RS—Surface runoff

This portion of precipitation is also called net rainfall or surface discharge, or the rainfall to the soil that is not infiltrated but reaches the lake directly through surface runoff. RS was calculated using the United States Soil Conservation Service curve number method for the study of small rural watersheds [23].

According to this method, the surface discharge is a function of precipitation P and a parameter S, which represents the quantity of water that can be stored in the watershed (or in the terrain). The parameter S is a function of the infiltration capacity (characterized by the minimum infiltration rate observed in soil without vegetation after a long wet period), the totality of the conditions (soil use, surface treatment, drainage), and the soil moisture content (or antecedent moisture condition, AMC). The parameter S is linked to another, non-dimensional parameter called runoff curve number, or simply curve number, CN. The CN value is determined using two different tables, one developed for agricultural and wooded areas [24] and the other for urban and kindred areas [25].

Considering the type of soil cover, use, and class, and the CN values for each category, we calculated the surface runoff for the watershed of Lake Candia. A reduction of 20% was applied in the category BUILT-UP AREAS and 50% in the category STREETS/ROADS to the net rainfall value obtained using the CN method [23] considering that a portion of rainfall in built-up areas and streets can be intercepted by grassland, vegetation, or drainage system through, for example, manholes [25].
