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. So far, lake evaporation has
been determined by indirect methods only and not measured
directly. Consequently, the governing factors of evaporation are
unknown. For the first time, long-term eddy covariance measurements
were performed at the western Dead Sea shore for a period of
1 year by implementing a new concept for onshore lake evaporation
measurements. To account for lake evaporation during offshore wind
conditions, a robust and reliable multiple regression model was
developed using the identified governing factors wind velocity and
water vapour pressure deficit. An overall regression coefficient of
0.8 is achieved. The measurements show that the diurnal evaporation
cycle is governed by three local wind systems: a lake breeze during
daytime, strong downslope winds in the evening, and strong northerly
along-valley flows during the night. After sunset, the strong winds
cause half-hourly evaporation rates which are up to 100

Since several years, the lake level of the Dead Sea declines by
over 1

In view of these environmental changes, resulting from the lake
level decline, more accurate estimates of the Dead Sea evaporation
are required

Map of the research area and location of the measurement site

The Dead Sea is a hypersaline terminal lake, located at the lowest
point of the Jordan rift valley. It is surrounded by the Judean
Mountains to the west and the Moab mountains to the east
(Fig.

To measure the energy balance components of the water surface, an
energy balance station (EBS) was installed directly at the
shoreline (Fig.

At the station the following meteorological variables were
measured and averaged over 10

As the station was located at the shoreline, the radiation
measurements of the lower half space represented the land surface
conditions. For the water surface they have to be calculated. The
applied method is explained in
Sect.

Measurement data from March 2014 until March 2015 were
analysed. To achieve the research aims, the following calculations
and methods were applied. The shortwave and longwave radiation
components of the lower half space were calculated. This is
presented in Sect.

The measurements of the radiation components of the lower half
space were not conducted directly over the water surface, but
over the land surface. Therefore, these two components had to be
calculated for the water surface. The reflected shortwave
radiation was calculated using literature values of the Dead Sea
albedo.

To calculate the sensible and latent heat flux from the wind,
temperature, and humidity data measured by the IRGASON, the eddy
covariance technique was used. This method uses the fluctuations
of the vertical wind velocity and temperature around a temporal
mean, here 30

Post-processing of eddy covariance data is essential as field
measurements generally do not fulfil all the theoretical concepts
and assumptions of the eddy covariance theory. In particular,
measurement limitations of the sensors, non-stationary conditions
over the averaging period, and horizontal heterogeneity
have to be considered

Spectral corrections were performed to account for the loss of
energy for high frequencies, due to path-length averaging and
limited sensor frequency response, following the approach after

The overall performance of the system was very good, and only
2.1

Selection of commonly used equations to calculate evaporation (Ev) in
mm

Through the installation of the EBS at the shoreline, flux data from
the water surface are only available for onshore wind conditions,
and all data for offshore wind conditions, i.e. wind directions
between 230 and 330

Overview of the sensitivity studies performed for the evaporation equations. Sensitivity studies applied to a method are marked with an X.

For the calculation of evaporation, several equations, based on
different physical approaches, exist. Each approach connects
evaporation to different meteorological parameters and is designed
for different time intervals, ranging from sub-daily calculations
to a time interval of at least 7 days. Four commonly used
indirect methods to estimate evaporation (Table

The first method is the aerodynamic approach after

In total, six sensitivity studies were performed. An overview of
the sensitivity studies and to which of the methods they are applied
to is given in Table

Daily precipitation (prec), 24

Wind conditions between

In the Dead Sea valley the measured average annual air temperature
was 26.5

Correlation coefficients for latent heat flux (LE) with wind speed
(

The footprint model showed that the fetch of the fluxes is over
land for wind directions between 230 and 330

Results of the stepwise linear regression model

Coefficients of the model equations to calculate latent heat flux (LE). The
equations have the general form:

In summary, the regression model

The calculation of the latent heat flux for offshore wind
conditions is especially important for the analysis of the diurnal
cycle of the latent heat flux, and also for its intra-annual
variation. The comparison of the mean diurnal cycles of the
measured fluxes with the cycles including the calculated values
for offshore wind conditions (corrected fluxes) shows that during
the day the differences are small
(Fig.

Median diurnal cycles of the measured latent heat flux (black lines)
and the latent heat flux corrected with the multiple regression model
for wind directions between 230 and 330

The latent heat flux is the dominating turbulent flux at the water
surface (Fig.

Box plot of daily evaporation rates. Red lines indicate medians, the edges of the boxes are the 25th and 75th percentiles, the whiskers extend to the most extreme data points not considered outliers, and outliers are plotted individually by red crosses.

For the calculation of daily and yearly evaporation, still-existing data gaps were closed, using the median evaporation rate of the corresponding time step of the respective month. The uncertainty due to this gap-filling method was estimated using the median absolute deviation, which is the median of the absolute deviations from the data's median.

In spring, evaporation rates steadily increase until a maximum
median evaporation of 4.3

With the comprehensive data set of the measurements, it is
possible, for the first time, to evaluate four of the commonly used
evaporation equations for their applicability for Dead Sea
evaporation on different timescales (30

Slope and offset of the regression lines between the evaporation estimates
calculated with the different equations and the evaporation measurements and
the corresponding correlation coefficient (

Correlation between estimated and measured daily evaporation rates for

Differences between the estimated daily evaporation rates calculated from the
28

The first equation is the aerodynamic approach after

The BREB method is first used in the simplified version shown in
Table

The Priestley–Taylor equation, as described in
(Table

The last equation tested is the Penman equation. In its original
form (Table

The calculation of the heat storage term as a linear function of
the net radiation results in

Another commonly used variation of the Penman equation is the
removal of the surface water temperature from the calculation of
the net radiation. This is tested in V4. However, in V4 the heat
storage term is still missing an thus does not result in reliable
evaporation rates (Figs.

The eddy covariance method is used for the first high-resolution,
direct evaporation measurements of the Dead Sea. The first aim of
this study was to present an applicable method to measure
evaporation with a shoreline station. The measurement strategy is
based on the installation of the station on a headland, surrounded
by water from 320

The second aim was to evaluate the diurnal and intra-annual
variability of Dead Sea evaporation. The annual Dead Sea
evaporation was found to be

For the prospective affordable long-term assessment of
evaporation, different equations to calculate evaporation were
tested for their applicability for the Dead Sea. The best
suitable, and also the only method applicable on sub-daily
timescales, is the aerodynamic approach. It is shown that the
consideration of the atmospheric stability in the calculations has
a negligible effect on the results. These results coincide with
results for Lake Kinneret

Metadata for all measurements are publicly available via

The surface water temperature

Dependency of the specific latent heat of vaporisation (

The latent heat of vaporisation and the activity of water

The molar latent heat of vaporisation,

The vapour pressure deficit for the regression approach is
calculated as follows. The vapour pressure deficit is defined as
the difference between the saturation vapour pressure above the
saline water,

The authors declare that they have no conflict of interest.

This article is part of the special issue “Environmental changes and hazards in the Dead Sea region (NHESS/ACP/HESS/SE inter-journal SI)”. It is not associated with a conference.

The current study was carried out in the framework of the Dead Sea Research
Venue (DESERVE) (