Water surplus
"1. Cumulative (positive) difference between precipitation and potential evaporation during a certain period. 2. Amount of water, in excess of demand, in a reservoir or supply system." (United Nations Publications, 2013)
Sources
United Nations Publications. "Glossary of Shared Water Resources (English-Arabic): Technical, Socioeconomic and Legal Terminology." (2013). DOI:https://dx.doi.org/10.18356/70b462ce-en