Impact of Bugun’ Reservoir on Groundwater and Soil: A Case Study from South Kazakhstan

The influence of the Bugun’ reservoir, created for irrigation in the semi-desert zone of South Kazakhstan, on groundwater and soils of the reservoir basin and its coast during long-term operation is analyzed. The rise in the level of groundwater on the banks of the reservoir and in downstream of the Bugun’ and Karazhantak dams in the process of the reservoir filling in 1963–1965 and further use for irrigation of more than 93,000 ha of land for growing cotton, grain, fodder, and fruit and vegetable crops was estimated. The leaching of toxic salts for plants into the groundwater of adjacent territories by seepage water from soils and rocks of the reservoir basin was calculated. The seasonally water-salt regime of soils and rocks of the aeration zone, as well as the regime of the groundwater level (10 days) and their mineralization (monthly) under conditions of strong seasonal fluctuations in the level of surface waters in the reservoir, was studied. A delay in the phases of rise and fall of the surface water level and groundwater level on the coast, reaching 2 months, depending on the distance from the reservoir, was revealed. Desalination of groundwater from salts is observed only in the bowl of the reservoir and on its banks, and in relief depressions, water mineralization (salt content) increased to 35 g/l. A strong salinization of coastal soils was noted in relief depressions and in the downstream of dams (up to 300–500 t/ha in a soil layer of 2 m). The satellite images (Landsat and Sentinel satellite images) reveal that the process of soil flooding (waterlogging) with groundwater and their salinization (salt concentration increase) continues for 60 years even after the reservoir has been filled. Geomorphological conditions, lithology of soils and existing rocks of the coast, and the winds direction contribute significantly to the intensive soil erosion process on the reservoir’s left bank. We found that the coastal destruction in the 60s–70s occurred at a rate of up to tens of meters per year, and later it began to slow down (averaging 7–11 m/year), due to the strengthening of the banks with stone filling and fragmentary overgrowth of wood-shrub vegetation. In the 2000s, it hardly exceeded 1–2 m/year. Nevertheless, in the last decade, climate change has caused changes in the reservoir filling regime in the winter-spring months and in the direction of winds, which has led to an increase in erosion processes. Monitoring erosion processes with satellite images and a quadrocopter (UAV) made it possible in 2020–2022 to timely identify the prominent erosion-prone areas and prevent possible reservoir destruction.