Abstract: Located in the western part of the Loess plateau, Gansu L. oess plateau has an area of 1.13×10⁵km², its climatic and vegetation areas include arid desert steppe area, semi-arid steppe area and scmi-humid forest-stepe area. The proprtion or atr y antang factor for agricultural de-local agriculture, therefore, moisture condition is the most important factor for agricultural development. This paper calculates and analyzes the features of annual and seasonal variation ofprecipitation, field evapotranspiration, and moisture deficit of crops and their impacts on the a-gricultural production in Gansu Loess plateau. The results show that the less quantity, big variability and heterogeneous seasonal distribution of precipitation restrict crop grow'tn serounit tinnbring about a low and unstable yield. The violent field evapotranspiration and less precipitation make the field moisture deficits during the growth periods of summer crops and autumn cropsbe up to 225~500 mm and 70~540 mm respectively, which are 0.8~3.7 times and 0.2~3.4 times higher than the precipitation during the same periods. The water niche suitabilities ofcrops are very low, only 0.16~0.65 and 0.19~0.68 during the growing season of crops asdaily mean temperatures ≥0℃ and ≥10℃, and that of winter wheat and spring wheat arelower than 0. 7 and 0.5 respectively during the growing season. The precipitation indexes ofcrop yield are small, they are 40%~82%, 18%～44%, 38%~94%, 0.3%～66% and 6%~73% for winter wheat, spring wheat, millet, corn and potao respectively.

Key words: Gansu loess Plateau, moisture, precipitation, evapotranspiration, precipitation index of crop yield