Seminar Ib

Modeling Evapotranspiration

Author: Ale² Satler

Adviser: doc. dr. Gregor Skok

Co-Adviser: dr. Gregor Gregori£

Ljubljana, junij 2016

Abstract

Evapotranspiration (ET) consists of two physical processes. Evaporation, which is vaporizationof liquid water from the surface to the atmosphere and transpiration process, which is evaporationof water through the small pores called stomata. Because it is di�cult to measure the amountof water that is evapotranspirated, we have several methods to estimate ET. FAO recommendedthe Penman-Monteith (PM) method as a standard method for calculation of ET. PM method usesthe reference surface for the estimation of ET, if we multiply it with crop coe�cient (Kc), we canestimate ET for a speci�c crop. Especially in summer time, crops are usually not grown under theoptimal soil water conditions. Water stress coe�cient (Ks) is used to describe the actual ET fromthe potential ET for a speci�c crop. Similar equations for the calculation of ET are used by theECMWF (European Centre for Medium-Range Weather Forecasts) meteorological model. Hence,The ECMWF ET should be compared with the actual ET, the comparison between the model ETand the reference ET is inaccurate.

Contents

1 Introduction 2

2 Evapotranspiration process 3

2.1 The Penman-Monteith method for calculating the reference Evapotranspiration (ETo) 52.1.1 Wind speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.2 Air temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.3 Air humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.4 Solar radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Crop Evapotranspiration under standard conditions (ETc) - Potential Evapotranspiration 82.3 Crop Evapotranspiration under soil water stress conditions (ETc adj) - Actual Evapo-

transpiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 The ECMWF water �uxes 10

3.1 Surface parameterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.2 ECMWF evapotranspiration equation . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Conclusion 12

1 Introduction

For the description of water loss from land surface to atmosphere, we often use the term evapotranspi-ration. Evapotranspiration consists of two physical processes, namely evaporation and transpiration.[1]

Evaporation is vaporization of liquid water from di�erent types of surfaces to the atmosphere.As shown in Figure 1, water can evaporate from lakes, rivers, sea surface, clouds and also from soil,which is radiated by the sun. Energy provided by the sun is required to change the liquid watermolecules to vapour. High air temperatures also help water to change state and as the mechanism ofevaporation continues, the surrounding air becomes more and more moist. If the atmosphere is calm,evaporation slows down or even stops. Hence, we need wind to blow the air �lled with vapour awayand bring new, drier air. [1], [2]

Figure 1: The hydrological cycle on Earth. Water can be cycled inside of the ocean or land partof the system or between the ocean and land. [2]

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Transpiration is a process by which liquid water in plants is taken up by the roots, transportedto the leaves and then released to the atmosphere as water vapour. The water inside the crop isevaporated through the small pores called stomata. Only a few percent of the transported wateris used for growth and metabolism, over 97% is lost through transpiration. Opening of the poresallows the di�usion of CO2for photosynthesis, transpiration also cools plants. As shown in Figure 2,transpiration contributes a minor share of evapotranspiration at the start of the vegetative season,but then grows rapidly and becomes much higher than evaporation from soil. Evaporation andtranspiration regularly occur at the same time, so there is very hard to distinct between these twoprocesses. [1], [3], [4]

Figure 2: The partitioning of evapotranspiration from sowing until harvest in correspondence toleaf area index (LAI). At sowing, when LAI is close to zero, transpiration is negligibly small. At fullcrop cover (maximum LAI) transpiration nearly 95% of evapotranspiration comes from transpiration.[1]

The di�erence between the amount of rain that falls over a certain area and evapotranspirationover the same area is very important in agriculture. Measuring the amount of precipitation is not verydi�cult, but it is hard to measure the amount of water that is evaporated and transpirated. Variousmethods for calculation of evapotranspiration have been developed over the last decades. In May 1990Food and Agriculture Organization of the United Nations (FAO) recommended the Penman-Monteithmethod as a standard method for calculation of evapotranspiration. The Penman-Monteith methodrequires four meteorological variables, which are measured at weather stations: air temperature, windspeed, air humidity and solar radiation. [1]

