Spatiotemporal patterns of water volume and total organic carbon concentration of agricultural reservoirs over South Korea

Lacking of available water quality data causes the limited understanding of the coupled dynamics of hydrologic and nutrient cycles in lakes and reservoirs and along river streams. This study conducts the rotated Principal Component Analysis (rPCA) of water volume and total organic carbon (TOC) concentration data from ∼2,200 agricultural reservoirs in South Korea to extract the major modes of their spatiotemporal variability. Over 2020–2022, the total TOC load in the reservoirs ranges between 1,165 and 1,492 tons (289 and 360 Mtons of water storage volume; 3.54 and 4.60 mg/L of TOC concentration). The first rPCA mode is a decreasing trend of water level (38% of the explained variance) and increasing trend of TOC concentration (27%) over the southern Korea region where the TOC concentration increased during the 2022 drought. The second rPCA mode is interannual variability of water level (25%) and TOC concentration (18%) over the central Korea region. This study found a marginal relationship between paddy field area and TOC concentration and their regime shift to high TOC concentration during the 2022 drought, which is a potential cause of the increase of TOC concentration in 2022. This study provides observational evidence of interactions between TOC concentration and water volume during a severe drought, suggesting a potential role change of agricultural reservoirs to carbon source.