Abstract:
The growing demand and increasing fresh-water scarcity urgently require
effective and sustainable management of water for irrigation to assure future requirements
of food and fiber production. The sustainable management uses advanced technologies to
improve water use efficiency. The computer-based irrigation models are globally
accepted as most consistent management tools for efficiently using irrigation water on
field.
An irrigation scheduling computer model (Mehran) is developed basically as an
irrigation research and management tool. The Mehran Model is an integrated decision
support system (DSS), which is designed using Visual Basic-6.0 platform to compute
Reference Evapotranspiration (ET o ) by FAO-56 Penman-Monteith method. Crop
Evapotranspiration (ET c ) and irrigation schedules are computed by the model using daily
soil-water balance and dual crop coefficients approaches. The model distinguishes soil
texture classes and different irrigation methods, and also considers 66 regional and
international crop types. The model computes soil-water profiles variation with respect to
the time and existing root depth of the crop. The model interface provides multiple
choices to user for designing irrigation schedules by (a) Real-time daily water balance, (b)
Planning on demand system, (c) Planning rotational system, and (d) Synchronizing on
demand and rotational (warabandi) systems.
The model has been field tested and validated on planning and management of
various irrigation schedules for cotton and wheat crops in Lower Indus Basin of Pakistan.
The field experiments at management allowed depletion (MAD) level of 55, 65 and 75%
were carried out for cotton crop, and at MAD level of 45, 55 and 65% for wheat crop. The
daily actual crop evapotranspiration (ET ca ) was observed through gypsum block readings
and a drainage lysimeter. The observed seasonal cotton crop actual Evapotranspirations
(ET ca ) in the experiments were 486, 413, and 397 mm and those computed by the model
iwere 504, 421, and 404 mm. Similarly the observed seasonal wheat crop ET c was 363,
359, and 332 mm, and those computed were 383, 369, and 355 mm. The crop water use
efficiency (WUE) determined in terms of seed-cotton yield per unit of land and per unit of
seasonal ET c were computed to be 6.0, 6.5, and 5.8 kg (ha mm) −1 . The corresponding
water use efficiencies (WUEs) for the wheat crops were obtained 14.1, 15.0 and 13.4 kg
(ha mm) −1 . The highest crop WUE was achieved with MAD at 65% for cotton crop and
at 55% for wheat crop’s experiments. The model averagely overestimated seasonal ET c of
cotton crop merely by 2.41% and 4.31% for wheat crop.
Weekly root depth and daily soil-moisture measurements were carried out, which
assisted in carefully monitoring effective root zone depth during experiments. When
practicing either scientific or traditional irrigation scheduling in the country, a seasonal
water amount of 370 mm is suggested for wheat crop, and 450 – 500 mm for cotton crop
to achieve optimum yield and WUE.
Statistical analysis (R 2 = 93%, T–test = 2.6, and F–test = 1481) showed good
correlation between the computed and actual seasonal ET c of the crops. The sensitivity
analysis on weather input parameters revealed that the model is most sensitive to
temperature variations on output of crop transpiration, soil evaporation and irrigation
water allocation.
The Mehran Model is found to be quite versatile, flexible, user–
friendly, and can be successfully used as a decision support system for irrigation
scheduling and management for general crops, specifically for cotton and wheat crops in
the Lower Indus Basin of Pakistan.