Trophic state changes of semi-arid reservoirs as a function of the hydro-climatic variability

Abstract

In semi-arid areas, such as the Northeast Region of Brazil (NRB), droughts lead to reduction of reservoir volumes and to increase of nutrient concentrations. The goal was to evaluate the trophic state index (TSI) of semi-arid reservoirs due to interannual variations of precipitation and stored volume. The study included 65 reservoirs in 8 NRB-based watersheds for 2008–2017. From 2013, NRB faced a severe drought that induced a significant reservoir volume reduction. Although the trophic state was already high before the drought (mostly eutrophic and hypereutrophic), the TSI of 91% of the reservoirs increased in the dry period. However, differently from the other watersheds, in which 100% TSI increase was observed for all reservoirs, the groundwater-rich and the lowest-water demand watersheds presented TSI decreases in 30 and 50% of the reservoirs, respectively. Thus, the impact of droughts on these two watersheds was less pronounced, as a potential result of groundwater flow with low-phosphorus concentration into the reservoirs of the former watershed, and lower nutrient loads/higher stored reservoir volumes of the latter one. Simple empirical equations, which are capable to estimate the mean TSI based solely on precipitation and reservoir volume, showed to be potential tools for integrated water management.