Dead Sea evaporation by eddy covariance measurements versus aerodynamic, energy budget, Priestley–Taylor, and Penman estimates
Abstract. The Dead Sea water budget is no longer in equilibrium. The lake level decline exceeds 1 m a−1 and causes severe environmental problems, such as a shifting of the fresh/saline groundwater interface and climatic changes. As the Dead Sea is a terminal lake, located in an arid environment, evaporation is the key component of the Dead Sea water budget and accounts for the main loss of water. However, the actual amount of evaporation as well as the governing factors are unknown. Therefore, for the first time, long-term eddy covariance measurements were performed for a period of one year, starting in March 2014. The total annual amount measured at this location was 994 ± 81 mm a−1. The median daily evaporation rate reaches 4.3 mm d−1 in July and only 1.1 mm d−1 in December. The wind velocity and vapour pressure deficit were identified as the main governing factors of evaporation throughout the year. Consequently, the local wind systems define the diurnal evaporation cycle. In the evening, strong downslope winds govern the wind field and cause evaporation amounts which are up to 100 % higher than during daytime, and also during the night evaporation rates are accelerated compared to daytime evaporation, due to strong northerly along-valley flows. Furthermore, a robust and reliable regression model is presented to calculate sub-daily and multiday evaporation values with a linear function of wind velocity and vapour pressure deficit. An overall correlation coefficient of 0.8 is achieved and the cross validation results in a prediction error of 4.8 %. Finally, indirect evaporation approaches were tested for their applicability for the Dead Sea and compared to the measurements. The aerodynamic approach is applicable for sub-daily and multi-day calculations and attains correlation coefficients between 0.85 and 0.99. For the application of the Bowen-Ratio-Energy-Balance (BREB) method and the Priestley–Taylor method, measurements of the heat storage term are inevitable to calculate evaporation on time scales up to one month. Without the heat storage term, the equations yield strong seasonal biases and over- or underestimate daily evaporation rates by up 100 %. The usage of an empirically gained linear function or a hysteresis model depending on the net radiation to estimate the heat storage term was not accurate enough to provide reliable evaporation amounts. The Penman equation was adapted to calculate realistic evaporation amounts, by using an empirically gained linear function for the heat storage term. The correlation coefficients are above 0.9, the daily mean difference is only 0.5 mm d−1 and the estimated annual amount is within the range of the measurement uncertainties. In summary, this study provides the first directly measured amounts of Dead Sea evaporation and applicable methods to calculate evaporation.