Surface Water Evolution (2001–2017) at the Cambodia/Vietnam Border in the Upper Mekong Delta Using Satellite MODIS Observations

Studying the spatial and temporal distribution of surface water resources is critical, especially in highly populated areas and in regions under climate change pressure. There is an increasing number of satellite Earth observations that can provide information to monitor surface water at global scale. However, mapping surface waters at local and regional scales is still a challenge for numerous reasons (insufficient spatial resolution, vegetation or cloud opacity, limited time-frequency or time-record, information content of the instrument, lack in global retrieval method, interpretability of results, etc.). In this paper, we use 17 years of the MODIS (MODerate-resolution Imaging Spectro-radiometer) observations at a 8-day resolution. This satellite dataset is combined with ground expertise to analyse the evolution of surface waters at the Cambodia/Vietnam border in the Upper Mekong Delta. The trends and evolution of surface waters are very significant and contrasted, illustrating the impact of agriculture practices and dykes construction. In most of the study area in Cambodia. surface water areas show a decreasing trend but with a strong inter-annual variability. In specific areas, an increase of the wet surfaces is even observed. Ground expertise and historical knowledge of the development of the territory enable to link the decrease to ongoing excavation of drainage canals and the increase of deforestation and land reclamation, exposing flooded surfaces previously hidden by vegetation cover. By contrast, in Vietnam, the decreasing trend in wet surfaces is very clear and can be explained by the development of dykes dating back to the 1990s with an acceleration in the late 2000s as part of a national strategy of agriculture intensification. This study shows that coupling satellite data with ground-expertise allows to monitor surface waters at mesoscale (<100 × 100 km2), demonstrating the potential of interdisciplinary approaches for water ressource management and planning.


