Modeling the impact of climate change on streamflow and major hydrological components of an Iranian Wadi system

Climate change has pronounced impacts on water resources, especially in arid regions. This study aims at assessing the impacts of climate change on streamflow of the Wadi Halilrood Basin which feeds the Jazmorian wetland in southeastern Iran. To simulate streamflow and hydrological components in the future periods (2030–2059 and 2070–2099), projections for the emission scenarios RCP4.5 and RCP8.5 from 11 global-regional climate models and two bias correction methods are used as input data for a hydrologic model that represents the daily streamflow with good accuracy (NSE: 0.76, PBIAS: 4.7, KGE: 0.87). The results indicate a slight increase of streamflow in January and March, due to the higher intensity of precipitation. However, according to the predicted flow duration curves, a decrease for high and very high flow and no remarkable changes for middle, low and very low flow is found under both emission scenarios for both future periods. Compared to the simulated hydrological components for the baseline, a slight increase of evapotranspiration of around 6 mm (4%) and 2 mm (<2%) for the midand end of the century is estimated, respectively. Moreover, a substantial drop of water yield of around 36 mm (63%) at midcentury and 39 mm (69%) at the end of the century are projected.

The impact of climate change on streamflow has been assessed in many parts of the world (e.g. Piao et al. () in China, Gizaw et al. () in Ethiopia, and Patil et al. Often climate change assessment studies focus on only one hydrological component such as streamflow. However, to gain insights into the variation of streamflow of the basin in the future, all relevant hydrological components should be considered (Uniyal et al. ). The long-term evaluation of the impacts of climate change on hydrological components such as streamflow, evapotranspiration, and water yield are necessary to support long-term water resources management and planning (Serrat-Capdevila et al. ; Uniyal et al. ). To achieve this goal, the specific objectives of this study are: (i) to assess the impact of climate change on streamflow, and (ii) to evaluate the variation of major hydrological components such as evapotranspiration and water yield in a Wadi system.

Study area
The Halilrood Basin is located in Kerman Province, Iran. It has an area of 7,224 km 2 ( Figure 1). Halilrood River is a major river in the province in terms of discharge, which feeds the Jazmorian Wetland. The water released from Halilrood River to the wetland is controlled by the Jiroft Dam.
The Jazmorian Wetland is a particularly valuable natural ecosystem. Recently, the wetland suffered from wind erosion, especially during the time of the year when the soil moisture is very low and potential evapotranspiration is

Measured climate and hydrologic data
The climate variables required to run the hydrologic model are daily precipitation (PCP) and temperature (TMP), solar radiation (SLR), humidity (HMD) and wind speed (WND) which are provided from nine climate stations and a synoptic station for the period 1979-2011. This period is defined as the baseline against which all future changes are compared. Observed streamflow data were available from 1993 to 2009. A five-year moving average is applied to visually show the temporal changes in PCP and TMP (as an average over all nine climatic stations) and streamflow (Supplementary Material, Figure A1). TMP has been increasing constantly since 1982, except for the last few years that show a slight decrease. There is a remarkable decrease in observed streamflow which correlates well with a decrease of PCP during the same period.
Global and regional climate model data  (Teutschbein & Seibert ). Therefore, to cope with the considerable deviation from the observed data shown in Table 1    should be noted that the method is superior to the SWAT weather generator, which was less successful when applied to the baseline period (KGE ¼ 0.71). We attribute the better performance of our approach to the fact that the variables are more consistently represented, as WND, HMD, and SLR are taken from the days with same weather conditions (in terms of precipitation occurrence) in the same month over the long-term period, and they represent valid values for a rainy or dry day (precipitation occurrence criterion) and for that time of the year (same month criterion).

