In many semi-arid and arid regions of the world, water saving strategies need to be implemented in the agricultural sector in order to increase the resilience to water scarcity. We investigate basin-scale hydrological impacts of possible irrigation technique improvements, considering extensive cotton fields in the Aral Sea drainage basin (ASDB), Central Asia. We use a distributed hydrologic model that combines basin-scale, calibrated discharge and evapotranspiration quantifications with experimental results of (on-farm) water application needs for different irrigation techniques. This allows for quantification of how return flows contribute to river discharge through coupled groundwater-surface water-systems at the basin scale, under different regional climatic conditions. Results show that an implementation of improved irrigation techniques can yield water savings that increase the discharge to the Aral Sea by between 1 and 6 km(3)/year. Such water savings could contribute to mitigation of the acute water scarcity in the lower ASDB. The basin-scale water savings are about 60% lower than corresponding on-farm reductions in irrigation water application, since water is re-used and, hence, return flows decrease when less water is applied. Spatial analysis of regional differences in climatic conditions shows that implementation of more efficient irrigation systems would result in much larger (up to a factor 4) water savings in the more arid downstream regions than in the colder, upstream mountainous regions.
