Accounting for the annual variability when assessing non-point source pollution potential in Mediterranean regulated watersheds

This work presents a proposal to classify land according to its potential as a source of non-point pollution within a Mediterranean watershed considering the interannual precipitation variability and the dam regulation effects. For this purpose, the Potential Non-point Pollution Index (PNPI) developed by the ANPA (Italian environmental Protection Agency) was modified to include the annual local behavior of precipitation as a new variable, leading to the local Annual APNPI. Both indices were computed in the Guadalquivir river (Spain), which has a drainage surface of 57,500 km2 and is highly regulated by more than sixty reservoirs. The results reflected the vulnerability along the Guadalquivir river in terms of the spatially variable nonpoint pollutant nature of its contributing watersheds. Annual average nitrate concentration values in the southern side exceeded by almost five times the average value of the northern side and showed a statistically significant power fit with PNPI with an R2 of 0.65. Long-term available nitrate data (1981/82–2006/07) at a monthly scale at the outlets of some watersheds allowed us to rank priority pollutant source areas within the watershed. Statistically significant power fits between annual average nitrate loads and the APNPI with 0.51–0.99 R2 values validated the utility of adding the variability of precipitation at an annual scale as a dynamic factor in the index. APNPI can constitute a simple dynamic classification index to assess the relative risk of NPS pollution across a large area, especially under data-scarce situations.