3) Based on the investigation and field experiment of water use efficiency (WUE) for main crops in Beijing, the results showed as follows: From the point of crop biomass, the WUE of spring corn was 30.4-31.8kg. hm-2.mm-1.The WUE of the other main crops in irrigated farmland in Beijing was spring peanut, alfalfa, spring soybean in order.
Considering the water use efficiency, salt balance and crops water requirement as a whole, the suitable quota of salt water irrigation was 120 m3/667m2. The quota at elongation, earing and milking period was 50 m3/667m2, 40m3/667m2 and 30m3/667m2.
The model is represented as followsP=R+aE c+U ,where P is observed precipitation, R is runoff, E c is computed evaporation, U is water stored in soil, α is a correction factor for computed evaporatin.
Through analysis of agriculture water resources and regularity of aridity on the loess Plateau, the authors thinks: the key period of water requirement of the main crops is just the period of lower soil moisture; the peak period of rainfall synchronize with evaporvation; the soil evaporation changes greatly;