Abstract:

The meteorological data of Dadu River Watershed of 30 years were used to calculate the reference evapotranspiration based on the newly revised FAO PenmanMonteith equation.Fortyfive scenes of MODIS LAI in 2007 were acquired at the same time.Based on the empirical relation between LAI and crop coefficient，the variation of crop coefficient in 2007 was obtained.The GIS Zonal analysis was conducted to estimate the annual variation of watershed mean crop water requirement.The analysis of the variation in source of crop water requirement was conducted based on the climate division by annual aridity index.Results show that the annual watershed mean crop water requirement in 2007 was 618.8 mm with monthly average of 51.6 mm.The crop water requirement is monomodally distributed in 2007 with a peak in summer (from June to August) of 221.2 mm that accounted for 35.7% of the annual amount.The crop water requirement in spring (March to May)，autumn (September to November) and winter (December to February) accounted for 30.4%、20.1% and 13.8% of the annual amount respectively.In rainy season (May to October)，the precipitation completely meets crop water requirement in both semihumid and semiarid areas.In arid season，while crop water requirement was mainly supplied by precipitation in semihumid region，irrigation was the main source of crop water requirement in semiarid region.While only aridity index imposed great influence on the precipitation percentage in the total amount of crop water requirement in semiarid region，other factors such as topography may also plays important role in precipitation percentage of crop water requirement in semihumid region.

Key words: MODIS/leaf area index/ crop/ ecological water requirement/Dadu river watershed