Calculation of water environmental capacity of ammonia-nitrogen and total phosphorus in Laodao River Basin of the Xiang River

The quantitative calculation of water environmental capacity is a prerequisite for the fine management of water environment. However, the parameters and their changes are hard to ascertain, especially in middle and small-scale catchments with insufficient datasets, which bring uncertainties to the calculation of water environmental capacity. The distributed hydrologic model and the one-dimensional (1-D) river network model can solve some parameters, thus improving the calculation accuracy of water environmental capacity. A representative catchment in the Chang-Zhu-Tan area, the Laodao River catchment, which is one of the first-level tributaries of the Xiang River Basin, was chosen. The distributed hydrologic model and the 1-D river network model were constructed, and then the water environmental capacity and remaining environmental capacity of different control units were solved, combined with the calculation formula of water environment capacity. The results showed that the distributed hydrologic model performed well in simulating streamflow, which provided vital hydrologic parameters necessary for calculating water environmental capacity. Second, the discharges of ammonia nitrogen exceeded water environmental capacity at control units of outlets of Jinjing and Baisha sections, while the other control units had pollutant discharges lower than their water environmental capacity. In the upper and lower reaches of the Laodao River Basin, the remaining environmental capacity of total phosphorus at the control units approximated 0, while in the middle reaches of the river, the remaining environmental capacity of several control units was negative, and some pollutants should be reduced. The study provided a reference method for calculating water environmental capacity in areas with a lack of data.