Introduction
Monitoring the spatial and temporal distribution of surface water resources is critical at the global, national and regional scales. This is particularly important in highly populated areas and in regions under climate change pressure such as the floodplains of Cambodia or Vietnam. The Mekong Delta in Southeast Asia (the third largest delta in the world) is a vast triangular plain of approximately 55,000 km 2 , most of it lower than 5 m above sea level. It supports dynamic agricultural systems of great socio-economic importance for the related countries. For instance, the Mekong Delta region covers only 12% of Vietnam but produces 50% of the rice produced in the country (with two or three harvests per year depending on the provinces), supports 50% of the fisheries of the country, and 70% of its fruit production [1]. However, this environment has witnessed major changes due to large scale human interventions, for example, the construction of a very dense network of canals, roads, and dykes (see Reference [2,3] for a description of water control infrastructure development in the Vietnamese Mekong Delta). Over the last two decades, several changes have impacted the surface water regime. Agriculture practices have evolved, and the construction of high dykes in Vietnam has caused considerable changes in the hydrodynamics of the Mekong River (see for instance References [4][5][6]).
While most papers focus on understanding the impact of upstream water development and dykes construction in the Vietnamese Mekong Delta, we propose to focus and analyse in situ dynamics in the Upper Mekong Delta, a mesoscale region that spreads over Cambodia and Vietnam. This area was selected because it is a floodplain that has witnessed very contrasted evolution over the last two decades. Its subdivision into six zones offers the opportunity to discuss contrasting flood dynamics in a similar hydro-ecosystem that has witnessed the strikingly different development trajectories. In Vietnam, and since the mid 1980s, the government has engaged in large scale water control and reclamation projects [3] aimed at intensifying agriculture and notably the production of rice. These have culminated in the construction of high dykes that supported the implementation of a triple-rice cultivation policy since the early 2000s, though with significant environmental and economic impacts such as depleting soil fertility and decreasing fishery productivity (see for instance Reference [2]). In contrast, in Cambodia, minimal investment in water control infrastructure (apart from the dredging of large earth drainage canals) has been made though this is progressively changing as sluice gates meant to further control floods are currently being built.
To understand the complex dynamics of floods and the evolution of water surface areas in this transboundary region, we propose to mix an expert understanding of local hydrological dynamics and land-and water-use practices acquired through in depth field visits and interviews with farmers and key informant interviews (with ministries, non governmental organisation and donors staff) with the use of global remote sensing products and expertise. Field-level expertise allows for providing sound interpretation of remote sensing results, while the latter allows putting field-level insights into a broader perspective or larger regional area. The coupling of these methods appears particularly interesting for characterising dynamics at mesoscale, especially in a transboundary region where field-based hydrological data can be politically sensitive and data sharing mechanisms fraught with challenges.
There is an increasing number of Earth satellites and resulting datasets monitoring surface waters. However, mapping surface water is still a challenge [7]. It is difficult to provide products with the accuracy required for a large range of applications (e.g., agriculture, disaster management, and hydrology). Each satellite observation has its own advantages and drawbacks (insufficient spatial resolution, vegetation or cloud opacity, limited time-frequency or time-record, information content of the instrument, lack in global retrieval method, and so forth).
Different SAR (Synthetic Aperture Radar) observations have been exploited to study floods and wetlands over the Mekong Delta (see Reference [8] for rice monitoring). Reference [9] used 60 Envisat ASAR observations during the years 2007-2011 to study the flood regime in the Delta. Thanks to the launch of the Sentinel-1A & B satellites by the European Space Agency, Sentinel-1 SAR observations are now regularly accessible for scientific purposes. Similar to previous SAR instruments, Sentinel-1 instruments show strong potential for detecting open water bodies at high spatial resolution [10,11]. In Reference [12], a methodology was introduced to detect surface water with Sentinel-1 SAR data within Cambodia and the Vietnamese Mekong Delta, over the 2014-present period. Reference [13] used MODIS data to investigate floods in Southern Cambodia from 2002-2013. However, no long-term SAR dataset exists so far, and other datasets are required in order to measure long-term changes.
Several methods were proposed to monitor surface water using VISible (VIS) and Near-InfraRed (NIR) images. Reference [14] applied a threshold on NIR reflectances of the NOAA/AVHRR observations to delineate lakes. Reference [15] used positive values of the Normalised Difference Water Index (NDWI) to classify water bodies. Reference [16] detected surface water by identifying the positive values of the Modification of Normalised Difference Water Index (MNDWI). Detailed lake morphometry over basins with large water level fluctuations can also be obtained from MODIS satellite observations [17]. Reference [18] produced a monthly-mean climatology of the water extent from 2000 to 2004 with a spatial resolution of 500 m, on a 8-day temporal resolution (by compositing all the clear data available during these eight days), using VIS/NIR Terra/MODIS data. Mapping paddy rice agriculture in South and Southeast Asia can be done by using multi-temporal MODIS images [19]. Reference [20] produced a new Global Surface Water Explorer (GSWE) dataset from Landsat imagery using a 32-year record, allowing for a description of the trends of surface waters and their occurrence. However, with 85% to 95% cloud cover during the wet season over the Mekong Delta [21], remote sensing methods derived from VIS/NIR images present some evident limitations. Vegetation is also an issue, as it makes the water detection difficult or impossible under canopy [7]. Reference [22] explored the limits of Landsat imagery for the monitoring of small water bodies with rapid changes. Reference [23] investigates the retrieval of remotely sensed hydrological variables like water level and their corresponding drought indices (i.e., TRMM Standardized Precipitation Index (TRMM-SPI) and GRACE Drought Severity Index (GRACE-DSI)) over the Mekong and the Yangtze Rivers Basins.
The Global Inundation Extent from Multi-Satellites (GIEMS) database combines satellite observations in the VIS, NIR, and passive/active microwaves at coarse resolution [24], and GIEMS-D3 at high spatial resolution [25], over a 15-year period (1993-2007). In Reference [7], it was shown that GIEMS-D3 is a good solution to obtain inundation at the global scale, with high spatial resolution, over cloud-covered and vegetated areas. This is a perfect complement to optical databases (e.g., Landsat) that highly underestimate surface waters over some tropical regions. However, the high spatial resolution of GIEMS-D3 is not a native satellite information but rather an indirect information from topography. It was noticed that over anthropized and very flat regions such as the Mekong Delta, GIEMS-D3 could be misled by the topography information, and that downscaling is difficult over these areas [25].
Since we are interested here by long-term and recent changes in the hydrological system at the Cambodia/Vietnam border, we chose to use the MODIS data that is available over 2001-2017. This VIS/IR data has also the advantage to be available on a 8-day compositing temporal resolution, which is important for considerations such as detecting the start or the end of the flood season. The methodology presented in Reference [18] is being used to process the 17-year record [26].
Section 2 presents the spatial domain with the six zones studied here, the MODIS surface water estimates, and some preliminary analysis. The spatio-temporal analysis is conducted in Section 3 using a principal component analysis. The long-term evolution of spatial patterns and the changes in the seasonality are interpreted based on ground expertise acquired by some of the authors who conducted frequent interviews with key informants and farmers in Cambodia and Vietnam between 2016 and 2019 in the context of a research project focused on understanding current and past land and water management practices in the Upper Mekong Delta (see http://deltasoutheastasia-doubt.com). Finally, conclusions and perspectives are provided in Section 4.

