TMDL for phosphorus and contributing factors in subtropical watersheds of southern China.

Water eutrophication, particularly that caused by phosphorus runoff, is of major concern in China due to the serious threats it poses to watershed environments. We investigated one forested and nine agricultural watersheds with areas of 9-5212 ha in a hilly region of Hunan Province in a subtropical region of southern China from 2010 to 2012 to study total phosphorus (TP) loads and contributing factors. The annual TP loads varied from 35.7 to 222.1 kg P km(-2) year(-1) among the different watersheds, with the rainy season of spring and summer accounting for 56.3-82.0% of TP loss. The highest total maximum daily load (TMDL, 0.5 kg P km(-2) day(-1)) and existing exported daily TP loads (DTPL, 1.8 kg P km(-2) day(-1)) were observed under high flow and moist flow conditions in the ten watersheds. However, the target daily reduction ratios for the DTPLs to reach the water quality standard of 0.05 mg P L(-1) varied little with flow condition in the stream but depended on the type of watershed, i.e., <50, <80, and 80-90% for forested, agricultural, and livestock-dominated watersheds, respectively. Gray relational analysis (GRA) suggested that livestock density was the most important factor for watershed TP load under various hydrologic conditions, while livestock density (LD), soil available phosphorous (SAP), cropland percentage, and mean shape index (SHMN) were notable factors for daily reduction rate (DRR) under high and moist flow conditions. Therefore, to protect the local watershed environments, watershed management approaches that include the regulation of livestock production are recommended as the most effective means of reducing P loads at the watershed scale in subtropical areas of southern China.