Hydrologic model
The SWAT model has been applied to simulate the hydrolo-

Climate model ensemble
The hindcasted and projected changes of annual tempera- This seasonality is also represented in the bias corrected    Although a substantial increase was expected for actual ET due to an increasing trend projected for TMP over the 21st century (Supplementary Material, Figure A2) and an associated increase of potential ET, the smaller amount of precipitation causes a water limitation and counterbalances this effect. The highest amount of average annual water loss by actual ET is simulated for DM under RCP8.5 (more than 10% increase compared to the baseline) at the mid-century (Figure 7(e)), when the increase of actual ET is less than 1% for LS-M. Due to the higher ET and less PCP in future, the amount of water leaving the catchment (WYLD) is projected to considerably decrease. This reduction increases at the end of the century when WYLD falls below 30 mm, around 73 and 56% reduction respectively for DM and LS (Figure 7(g)). The results of the historical simulation indicate that almost half of the total amount of water entering the main channel originates from the lateral flow (30 mm), around 10 mm from surface runoff, and 5 mm from groundwater (Figure 7(b)). Therefore, LATQ and GWQ have the highest and smallest contribution to the stream. The remarkable reduction in WYLD is mainly mirrored in SURQ and GWQ in the future.
While a zero contribution (from 5 mm in the baseline to zero, 100% reduction) is simulated for GW in future for both bias corrected methods (Figure 7(b), 7(d), 7(f), and 7(h)), surface runoff is also estimated to be negligible (from 10 mm in the baseline to 1.1 mm, 90% reduction).
Lateral flow, with 30 mm originally being the main contributor, is less than 20 mm for LS (more than 30% reduction) and less than 15 mm for DM (more than 50% reduction) in both future periods (Figure 7(b), 7(d), 7(h), and 7(f)). As expected, the reduction of LATQ is higher at the end of the century under RCP8.5 (more than 50% reduction for both LS and DM) (Figure 7(g)).
In summary, the availability of water is lower when applying DM as compared to LS. Also, the Halilrood Basin is mainly covered by bare land and soils with low available water capacity, where the reduction in rainy days is problematic for these types of soils, especially for rainfed agriculture. Therefore, more intense precipitation events are expected to intensify the consequences of floods estimated for the future.
The similar seasonality of precipitation and streamflow shows a water-limited system where flow conditions are strongly linked to the precipitation regime, which is typical for torrential rivers in dry regions (Pumo et

CONCLUSIONS
In this study, a hydrological model (SWAT) has been applied to assess the potential impacts of climate change on streamflow conditions and major hydrological components in two different time slices (2030-2059 and 2070-2099) in the Halilrood Basin, Iran.
Our findings have shown that climate change has substantial effects on streamflow. For the two future periods, climate change scenarios projected a pronounced reduction in the mean annual water yield, which mainly reflects the changes in precipitation. The alteration of streamflow is mainly occurring in very high and high flow segments of the flow duration curve. The reduction in streamflow becomes larger towards the end of the 21st century. Besides future precipitation and streamflow reductions, which is a common outcome for dry regions of Iran, actual evapotranspiration is expected to slightly increase in the future while the amount of water leaving the basin (water yield) is expected to strongly decline. According to our simulations, surface runoff, groundwater, and lateral flow are predicted to decrease substantially at mid-and end of the century.
These strong reductions are due to both the effects of climate change and the unchanged water withdrawal assumed in our modeling approach, which is likely to further increase in the future and exacerbate the impacts of climate change.
The remarkable reduction in water yield and consequently in streamflow coincide with a slight increase in evapotranspiration which will lead to a decrease of the water being released to the wetland. Therefore, a decrease of the surface area of the Jazmorian wetland and an increase of wind erosion rates are expected.
This study has demonstrated that the hydrological response of the basin to climate change is strongly dependent on the considered emission scenarios and bias correction methods. The reduction in water yield is 10% more under RCP8.5 in comparison to RCP4.5 at the end of the century.
Also, a different response to climate change is found for the different bias correction methods, where the reduction in water yield is higher for raw and DM models compared to LS models. Therefore, we recommend to include multiple bias correction methods for climate change studies in arid regions. The future reduction in water yield is robust since it is observed under both RCP4.5 and RCP8.5, both bias correction methods and the raw data. Therefore, a sustainable strategy needs to be developed to mitigate the negative impact of climate change on future water resources.

ACKNOWLEDGEMENTS
The financial support was provided for Nariman Mahmoodi

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.