Abstract
In recent decades, the increasing nutrient contamination in several water bodies across the globe has made it necessary to revisit, study, and understand all the mechanisms that contribute to the increasing contamination levels to protect the water bodies. Precipitation-induced surface runoff could be one of the significant contributors to controlling water quality in inland water bodies by regulating nutrient movement across the contributing watersheds where the land use is mainly driven by precipitation, such as the tropical monsoon climates of Southeast Asia. Current literature does not provide sufficient information to understand the role of precipitation in controlling the water quality under these conditions. Hence, the interactions between precipitation and nutrient transport need to be studied to mitigate the ill effects of the contamination. As a case study, this work used the Soil and Water Assessment Tool (SWAT) hydrological model to assess these interactions in Nagarjuna Sagar (NS) and the contributing watersheds from the Krishna River basin. The model was calibrated for the entire Krishna River basin using the flow and in-stream nitrate concentration values measured at the Wadenapally gauge station. The model was used to study the contaminants produced from the NS contributing watershed from 2007 to 2017, during which the basin recorded flood and drought conditions. Since Total Nitrogen (TN) influences the nitrate concentration in streams, TN production was used as a reference for contamination output from the contributing watershed. The results showed that the land use in 2013 and 2017 had similar land use conditions and statistics, but the contribution watershed saw higher production of TN during 2013 due to good rainfall, and 2017 saw reduced output as the precipitation decreased. When the precipitation values were mutually exchanged, the resulting TN output from 2013 and 2017 years was comparable to the original TN yield from 2017 and 2013, respectively. This shows that precipitation essentially controls the production of the contaminants in the contributing watershed.