Abstract
Various indicators are used for evaluating the performance of different aspects of an irrigation system, and assessments also differ in terms of the types of performance indicators used. This paper describes a GIS-based assessment system which utilizes a new concept and evaluated the inadequacy of a widely used Relative Water Supply (RWS) concept to characterize the irrigation delivery performance for a rice irrigation system as the season advances. Development of this GIS-based assessment system resulted in the creation of new indicators, viz., the Rice Relative Water Supply (RRWS), Cumulative Rice Relative Water Supply (CRRWS) and Ponding Water Index (PWI). These indicators were determined from field tests and evaluated in a Malaysian Tanjung Karang Rice Irrigation Scheme (TAKRIS). The RWS concept was found to be inaccurate for characterizing the oversupply condition on irrigation deliveries for rice irrigation; and difficult to correctly quantify the oversupply condition for irrigation supplies. Besides, it was found that the RRWS indicator can distinctly characterize the oversupply condition for RRWS > 1.0 and undersupply condition for RRWS < 1.0 on irrigation delivery for any given period. A value of 1.0 for RRWS indicates an irrigation delivery that matches perfectly the actual field water demand. This study presents a cumulative RRWS plot that provides important information on irrigation supplies for any given time interval for management decisions. An increasing slope in the actual CRRWS curve with CRRWS = 1.0, means that irrigation supply can be slightly curtailed in the next period. On the other hand, if the slope is negative, supply has to be increased. If a computed CRRWS line follows the CRRWS = 1.0 line, it means that irrigation deliveries are perfectly matched with the field water demand. A graphical user-interface was developed for structuring the assessment tool within an ArcGIS platform. The system can instantly provide information on the uniformity of water distribution and the shortfall or excess, and provides vital information in terms of decisions that need to be made for the next period. The system helps to maintain continuous updating of input and output databases on real field conditions. Results are displayed on the computer screen together with color-coded maps, graphs and tables in a comprehensible form. The system is likely to be adopted for evaluating various water allocation scenarios and water management options. It can also be used as an analytical and operational tool for irrigation managers.
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