CALCULATION OF EVAPOTRANSPIRATION BASED ON SOLAR RADIATION INCOME
MODELING IN MOUNTAINOUS AREAS
I. Meszaros, P. Miklanek
Institute of Hydrology, Slovak Academy of Sciences, Racianska 75, 831 02 Bratislava, Slovakia
e-mail: mailto:meszaros@uh.savba.sk
Mountainous areas are typical for their complex relief and great elevation changes. Solar energy
income and evapotranspiration are hardly to measure in mountainous environment. Especially the
practical possibilities of direct measurement of the energy income, as the most important parameter
for calculating different elements of water balance equation, like evapotranspiration, snowmelt etc. on
the slopes is very limited.
This study comes with the proposing of methodology for both, solar energy income modelling and
evapotranspiration modelling in mountainous areas. For that reasons the SOLEI model which can
calculate potential, as well as actual energy income to the slopes with different orientation and taking
into account the shadowing of the surrounding topography was developed.
Spatial estimates of potential evapotranspiration are in the SOLEI model calculated in two steps. First,
the solar energy income to the watershed is simulated. The main inputs for this calculation are the
digital elevation model and the albedo map derived from the land use grid map in the same matrix
shape. If the actual energy income is simulated, the time series of relative sunshine duration are
necessary. In the next step, potential evapotranspiration is calculated according the Penman-Monteith
equation based on solar energy income calculation. For the evapotranspiration modelling the
supplementary meteorological data are required, as well.
The contribution presents results of calculation of potential evapotranspiration based on actual solar
energy income modelling obtained in the Jalovecky creek experimental hydrological basin in the
Western Tatra mountains. Its area covers 22.2 km 2. and the elevations range from 800 to 2178 m
a.s.l.. This region is a typical high mountain area of the central Europe (Fig. 1).
Cervenec
Fig. 1
Parichvost
Jalovecky creek basin and stations (Cervenec and Parichvost), where direct measurements of
incoming solar radiation were carried out.
Actual solar radiation income was calculated for the year 1998 and compared to the direct
measurement of global radiation income during the same period. Comparison of measured and
simulated daily sums of incoming solar radiation for selected time period showed that SOLEI precisely
simulates the solar energy income.
Example of calculated raster maps of actual energy income are shown in Fig. 2. The mean monthly
energy income during the year 1998 varied from 28 225 Wh.m-2 to 110 254 Wh.m-2.
March 1998
Fig. 2
Solar energy income [Wh.m-2]
April 1998
May 1998
Calculated raster maps of actual energy income for March, April, May 1998.
Based on actual solar energy income potential evapotranspiration according the Penman-Monteith
equation was calculated. Potential evapotranspiration was calculated for the year 1998 in daily step.
It’s values varied between 20 mm.month-1 (January 1998) and 103 mm.month-1 (June 1998). The
example of calculated raster map for the May 1998 is shown in the Fig. 3.
[mm]
140
120
100
80
60
40
20
10
5
0
Fig. 3
Raster map of calculated potential evapotranspiration for May 1998.
In this contribution, we have presented an effective and efficient algorithm for the computation of the
solar energy income and evapotranspiration in topographically varying terrain. Based on raster maps
of elevation, vegetation and possibly cloud cover, SOLEI model simulates the potential and actual
solar energy income and potential evapotranspiration in the form of raster maps, as well as time series
for selected points. Comparisons between measured and computed daily sums of incoming solar
radiation and mean hourly intensities for selected cloudless days in Jalovecky creek basin showed
very good agreement between simulations and direct measurements. Important input for
evapotranspiration calculation is albedo raster map which enables to take into account different
vegetation cover in different parts of the basin and to analyse land use impact on evapotranspiration
process.