for Photonic Integrated Circuits: Sb2S3 and Sb2Se3[END_REF]. Being able to fully amorphize the PCM film before subsequent controlled recrystallization would enable reversible multi-level phase modulation.

Conclusion

We have demonstrated the applicability of the emerging PCM Sb 2 S 3 as a phase shifter. By patterning a patch of Sb 2 S 3 on top of one of the arms of a Mach-Zehnder interferometer we were able to measure the phase shift due to the change of refractive index of the PCM as it crystallizes without optical absorption. For future applications, an integrated micro-heater could be used to control the state of the Sb 2 S 3 allowing for reversible phase modulation.

Fig. 1

 1 Fig. 1 Optical micrograph of the patterned Sb 2 S 3 film a) before and b) after annealing

Fig. 2

 2 Fig. 2 Change in refractive index and extinction coefficient of Sb 2 S 3 upon crystallization measured by ellipsometry Fig. 3 Transmission spectra of the Mach-Zehnder interferometer p artially covered with Sb 2 S 3 after consecutive annealings at different temperature