Assessing the Effects of Climate Change on Hydrological and Economic Conditions of South Karkheh Sub-basin

Water resources are under increasing pressure from climate change, population growth, and socio-economic development. Recent assessments have revealed that climate change accounts for about 20% of the increased water scarcity. Due to the impacts of climate change on water resources, agriculture is one of the most important sectors of the economy affected by this event. Accordingly, food security threatened by climate change is one of the most important challenges of the 21st century to provide sufficient food for population growth. Because of the abundant effects of climate change and water scarcity on farmers' welfare, different strategies have been adopted to adapt to these changes. Adaptation can be defined as: “adjustment in ecological, social or economic systems in response to actual or expected climatic drivers and their effects or impacts. Climate change phenomenon has occurred in Iran during recent decades and it will continue in the future with greater intensity based on the seventh report of the Intergovernmental Panel on Climate Change. In other words, Iran’s aridity has been exacerbated by the climatic change to pose serious threats to its water resources and the ability to meet its residential, environmental, industrial, and agricultural water demands. In this regard, improved irrigation efficiency is vital for alleviating water scarcity and can be an adaptation strategy to climate change for this country due to water losses account for at least 45% of the water used in Iran's irrigated agriculture, which arises from inefficient irrigation systems.

The Karkheh River Basin (KRB) is one of the largest watersheds in Iran after Karoon and Dez. It is located on the western border of the country in a strategic position. This watershed consists of five sub-basins, namely Kashkan, Qarasou, Gamsiab, Seymareh, and South Karkheh. The Karkheh River Basin deal with water shortage through the frequent droughts in the region over the past decades, leading to major water deficit in downstream of the basin. Also, Climate change is expected to exacerbate the current water problems in KRB. Such ways that, 10-30% decrease in the runoff by mid-century is estimated in this basin thanks to less precipitation and higher evapotranspiration. The aim of current research is to evaluate the potential effects of climate change on irrigated agriculture and irrigation efficiency improvement strategy for adaptation in South Karkheh sub-basin using a hydro-economic model. This model integrates the strengths of the Positive Mathematical Programming (PMP), and Water Evaluation and Planning (WEAP) as a hydrological model. The MABIA module within WEAP was also used to simulate the impacts of climate change scenarios and water availability scenarios on crop yield. The hydro-economic model simulation starts with run WEAP-MABIA model for hydrological simulation of South Karkheh sub-basin. For this purpose, Model calibration was carried out using the PEST method. The Zahraei’s climate change scenarios (2015) were then used as input to the WEAP-MABIA model. Finally, results of the WEAP- MABIA were used as input to the Positive Mathematical Programming model to investigate the effects of climate change scenarios and adaptation strategy (improvement of irrigation efficiency) on the crop patterns and farmers’ profit. Required data for this study were collected from various sources. The data related to the Hydrological aspect was obtained from Khuzestan Water and Power Authority (KWPA) and Khuzestan Meteorological Office for the base period 1960-2018. Also, the required economic data were obtained through interviews with 180 farmers which randomly selected in Karkheh basin and Agriculture Organization of Khuzestan province.

The results of the WEAP model show that climate change will decrease the availability of water in agricultural areas of south Karkheh sub-basin. But the impact of climate change varies across regions. So that the water available in Payepol and Karkheh-Noor is reduced by 8.29% and 32.76% respectively. The MABIA method results also indicated that the B2 climate change scenario in the 2018–2050 periods may have a severe impact on crop yields and irrigation water requirements.The MABIA method results  also showed that the climate change scenario will likely lead to crop yield decreases, ranging between 1.5% and 34.05%. These changes in crop yields would be accompanied by increases in irrigation water requirements ranging between 1.11% and 22.43%, for all crops. A strategy to improve irrigation efficiency will mitigate the effects of climate change on the performance of different crops (especially rice crops). The results show that applying this strategy does not have a significant impact on the pure water requirement (actual evapotranspiration of products). In fact, applying this scenario (improving irrigation efficiency by 20%) will reduce gross water requirement by preventing water loss. Finally, results of the Positive Mathematical Programming showed that the total irrigated crop area is reduced under climate change by 17.93%. Therefore, Total agricultural profits decrease by 44% under climate change, compared to the reference scenario. The Karkheh Noor irrigation area faces the largest reductions in crop area. Also, this area shows the largest reduction in agricultural profits (59.9%). In contrast, the smallest decrease in crop area and agricultural profits have occurred in Payepol. But the reduction of water use due to the improving irrigation efficiency will increase the profit of agricultural activity in the Karkheh sub-basin by 340 billion Rials.

This research has tried to contribute to the analysis of climate change impacts and adaptation by addressing the processes that occur at the sub-basin level. The Karkheh sub-basin case study, an illustrative example of critical water and climate interactions, permitted to illustrate climate change vulnerability and adaptation. The results of this study showed that Climate change can affect South Karkheh sub-basin's agriculture through changes in water resource flows and allocations. These changes impose remarkable economic costs on farmers. But the application of on-farm adaptation option via investment in improved irrigation efficiency technology does reduce the impact of climate variability. Therefore, this research demonstrates that the implementation of such a policy, can support adaptation and reduce the risk faced by farmers in light of climate change.