Estimating evaporation in semi-arid areas facing data scarcity: Example of the El Haouareb dam (Merguellil catchment, Central Tunisia)

Abstract

Study region: The El Haouareb dam (Merguellil catchment) in central Tunisia, which is typical of semi-arid environments. Study focus: Most estimates of evaporation from water bodies located in semi-arid environments suffer from the lack of data, or biased field measurements. It is thus important for hydrologists to assess the relative performance of the various available methods used to estimate this water loss, as well as their uncertainties. We confronted physical approaches based on contrasted theoretical formulae (Dalton, simplified BREB, Penman) and geochemical approaches based on mass conservation (stable isotopes and chloride). We compared the results with Colorado pan measurements, and tested the methods’ sensitivity to various physical parameters and data gaps. New hydrological insights for the region: In this region, where mean annual rainfall is 300 mm, estimates of evaporation of the El Haouareb Dam lake ranged from 1400 to 1900 mm a−1, depending on the method and the year. The Penman approach was found to be the most robust and gave an annual mean of 1600 mm a−1. Evaporation values were refined by combining results from the different methods. Mean interannual evaporation was estimated to be 1700 mm a−1, with an uncertainty of 15%. From this work, we propose an annual Colorado pan conversion coefficient of 0.8 which can be adjusted, 1.0 for spring and 0.76 for the rest of the year. Keywords: Semi-arid areas, Evaporation, Water budget