Effects of shallow water table on capillary contribution, evapotranspiration, and crop coefficient of maize and winter wheat in a semi-arid region

A lysimeter experiment was conducted during 19866—96 to study the impacts of groundwater tables on the capillary contribution, evapotranspiration, and crop coefficient of maize and winter wheat grown in a semi-arid region in loess loam soils. The depth of groundwater table was set to 0.5, 0.8, 1.0, 1.2, 1.5, 2.0, and 2.50 m, respectively. The results showed that the rate of capillary contribution from groundwater to crop root-zone was influenced mainly by the depth of the water tables. The daily variation in capillary contribution was not the same as pan evaporation; the peak was delayed when the water table was >0.8 m, and the time of delay increased with the depth of water table. The crop evapotranspiration was decreased with increasing groundwater table in the early growth period and harvest period. The maximum evapotranspiration occurred at 1.2 m groundwater table in the other periods. Values of crop coefficients (K c ) were estimated based on the measured evapotranspiration (ET) and reference crop ET computed by the modified Penman method. The estimated K c was significantly different from the values computed and used in the region in the absence of groundwater table effects, and it varied markedly with groundwater tables. Relationships between the crop coefficient and the depth of groundwater table were developed using mean crop coefficients derived from multi-year data. It was found that linear model was better for the period Octobermp;mdash;February in the winter wheat growing season and June in the summer maize growing season. The polynomial model was suitable for the period March;mdahs;June in the winter wheat growing season and from July to October in the summer maize growing season.