18 Upscaling Water Productivity in Irrigated Agriculture Using Remote-sensing and GIS Technologies

Reliable information on water depletion for agricultural production is much needed when freshwater resources are getting scarcer. This is the case in the irrigated Indus basin. Despite their importance, data required to monitor the productivity of the land and water resources over vast areas are usually not avail-able or accessible. Satellite measurements from the National Oceanic and Atmospheric Adminstration (NOAA) weather satellite are combined in this study with ancillary in situ data into a geographic informa-tion system (GIS). Remote-sensing measurements are converted to crop yield, to actual evapotranspiration and, indirectly, to net groundwater use. The GIS data consist of canal-water deliveries and rainfall records. For each of the canal commands, the productivity of water is calculated. Large variability in the data is found from the different canal commands in the Indus basin. It is concluded that water productivity is controlled more by crop yields than by the water input. The spatial variability of productivity per unit water diverted is greater than per unit depleted. This can be ascribed to wide variations in the relationship between canal-water supply and actual evapotranspiration. This is an issue covered by classical irrigation efficiencies. Upscaling of water productivity for the Indus basin was achieved by aggregating the various canal command areas from the upstream end of the system downwards. The results show that the produc-tivity of water tends to a constant value at a spatial scale of 6 million ha and higher. At that scale, water diversion and water depletion are equal, which implies that groundwater systems, to a large extent, regu-late losses and reuse of water resources. The Indus basin is an example of substantial groundwater recy-cling and this needs to be taken into account in analytical frameworks of water productivity