Introduction
The limited water resources and heterogeneous distribution of precipitation in different parts of Iran on the one hand and rapid population growth and the increasing water demand on the other hand, have made optimal utilization and management of available water resources as one of the most important issues in water resources studies. Therefore, in order to achieve optimal water resources planning and management, it is necessary to know the components of hydrological phenomena such as precipitation and runoff. In addition, in order to proper management of water resources in the future, investigating the changes in watershed runoff and the input discharge to the dam reservoir under the impact of climate change is of utmost importance. The main purpose of this study is to investigate the probabilistic effects of climate change on runoff by analyzing uncertainty.
Materials and Methods
The studied area is the Roodzard River basin in Khuzestan province. The watershed area to the site of the Jereh dam is about 810 km2. The main parts of this watershed are mountainous, so that the highest point is about 3300 m and the average elevation is 1190 m above sea level. The Jareh storage dam has been created downstream of this basin to provide the water needed for Ramhormoz plain with an area of about 22000 hectares, to control the floods of the Roodzard river flood and to generate the hydroelectric power equal to 48 Giga watt hours per year. Therefore, assessing of the effect of climate change on water resources in the basin is a very important issue for optimal management of dam reservoir.
There are various methods to study the effects of climate change, the most valid being the output of atmospheric general circulation (GCM) models. In this study, the combination of 14 general circulation models under two emission scenarios A2 and B1 from the SRES (Special Reports on Emission Scenario) scenarios collection of the fourth report of the IPCC were used for simulating the climatic variables in the next period (2025-2054) compared to the baseline period (1971-2000). The weighting method of MOTP was used for combining general circulation models. Although the GCM models are powerful tools for assessing the effects of climate change but they have a low spatial accuracy, which is one of the weaknesses of these models. To overcome this weakness in this study, the combination of Change Factor and LARS_WG were used for micro-scaling. Also, in order to evaluate the effect of climate change on runoff, the distributed hydrological model of WetSpa was used to simulate rainfall-runoff process in this watershed.
Discussion and Results
The results of assessing the effects of climate change in future periods on monthly temperature indicates an increase in all of the months in the studied basin. Unlike temperature, the expected average monthly precipitation changes in future periods are irregular, as it has estimated incremental changes in some months and decreasing changes in some other months. The results of weighting the GCM climate models showed that the HadCM3 model in simulating the rainfall and the ECHAM5-OM model in simulating average temperature had the best performance. The results of evaluating the performance of LARS-WG model in simulating climate parameters and WetSpa model in simulating runoff indicated that both models had a good performance. The results of investigating the effects of climate change on runoff, indicated a reduction in annual runoff under all three probability levels emission scenarios. So that the changes in annual runoff under the A2 scenario for combination scenarios of IS, MS and CS are estimated to be reduced 8.49, 14.53 and 20.17 percentage, respectively, while these changes under the B1 scenario for combination scenarios of IS, MS and CS are estimated to be reduced 5.3, 12.36 and 17.36 percentage, respectively.
Conclusions
According to the results of this study, it can be said that the Roodzard River basin will experience more dry conditions due to climate change. Thus, according to the predicted changes of runoff in future periods and also because of the limited available water resources across the country, in order to reduce the negative effects of climate change in the future, the water resources managers and planners should consider the climate change effects in their decisions to cope with water scarcity in the future. For this purpose, they should use adaptation strategies, including improving the performance of water resource systems, changes in cultivation pattern in this area, more accurately management on the release of dam reservoir and changing the operation of the Jereh dam in accordance with climate change effects.