Separating the impacts of climate change and human activities on runoff reduction in Karkheh basin by hydrologic sensitivity analysis method

In the wake of global and regional climate change and heightened human activities, runoff from some rivers in the world, especially in the arid and semi-arid regions, has significantly decreased. During the past decades, due to the climate warming and the significant regional precipitation variation coupled with strong human activities such as the drastic agricultural and industrial development, soil and water conservation projects and water conservancy projects, more attention has been given to assess the impacts of climate change and human activities on runoff change. These variations in runoff have been an urgent challenge for water resources planning and management. In response to this challenge, one of the tasks for researchers is to identify the different roles of climate change and human activities on the nonstationarities in runoff. Both deterministic rainfall–runoff models and statistical methods have been proposed to assess the impacts of climate change or human activities on runoff. Climate change and human activities are the two main factors affecting runoff change in basins and the separating their impacts is of great importance for land use planning, water resources management and social, economic and political activities. Identifying and evaluating these factors and their contribution to reducing surface runoff is important for the sustainable development of water resources and for reviewing past and future calculations, plans and designs.

In this study, the contribution of the impacts of climate change and human activities on runoff reduction in Karkheh basin has been separated by hydrologic sensitivity analysis method. Hydrological sensitivity can be described as the percentage change in mean annual runoff in response to the change in mean annual precipitation and potential evapotranspiration. The Kaekheh basin is an important transboundary river basin whose environmental and economic significance, as well as future food security and hydropower production challenges, have been recognized nationally and internationally. The area of the Karkheh basin up to the Jelogir hydrometric station is approximately 40,000 square kilometers, of which about 55.5% are in mountainous areas and about 44.5% are plains and foothills. The common time period between the studied variables including daily streamflow in the hydrometric station and the air temperature and precipitation in the synoptic stations from 1990 to 2020 was considered. streamflow naturalization was done by adjusting agricultural water withdrawals, associated return flows and operational reservoir upstream of the Jelogir station. The Thiessen polygon method was used to estimate the average rainfall in the basin. Also, the arithmetic mean method was used to estimate the average air temperature and potential evapotranspiration at the basin. Hargreaves method was used to estimate potential evapotranspiration in synoptic stations. The trend in the annual values of precipitation, runoff, air temperature runoff and potential evapotranspiration from 1990 to 2020 was determined by Mann-Kendall test. Also, change points in the annual runoff were detected through Pettitt’s test and the precipitation–runoff double cumulative curve method. The Pettitt’s test is a non-parametric approach to determine the occurrence of a change point. It has been commonly used to detect changes in the hydrological series as well as climatic ones. The double cumulative curve is the plot of the accumulated values of one variable against the accumulated values of another related variable for a concurrent period.

The results show that runoff shows a decreasing trend at the significance level of 0.01, while no significant trend is observed in the amount of precipitation. The average annual air temperature and evapotranspiration show the potential of increasing trend at the significance level of 0.01. The rate of the annual runoff, potential evapotranspiration and air temperature trend is equal to -1.96, 2.71 and 0.062 mm per year, respectively, at a confidence level of 99%. The results show that the change-point in the annual runoff series based on the Pettitt’s test and the precipitation–runoff double cumulative curve method occurred in 1999. Therefore, the pre-change period and post-change period are before and after 1999, respectively. The average annual runoff in the post-change period has decreased by 34.4% compared to the pre-change period. The results of hydrologic sensitivity analysis method show that climate change and human activities have reduced runoff by 36.2% and 63.8% in the Karkheh basin, respectively. The results of this study can provide a reference for the development, utilization and management of the water resources and environment protection. The results of the present study can supply a reference to regional water resources management and planning. Water and soil conservation is not the sole purpose and means of development, at the same time, a practically possible proposition in term of increasing the produced runoff can be put forward for local managers to reasonably arrange the local actions, synthetically considering the sustainable development in the regional water resource and ecological environment.