Abstract: The study of crop water demand during the whole growing period is an important basis for rational irrigation and optimal allocation of water resources.The Sanjiang Plain is an important grain production base in Heilongjiang Province,China,which has a lot of water for agricultural irrigation.Therefore,it is urgent to carry out a quantitative and fine research on water demand of farmland crops so as to provide a scientific basis for improving irrigation efficiency.Based on the daily data from 22 meteorological stations and[JP] China’s regional surface meteorological factor data sets during from 2000 to 2015,the PenmanMonteith model,provided by the international food and agriculture organization (FAO),and the segmented singlevalue average crop coefficient method were used to estimate the daily crop water demand and net irrigation water demand in the growing seasons (from May to September) of rice,corn and soybean crops in the Sanjiang Plain,and to analyze the change rules of the water demand of main crops in the Sanjiang Plain.The path analysis method was then used to study the change causes of the water demand of main crops.The results showed as follows: (1) The average annual reference crop evapotranspiration in the Sanjiang Plain in the 16 years period was 537.4 mm,and the average daily was 3.5 mm,showing a fluctuation reduction trend,and the fluctuation range was 447.3-602.8 mm. (2) The water demand of rice,corn and soybean during the whole growing period showed a singlepeak trend of increasing first and decreasing later.The water demand of rice at tillering stage was the largest with the amount being 177.1 mm.The water demand of corn at sevenleaf stage was the largest with the amount being 99.7 mm.The water demand of soybean at pod bearing stage was the largest with the amount being 96.1 mm. (3) For rich and soybean,the effective rainfall mainly happened in the middle and late stages of their growth;while for maize,the effective rainfall was evenly distributed in the whole growth period.The net irrigation water requirements of rice,corn and soybean are respectively 195.4 mm,130.8 mm and 72.2 mm,and the degree of dependence on irrigation for the three crops formed a descending order as rice>corn>soybean.(4) Reference crop evapotranspiration is influenced by various meteorological elements,and its correlation with sunshine hours,air temperature and net radiation is better,and its mean correlation coefficients are 0.85,0.61 and 0.89 respectively;the reference crop evapotranspiration was negatively correlated with relative humidity and precipitation,and the average correlation coefficients were -0.73 and -0.53,respectively. (5)According to path analysis,the direct path coefficient results show that the main factors affecting crop water demand in the Sanjiang Plain are net radiation,air temperature and sunshine hours.It can be seen from the indirect path coefficient of the influence factors that the influence factors restrict and influence each other,so the change of water demand of rice,corn and soybean in the Sanjiang Plain is affected by the combinations of the influence factors.This study provides a long time series analysis of daily crop water demand characteristics in the Sanjiang Plain,aiming to provide a reference for rational irrigation of crops in the Sanjiang Plain in order to realize optimal allocation and efficient utilization of water resources.

Key words: Sanjiang Plain, crop water requirement, net irrigation water requirement, PenmanMonteith model, path analysis