Modeling water quality in a tropical reservoir using CE-QUAL-W2: handling data scarcity, urban pollution and hydroclimatic seasonality

Abstract

ABSTRACT This study applies a 2-D hydrodynamic model (CE-QUAL-W2) for simulating water quality dynamics in a tropical reservoir located in Fortaleza, Ceará, Brazil. While rainfall concentrates basically in the first semester, this reservoir receives untreated sewage from an urban catchment throughout the year. To deal with data scarcity, model simplifications are justified and several adjustments are carried out, so that only the parameters temperature (T), dissolved oxygen (DO), chlorophyll a (Chla) and phosphate (PO4) are kept in the modeling process. Additionally, different assumptions are performed regarding the time-evolution of reservoir inlet concentrations: constant values, step and linear variations. The results indicate that the simplified model can predict well the seasonal variations of T, DO, Chla and PO4. The best fitting between model results and measurements are obtained with the assumption of linear variation in inlet concentrations, followed by the assumptions of constant values and step variation. Moreover, the results reveal that while PO4 presents a complete mixing behavior with a clear increase in concentration from the wet to the dry season, T, DO and Chla show an alternating stratification-destratification patter during the day-night but without relevant variations throughout the year. Model simulations of different scenarios also indicate a significant reduction in Chla concentration in the second semester, but external load reduction has a stronger impact on model outputs than hydroclimatic variability. The modeling approach developed in the present study is proposed as a simple way to cope with data scarcity, urban pollution and hydroclimatic seasonality in tropical reservoirs.