Study Domain
We focus here on six zones along the Cambodia/Vietnam border in the Upper Mekong Delta. These six zones are represented in Figure 1 and described in Table 1: Closest to the river bank, vegetables and fruit trees can be cultivated (see Figure 2F). The rehabilitation of transversal canals (called Preks), drawing water from the Bassac and the Mekong, allows for an intensification of agriculture and a progressive shift to two rice cultivation seasons while capture fisheries are still an important activity when the flood recedes (November-February).

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Zone 3C: This zone is located in the Prey Veng province of Cambodia. It is slightly more elevated than the two preceding Cambodian zones and faces drought conditions during the hot months of the year (March-May), see Figure 2D). For the most part, farmers engage in a single season of rice cultivation when the flood recedes (December-March), with occasional areas where double rice cultivation can be found, supported by groundwater extraction (wells, see Figure 2B) or collective pumping systems that supply surface water from a dense network of earth canals to fields in May-June before the flood comes. The Phu Tan district, where the North Vam Nao Water Control project was implemented during the 2000s, is the only area of the Vietnamese Mekong Delta where the policy to enforce a 3-3-2 crop cycle has been adhered to Reference [27] (The "3-3-2 crop cycle" is a term indicating that farmers engage in three crop cycles for 2 years in a row and for only 2 crop cycles the third year. During that third year, water "is allowed" to enter the fields through control flooding to flush field and bring fertile sediments. Compartments are flooded on a rotational basis. The advantages and drawbacks of this policy have been strongly debated, see for instance Reference [27,28]). This is made possible because the "island" is surrounded by a high "ring dyke" and sluice gates are operated by a dedicated office according to a strict schedule, see Figure 2H. • Zone 3V: This zone constitutes part of what is commonly known as the "plain of reeds". Located in Dong Thap province, the plain of reeds is one of the last areas of the Mekong Delta to have been reclaimed for agricultural purposes due to sociopolitical and ecological challenges [29]. Most of the zone is protected by low-dykes built in the 1990s (also called August dykes because they allow delaying the flood to harvest the summer-autumn crop) and farmers generally engage in double rice cultivation.
As can be seen in Table 1, there are thousands of pixels in each one of the six zones. These pixels are no independent from each other so we do not have thousands of independent pixels. However, the results obtained in the following appear to be statistically significant. In particular, retrieval errors from the MODIS database (e.g., misclassification between dry, mixed and water pixels) should compensate each other.   Table 1). Zone 1 is in blue, zone 2 in green, and zone 3 in yellow. Cambodian zones are in a lighter colour than Vietnamese ones. The Cambodia/Vietnam border is indicated in yellow. This full domain represents 21,618 MODIS pixels (500 m × 500 m).

