Actual evapotranspiration and crop coefficients of wheat (Triticum aestivum) under varying moisture levels of humid tropical canal command area

Wheat (Triticum aestivum L.) is the most important cereal crop after rice in India. Optimum schedule of irrigation for wheat and its response to irrigation based on the ratio of irrigation water and cumulative pan evaporation (irrigation depth (IW):CPE at 1.2, 0.9 and 0.6) was studied in a field experiment during the winter seasons of 1989-1990 and 1990-1991. The site had a sandy loam (Typic Fluvaquent) soil and shallow water table, located in the humid tropical canal command area at the University Experimental Farm, Memari (23degrees1'N, 88degrees5'E and 21.34 m above mean sea level) in West Bengal, India. The yield of wheat, weekly and seasonal actual evapotranspiration (ETa) from water balance equation, crop coefficients (K-c), soil water stress coefficients (K-s), water extraction patterns, water use efficiency (WUE) and water application efficiency (A(e)) were determined. Maximum profile water depletion and ETa of wheat occurred in the wetter moisture regime (1.2 IW:CPE). The average daily ETa varied from <1 mm per day in the early growing period to a maximum value of 2.72 mm per day at 9-14 weeks after sowing (WAS) under 1.2 IW:CPE when leaf area index (LAI) was >3. During crop development to mid season (4-14 WAS), 1.2 IW:CPE showed 9-15% increased ETa than 0.9 and 0.6 IW:CPE. The seasonal ETa of wheat with IW:CPE of 1.2 was 238.2 and 261.95 mm during 1989-1990 and 1990-1991, respectively. Maximum roots were concentrated in the top 150 mm layer (57% of root length density) and maximum root water extraction efficiency (U-z) was observed at 600-750 mm depth under the drier soil moisture regimes of 0.9 and 0.6 IW:CPE, respectively. The yield increased significantly with increasing levels of irrigation. IW:CPE ratio of 1.2 produced 13 and 21% more yield, and 8 and 14% improved WUE than 0.9 and 0.6 IW:CPE, respectively. Water application efficiency (A(e)) was noticed higher (79.85%) under 0.6 IW:CPE irrigation regime. Precise information of K-c, which is required for regional scale irrigation planning is lacking in developing countries. The estimated values of K-c for wheat at four crop growth stages (initial, crop development, mid season and maturity) were 0.33, 0.82, 1.08 and 0.64, respectively which were identical to those suggested by the FAO indicating need for generating these values at the local/regional level. The K-s values of 0.6 IW:CPE decreased steeply and experienced more stress condition than 0.9 IW:CPE. (C) 2002 Elsevier Science B.V. All rights reserved.