## Materials and Methods
### Modeling Future Impacts of Climate Change on Hydrological Processes
The sub-basin element in HEC-HMS model conceptually represents interactions of infiltration, surface runoff, and subsurface processes. Priestly-Taylor method was used to compute the potential evapotranspiration, _ET_ (mm/day) as mostly used in continuous simulation using HEC-HMS [19]. The model is expressed as [35]:  
\[ET=\frac{\alpha}{\lambda}\frac{\Delta}{\Delta+\gamma}(R_{n}-G), \tag{7}\]  
where, \(\alpha\) = 1.26, \(\lambda\) is latent heat of evaporation (MJ/kg), \(R_{n}\) is net radiation at crop surface (MJ m\({}^{-2}\) day\({}^{-1}\)), \(G\) is soil heat flux density (MJ m\({}^{-2}\) day\({}^{-1}\)), \(\Delta\) is slope vapor pressure curve (kPa \({}^{\circ}\)C\({}^{-1}\)), and \(\gamma\) is Psychometric constant (kPa \({}^{\circ}\)C\({}^{-1}\)). The Deficit and Constant Loss (DCL) method was used to compute the runoff-volume (loss). It uses one layer to account for continuous changes in moisture and requires only four parameters namely initial deficit, maximum deficit, constant rate, and percentage of impervious area. The method is simple, requires lesser-input parameters and can be applied for event-based and long-term simulations [36] and applied by many studies [19; 37; 38; 39]. The DCL parameters were obtained from the land use and soil grids of the area using HEC-GeoHMS, a GIS extension of HEC-HMS. The DCL method was combined with simple canopy and surface methods. The canopy method accounts for precipitation intercepted by plants from one storm to another, which subsequently evaporates and the water extracted by the plants through the process of transpiration. The surface method on the other hand accounts for the maximum amount of water that held on the soil before surface runoff begins. Soil Conservation Service Unit hydrograph (SCS-UH) method was used for the transformation of precipitation excess into point runoff. The method is applicable for long-term simulation [19]. In this method, unit hydrograph (UH) discharge, \(U_{t}\) is given as a ratio to the UH peak discharge, \(U_{p}\), for any time t, a fraction of \(T_{p}\), the time to UH peak [36]:  
\[U_{p}=C\frac{A}{T_{p}}, \tag{8}\]  
\[T_{p}=\frac{\Delta t}{2}+t_{lag}, \tag{9}\]  
where, \(A\) is the watershed area, \(C\) is the conversion constant (2.08), \(\Delta t\) is duration of excess precipitation, and \(t_{lag}\) is the basin lag. The input parameter for this method is the basin lag, which is 0.6 times the time of concentration \(t_{c}\) of the flow. The \(t_{c}\) was obtained using HEC-GeoHMS and adjusted through calibration process. The baseflow in each sub-basin was modeled using constant monthly-varying method. In this method, the program represents baseflow as a constant flow but may vary monthly. The monthly flows were approximated from the long-term series of monthly-observed flow data and were added to the direct runoff, computed from rainfall for each time step of the simulation. The channel flow was computed using Muskingum routing method as widely adopted by many studies [19; 37; 38; 39] because of its wider considerations to most conditions of watershed and it accounts for streamflow attenuation. The method consists of two parameters; the travel time through the reach, \(K\) and a weighing factor, X. The parameters \(K\) and X were estimated using the channel geometry. The travel time, \(K\) (h) was computed using equation 10 [40]:  
\[K=\frac{L}{3600V_{w}}, \tag{10}\]  
where \(L\) is the reach length (m) and \(V_{w}\) is the flood wave velocity (m/s). Historical discharge records were used for evaluating the performance of the HEC-HMS model. Manual and automatic calibration techniques were applied to optimize model parameters. Discharge data for 18 years (1981-1998) was used for model calibration and 8 years (1999-2006) for validation with 5 years of warm up period in each.