Long-Term Retrieval Using MODIS Instrument
In this study, atmospherically corrected land surface reflectance 8-day Level 3 global 500 m products from MODIS/Terra (MOD09A1) are used to estimate surface water extent at 500 m spatial resolution, for the 2001-2017 period. The MOD09Q1 product at a 250 m resolution could had been used but there is no blue band in this case so the 500 m resolution of the MOD09A1 was chosen instead. The MCD43A4 product could had been used at the daily time resolution but this was not necessary in this study, the 8-day resolution is a good compromise for the present analysis. Each MOD09A1 image was created by selecting the best Level-2 Gridded observation during an 8-day period on the basis of high observation coverage, low view angle, absence of clouds or cloud shadow, and aerosol loading [30]. As we have one MOD09A1 estimate every eight days, in the following and for simplicity purpose, we will sometimes call "week" the 8-day time step of the MODIS dataset. MODIS/Terra satellite observations are distributed free of charge for research and educational purposes from the NASA EarthData Hub at https://search.earthdata.nasa.gov/search.
Reference [18] introduced a methodology using MODIS observations to detect surface water over the Lower Mekong Basin for the 2000-2004 period. It was then applied in Reference [26] to obtain a 17-year long time record (2001-2017) period, which we apply to the transboundary Upper Mekong Delta in this paper. This methodology uses low values of water indices to detect the presence of surface water extent. Here, a summary of the methodology is briefly presented but other details should be searched for in Reference [18]. First, the Enhanced Vegetation Index (EVI, Equation (1)), the Land Surface Water Index (LSWI, Equation (2)), and the Different Value between EVI and LSWI (DVEL, Equation (3)) are calculated at the pixel level from the original atmospherically corrected surface reflectance data derived from the MOD09A1 8-day composite products.
where RED, NIR, BLUE, and MIR are MODIS surface reflectances in the Visible band 1 (RED: 620-670 nm), NIR band 2 (NIR: 841-876 nm), Visible band 3 (BLUE: 459-479 nm), and MIR band 6 (MIR: 1628-1652 nm), respectively. Cloud-covered pixels (where surface reflectances the blue band ≥ 0.2) are filled using a linear interpolation before a simple weight function is applied to smooth all water indices [26]. All pixels with smoothed EV I ≥ 0.3 are classified as non-flooded pixels. Water-related pixels are marked if smoothed DVEL <= 0.05 and smoothed EV I ≤ 0.3, or if smoothed EV I ≤ 0.05 and smoothed LSW I ≤ 0. Next, water-related pixels are sub-classified into "water pixels" or "mixed pixels" by applying a threshold on the smoothed EV I. If smoothed EV I ≤ 0.1, these pixels are set as WATER pixels, meaning that 100% of its surface is inundated (e.g., open water bodies, rivers, lakes or flooded areas). If smoothed 0.1 < EV I ≤ 0.3, then these pixels are set as MIXED pixels, meaning that only a part of its surface is inundated and the inundation ratio is unknown (e.g., rice paddies, pixels with half water only, spare vegetation over standing water, and so forth). At the 500 m spatial resolution, MODIS instrument has limitations to detect water bodies with a size smaller than its spatial resolution. In addition, water under aquatic vegetation or canopy is mostly invisible from the optical MODIS sensor. However, the most important advantage of the MODIS products is its ability to provide continuous observations at good temporal resolution (8-day) suitable for both global and regional monitoring applications, over almost the last two decades. Sap is permanently in the water state, meaning that it is 100% covered by water during all the time steps of the long-term record. These three maps provide the overall surface water statistics in the whole Lower Mekong Basin but no long-term trend information is provided here as this statistics mixes what happened in the beginning and in the end of the time record. Figure 4 represents the time evolution from 2001 to 2017 of the number of mixed, water and total (mixed + water) pixels over the six zones represented in Figure 1 and described in Table 1. Values oscillate between zero (no inundation) and 100% (zone fully inundated). The long-term evolution over Cambodia zones 1C, 2C and 3C appears relatively stable (except for a decrease in the last three years in 3C), but inter-annual variabilities can be strong, for both mixed and water areas, especially in subdomains 2C and 3C. In comparison, long-term trends are very pronounced over Vietnam. In particular, zones 1V and 2V display a significant decrease in the number of water or total pixels. Zone 3V has more stable inundation, except for the last years (2015-2017) because this region has witnessed less infrastructure development. It can also be noted that zone 1V has a higher number of mixed pixels during 2011-2017 when compared to the earlier period, especially during the beginning of the water season. Figures 3 and 4 provide indications on spatial and temporal changes of wet surfaces respectively. In the following sections, we will combine these two types of information to provide further understanding of flooding dynamics.

