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 ∼2200 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 assoicated with 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 associated with 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 was a potential cause of the increased TOC concentration in 2022. This study provided observational evidence of interactions between water volume and TOC concentration during a severe drought, suggesting a possible shift of the role of agricultural reservoirs to carbon source.

中文翻译：

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韩国农业水库水量和总有机碳浓度时空格局

缺乏可用的水质数据导致对湖泊、水库以及沿河溪流的水文和养分循环的耦合动态的了解有限。本研究对韩国约 2200 个农业水库的水量和总有机碳 (TOC) 浓度数据进行旋转主成分分析 (rPCA)，以提取其时空变化的主要模式。 2020-2022年，水库总TOC负荷范围为1,165至1,492吨（蓄水量289至360吨；TOC浓度3.54至4.60 mg/L）。第一个 rPCA 模式与韩国南部地区水位下降趋势（解释方差的 38%）和 TOC 浓度上升趋势（27%）相关，该地区的 TOC 浓度在 2022 年干旱期间增加。第二种 rPCA 模式与韩国中部地区水位 (25%) 和 TOC 浓度 (18%) 的年际变化相关。这项研究发现，稻田面积与 TOC 浓度之间存在边际关系，并且在 2022 年干旱期间，稻田面积向高 TOC 浓度转变，这是 2022 年 TOC 浓度增加的潜在原因。这项研究提供了水量与 TOC 浓度之间相互作用的观测证据。严重干旱期间 TOC 浓度，表明农业水库的作用可能转向碳源。