Abstract: Water use efficiency (WUE) is an important indicator to link the coupling between ecosystem water and carbon cycles. Understanding the spatiotemporal variation of ecosystem WUE and its influencing factors is essential for optimizing the integrated management of regional water and carbon resources and improving ecosystem productivity. With remote sensing data from MODIS, meteorological data from CRU, and land use data from CCI, the spatial distribution and temporal variations of ecosystem WUE in Ili River-Balkhash Lake Basin were investigated by using MK test and Sen’s trend analysis. The factors influencing WUE were further detected by partial correlation analysis. The results showed that the multi-year average values of annual, spring and autumn WUE illustrated same change patterns from the southeast to northwest, with a trend of increasing at first, then decreasing, and increasing at last. However, WUE values in summer displayed an opposite spatial variation as described above and WUE values of most areas in winter were close to zero. WUE values were highest in summer, followed by spring, autumn, and winter. The average annual WUE value over the whole basin increased slightly with time, with significantly increasing annual WUE in the lower reaches. The summer WUE values decreased with time in more than 90% of the basin, especially in the areas near Balkhash Lake, Ili River Valley, and Tianshan Mountains. Moreover, the annual WUE values in the upper reaches of the basin were obviously higher than those in the middle reaches, and the annual WUE values were lowest in the lower reaches of the basin. The interannual variations and trends of annual WUE values in the upper, middle and lower reaches of the basin were not significant. The rank of summer WUE values in the upper, middle and lower reaches of the basin was absolutely reverse to that of annual WUE values. The summer WUE values in the upper and lower reaches of the basin showed a significant decreasing trend, while the decreasing trend in the middle reaches of the basin was not significant. The intra-annual distribution of WUE in the basin was extremely uneven, with a peak value in August. In terms of influencing factors, gross primary productivity and precipitation showed stronger influences on WUE than evapotranspiration and temperature, respectively.

Key words: water use efficiency, Ili River-Balkhash Lake Basin, MODIS, spatiotemporal evolution.