Modelling Water Allocation for a Run-of-the-River Rice Irrigation Scheme Using GIS

Water is becoming increasingly scarce and it is important to find methods to reduce water consumption. It is a well-known fact that in most run-of-the-river irrigation projects, the water demands seldom satisfactorily match the erratic flows in the river. In large-scale irrigation schemes like the Tanjung Karang Rice Irrigation Scheme, allocation of water resources is more difficult due to the variability of water use, typical irrigation supply and the pattern of water availability. Due to unreliable water supply in the absence of storage reservoir for the scheme, there is a need for daily prediction and allocation of the available water resources equitably and efficiently to improve the system management. A study was attempted to develop Geographical Information System (GIS) integrated with water allocation model for ensuring equitable supply and efficient use of available water resources. A mathematical model was developed for the equitable and allowable water delivery through tertiary canals. The relationship was also developed for determining the available daily discharge for irrigation supply at Tengi River Headworks (TRH) in the main canal with respect to the upstream discharge at the Bernam River Headworks (BRH). The discharge variation regression equation was used to compute discharges downstream using known discharge records at the upstream point. It was found to be reliable for estimating daily available discharges for irrigation supply at the downstream point. New performance indicators were also developed to evaluate the irrigation delivery performance. The crop-water relationship model was developed based on different water allocation rules under field condition. A water balance model was studied. The model is able to determine the crop water requirements of rice at specific periods. The Penman-Monteith method was applied for estimating daily reference crop evapotranspiration. Autoregressive model was applied to forecast reference crop evapotranspiration and water diverted to the irrigation system at the upstream of the scheme since daily records were not available. A First-order Markov chain model was used to simulate the occurrence of rainfall, and a skewed normal distribution was applied to fit the amount of rainfall for a rainy day. Results obtained showed that these stochastic models could satisfactorily be used to generate daily records for the area. A comprehensive modelling framework was developed for equitable water allocation in tertiary units considering the optimal utilisation of limited water resources as the season advances. It was embedded into the user-friendly interactive tool known as Rice Irrigation Management Information System (RIMIS). ArcObjects and Visual Basic for Application (VBA) programming languages were used to develop RIMIS inside the powerful ArcGIS 8.3 software. RIMIS is comprised of six modules, several sub-modules and functions. The “Detailed Scheme Information” module describes a new framework for creating generic graphical user interfaces with ArcObjects and VBA for database access of spatial and non-spatial data recorded in a geodatabase. It provides appropriate information interactively for irrigation engineers or water managers towards solving the irrigation water management issues as the season advances. The “Equitable Irrigation Deliveries” module simulates irrigation deliveries incorporated with allowable and design irrigation supply based on the tertiary canal’s command areas, stochastic daily rainfall and evapotranspiration and available water resources for irrigation supply. It allows the manager to enhance the decision-making for delivering the right amount of water to the fields for the upcoming irrigation period. The “Optimisation of Irrigation System” module is intended for optimal utilisation of available water resources through proportional allocation to all irrigable areas. The simulation model was tried with different parameters and criteria using optimal allocation policy. The “Monitoring Irrigation Deliveries” module gives information on the uniformity of water distribution and the shortfall or excess; and what decisions to adopt for the next day. The “Post-Season Analysis” module uses daily or periodic information to evaluate the season’s irrigation management performance. The “Hydro-climatologic Simulation Module” integrates several sub-modules for hydrological simulation models. The RIMIS is a new development in Malaysia. It was evaluated for its decision-making capability on equitable water allocation along with trade-offs in water use by the scheme for two irrigation seasons in 2003 and 2004. The RIMIS was found to be practically acceptable and an effective tool for providing a more equitable distribution of available water resources for irrigation supply or more crops with less water.

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