Agro-hydrological water management frameworks help 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 crop growing seasons, but also poor water management decisions. The Tanjung Karang Rice Irrigation Scheme in Malaysia has yet to model agro-hydrological systems for effective water distribution under climate change impacts. A climate-smart agro-hydrological model was developed using Excel-based Visual Basic for Applications (VBA) for adaptive irrigation and wise water resource management towards water security under new climate change realities. Daily climate variables for baseline (1976–2005) and future (2010–2099) periods were extracted from 10 global climate models (GCMs) under three Representative Concentration Pathway scenarios (RCP4.5, RCP6.0, and RCP8.5). The projected available water for supply to the scheme would noticeably decrease during the dry season. The water demand in the scheme will differ greatly during the months in future dry seasons, and the increase in effective rainfall during the wet season will compensate for the high dry season water demand. No irrigation will therefore be needed in the months of May and June. In order to improve water distribution, simulated flows from the model could be incorporated with appropriate cropping patterns.

Spatial and temporal climatic variability influence on the productivity of agricultural watershed and irrigation systems. In a large irrigation system, the quantification and regulation of the flow at different locations of the channel is quite difficult manually, leading to a poor delivery of supply and demand. Water shortage is a crucial issue due to mismatch between available water and demand at intake point of Tanjung-Karang Irrigation Scheme. This study assessed the potential impacts of climate change on basin outflow for 2010–2039, 2040–2069, and 2070–2099 to the baseline period (1976–2005) and used it as input hydrograph to simulate river discharge. A Hydrologic Engineering Corps Hydrologic Modeling System (HEC-HMS) model driven by projections from ten global climate models (GCMs) with three scenarios (Representative Concentration Pathways (RCPs) 4.5, 6.0, and 8.5) used to simulate the outflow and the Hydrologic Engineering Centers River Analysis System (HEC-RAS) model applied for hydraulic modeling. The projected seasonal streamflow showed a decreasing trend for future periods. The average available irrigation supply for historical period is 15.97 m³/s, which would decrease by 12%, 18%, and 21% under RCPs 4.5, 6.0, and 8.5, respectively. Projected irrigation supply showed oversupply and undersupply to the required supply during the growing season. Simulated discharge could therefore be incorporated into cropping practices to boost the sustainable distribution of water under the new realities of climate change in the future.

In the face of the prevailing challenges of limited water for irrigated agriculture, the knowledge of crop coefficients for use in irrigation, and the yield response to moisture stresses become pertinent for developing strategies to improve agricultural water productivity. Field experiments were conducted to evaluate the crop coefficients and yield response factors of UC 82B tomato subjected to soil moisture stresses on growth-stage basis. The irrigation treatments investigated were: a full irrigation (application of 100% ETo), and three soil moisture deficit levels (20%, 40%, 60% ETo) imposed at the vegetative, flowering and maturity growth stages, in successions. The mean crop coefficient (Kc) was highest (0.99) during the mid-season under full irrigation, and lowest (0.47) during the vegetative stage under 60% ETo soil moisture deficit, while the mean value across the entire crop growth stage was 0.88. The yield response factors (Ky) were 1.26 and 1.30 for the 2017/2018 and 2018/2019 seasons, respectively. The mean Ky of was 1.28 for the entire growth cycle, implying high sensitivity of the tomato cultivar to water deficits, and that yield reduction is proportionally larger when water used is reduced because of stress. Full irrigation at the maturity stage is recommended.