Regulation of water in the Mahaweli project systems

Abstract

Regulation of water resources systems has become increasingly complex during the recent decades and there is evidence that the process will become more so in future. In the Mahaweli Project Systems, especially in a country short of natural resources this task will call for sophistication in planning the system for the future with consideration being given in complete allocation of available resources for maximization of area under cultivation and generation of electrical energy. However, the average planner and the decision maker has been provided with very few new tools and techniques to compensate for the added complexities. Once the water is made available for irrigation of lands, its use is limited unless water management aspects at field level are not implemented. In order that the results be successful, careful consideration of every aspect of water management including the cultural practices is necessary. In arriving at acceptable management practices, a brief review of ancient irrigation systems management practices are discussed in order to establish continuity with past traditions .. The specific objective is the determination of the most efficient use of the water resources of Mahaweli and adjacent river basins within the limits- of constraints, fixed with regard to the priority to meet certain demands. This lays emphasis on minimizing the loss due to water deficits as regards the water demands for irrigation and energy and capacity requirements for hydro-power during the dry years and making a balance between water demands and flood control during wet years. A large number of components, variables and relationships define the system. In order to regulate water in such a system to achieve this specific objective a technique that is adequate to consider all the complexities has been devised. The method of System Analysis is used in this investigation to determine the most efficient use of the limited water resources available within the Project Systems. Systems Analysis in a sense, is a method of integrated thinking, and conceptually there is very little difference between mental and mathematical models. Systems analysis cannot replace experience, in fact, it augments it. The water balance study used to determine the availability of water to meet the demands within the system, necessitated the reviewing and processing of hydrological data for INFLOWS and factors affecting the consumptive use for DEMANDS. The basic sub-systems used to build up the Macro System of water use and allocation are described in detail. The methodologies used include basic operational rules, priority for irrigation of certain areas, rule curves, probability criteria for inflows, trial and error process and successive approximations. A rather detailed review was made of the crop water requirements and the diversion requirements for irrigated perimeters. The efficiencies of the irrigation applications, cropping patterns etc. are based on past experience as described herein. As the storage reservoirs are operated for energy generation the other river systems which generate major portion of electrical energy outside the Mahaweli Basin are also considered. The model was prepared to simulate the integrated operation of all the hydraulic structures and power stations using available input data for INFLOWS, DEMANDS, and POWER TARGETS. The model is run for each year in a historical hydrologic period of 28 years with time step as one week for specified system characteristics and selected operational rules. This is repeated for different combination of reservoirs, power stations, irrigated perimeters and different operational rules. Sensitivity calculations were done to ascertain the validity of the assumptions and limitations of the analysis. Further studies and investigations are recommended based on the results of these calculations. The results of the water balance study are presented for different combinations of reservoirs and irrigation areas. Study reveals that the total area that can be successfully cultivated without any reservoirs to be about 150,000 ha. of land while the development of all the reservoirs will make it possible to cultivate about 350,000 ha. of land. The total annual energy production will be about 2650 GWh.