DETERMINATION OF ACTUAL CROP EVAPOTRANSPIRATION (ETC) AND DUAL CROP COEFFICIENTS (KC) FOR COTTON, WHEAT AND MAIZE IN FERGANA VALLEY: INTEGRATION OF THE FAO-56 APPROACH AND BUDGET

Abstract

Determination of the actual crop evapotranspiration (ETc) during the growing period is important for accurate irrigation scheduling in arid and semi-arid regions. Development of a crop coefficient (Kc) can enhance ETc estimations in relation to specific crop phenological development. This research was conducted to determine ETc values as well as daily and growth-stage-specific Kc for cotton, winter wheat and maize for silage at fields in Fergana Valley (Uzbekistan). The soil water balance model - BUDGET with integration of the dual crop procedure of the FAO - 56 was used to estimate the ETc and separate it into evaporation (Ec) and transpiration (Tc) components. An empirical equation was developed to determine the daily Kc values based on the estimated Ec and Tc. The Kc determination and comparison to existing FAO Kc values were performed based on 10, 5 and 6 study cases for cotton, wheat and maize, respectively. Mean seasonal amounts of crop water requirement in terms of ETc were 560 ± 50, 509 ± 27 and 243 ± 39 mm for cotton, wheat and maize, respectively. Estimated ETc for these crops were 1.10 - fold, 1.09 - fold and 0.73 - fold of recommended irrigation norm according to currently used hydromodule zoning (GMR) under semi-hydromorphic reclamation regime in Fergana province. The growth-stage-specific Kc for cotton, wheat and maize was 0.15, 0.27 and 0.11 at initial; 1.15, 1.03 and 0.56 at mid; and 0.45, 0.89 and 0.53 at late season stages. These Kc values correspond to those reported by the FAO - 56. Development of site specific Kc helps tremendously in irrigation management and furthermore provides precise water applications in the region. The developed simple approach to estimate daily Kc for the three main crops grown in the Fergana region was a first attempt to meet this issue.