Effects of climate change and human activity on lake drying in Bakhtegan Basin, southwest Iran

Bakhtegan and Tashk saline lakes with a total area of 1455 km2, as the second largest lakes in Iran, have been desiccating at an alarming rate since 2008, threatening the environment and ecosystem. In the absence of sufficient data, the relative impacts of human activity and climate change on the lakes shrinking remain as yet undetermined. In this study, the seasonal changes in the lakes water surfaces and area of irrigated lands at the Bakhtegan Basin during the past 37 years were obtained by analyzing 254 satellite images. The calculated drought indexes revealed that both rainfall and river discharge have experienced the most severe droughts since 2008. The applied Mann–Kendall test shown a general warming with a slope of 0.46 °C/decade in the basin, as well as decreasing trends in rainfall, river discharge, groundwater levels and stored water in dam reservoirs. During the 1987–1998, the area of irrigated lands has increased from 1253 to 2506 km2 throughout the basin. By beginning of drought in 2008 until 2017, whereas a major reduction (about 580 km2) in irrigated lands was detected in plains located at downstream part of the basin, a significant expansion (about 290 km2) of agricultural lands was observed at the upstream plains, where the lands were mainly cultivated with the rice. Based on a provided hydrologic water balance, it was concluded that the agriculture expansion in the upstream part of basin was a major factor in the lakes shrinking and consequently, the hypothesis ascribing climate change and dam construction as the only responsible was rejected.