Assessing the integrated water resources development potential of Wadi systems in Iran and their vulnerability to climate change
Similar to other Middle Eastern countries, Iran is experiencing a serious water crisis. Water resources become increasingly stressed in the Wadi regions of central and eastern Iran. Assessing water resources there is thus essential due to growing water demand and possible effects of future climate change. Due to the lack of readily available surface water resources, different water use systems (WUSs) exist that provide water mainly for drinking, irrigation and further domestic use. In addition, many soil and water conservation measures (SWCMs) have been established to slow down the velocity of water and to increase infiltration and percolation rates. These alter hydrological components in Wadi regions that need to be considered. Such characteristics particularly apply to the Wadi basin Halilrood in central Iran. Halilrood River is the largest river in terms of discharge in the Kerman Province and the major water provider for the downstream Jazmorian wetland. The Soil and Water Assessment Tool (SWAT) is employed to model the hydrological processes of the Wadi. WUSs and SWCMs are implemented into the hydrological model to simulate streamflow and major hydrological components under current and future climate change conditions. Moreover, groundwater sustainability and possible alterations in the ecological flow regime are evaluated by taking both climate change and population growth into account. Results of the SWAT model simulation show that the hydrological processes of the Wadi system are better represented when WUSs and SWCMs are implemented. The improvement in model performance varies for different segments of the hydrograph. Peak and mean flows are simulated more accurately due to the improved depiction of infiltration rates and the slower release of water to the channels. The investigation of hydrological components reveals that the contribution of surface runoff and groundwater flows to the streams decrease in all sub-basins with WUSs. On the contrary, a higher contribution of groundwater to the streams is shown in most sub-basins with SWCMs. In sub-basins with both WUSs and SWCMs, groundwater contribution increases or does not show any change. To assess the impact of climate change on the water resources in the near and far future (2030-2059 and 2070-2099) in comparison to the baseline period (1979-2009), the projections from 11 climate models and two bias correction methods (LS: linear scaling and DM: distribution mapping) for two Representative Concentration Pathways (RCP4.5 and RCP8.5) are used as input data for the calibrated hydrologic model. The results indicate a slight increase of streamflow in winter season for both RCPs and both bias correction methods, due to higher precipitation intensity. Besides that, a shift is simulated in the timing of the seasonal peak-flow. This is due to increases in temperature and changes in the precipitation pattern. The Halilrood Basin is expected to be vulnerable to climate change as different segments of the flow duration curve (FDC) show increasing variability that can also be interpreted as an alteration of the future flood and drought extremes. A decrease for very high and high flows is projected under both RCPs. Climate change is causing a slight increase in evaporation and less available water for infiltration and percolation, which eventually leads to zero contribution of groundwater to the main channel. The impacts of climate change and growing water demand on the sustainability of groundwater use and the hydrologic regime of the Wadi are analyzed by linking the SWAT model results to the Indicators of Hydrologic Alteration (IHA) and the Range of Variability Approach (RVA). An unsustainable water resources situation (groundwater recharge is equal or greater than groundwater Demand) is expected in the future for the vast majority of sub-basins with WUSs. Due to the imbalance between the groundwater recharge under climate change and estimated groundwater demand in the future, a decline of groundwater levels is anticipated for the entire Halilrood Basin. This is not only resulting in unsustainable groundwater use, but also changes the hydrologic regime and poses a significant threat to downstream ecosystems. The presented modeling framework is a useful approach, providing beneficial information on the water resources of Iranian Wadi systems and their vulnerability to climate change and population growth. The results of this research can contribute to long-term planning there, which is required for a sustainable water resources management under changing future conditions.