## 3 Results and Discussion
### Projection of Available Supply for Irrigation under Climate Change  
A DEM of the area was used to generate and extract the geometry of the river, starting from the water source at SKC gauging station to the TRH at the intake of the scheme. The river reach was divided into about 59 cross-sections, perpendicular to the flow direction from Upper Bernam watershed outlet to the TRH as shown in Figure 8.  
Figure 7: Projected mean monthly streamflow for three future periods of 2010–2039, 2040–2069, and 2070–2099 relative to the baseline period of 1976–2005 using three Representative Concentration Pathway (RCP) scenarios (RCPs 4.5, 6.0, and 8.5). Figure 8: River geometry traits at the study location: (**a**) view of conveyance system from Bernam River Headworks (BRH) to Tengj River Headworks (TRH) (Map Data ©Google Map, 2019); (**b**) cross-sections at Bernam River/Feeder canal; and (**c**) cross-sections at Tengj River. The GIS extension of HEC-RAS model (HEC-GeoRAS) was used to extract the bathymetry of each cross-section, the distance between two adjacent cross-sections. Each section is represented by the coordinates (X,Y) where X (corresponding to station value), is the abscissa measured starting from a point chosen on one of banks and Y (corresponding to elevation value), is the ordinate measured starting from a horizontal plane of reference. The adjusted values of coefficient of Manning's roughness used are 0.045 for the main channel and 0.05 for both left and right overbanks. The contraction coefficient was fixed to 0.1 and the coefficient of expansion was fixed to 0.3. In both calibration and validation periods, the daily observed and simulated discharge from the model were compared. Results of R\({}^{2}\), NSE, and PBIAS during the calibration and validation periods were 0.73, 0.72, and 0.81 and 0.70, 0.65, and 1.79, respectively. These show that the simulation is satisfactory since the values are greater than 0.5 [51]. In addition, the model was able to optimally captured low and peak flows for most of the days. Figure 9 shows the projected annual available discharge for irrigation supply at the scheme intake. The available discharge depends on the streamflow from the basin outlet. The interquartile range, the median and average values of available water supply for the future years would be smaller than that of past years. The decrease in supply is more pronounced during the 2080s period under the RCP 8.5 scenario. The average supply for historical (1976-2005) period is 15.97 m\({}^{3}\)/s, which would decrease by 12%, 18%, and 21% under RCP 4.5, 6.0, and 8.5, respectively. Therefore, based on these findings, the projected available supply by the model for different scenario could be integrated with adaptive cropping patterns to enhance future water allocation. For instance, a recommended supply could be adopted, which enables proportionate distribution of water among the tertiary canals through proper control of the off-take structures gate. Thus, a factor can be introduced (considering the rice's water-sensitive growth stages) to achieve an equitable supply. The factor depends on the management and operational decisions in the command area with respect to the available water for irrigation. With the advancement of irrigation season, the recommended irrigation supply (allowable supply based on available water) for tertiary canals is proposed. This adaptive strategy could be useful for most irrigation schemes operating on run-of the river, with limited or no water storage facility. Figure 10 illustrates the average seasonal future changes in available supply for the wet/main irrigation season (July-December) and dry season (January-June) relative to the baseline period. The mean changes for RCPs 4.5, 6.0, and 8.5 are \(-\)7.94% (\(-\)7.86%), \(-\)10.30% (\(-\)9.84%), and \(-\)12.42% (\(-\)12.26%) during the dry (and main) seasons, respectively. Generally, the seasonal supply is projected to decrease for RCP 4.5, RCP 6.0, and RCP 8.5 scenarios.