Evaporation in a Mediterranean environment by energy budget and Penman methods, Lake Baratz, Sardinia, Italy

F Giadrossich,M Pirastru,M Niedda,D Cohen

doi:10.5194/hess-19-2451-2015

Abstract

Abstract. In Mediterranean environments, evaporation is a key component of lake water budgets. This applies to Lake Baratz in Sardinia, Italy, a closed lake that almost dried up in 2008 after a succession of years with low seasonal rainfall. We used the energy budget method and Penman's equation to estimate evaporation over Lake Baratz. We measured, using a raft station, water temperature at the surface, at 1, 2, 4, 6 m depth and at the bottom of the lake, as well as air temperature, relative humidity, wind speed, and net radiation over a period of 3 years. We also compared Penman's equation and the energy budget method in two other climatic zones using published data. Our results indicate that mean yearly evaporation over Lake Baratz was 950 mm. On an annual scale, evaporation estimated by Penman's method omitting heat storage as is usually done was 18% higher than by the energy budget method that included heat storage, with monthly differences ranging between −38 and +60%. Including the heat storage term in Penman's equation changed the monthly values but did not change the yearly value significantly. Solar radiation and heat storage were found to be the most important energy fluxes to and from the lake and had the greatest effect on evaporation rates for the energy budget method. The bias between the two methods has a seasonal cycle due to the storage and release of energy from the lake. Energy advected to and from the lake by precipitation, surface water and ground water had minor effect on evaporation rates. Lake Baratz, like other lakes in a Mediterranean environment, is particularly sensitive to the summer hot and dry climate. In contrast, we found that rates of evaporation estimated from Penman and the energy budget methods over tropical African lakes were nearly constant over the entire year and the difference between the two methods smaller. Difference between the two methods for North American lakes is also smaller probably owing to the ice-cover season and to lower radiation and lower temperatures during summer.

Highlights

Lakes in semi-arid regions like around the Mediterranean Sea are extremely sensitive to changes in climate and land use
This applies to Lake Baratz in Sardinia, Italy, a closed lake that almost dried up in 2008 after a succession of years with low seasonal rainfall
We found that rates of evaporation estimated from Penman and the energy budget methods over tropical African lakes were nearly constant over the entire year and the difference between the two methods smaller

Summary

Introduction

Lakes in semi-arid regions like around the Mediterranean Sea are extremely sensitive to changes in climate and land use. Recent increases in drought frequency (Hoerling et al, 2012), often exacerbated by excessive water extraction either directly from lakes or from aquifers beneath, have caused lake levels in the region to decrease significantly (Myronidis et al, 2012; Gianniou and Antonopoulos, 2007). Lake Baratz in Sardinia, Italy (Fig. 1), suffered significant changes in its water level over the last 10 years and almost dried up in 2008 because of a succession of years with low seasonal rainfall (Fig. 2). This decrease in precipitation together with water losses by evaporation and infiltration were not balanced by other water influx.

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Results