Influence of agricultural land use on limnology of Sri Lankan reservoirs during rainy season

Abstract

Reservoir limnology is closely linked to land use and land cover (LULC) in their upper catchments, particularly during peak rainy-season runoff. Understanding these interactions is key to sustainable catchment and reservoir management. This study investigates how agricultural land use influences water quality (WQ) and sediment quality (SQ) in ten Sri Lankan reservoirs: Karavita-wewa (7.57°N, 79.85°E), Kiulekada (8.32°N, 80.81°E), Kottukachchiya (7.93°N, 79.95°E), Mahawilachchiya (8.47°N, 80.20°E), Mahagal-wewa (6.40°N, 81.04°E), Pudumurippukulam (9.36°N, 80.35°E), Unnichchai (7.61°N, 81.54°E), Urusita-wewa (6.33°N, 80.93°E), Vadamunai (7.90°N, 81.27°E), and Vijayakatupotha (7.72°N, 79.90°E). Bimonthly surface water and sediment samples were collected from each reservoir between February 2023 and April 2024, covering before-rainfall, during-rainfall, and after-rainfall periods. WQ parameters included temperature, dissolved oxygen (DO), conductivity, pH, oxidation-reduction potential (ORP), Secchi depth, chlorophyll-a, alkalinity and hardness, measured using YSI-ProDSS and APHA standards. SQ parameters included ORP and pH, while total nitrogen and phosphorus were measured in both water and sediment samples using sodium salicylate and ascorbic acid methods. Catchment characteristics, LULC and reservoir morphometry (surface area and perimeter) were derived using QGIS. Statistical analyses were conducted in RStudio using Pearson’s correlation, Shapiro-Wilk tests and t-tests. Agricultural area (AA) showed strong positive correlations (p<0.001) with built-up area (r = 0.96, R² = 0.92) and water extent (r = 0.87, R² = 0.76), however a negative correlation with forest cover (r = –0.69, R² = 0.48). During and after rainfall, significantly higher conductivity, hardness, and alkalinity (p < 0.001) were observed in reservoirs with higher AA:SA ratios. After-rainfall, sediment phosphorus was positively correlated with AA:SA (r = 0.70, R² = 0.49) and alkalinity (r = 0.59, R² = 0.35) and negatively correlated with DO (r = –0.75, R² = 0.56), all with p < 0.001. These results demonstrate that agricultural runoff is a major driver of limnological changes, leading to elevated nutrient and dissolved ion concentrations in reservoir waters. Although water is essential for agriculture, intensive land use, if not properly managed, may compromise reservoir health. The findings underscore the urgent need for integrated agricultural watershed and reservoir management to preserve water quality and ecosystem function.