1 AIT Asian Institute of Technology

Optimal water allocation at the tertiary levels of irrigation projects

AuthorRamos, Edwin P.
Call NumberAIT Diss. no. WA-91-06
Subject(s)Irrigation water--Mathematical models

NoteA dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Engineering and Technology
PublisherAsian Institute of Technology
Series StatementDissertation ; no. WA-91-06
AbstractIn large irrigation systems. where water is delivered over a wide area with varying climate, soil. and crop conditions. the water allocation at the tertiary unit level is an important component to improve the system management. In this study. models were developed to determine the water allocation to crops over the growing season in the tertiary units of large irrigation projects. The models are based on biological, hydrological and optimization submodels. The biological or the water-production function submode! relates the yield of crops to the actual evapotranspiration. The hydrological submode!. which constitute the water balance model determines the actual amount of water used by the crop in relation to the soil moisture. rainfall and irrigation decisions. The optimization submode!, which is based on dynamic programming is used to determine the best irrigation decisions. i.e .• the optimal intraseasonal water allocation. by maximizing the weighted relative yield of the crops, given the state of the system. The state of the system is defined by the amount of water remaining for allocation and the soil water status of the crops at the beginning of any growth stage. The weighted relative yield was computed by introducing weights to the relative yields based on the maximum yields and corresponding price of the individual crops. The available water is limited by the capacity of the turnout serving the system and the water delivery schedule being followed. Both deterministic and stochastic approaches have been adopted in the optimization model. The uncertainty of the soil water status of the crops is included in the stochastic model. In both models. a factor a. which defines the maximum proportion of the available water in each growth stage that can be allocated to all crops is introduced. The water allocation models are capable of determining the amount of water needed by the crops in the tertiary units command area for biweekly and weekly durations of growth. Based on the models. a step-wise procedure was developed which can be used for determining water allocation and studying existing ones and their effect on crop production. An area allocation model was developed to determine the best area under each crop considering the optimal water allocation derived using the water allocation models and the weekly water delivery schedules. The models were applied in two selected tertiary units of the Mae Klong Irrigation Project in Thailand. It was observed that the models can determine. at a given rainfall probability of exceedance. the minimum level of seasonal water supply and its corresponding distribution throughout the growing season which will result to the maximum yields of the crops. This will help irrigation operators limit water deliveries when water is unlimited. In case of limited water supply. the models can predict the best options of allocating the given amount of water and the expected return from the crops. Results indicate that although the stochastic model formulation is useful and feasible. the effect of uncertainty in soil water status of crops on irrigation decisions are minimal. as compared to those obtained in the deterministic model. The area allocation model is capable of assessing the cropping pattern and area under each crop given the weekly water deliveries and the corresponding optimum weekly water allocations of the crops.
Year1991
Corresponding Series Added EntryAsian Institute of Technology. Dissertation ; no. WA-91-06
TypeDissertation
SchoolSchool of Engineering and Technology
DepartmentOther Field of Studies (No Department)
Academic Program/FoSWater Resources Research Engineering (WA)
Chairperson(s)Murty, V.V.N ;
Examination Committee(s)Harboe, Ricardo ;Demaine, Harvey ;Loof, Rainer ;Helweg, Otto J. ;
Scholarship Donor(s)Government of Australia;
DegreeThesis (Ph.D.) - Asian Institute of Technology, 1991


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