Abstract: Based on the 1961-2008 daily observation data from 17 meteorological stations in the inland river basins in Hexi region, the daily reference crop evapotranspiration (ET₀) in the basins was computed by Penman-Monteith equation, and the spatiotemporal characteristics of seasonal and annual ET₀ were studied by GIS and IDW inverse-distance spatial interpolation. In 1961-2008, the mean annual ET₀ (700-1330 mm) increased gradually from southeast to northwest across the basins. The high value of mean annual ET₀ in Shule River basin and Heihe River basin declined significantly (P<0.05), with the climatic trend rate ranged from -53 to -10 mm·（10 a）^-1, while the low value of mean annual ET₀ in Shiyang River basin ascended slightly. The ET₀ in the basins had a significant annual fluctuation, which centralized in Linze and decreased toward northwest and southeast. The ET₀ in summer and autumn contributed most of a year, and the highest value of ET₀ all the year round always appeared in Shule River basin. The climatic trend rate was in the order of summer > spring > autumn > winter. Wind speed and maximum temperature were the primary factors affecting the ET₀ in the basins. Furthermore, wind speed was the predominant factor of downward trend of ET₀ in Shule and Heihe basins, while maximum temperature and sunshine hours played an important role in the upward trend of ET₀ in Shiyang basin.

Key words: reference crop evapotranspiration, spatiotemporal variation, Penman-Monteith equation, inland river basins in Hexi region, land use type, ecological security baseline, ecosystem service value, food security.