Assessing the Influence of Precipitation on the Modulation of the Water Quality Index in a Forested Catchment: The Case of a Tropical Montane Forest in Brazil

Understanding the effects of precipitation on freshwater properties in different environmental contexts can make an important contribution to maintaining the quality of hydrological resources. The present work aimed to assess the influence of rainfall events on the modulation of stream Water Quality Index (WQI) and its parameters in a basin predominantly covered by a tropical montane forest. The focus was on the post-stormflow legacy. The main parameters that varied correlated to precipitation were turbidity, total dissolved solids, and dissolved oxygen. However, these parameters varied within a small range of values and had little effect on WQI. Total phosphorus did not correlate with the precipitation. Nitrates had a very weak correlation and inverse. There was no seasonal trend for WQI due to weak modulation of parameters influenced by precipitation. In this way, the natural forest cover of the basin protects the stream from the arrival of pollutants in large quantities by hindering the runoff formation. It is the main factor contributing to a high water quality index and provides some protection against quality modulation caused by precipitation. Even at relatively low values, thermotolerant coliforms were the parameter most affecting WQI, without correlating with precipitation; they were controlled by tourism dynamics, warning about the impacts from land use/cover changes since anthropogenic cover represented less than 1% of the basin area. However, it must be considered that the Coliforms protagonism was reinforced by the low modulation of the other parameters.