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Changed ambient conditions in the Aral Sea Drainage Basin (ASDB) in Central Asia have led to drastically decreased river discharges into the Aral Sea during the twentieth century. This decrease has in turn led to the still ongoing Aral Sea desiccation and to particularly adverse environmental effects, in terms of both affected number of people and degree of environmental degradation in the ASDB. We have used a distributed basin-scale hydrological balance modeling approach for estimating the relative influences of agricultural irrigation and climate change, respectively, on observed decreases of river discharges in the ASDB. Results show that water losses through evapotranspiration increased as a result of higher temperatures in the basin after 1950. However, these increases in evapotranspiration loss due to rising temperatures alone are smaller than the water gains caused by increased precipitation in the ASDB over the same time period. Climatic changes can therefore not at all have contributed to the observed drying of the rivers in the basin, at least not so far. By contrast, the evapotranspiration loss increases from the expanded agricultural irrigation in the area can fully explain the decreased river discharges and the present water scarcity in the ASDB. We further show that the largest increase (1.85°C) in seasonal average temperature in the basin has occurred in the winter, whereas the smallest increase (0.69°C) has occurred in the summer. This result is consistent with a surface temperature cooling effect of intense irrigation in the summer, which should have increased since the 1950s due to the evapo-transpiration increase implied by the major irrigation expansion in the ASDB. [ABSTRACT FROM AUTHOR]