The Effect of Mid-term Changes in Climate and Land Use on Flow Reduction in Karkheh Catchment

Investigating future flow changes is a very important factor in policy-making and determining the strategy of a catchment. Two very important parameters affecting the flow are climate change and land use change. How these two parameters affect the water flow is due to the effect that each of them has on the hydrological cycle. The present study investigates and evaluates the effects of climate change and land use change on surface flow in one of the catchments in southwestern Iran, Karkheh. This catchment is known as one of the most important and strategic catchments for food security and electricity generation in Iran. This catchment is one of the most critical and strategic catchments for providing food security and electricity generation in Iran.

The present study uses the soil and water assessment model (SWAT) to investigate the impact of climate change and land use on the Karkheh catchment. A total of 15 scenarios have been studied. These scenarios, firstly investigate the changes on each climate and land uses separately, and then the simultaneous impact of these two parameters in the mid-term on the surface flow is analyzed. Climate change has been studied using RCP scenarios and general atmospheric circulation (GCM) models for two different GCMs, CMCC-CESM and MRI-CGCM3 in the 4.5 and 8.5 series over the mid-term (2040 to 2060). The changes applied to land use in two different cases are based on Iran's future policies. Changes in these policies have been made using the Sixth Development Plan, the 550,000-hectare Plan, the Land Management Plan from an agricultural perspective, and the population growth rate in the region according to the scenarios considered in the plan of the Statistics Research Institute of Iran.

To calibrate the model, 9 parameters and two objective functions NSE and R2 were used in 6 hydrometric stations. Based on the findings of this study, the highest sensitivity of the model is to the CN2 parameter. In model calibration and validation, all stations have values above 0.5 for the intended objective functions. Changes in precipitation and temperature are other findings of this study. Based on the results obtained from the analysis of precipitation data, a decrease in precipitation is visible in all months, and the highest drop is related to the autumn season, with a decline of 7.83 to 8.23%, and the lowest reduction is dedicated to 8.5 scenario.

The results of climate change illustrate that in the mid-term, the most changes will be achieved in the RCP8.5 scenario which include a 14.3 to 22.8% reduction in runoff. The results obtained from land use changes based on future policies in Iran show a maximum decrease of 7% in the average monthly runoff. If the changes in climate and land use are studied simultaneously based on the average flow rate, it can be concluded that in the RCP8.5 scenario, the most change will occur.