Spatio-Temporal Analysis of the Long-Term Variability of Surface Waters
A Principal Component Analysis (PCA) of the surface water area will first be performed to facilitate the space/time analysis of the raw database.

Principal Component Analysis
First of all, an important choice needs to be made. First, the PCA can be done individually in each one of the six considered zones of Section 2.1 and Table 1. In this case, the PCA can be more precise because the analysis will be better suited for each zone, and will be less ambiguous because inundation variabilities from different areas would be less mixed. Second, the PCA can also be done simultaneously on the whole domain of the six zones. The advantage of this later approach is that the extracted components are then applicable to the six zones, and the inter-comparison of the zones is made easier. In this paper, we chose to use the second strategy.

PCA Methodology
PCA is a very general statistical methodology that intends to describe the variability of multivariate observations using a new base defined by a limited set of patterns. PCA has been widely used in geophysics to isolate important spatial patterns, called Empirical Orthogonal Functions (EOFs) in this context (e.g., References [31][32][33]).
Let X be the inundation maps from MODIS in binary values (i.e., 0 for no inundation and 1 for inundation). A spatial map of inundation will be noted X(t), for "week" t (with 1 ≤ t ≤ T = 17 years × 46 "weeks" = 782 "weeks"). The inundation time series for pixel i will be noted X i , for pixels i = 1, . . . , P (P = 21,618 pixels in this application, see Table 1). PCA is used to decompose the X space-time variability into a limited number of simple components [31]. Since the spatial dimension P = 21, 618 is much larger than the time dimension T = 782, it is highly recommended to perform the PCA in the time space. The PCA decomposition allows for the estimation of two matrices: C, the P × T matrix of the components (or scores), and F the T × T matrix with the T-dimensional temporal base function { f k ; k = 1, . . . , T}. The PCA is performed here on the covariance matrix, meaning that the input data is not normalised. Matrix F describes the axis of a new space of dimension T, and C are the coordinates of the data in this new space.
The temporal decomposition can be written: where the sign ":" in a matrix coordinate indicates that all the lines or columns are considered. In the third line of Equation (4), instead of using the full T PCA components, only the first K components are used (K ≤ T). This allows to describe any time series using a limited number K (K = 4 in the following) of temporal base functions but of course, part of the variability will be lost (i.e., higherorder components). The interpretation of PCA components is not always trivial because physical processes can require more than one PCA component to describe their complexity [34][35][36], and the mixing of the components can be non-linear and involving higher-order statistics [37]. The PCA technique will be used here to assess the spatio-temporal variability of surface waters in a more robust way than by performing a time series analysis at each pixel. Figure 5 represents the first four temporal base functions { f k = F(j, :); k = 1, . . . , 4}. The percentage of variance explained by each one is also indicated. Together, the first four components explain 81% of the variability in the MODIS dataset. Figure 6 represents the corresponding spatial components C(:, k) of the first four components.

