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 1986-96 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 l oess 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 m ainly by the depth of the water tables. The daily variation in capillary co ntribution was not the same as pan evaporation; the peak was delayed when t he water table was >0.8 m, and the time of delay increased with the depth o f water table. The crop evapotranspiration was decreased with increasing gr oundwater 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 ev apotranspiration (ET) and reference crop ET computed by the modified Penman method. The estimated K-c was significantly different from the values comp uted and used in the region in the absence of groundwater table effects, an d it varied markedly with groundwater tables. Relationships between the cro p coefficient and the depth of groundwater table were developed using mean crop coefficients derived from multi-year data. It was found that linear mo del was better for the period October-February in the winter wheat growing season and June in the summer maize growing season. The polynomial model wa s suitable for the period March-June in the winter wheat growing season and from July to October in the summer maize growing season.