Abstract: To better assess water use status in irrigated agriculture and understand irrigation water use efficiency (WIUE) at different spatial scales,an improved distributed hydrological model,soil and water assessment tool (SWAT),is used to smiulate hydrological processes and the cropy ields in the irrigation region of the third main canal in Zhanghe irrigation district.The nine nesting scales is adopted to calculate the WIUE and to result in five IWUE indexes.The variation of five assessment indexes w ith the nine nesting scales is analyzed.Results show that one of theindexes,the irrigation water productivity,is greatly influenced by the spatial temporal distribution of precipitation as well as other weather conditions in the irrigation region.Its values increase with nesting scales.This scale character can be approximately represented by a power function that is smilar to the fractal principle.Another important index,the water drainage ratio,achieves the maximum value at the intermedium nesting scale.However,the scale character of the water drainage ratio is insignificant as this index is quasi-stationary across all nesting scales.Therefore,the power function can be used to scale up/down the irrigation water productivity.The greatest potential for water saving practice in this irrigation region is achievable at the intermedium nesting scale.