PCA Results
First component-The first component (representing 69.3% of the surface water variability) is a low-frequency time evolution that includes the seasonal cycle and some inter-annual variabilities such as a strong decrease of the total inundated area in 2015 or a shorter inundation season in 2012. A general decreasing trend can be noticed. This component shows that there is a well-identified seasonal cycle, but also strong inter-annual variations. It is not surprising that this inundation-related component appears to explain a large part of the variability in the data since the spatial domain under study is a large-scale floodplain. Of course, all the pixels in the study domain do not follow exactly the temporal patterns of this first base function and additional components are necessary to describe smaller or higher-order differences. These differences will be expressed in terms of anomalies with respect to this first base function. In Figure 6, this first component is positive for most regions, except two large areas in the Northeast of Zone 3C and in the Northwest of Zone 1C that have pixel values close to zero due to higher elevation.
Second component-The second base function in Figure 5 (i.e., 6.03% of the variability) has an annual behaviour, with a main downward peak in the middle of the inundation season, and two secondary peaks (mainly positive) at the beginning and the end of the season. This annual behaviour is used to modulate the quite regular seasonality of component 1, by increasing or decreasing the amplitude of the season, and by shortening or extending the duration of the season. This component is therefore used to describe the kurtosis (i.e., flatness) of the inondation season. The two peaks at the beginning and the end of the season can also be used to describe two "mixed" pixel peaks, corresponding to the watering of rice fields (this will be discussed below). In addition, component 2 has also a positive long-term trend, from lower to higher values. When this second component is positive, the seasonal inundation amplitude is lowered in the early years, and increased in the latest years; and the opposite is true when this component is negative. The spatial pattern of EOF 2 ( Figure 6) displays strong contrasts: (1) high negative values in Zone 3C and the west of Zone 1C; this is representative of a long-term decreasing trend in inundation areas that are slightly more elevated; and (2) positive values in areas that have witnessed an increasing trend in inundation (eastern part of Zone 1C, Zone 2C, and western part of Zone 3C. Some positive values can also be observed in the eastern part of Zone 3V. Since both positive and negative trends can be observed in the three Cambodian zones, trends are not easy to analyse when doing zone-averaging as in Figure 4. The meaning of these positive and negative trends will be discussed in the following section. Third component-The component 3 (3.62% of the variability) displays a strong temporal negative trend (see third base function in Figure 5). When positive, the amplitude of the season decreases with time. As for component 2, this component has a peak at the start and at the end of the inundation season, so this component is used to modulate the inundation duration. This component acts almost only in Vietnam (see EOF 3 in Figure 6), reflecting particularly well the construction of water control infrastructures. This component is therefore used to represent the decreasing trend of surface inundation in Vietnam due to the construction of low and high dykes over the last two decades.
Fourth component-Component 4 represents only 1.93% of the surface water variability in the MODIS record ( Figure 5). This component has higher temporal frequencies during the year than the previous components. Furthermore, the variability it represents is more irregular over the years. For instance, the anomalies in this base function are positive in the earlier time record, negative in the 2007-2011 period, and positive again towards the end of the record. The spatial EOF 4 in Figure 6 is linked to EOF 2 but with a lower spatial extent. This suggests that this component is used to introduce a delay/progression of the inundation season, mostly in Cambodia and possibly in relation to land use change.   Overall, the PCA representation of surface waters is able to synthetise the variability present in the MODIS database. This PCA represents first (1) the seasonality of inundation with some inter-annual variability, (2) a natural increasing or decreasing trend in Cambodia, and (3) decreasing trend in Vietnam related to anthropic activities.

Long-Term Evolution of the Spatial Distribution of Surface Waters
The satellite observations can be used to analyse the spatial pattern of the long-term trends described above in Figure 7. These figures represent the changes in the frequency (between 0 to 1) of mixed, water and total pixels, when averaged in the first (2001)(2002)(2003)(2004)(2005)   For water pixels, the decrease is again noticeable in the Cambodian zones, but the decrease is much more marked over Vietnam (Figure 7-Center). This is particularly true in Zones 1V and 2V where water pixels have almost disappeared. This striking change is linked to the construction of high dykes and water control infrastructure (see Figure 2H) that started in the early 2000s but accelerated from 2008 onwards. In Zone 3V, the water pixel frequency has decreased due to the construction of dykes too, but water pixels still represent 20 to 30% of the area with flooding occurring over large field-shaped structures. There is a particular V-shaped area in the south of Zone 3V (longitude = 105.55 • and latitude = 10.7 • ), which corresponds to a protected area where cultivation has been forbidden and vegetation allowed to grow back naturally.
As a consequence, the total pixel frequency has also decreased ( Figure 7-Bottom), mainly over the Vietnam zones, in a very significant way.  In contrast, the changes in Vietnam are much more significant and results from man-made intervention can be seen, especially on Zones 1V and 2V (zone 3V is less anthropized and is more heterogeneous).

