# Climate-Smart Agro-Hydroological Model for a Large Scale Rice Irrigation Scheme in Malaysia
## Materials and Methods
### Water Demand Estimation  
Several parameters are required to estimate paddy water demands under climate change scenarios. These include the historical and GCM-projected daily hydro-meteorological data, which are needed for estimating reference evapotranspiration, and irrigation requirements of paddy fields and effective rainfall, which need to be estimated using a water balance model. Other related data are crop coefficient, seepage-percolation rate, water application and conveyance losses, and irrigation command area. The daily water demand for the command area was calculated as:  
\[\mathrm{IR}_{\mathrm{t}}=\left\{ \begin{array}{cc}\left(\mathrm{ET}_{\mathrm{c },\mathrm{t}}-\mathrm{ER}_{\mathrm{t}}+\mathrm{SP}_{t}+\mathrm{RP}_{t}\right)& \mathrm{if}\ \mathrm{RP}_{\mathrm{t}}\ \ \mathrm{WDmax}\\ 0\,&\mathrm{if}\ \mathrm{RP}_{\mathrm{t}}>\mathrm{WD}_{max}\end{array} \right. \tag{12}\]  
where \(\mathrm{IR}_{\mathrm{t}}\) is the irrigation requirement of the rice crop on the t-th day (mm/day); \(\mathrm{ET}_{\mathrm{c},\mathrm{t}}\) is the water requirement by the rice crop on t-th day (mm/day); \(\mathrm{ER}_{\mathrm{t}}\) is the effective rainfall on the t-th day (mm/day); and \(\mathrm{SP}_{\mathrm{t}}\) is the seepage and percolation losses in a rice field (mm/day). A \(\mathrm{WD}_{\mathrm{max}}\) of 100 mm is usually the standard practice during the normal irrigation supply in Malaysia; however, it varies from 50 mm to 100 mm during irrigation supply periods. \(\mathrm{RP}_{\mathrm{t}}\) is the required ponding water depth on the day. A reference evapotranspiration (\(\mathrm{ET}_{\mathrm{o}}\))-crop factor (\(\mathrm{K}_{\mathrm{c}}\)) approach was used to determine the crop requirement \(\mathrm{ET}_{\mathrm{c}}\) (mm day\({}^{-1}\)) as expressed in Equation (12). The \(\mathrm{K}_{\mathrm{c}}\) values for a Malaysian rice variety were used [37]. \[\mathrm{ET}_{\mathrm{c}}=\mathrm{ET}_{\mathrm{o}}\times\mathrm{K}_{\mathrm{c}} \tag{13}\]  
The \(\mathrm{ET}_{\mathrm{o}}\) (mm day\({}^{-1}\)) was determined using the FAO-56 Penman-Monteith (PM) model [38]:  
\[\mathrm{ET}_{\mathrm{O}}=\frac{0.408\Delta(R_{n}-G)+\gamma^{\prime}\frac{900}{ \mathrm{T}+273}u_{2}(e_{\mathrm{s}}-e_{\mathrm{s}})}{\Delta+\gamma(1+0.34u_{2})} \tag{14}\]where ETo is standardized reference crop evapotranspiration for short grass (mm d\({}^{-1}\)); Rn 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 temperature at 2 m height (\({}^{\circ}\)C); u\({}_{2}\) is wind speed at 2 m height (m s\({}^{-1}\)); e\({}_{\rm s}-\)e\({}_{\rm a}\) is saturation vapor pressure deficit (kPa); \(\Delta\) is the slope vapor pressure curve (kPa \({}^{\circ}\)C\({}^{-1}\)); \(\gamma\) is a psychrometric constant (kPa \({}^{\circ}\)C\({}^{-1}\)); and 900 is a constant factor. The projected climate parameters, extracted and downscaled from 10 different GCMs based on three RCP scenarios, were used as basic inputs in the reference ET model for calculating future ETo series for the period to 2099. Effective rainfall (ER) was derived from the projected rainfall (RF) based on GCM and RCP scenarios. It was calculated based on 1 in 5-year rainfall analysis using the FAO method as:  
\[{\rm ER}=\left\{ \begin{array}{ll}0.6{\rm RF},&\mbox{for RF }<\,200\,{\rm mm/month}\\ 0.3\,({\rm RF}+200),&\mbox{for RF }>\,200\,{\rm mm/month}\end{array} \right. \tag{15}\]  
In this study, average seepage and percolation rates between 2-3 mm/day were 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 [39] and presented in Table 1. The schedule is designed for rice MR84, which is the widely adopted rice variety in Malaysia, with growth duration of 120-125 days. Wet direct seeding method is practiced in the irrigation scheme.