First evaporation measurements at the Dead Sea

Fig.1: Onshore Eddy covariance station to measure evaporation from the Dead Sea water surface. This station is located at 429 m below mean sea level, and, thus, the lowest meteorological station on earth. B. Deny, KIT, IMK-TRO.	Fig.2: Median diurnal cycles of the measured evaporation (black lines), including evaporation from the land surface for offshore wind conditions. Evaporation from the water surface (red line), where evaporation for offshore wind conditions (wind directions between 230° and 330°) was calculated with the multiple regression model.

Each year the water level of the Dead Sea drops by more than one meter and the water surface decreases significantly. Hereby, evaporation is a decisive factor. However, reliable, direct measurements of evaporation have not been available so far.

Researchers of the Institute of Meteorology and Climate Research led by Dr. Ulrich Corsmeier have developed a new concept for carrying out measurements of lake evaporation at the shore of the Dead Sea. During onshore wind conditions, evaporation could be measured directly, but during offshore winds no direct evaporation measurements from the water surface were available. A multiple regression model, developed by Dr. Jutta Vüllers, together with the relevant influencing variables of evaporation, namely wind speed and water vapor saturation deficit, were used to calculate evaporation for offshore wind conditions. Thus, the limitations of a shoreline station for lake water evaporation were overcome.

The measurements showed that evaporation mainly depends on three prevailing local wind systems, namely the lake breeze during the day, strong downslope winds in the evening, and strong along-valley northerly winds at night. In contrast to the diurnal cycle of evaporation in our latitudes, the maximum of evaporation occurs at the Dead Sea during the evening and night. This is due to the high wind speeds of the evening downslope winds and the nocturnal valley winds. The total evaporation for the period from March 2014 until March 2015 was 994±88 mm a-1. Average daily evaporation rates were 4.3 mm d-1 in July and only 1.1 mm d-1 in December.

In addition, evaporation formulas were evaluated which are based on meteorological input parameters, and which can be used to estimate evaporation. The best matches were found with the so called aerodynamic approach, which only takes wind speed and saturation deficit into account.

These results are important for a thorough analysis of the water balance of the Dead Sea. This is necessary to evaluate the consequences of a negative water balance for the climate of the region and to estimate the ecological and economical impact of the degradation of the coastal area and the production of minerals.

Our results are published here: https://www.hydrol-earth-syst-sci.net/22/1135/2018/

[Working group: Convective Systems]