Climate-smart agro-hydrological model for the assessment of future adaptive water allocation for Tanjong Karang rice irrigation scheme

Agro-hydrological water management framework helps to integrate expected planned management and expedite regulation of water allocation for agricultural production. Low production is not only due to the variability of available water during the crop growing seasons, but also due to poor water management decisions, such as not considering the available water for irrigation. Climate-smart agro-hydrological model can be a robust solution for wise water management decisions in a large-scale irrigation scheme to cope with the risk of water and food security under the new realities of climate change. The Tanjung Karang Rice Irrigation Scheme has yet to model agro-hydrological systems for effective water distribution under climate change impacts. The study aimed to develop a climate-smart agro-hydrological model in the context of adaptive water allocation under the risk of climate change for a large-scale rice irrigation scheme. In this study, daily climate variables for baseline (1976-2005) and future 2020s (2010-2039), 2050s (2040-2069) and 2080s (2070-2099) periods were extracted for ten global climate models (GCMs) under three Representative Concentration Pathways (RCPs) scenarios (RCP4.5, RCP6.0, and RCP8.5). Climate variables then downscaled to a local station using Climate-smart Decision Support System (CSDSS) in the MATLAB environment. Two hydrological models Soil Water Assessment Tool (Arc-SWAT 2012) and Hydrologic Engineering Corps Hydrologic Modeling System (HEC-HMS 4.2) simulated climate change impacts on hydrological processes in Upper Bernam River Basin (UBRB). The Hydrologic Engineering Center’s River Analysis System (HEC-RAS 5.0) hydraulic model used to compute available discharges for the main water conveyance system from the Bernam River Headwork to Tengi River and at the key points in the main canal. The impact of climate change on potential basin streamflow was evaluated using the validated HEC- HMS model. Based on design parameters, the inflow and release patterns for the newly built reservoir were assessed with the need for irrigation water demand and available water for supply under future climate change. Finally, Climate-smart agro-hydrological model was developed using Excel-based Visual Basic for Application (VBA) to analyze and visualize climate and hydrological knowledge for wise adaptive water management practices under new climate change realities. The statistical results of the model evaluation in the watershed both during the calibration (p = 0.014) and validation (p = 0.022) indicated that HEC-HMS performed better compared to Arc-SWAT model. The R², NSE, PBIAS and RSR for HEC-HMS are 0.74, 0.71, 4.21 and 0.37; and 0.71, 0.69, 5.32 and 0.31 while that of SWAT are 0.67, 0.62, -5.4 and 0.64; and 0.64, 0.61, -4.2 and 0.65, respectively during the calibration and validation periods. The projected temperature will increase under scenarios with the largest changes of 1.97 ᵒC and 2.08 ᵒC, respectively for mean maximum and minimum temperatures during the off-season period (January-June) in the most severe scenario (RCP8.5). Projected rainfall may have normal fluctuations, increasing in the main-season and decreasing in the off-season with higher (average increase of 2.4% and decrease of -3.7%) rate in the most severe scenario (RCP8.5). The projected climate patterns indicate that the water availability for irrigation is expected in the future to be more critical during the off-season period. Future streamflow at UBRB decreases in all future periods (2010-2099) during the main and off-seasons. However, the changes is more pronounced during the off- season, with a decrease of -9.14% under the worst-case scenario (RCP8.5). Projected future hydro-climatic variables show that the basin may likely to experience tremendous pressure in the late century (2070-2099) particularly during the off-season months. The analysis of water allocation in the scheme show imbalance between the scheme water demand and the available water for supply across the seasons. The scheme is under-supplied from January to March, and over-supplied from April to June during the off-season. In the main-season, there is shortage water supply from July to September, as well as excess supply from October to December, which runs as waste. Evaluation of the newly constructed reservoir in the area, to store excess water for use during water shortage shows that its capacity is inadequate. Therefore, to have effective water allocation in the scheme, provision of the additional reservoir(s) is highly recommended. The developed agro-hydrological model is user-friendly, can visualize and analyze daily, weekly, monthly and seasonal streamflows at various sections of the river, available water for supply into the scheme, scheme water demand and reservoir inflow/release/storage patterns for the baseline and future periods. The model allows water management authorities to explore water allocation alternatives under new realities of climate change.

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