Transforming method from point to area of crop evapotranspiration in Shiyang River Basin in an arid region of Northwest China

DEM-based and GIS-assisted methods were employed to estimatethe temporal and spatial variation of spring wheat evapotranspiration(ET_c) in Shiyang River Basin. Based on the data collected from eight meteorologicalstations in the Basin in recent 50 years, the reference crop evapotranspiration(ET0) was calculated with the Penman-Monteith equation recommended by FAOin 1998. Crop coefficient(K_c) was calculated from selected experimental data for spring wheat evapotranspiration. In recent 50 years, ET_c values were gently increased in Gulang and Tianzhu in the upper reaches, while inLiangzhou in the middle reaches, the decreasing trend was greatly significant, and in other regions, the decreasing trend wasnot so significant. The multiple regression equation between ET_c and altitude, latitude, and aspect of slope was established.Spatial distribution of spring wheat ET_c in the Basin was obtained with Arcview3.3, ArcGIS9.0 and Visual Basic 6.0, and itsspatial variations in different hydrological years of 25%, 50% and 75% were analyzed. The results show that, spatial variation of ET_c was significant(from270 mm to 591 mm) with the trend increasing from mountainous area to oasis plain in the Basin. The relative error between the calculated and estimated values of ET_c was within 11.1%.