Water balance approach for development of irrigation scheduling of wheat crop in Badin [Pakistan]

Study was conducted to devise irrigation scheduling of wheat based on water Balance approach using two predominant soil textural claws of Badin district. The input in the water Balance includes: meteorological data of district Badin (Temperature, relative humidity, wind speed, sunshine hrs and rainfall data for 20 years, soil textures and their water characteristics (Field capacity, saturated moisture content and permanent wilting point), crop season, maximum root depth, root depth as a function of time, potential evapotranspiration and actual evapotranspiration. Weibul probability method was used on 20 years climatic data of district Badin to get dependent climatic values which may occur 3 out of 4 years (75% probability). These dependent climatic values were then used for determination of dependable values of reference crop evapotranspiration using both Hargreaves and modified penman methods. Potential evapotranspiration values were calculated by multiplying reference crop evapotranspiration with crop coefficient values. It was found that on calibration of estimated Hargreaves potential evapotranspiration values to those of modified Penman method, following simple modified Hargreaves equation ,Etp=C (Tav+17.78) Rs., was developed. Crop coefficients for wheat crop in Badin district have been evolved for the growing period of the crop, using climatological and lysimetric data for greater accuracy in a given set of conditions. Relatively simple and explicit empirical equation to determine potential evapotranspiration was developed based on dependable climatological data predicted from 20 years climatic record of district Badin. The equation in its simplest form is ,Etp=0.204 Rs + 0.005 Vw+0.6 (Tav/Rh) Rs. To check the accuracy, reliability and efficiency of authors equation, a comparison was made with the values of potential evapotranspiration estimated by modified Penman and calibrated Hargreaves equation. The comparative results reveal that authorþs equation contain less mean absolute error, which confirms the reliability and efficiency of authorþs equation. Authorþs empirical equation is definitely simple and a useful for practicing irrigation engineer to estimate potential evapotranspiration compared to a complicated and cumbersome modified Penman equation. Irrigation intervals showed decreasing trend in both sandy loam and silty clay soils as the season progressed. Management allowable depletion levels (MAD) was found to have great influence on irrigation schedules. As the MAD level increased from 50% to 70%, the number of irrigation decreased from 7 to 4 in sandy loam soil and 5 to 4 in silty clay soil; the average irrigation interval increased from 17 to 24 days in sandy loam soil and 17 to 39 days in silty clay soil and the irrigation water requirement decreased from 421 to 340 mm in sandy loam soil and 444 to 363 mm in silty clay soil. Evaluation assessment performance (EAP) of various irrigation schedules of wheat on both the soils of district Badin indicated that irrigation schedule based on 50% MAD level gave the best results in terms of maximum relative crop production followed by MAD levels of 60% and 70%.