Water resource management measures by co-regulating water quality and water quantity for plateau watersheds in Southwestern China

Water pollution increases the volume of wastewater needing treatment in the watershed. It directly affects drinking water safety and poses significant economic risks to agriculture and manufacturing. Therefore, the realization of clear water and sewage diversion in the watershed is an important way to achieve water environmental protection in the watershed. Here, we divided the Fengyu River watershed, an agricultural basin in southwestern China, into regions including clean water regions (CW), highly concentrated wastewater regions (HCW), and low concentration wastewater regions (LCW) based on plateau watershed land use and topography. This study coupled TN, NO3--N, and NH4+-N to characterize the water quality, and used the SWAT model and ECK filter to simulate water quantity and ecological baseflow, constructing a water quality and quantity co-regulation framework. Meanwhile, we further revealed the contribution of different regional nitrogen sources based on dual stable isotope (N-15-NO3-/O-18-NO3-) and MixSIAR models. Results indicated that the nitrogen fertilizer (NF) and manure & sewage (MS) as important sources of NO3- in the watershed, their different contributions between regions exacerbate N variation, and the ordering among various regions is HCW (35.79%) > LCW (24.94%) > CW (11.54%). By co-regulating water quality and quantity and realizing clean and sewage diversion, 45.51% of river N can be recycled. Clear water with TN concentration below 0.5 mg/L, 50.5% of which can be directly discharged into lakes, to achieve the stability of the ecological base flow quality of rivers (China Environmental Quality Standard for Surface Water, TN < 1 mg/L) and the purification of lake water resources. Thus, co-regulating water quality and quantity is suitable for plateau watershed water environment protection key development orientation.