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.
