###### Abstract  
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 m\({}^{3}\)/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. modeling; HEC-HMS; HEC-RAS; climate change; irrigation; water resources; adaptive water allocation 2020_00_0000-0000-0000-0000-0000-00000-0000-0000-0000-0000-0000-0000-00000-00000-00000-00000-00000-00000-000000-00000-00000-00000-000000-000000-000000-000000-000000-000000  
availability and its coherent quantification in the respective river basins and also on a global scale have been a great challenge [3,4]. Variations in temperature and rainfall seem to be the most widely reported problems plaguing predicted climate change in the next century [5]. However, these variations differ significantly across the place in the world. For example, a catchment located in a temperate climate could respond differently to the impact of climate change compared to a catchment situated in a mountain climate [6]. Climate change has a significant effect in altering rainfall patterns in humid tropics [7]. Therefore, the change in rainfall patterns is more significant than in the climate change temperatures in a tropical region like Malaysia [8]. Although Malaysia is blessed with abundant water resources, many regions suffer water shortages because of spatial and temporal variability of precipitation, which resulted in incremental demand for water resources [2]. Consequently, rivalry for actual water supplies is now more crucial. Most of the country's rice farming systems rely mainly on river streamflow. However, due to insufficient flow in the rivers, particularly during the dry season (January to June), some rice schemes incorporate reservoir to river headwork, to meet water requirements for irrigation. Although water storage is a good option in the global water cycle and climate change [9], the design and operation of these storage facilities should be linked with the projected available water resources for adaptive water allocation. Several research works were carried out to understand the variability of streamflows due to climate change [10, 11, 12, 13, 14, 15] with mixed results in the streamflow variability, depending on the region and season. For instance, a decrease in annual streamflow was observed [5,16]. A similar trend was obtained when the hydrologic impacts of climate change were assessed on the runoff trend in Klang Watershed, Malaysia [17]. The results predicted annual mean streamflow for future periods to decrease, except the period 2080s.