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For instance, the dynamics in Zone 1C have not changed much except that water pixels are delayed by a few weeks. The mixed pixels peak are less pronounced in 2012-2016 than in 2001-2005 but still mimic the two cropping season in the area (with land preparation occurring in May and December, respectively, see Figure 2E). The water pixels season length seems to be shorter with a later start and a shorter duration, and amplitude is slightly lower ( Figure 2E) shows the flooded landscape of the area in September 2018).

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Zone 2C has also witnessed little change: the mixed and water pixels have a peak that is delayed by a few weeks, and the two peaks of the mixed pixels are less pronounced, as if the onset of the flood is more progressive while the recession of the flood is more rapid.
• Zone 3C has trends more difficult to analyse due to the heterogeneity of the large area (6267 pixels). The water pixel area has dropped significantly (by 20% in the season maximum) and the mixed pixel peak seems to occur earlier but to be less pronounced.

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Zone 1V shows a decrease in the number of water pixels by about 70%. Over one year, the total number of mixed pixels has not changed much but a first pick in the season in April has appeared. This peak marks the preparation of rice fields through flooding by the use of pumps.

Interpretation/Discussion
In Cambodia, the analysis shows some positive and negative trends (in terms of frequency of mixed and water pixels; see Figure 7). Most of the study area is characterised by a negative trend (i.e., a decrease in the spatial extent of surface water and in the duration of the flood season). This overall decreasing trend has come together with a delay of the onset of the flood by about two weeks, which we may attribute to large scale drainage works conducted in Zone 1C and the south of Zone 2C notably (Figure 8) as well as delayed river flows in long term MRC rime records (www.mrcmekong.org). The MODIS analysis seems to confirm interviews with farmers who frequently indicated that the flood comes and recedes faster in recent years than 15 years ago. This can be attributed to the combined effects of effective drainage and sluice gates operation in Vietnam-sluice gates remaining close to protect the August harvest as was the case in 2017 when the flood peaked relatively early.
Some areas in Cambodia (e.g., extreme north of Zone 1C and center of Zone 2C; Figure 8) have witnessed a positive trend (i.e., an increase of the flooded area and of the duration of the flood season). This may rather be linked to deforestation and land reclamation (discovering surface waters that were previously hidden by the natural vegetation at least for part of the year) than to an actual amplification of the flood. Many farmers and key informants we interviewed indeed mentioned that the natural vegetation had been progressively cleared and land converted to rice fields over the last 10 years when private fishing concessions wee cancelled and regulations to accessband use land for cultivation not as strictly enforced as before. It would be interesting to complement these farmers insights with an historical land use/land cover analysis. Such increase in the surface water area constitutes, therefore, an artefact of global remote sensing products.
The overwhelming trend in Vietnam is one of a lower amplitude and duration of the flood. This can be clearly attributed to the extensive construction of low and high dykes. The high dykes have been built higher than the maximum river water level in an attempt to totally flood-proof agricultural lands and allow triple rice cultivation. This decrease is particularly noticeable in Zones 1V and 2V, which show internal differences that can be linked to the dates at which the dykes were built. For instance, the western parts of Zones 1V and 2V were already protected from floods in the early 2000s (in blue, Figure 7 left side) while the eastern parts have become insulated from flood later (shift from yellow to blue in Figure 7) (see References [2,5] for a detailed discussion of the dynamics of dykes construction in Vietnam). Similarly to Cambodia, the duration of the flood season has decreased with a later onset and an earlier recession, marking an increased control of the flood through human engineering.
Two other trends are worth mentioning. First is the existence of a new peak of mixed pixels around April in Zone 1V and 2V in addition to the two pre-existing peaks that occur at the end of August and at the end of December. This may correspond to a period where farmers water their fields before sowing their third rice crop now protected from the floods by high dykes. In Cambodia, these peaks occur in May and in November, the two periods during which farmers water their field for rice cultivation. Second, there are small patches of mixed pixels (in the center south of Zone 3V and the center east of Zone 1C); these correspond to protection and conservation areas where the natural vegetation is still "allowed to grow" (the national park of Tràm Chim in Vietnam and the Boeung Prek Lapouv in Cambodia). In both instances, the extent of mixed pixels has decreased when comparing the two periods analysed (2001-2005 and 2012-2016) pointing out to the fact that these areas are under human-pressure despite their protected status (precipitation trend is a second-order here considering the strong human intervention). This would also require a more detailed analysis in these regions, including the monitoring of the vegetation and precipitation.

