Seasonal Changes in the Riverine Dissolved Organic Carbon in the Permafrost Regions on the Qinghai-Tibetan Plateau

Permafrost degradation can thaw previously frozen organic matter, which can result in decomposition or trans-regional export along with runoff. So far, little is known about the water quality in the permafrost regions on the Qinghai-Tibetan Plateau. We investigated the temporal variation of dissolved organic carbon (DOC) in the rivers of the Three Rivers' headwater region. The results showed that the average monthly DOC concentrations of 47 rivers in the Three Rivers' headwater regions ranged from 2.92 mg/L to 6.82 mg/L, with an average of 4.39 mg/L. From the spring to the beginning of summer, DOC concentrations increased sharply. During this period, the monthly mean temperature raised from -11℃ to 2℃, and reached the maximum value. Then, DOC concentrations decreased rapidly as the temperature rising the highest temperature from 2℃ to 13℃. From summer to winter, the average DOC concentrations decreased gradually during the temperatures decreasing from 13℃ to -11℃, and the lowest value was recorded in November. There were a great variance in the seasonal DOC export fluxes, ranging from 0.406 kg/(km²·d) to 11.022 kg/(km²·d) among the rivers under different land cover types. DOC export fluxes were positively correlated with average runoffs in the watersheds. The highest DOC export was recorded in the spring snowmelt period and the rainy summer.