Mapping and assessing water use in a Central Asian irrigation system by utilizing MODIS remote sensing products

Kurzfassung

Spatial and temporal patterns of water depletion in the irrigated land of Khorezm, a region located in Central Asia in the lower floodplains of the Amu Darya River, were mapped and monitored by means of MODIS land products. Land cover and land use were classified by using a recursive partitioning and regression tree with 250 m MODIS Normalized Difference Vegetation Index (NDVI) time series. Seasonal actual evapotranspiration (ETact) was obtained by applying the Surface Energy Balance Algorithm for Land (SEBAL) to 1 km daily MODIS data. Elements of the SEBAL based METRIC model (Mapping Evapotranspiration at high Resolution and with Internalized Calibration) were adopted and modified. The upstream–downstream difference in irrigation was reflected by analyzing agricultural land use and amounts of depleted water (ETact) using Geographical Information Systems (GIS). The validity of the MODIS albedo and emissivity used for modeling ETact was assessed with data extracted from literature. The r2 value of 0.6 indicated a moderate but significant association between ETact and class-A-pan evaporation. Deviations of ETact from the 10-day reference evapotranspiration of wheat and cotton were found to be explainable. In Khorezm, seasonal maximum values superior to 1,200 and 1,000 mm ETact were estimated for rice and cotton fields, respectively. Spatio-temporal comparisons of agricultural land use with seasonal ETact disclosed unequal water consumption in Khorezm. Seasonal ETact on agricultural land decreased with increasing distance to the water intake points of the irrigation system (972–712 mm). Free MODIS data provided reliable, exhaustive, and consistent information on water use relevant for decision support in Central Asian water management.