The contribution by the water table to crop water use
The contribution by the water table to crop water use was evaluated in the absence of surface water application from lysimetric studies in a glassliouse during 1988, 1989 and 1990. The water table contribution was measured for beans, barley and lettuce in the presence of constant water tables 60, 90 and 120 cm deep. The water table contributed to about 27.0, 16.4 and 11.4% of evapotranspiration of barley with water tables 60, 90 and 120 cm deep, respectively. The contribution in lettuce was found to be 34.7, 13.5 and 6.0% for the 60, 90 and 120 cm water tables, respectively. The water table could not contribute to the evapotranspiration of beans because the initial soil moisture suction profile was not in equilibrium, and there was always a zero-flux plane above the water table. Capillary upward flux from the water table was also measured using Darcy's equation and by direct measurement. For this, unsaturated hydraulic conductivity was determined in the laboratory from diffusivity over a wide range of moisture content. Conductivity values were also evaluated in situ using Darcy's equation. In situ and laboratory conductivity values were well fitted by Gardner's (1958) conductivity function but not by that of Rijtema (1965). Root water uptake was evaluated using the extraction-term approach. A very small proportion of roots near the water table was absorbing water from the capillary fringe iii the case of a deep-rooted crop (barley) for all water table depths. Lettuce, a shal1ow-rootd crop, was absorbing water from the water table although roots were confined to the top 5 cm depth for all water table depths. A simulation model (CAPROW) was developed to account for capillary rise from constant water tables. The model can also predict soil moisture content, root water uptake and inflow to roots provided soil physical parameters and relevent data are known. Parameters needed to run the model were determined from the bean experiment with the water table at 60 cm depth. CAPROW was used to simulate results for water tables at 90 and 120 cm under three different crops. Model predictions of soil moisture contents at harvest agreed well with the measured values. The predicted cumulative upward flux in barley and lettuce under two different water table treatments agreed closely with the measured values. The contribution by the water table to water use by barley was found to be 16.4 and 11.4% for 90 and 120 cm water table depths, respectively. Corresponding simulated values were 15.5 and 10.4%. For lettuce, measured contributions from the water table to evapotranspiration were 13.5 and 6.0%. Corresponding simulated values were 15.7 and 6.7%.