Improving methods for estimating small reservoir evaporation in the Brazilian Savanna

Abstract

The Brazilian Savanna biome is the main agricultural region of Brazil and is facing a growing water shortage and conflicts that tend to expand to new areas and intensify in those already stressed, which is hindering the economic development of the region. Small reservoirs play an important role in supporting the local economy in the savanna areas of Brazil. Hundreds of small reservoirs have been built in the last twenty years in the region. They were constructed by individual farmers or projects funded by different agencies without any coordination and knowledge that the reservoirs are part of a complex hydrologic system. The lack of reservoir basic information and appropriate methods to quantify water supply, demand and losses are hindering efficient water management. In this context, evaporation is a non beneficial loss to the water system and should be better quantified. The present study aimed to improve methods for estimating small reservoir evaporation in the Brazilian savanna region. The study area was the Buriti Vermelho watershed, where the evaporation were measured in Class A pans installed inside and outside of a small reservoir. Pan coefficients were calculated on a monthly, seasonal and annual basis. In addition, equations for estimating evaporation based on climatic variables were developed. The pan coefficients values varied throughout the year from 0.72 to 0.92, with mean absolute error (MAE) ranging from 0.32 to 0.52 mm d−1. The two best climatic equations presented MAE of 0.44 mm d−1, while equations that used solar radiation and relative humidity as input also presented good performance.