Conclusions
The MODIS flood database developed and used here enables the monitoring of seasonal, inter-annual, and long-term trend evolution of the surface water distribution in the floodplain of the Upper Mekong Delta at the Cambodia/Vietnam border. There are clear differences in the distribution of these changes between Cambodia and Vietnam, over the 2001-2017 record. In Cambodia, changes detected by visible/infrared observations can be misleading because some of them are related to deforestation and land reclamation instead of a change in water presence. In contrast, over Vietnam, very significative negative trends in the extent and duration of flooding can be observed. Climate change may explain some of these changes, in particular for precipitation, but they are mainly the result of land use and water management changes that have occurred over the last twenty years. In Vietnam, notably, the construction of high dykes and drainage networks has induced a large decrease in the extent and duration of flooding so as to support triple rice cultivation.
The 500 m spatial resolution of the MODIS data appears to be compatible with the heterogeneity observed in the field. Satellite data, especially when combined with ground truth expertise, can contribute to better monitoring and understanding hydrological dynamics at such mesoscale ( 100 × 100 km 2 ), hence inform water managers and policy makers involved in designing water control infrastructure projects to support agricultural intensification. Visible data are an essential long-term source of information but this type of observation (MODIS as well as Landsat and Sentinel-2 data) is limited by cloud cover and vegetation presence. It is very important to keep these two limitations in mind when using such visible-based databases. Furthermore, satellite observations need to be complemented by ground-truth expertise in order to interpret the results correctly. Our results-and notably our ground knowledge-allows putting in perspective Pekel's analysis [20] which points to an increase in the frequency of flooding in the Upper Mekong Delta of Cambodia, which we attribute at least partly to deforestation and land reclamation rather than actual change in flood dynamics.
This study also stresses that, for any long-term analysis, older satellite missions such as MODIS or Landsat will continue to be necessary: newer and more precise missions such as SWOT (to be launched in 2021) will not be able to describe the long-term evolution of continental hydrology due to climate change or water management. Satellite observations are a promising tool to build a historical inundation record where and when ground observation is missing. Each satellite observation has its own advantages and drawbacks. We suggest to combine all sources of information to benefit from the quality of each of them. Once reliable global SAR estimates become available-from ENVISAT, Sentinel-1 or SWOT [38]-they can be combined with historic GIEMS/MODIS/Landsat estimates to build a very long record of inundation areas back to 1978, which, when coupled with ground level expertise will allow to understand in situ dynamics at meso-and even local-scale. A perspective of this work would be to analyse other parts of the Mekong River system. It could be used to study the long-term evolution of water bodies in the coastal area of the Mekong Delta, where shrimp farming has significantly expanded over the last two decades. Vegetation occultation would be less of a concern there. In this study, we show that the consequences of a national policy to control water (construction of dykes and drainage canals in Vietnam) can be monitored using global remote sensing database such as MODIS. The construction of water control infrastructures has had some advantages for agriculture allowing to have up to three rice harvests a year but it also brought some difficulties such as the demise of capture fisheries and the decrease of soil fertility due to a lack of silt brought by the natural flood, which made necessary the use of chemical fertilisers. The relevance of such policy is now being questioned and further analysis will be needed to measure its long-term impacts. This is particularly relevant as Cambodia tends to follow the path followed by Vietnam over a decade ago and invests significantly in water control infrastructure projects. Before investing in such a strategy, conducting a long-term sustainability and trade-off assessment is needed. We have shown in this study that satellite observations, although coarse in spatial (500 m) and temporal resolution (8 days), can help monitor these changes in the long-term. It can therefore be one of the tools to help study, improve our knowledge, calibrate and validate hydrological models. In the future, satellite data could also be assimilated into hydrological models to improve their reliability and prediction ability. Finally, such study that cross boundaries can provide useful insights when data sharing mechanisms between countries are fraught with challenges.