Effects of rainfall and land use on river nutrient dynamics in the Brazilian semiarid region

Abstract

This paper investigated the effects of rainfall and land use on the dynamics of total phosphorus (TP) and total inorganic nitrogen (TIN) in the Brazilian semiarid region. The study covered eleven watersheds within an area of 142,381 km². TP and TIN were monitored between 2013 and 2018 at 92 river water quality monitoring sites. The monthly rainfall (Rn) was obtained from 575 rain gauges. Pearson’s correlation (R) between Rn and nutrient concentration was performed. The correlation patterns were also analysed based on land use data: urban area (%), agricultural field area (%), demographic density (inhabitants/km²), sewer system coverage (%), and reservoir density (reservoir/km²). Backward stepwise regression was carried out to identify predictors of nutrient concentrations. The results revealed that in watersheds with high demographic density, the rainfall is not the most influential factor in the concentration of nutrients (marginal effect). However, in regions with greater accumulated rainfall and lower reservoir density, the rainfall was related to a linear increase in nutrient concentrations (positive effect) (R > 0.8). On the other hand, in watersheds with less accumulated rainfall and greater inter-basin hydrological disconnection due to a high-density reservoir network, there was a linear decrease in nutrient concentration (inverse effect) (R<-0.5). In the backward stepwise regression, sewer system coverage and Rn had the greatest influence on TP, and the urban area was the strongest predictor of TIN. These findings provide valuable information for water quality management in drylands with strong human-induced impacts, similar to the Brazilian semiarid region.

Chlorophyll-a prediction in tropical reservoirs as a function of hydroclimatic variability and water quality

“…In addition, the existence of several smaller reservoirs with lower hydrological e ciency upstream of strategic reservoirs leads to a reduction in their volumes; Banabuiú, for example, reached ). As for SC, the high concentrations can be attributed to the fact that the reservoir monitoring site is located immediately downstream of stabilization pond systems (Freire et al, 2021(Freire et al, , 2022.…”

“…In general, the reduction in nutrient load (mainly TP) due to the dilution effect occurs when the predominant sources of pollution are punctual. However, in the case of predominantly diffuse sources, the release of TP retained in the sediments causes an increase in the load proportional to the in ow (Coffey et al, 2019;Freire et al, 2022;Lima Neto et al, 2022). In fact, the reservoirs in which increased precipitation intensity had a nutrient 'enrichment effect' are mainly located in rural basins, where non-point sources of pollution are predominant, originating from soil and agriculture.…”

“…The greater sensitivity of Castanhão to precipitation, compared to Orós, can be attributed to the low coverage of sewage treatment services in the basin where it is located, which, according to Freire et al (2022), has a coverage of only 4.2%, the worst in the state.…”

“…Freire et al (2022), investigating the in uence of precipitation on nutrient concentration in the Ceará basins, also identi ed positive and negative relationships between TP and P in different watersheds. During precipitation events, surface runoff increases, and more sediment is transported, resulting in an excessive nutrient load (M Lira et al, 2020)…”

Chlorophyll-a prediction in tropical reservoirs as a function of hydroclimatic variability and water quality

“…In general, the reduction in nutrient load (mainly TP) due to the dilution effect occurs when the predominant sources of pollution are punctual. However, in the case of predominantly diffuse sources, the release of TP retained in the sediments causes an increase in the load proportional to the in ow (Coffey et al, 2019;Freire et al, 2022;Lima Neto et al, 2022). In fact, the reservoirs in which increased precipitation intensity had a nutrient 'enrichment effect' are mainly located in rural basins, where non-point sources of pollution are predominant, originating from soil and agriculture.…”