Evapotraspiration is also calculated by the numerical weather prediction (NWP) models. Complexsurface parameterization is used to describe surface �uxes of energy and water. In seminar, focus willbe on ECMWF (European Centre for Medium-Range Weather Forecasts) meteorological model andits calculation of evapotranspiration. [5]

2 Evapotranspiration process

The main mechanism by which moisture is transferred from the atmosphere to the surface is precip-itation, transfer in the opposite direction is evapotranspiration. Approximately 80% of moisture isevaporated from the sea, land surface evaporation represents only 20%, including evaporation fromrivers, lakes, moist soil and from water intercepted by plants. Units used for the evapotranspirationrate are mm per time unit (mm refer to the amount of evapotranspirated water per area in time unit,so �real� units are kg m-2time unit -1). The time unit is usually a day, a month or an entire vegetativeseason.[2]

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Figure 3: Weighing lysimeter from Switzerland. The vegetation outside of the lysimeter and thevegetation from the inside should perfectly match in order to provide accurate measurements. [6]

We have speci�c devices for measuring evapotranspiration, lysimeters, but they are expensiveand can only be operated by well-trained professionals. In weighing lysimeters, plant is grown in alarge isolated soil tank and water loss can be easily measured by the change of mass. One weighinglysimeter in Slovenia is in Kle£e near Ljubljana. As we can see in Figure 3 these instruments are verylarge and not easy to install. [1]

The term reference evapotranspiration (ETo) is used for the evapotranspiration from the referencesurface. The reference surface is well watered short grass with the height of 0.12 m, the albedo of0.23 and the surface resistance of 70 sm-1 and is calculated from the Penman-Monteith equation. Thesurface resistance (rs) is the resistnace of water vapour �ow through stomata openings, total leaf areaand soil surface.

Usually the surface is not covered with short grass. Hence, evapotranspiration for the speci�c cropshould be calculated. So-called crop evapotranspiration under standard conditions (ETc) includes thecrop coe�cient Kc (dimensionless), which is unique for each crop and is changing during the growth.As shown in Figure 4, ETo is multiplied by the Kc coe�cient for the estimation of the potentialevapotranspiration. Crop evapotranspiration under standard conditions (ETc) is, in a way, potentialevapotranspiration, because the crop is grown under optimum agronomic and soil water conditions.ETc is actually the maximum amount of water that could be evaporated and transpirated into theatmosphere.

During dry and warm periods of the growing cycle the evapotranspiration is more extensive thanprecipitation. The crop becomes water stressed and the evapotranspiration rate slows down. Thisis described by the actual evapotranspiration or the crop evapotranspiration under soil water stressconditions (ETc adj), which includes the water stress coe�cient Ks (dimensionless). [1]

Figure 4: At the top is schematic demonstration of estimating the ETo with meteorologicalobservations and the reference surface. In the middle is the ETc, ETo is multiplied by the Kc factor.At the bottom we can see the crops, which are water stressed. Hence, ETo is multiplied by the Kc

and the Ks factors. [1]

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2.1 The Penman-Monteith method for calculating the reference Evapotranspi-ration (ETo)

The reference surface presents actively growing green grass of uniform height, entirely shading theground and with a decent water supply. Many studies have been done so far to evaluate the per-formance of di�erent methods for calculating ETo and the Penman-Monteith method has shown thebest result among them. It uses the reference surface to avoid complications of local calibration whichrequires extensive and expensive studies. It is a method that can accurately estimate ETo for variousclimates around the world and can be used even when meteorological data is missing. [1]

The Penman-Monteith method combines energy balance with the mass transfer and it is a so-called combination method for calculating reference evapotranspiration. It contains many di�erentparameters:

λETo =∆ (Rn −G) + ρacp

es−eara

∆ + γ(1 + rs

ra

) (1)

ETo= reference evapotranspiration [kg m-2 day-1 or mm day-1]

λ = latent heat of vaporization

ρa = mean air density at constant pressure

cp = speci�c heat of the air at constant pressure, 1013 [J kg-1K-1]

