In order to identify suitable water regime(s) for irrigation management of rice in soils with shallow and medium water tables (SWT and MWT), six water regimes (IC, IC1, IC3, IC5, IC7 and I0) ranging from continuous submergence of 5 ± 2 cm water (C) to completely rainfed (I0) were tested. In the IC1, IC3, IC5 and IC7 regimes 7 cm irrigation was applied, respectively 1, 3, 5 and 7 days after the water vanished from the surface of the soil. During the entire rice growing season, the depth of the water table varied from 0.7 to 92.3 cm under SWT and from 12.6 to 126.3 cm under MWT conditions. To maintain the IC regime, as compared to IC1 to IC7, an additional amount of 10 to 20 cm water under SWT and 25 to 40 cm water under MWT conditions was required. As compared to MWT, water use efficiency (wue) under SWT was 6.9% higher. The groundwater table contribution (gwc) towards evapotranspiration (et) was 27.1% higher under SWT condition as compared to MWT. Percolation losses (pc) were lower by 20.4 and 23.6% under SWT and MWT conditions, respectively, as compared to those under the IC regime. Grain yield, et, pc and surface run-off (ro) losses were found in the order IC > IC1 > IC3 > IC5 > IC7 > I0, whereas wue followed the reverse order. In terms of yield, the soil water regimes could be put in the order: IC > IC1 ≈ IC3 > IC5 ≈ IC7 > I0. Grain yield was higher under IC but differences among IC, IC1 and IC3 regimes were narrow (141-490 kg ha-1 under SWT and 413-727 kg ha-1 under MWT conditions). The results demonstrated that under the conditions of the Tarai region where the water table remains shallow (< 30 cm) for most part of the rice growing season, optimum yield (55-60 q ha-1) with high wue (3.21-3.67 kg ha-1 mm-1) can be obtained by intermittent irrigation 3 to 5 days after the water vanished from the surface of the soil under SWT and 1 to 3 days under MWT conditions instead of continuous submergence, a practice traditionally followed in the Tarai region. © 1990.
