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.
EFFECTS OF DROUGHT ON OPTIMIZING NITROGEN USE OF WINTER WHEAT IN A SEMI ARID REGION