WATER-USE EFFICIENCY OF IRRIGATED WHEAT IN THE TARAI REGION OF INDIA

Experiments were conducted during the winter seasons of 1983-1984 and 1984-1985 to identify suitable irrigation regimes s for wheat grown af ter rice in soils with naturally fluctuating shallow water table (SWT) at a depth of 0.4 to 0.9 m and medium water table (MWT) at a depth of 0.8 to 1.3 m. Based on physiological stages, the crop was subjected t o six irrigation regimes viz., rainfed (I-0); irrigation only at crown root initiation (I-1); at only crown root initiation and milk (I-2); at crown root initiation, maximum tillering and milk (I-3); at crown r oot initiation, maximum tillering, flowering and milk (I-4); and at cr own root initiation, maximum tillering, flowering milk and dough (I-5) . Tube-well water with an EC <0.4 dsm(-1) was used for irrigation. Bas ed on 166 mm effective precipitation during the cropping season, 1983- 1984 was designated as a wet year and 1984-1985 with 51 mm as a dry ye ar. The change in profile soil water content Delta W (depletion) in th e wet year was less (23%) under SWT and 10% under MWT as compared to t he dry year. The ground water contribution (GWC) to evapotranspiration (ET) was 58% under SWT and 42% under MWT conditions in both the years . The GWC in the wet year was 20% under SWT and 23% under MWT. Of the total net water use (NWU), about 85% was ET and 15% drainage losses. T he NWU was highest (641 and 586 mm) in I-5 under SWT and MWT condition s, respectively, but not the yield (5069 kg ha(-1)). Compared to I-5, NWU in I-2 treatment decreased by 10% in the wet and 25% in the dry ye ar. A similar trend was observed in the I-3 treatment under MWT condit ion. However, there was no statistically significant difference betwee n yields of the I-1 to I-5 treatments of either water table depth duri ng the wet year. This was also true during the dry year for the I-2 to I-5 treatments. Under SWT, in I-2, the grain yield was 5130 kg ha(-1) and under I-3 regime, 5200 kg ha(-1). Under MWT in I-3, the yield was 5188 kg hal and under I-4 regime, 5218 kg ha(-1). Thus it appears tha t in the Tarai region where the water table remains shallow (<0.9 m) a nd medium (<1.3 m) for most of the wheat growing season applications o f more than 120 and 180 mm irrigation under SWT and MWT conditions, re spectively were not necessary. Irrigation given only at crown root ini tiation and milk stages under shallow water table conditions, and at c rown root initiation, maximum tillering and milk stages under medium w ater table conditions, appears to be as effective as frequent irrigati ons.