Impacts of water quality on water availability of reservoirs in the state of Ceará

Abstract

This study aims to analyze the impacts of water quality on the water availability of reservoirs using the total phosphorus (TP) concentration as an indicator. Three reservoirs in the State of Ceará were investigated for this purpose: Acarape do Meio (29.6 hm3); Aracoiaba (162 hm3); and Pentecoste (395.6 hm3). For estimating the TP load yielded in the studied catchments, equations that correlate census data on animal, agricultural and human production with TP gross production were applied. The TP balance in the three reservoirs was carried out based on the Chapra equation for complete mixing, considering periods with different hydrological regimes: wetter period (2004 to 2010); and period with prolonged drought (2011 to 2020), indicating a decay relationship of TP concentrations for larger stored volumes. This study proposed the introduction of the PIR (Phosphorus Input Ratio) concept, which refers to the ratio between the TP input load into the reservoir and the gross TP load yielded in the catchment. Considering that the hydrological regime can influence the TP transport conditions, a correlation between PIRs and runoff coefficients was established, presenting a good adjustment. The effects of including water quality restrictions regarding TP content on water availability at the reservoirs were also addressed. TP concentrations at the reservoirs can negatively impact water availability subject to quality constraints. Therefore, the adoption of techniques to reduce the production and export of TP from the study areas is extremely important for making water available within the quality standards required.