## Materials and Methods
### Water Demand Estimation  
To compute the daily water demand in the paddy area, a water balance model for a paddy field was employed, which is given by [28]:  
\[SW_{t+1}=SW_{t}+IR_{t}+ER_{t}-ET_{t}-SP_{t}-DR_{t} \tag{16}\]  
where, \(SW_{t+1}\) and \(SW_{t}\) are the standing water depths in the field for the (\(t+1\))th and \(t\)th days, respectively. \(IR_{t}\), \(ER_{t}\), \(ET_{t}\), \(SP_{t}\), and \(DR_{t}\) are the applied irrigation, effective rainfall, crop evapotranspiration, seepage/percolation losses, and surface drainage requirement, respectively, on the \(t\)th day. These were determined from routine measurement in the scheme. An optimum (\(SW_{opt}\)) depth of 50 mm and a maximum (\(SW_{max}\)) of 100 mm were considered for irrigation application. When water in the field on the (\(t+1\))th day, \(SW_{t+1}\), exceeds \(SW_{max}\), the excess water was considered as the drainage volume. The components of Equation (16) were calculated daily to estimate the water depth at the end of the irrigation period. Similarly, the depth of water at the end of a subsequent day was computed. Finally, the depth of irrigation requirement for the subsequent day is computed by (\(SW_{opt}-SW_{t+1}\)) or (\(SW_{max}-SW_{t+1}\)) and the drainage requirement by (\(SW_{t+1}-SW_{max}\)). A reference evapotranspiration-(\(ET_{o}\))-crop factor (\(K_{c}\)) approach was used to determine the daily crop requirement \(ET_{t}\) (mmday\({}^{-1}\)) as expressed in equation 17 [28]. The \(K_{c}\) values for Malaysia rice variety were used [46]. \[ET_{t}=ET_{o}\times K_{c} \tag{17}\]  
The \(ET_{o}\) (mm day\({}^{-1}\)) was determined using FAO-56 Penman-Monteith (PM) Model [47]:  
\[ET_{O}=\frac{0.408\Delta(R_{n}-G)+\gamma\frac{900}{T+273}u_{2}(e_{s}-e_{a})}{ \Delta+\gamma(1+0.34u_{2})} \tag{18}\]  
where: \(ET_{o}\) is standardized reference crop evapotranspiration for short grass (mm d\({}^{-1}\)), \(R_{n}\) is net radiation at crop surface (MJ m\({}^{-2}\) day\({}^{-1}\)), G is soil heat flux density (MJ m\({}^{-2}\) day\({}^{-1}\)), T is air temperatureat 2 m height (\({}^{\circ}\)C), \(u_{2}\) is wind speed at 2 m height (m s\({}^{-1}\)), \(e_{s}-e_{a}\) is saturation vapor pressure deficit (kPa), \(\Delta\) is slope vapor pressure curve (kPa \({}^{\circ}\)C\({}^{-1}\)), \(\gamma\) is the psychometric constant (kPa \({}^{\circ}\)C\({}^{-1}\)) and 900 is a constant factor. The projected climate parameters, extracted and downscaled from ten different GCMs based on three RCP scenarios were used as basic inputs in the Reference ET model for calculating future \(ET_{o}\) series for up to 2099 period. Effective rainfall on the \(t\)th day \(ER_{t}\) (mm) was derived from the projected rainfall (RF) based on GCMs and RCP scenarios. It was calculated based on 1 in 5-year rainfall analysis as [48]:  
\[ER_{t}=\left\{ \begin{array}{rl}0.6\text{RF},&\text{for RF }<200\text{ mm/month}\\ 0.3\text{ (RF}+200),&\text{for RF }>200\text{ mm/month}\end{array} \right. \tag{19}\]  
In this study, an average seepage and percolation rates between 2-3 mm/day was adopted based on many field tests conducted by the Ministry of Agriculture, Malaysia. Pre-saturation is a significant initial phase before rice is planted. In this study, the water requirement was calculated in accordance with the recently recommended cropping irrigation schedule for the scheme, as reported by [49] and presented in Table 1. The schedule is designed for rice MR84, which is Malaysia's widely adopted rice variety with a growth duration of 120-125 days. In the irrigation scheme, wet direct seeding system is implemented. Water for pre-saturation is allocated to the irrigation blocks of the four ISAs (each staggered by one month) at a rate of 18.5 mmday\({}^{-1}\)ha\({}^{-1}\) for 14 days, followed by land preparation and drainage of excess water, if there was any.