Rn= calculated net radiation at the crop surface [MJ m-2 day-1]

G = soil heat �ux density at the surface [MJ m-2day-1]

T = calculated mean daily air temperature

es= calculated saturation vapour pressure at 2 m height

ea= mean actual vapour pressure at 2 m height

∆ = slope of the saturation vapour pressure-temperature curve [kPa K-1]

γ = psychrometric constant [kPa K-1]

rs = surface resistance [s m-1]

ra = aerodynamic resistance [s m-1]

The aerodynamic resistance (ra) describes the resistance of the crop from the vegetation upwardand includes friction from air �owing over the surface. ra is a function of the measured wind speedat the height z, z is usually 10 m, and a function of the crop height. With the Penman-Monteithequation we can compare evapotranspiration among regions all around the world and in di�erentperiods of year. [1]

First we have to de�ne atmospheric pressure P at the station elevation:

P = p0

(1 − L ∗ z

T0

) gMRL

(2)

where P is the atmospheric pressure, z is elevation above the sea level, p0 is the sea level standardatmospheric pressure, 101.325 kPa, T0 is the sea level standard temperature, 288.15 K, g is thegravitational acceleration, 9.81 m s-2, M is the molar mass of dry air, 28.97 kg kmol-1, L is the

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atmospheric temperature lapse rate, 6.5 K/km and R is the universal gas constant, 8314 J kmol-1

K-1. [1]

Next parameter is psychrometric constant γ, which is not a real constant. It is connected toatmospheric pressure as γ =

cpPελ . cp and λ are given by (1), P by (2), ε is the ratio of molecular

weight of water vapour to dry air, 0.622. [1]Other variables in (1) are calculated from observations at meteorological stations and described

in the next four subsubsections.

2.1.1 Wind speed

For calculation of evapotranspiration in (1), a wind speed at the height of 2 m (u2) is needed. Weassume a logarithmic wind speed pro�le:

u2 = uzln(z2zo

)ln(zz0

) (3)

where u2 is wind speed at 2 m above the surface, z2 is the height above the ground for u2,2 m, uz is the measured wind speed at the height of z, z is the height of measurement and z0 isthe roughness length governing momentum transfer. For the reference crop height (h = 0.12 m)z0 = 0.123 ∗ h = 0.02476 m . Daily average wind speed is used for the calculation of referenceevapotranspiration. [1]

2.1.2 Air temperature

Air temperature is measured at 2 m above the surface. We calculate the mean daily temperature Tused in the Penman-Monteith method simply as the arithmetic mean of the daily maximum temper-ature Tmax and the the daily minimum temperature Tmin. [1]

Figure 5: Saturation vapour pressure as a function of temperature. The slope of the curve is smallat temperatures between 0 °C an 10 °C, but at higher temperatures it is much larger. At temperaturesabove 30 °C a small change in temperature, results a big change in saturation vapour pressure. [1]

The daily maximum and minimum temperature is also used for calculation of saturation vapourpressure (es) in (1). The amount of moisture that can be contained in the air depends on temperature.

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When moisture reaches the maximum point, the air gets saturated. Saturation vapour pressureat certain temperature is shown in Figure 5. From the mean daily temperature another part ofthe Penman-Monteith equation can be calculated. The slope of the saturation vapour pressure-temperature curve [kPa K-1] is given by the Clausius�Clapeyron relation:

∆ =desdT

=λ(T )esRvT 2

(4)

where λ is the latent heat of vaporization, which is dependent on temperature, T is the meandaily air temperature, es is the saturation vapour pressure and Rv is the speci�c gas constant forwater vapour, 461.5 J/kgK. [7]

2.1.3 Air humidity

Relative air humidity (RH) is measured with hygrometers where human hair is used to detect changesin air humidity. The length change is recorded on hygrograph and the daily maximum relativehumidity (RHmax) and the daily minimum relative humidity (RHmin) can be read from it. The actualvapour pressure ea can be calculated from relative humidity together with minimum and maximumtemperature :

ea =e0(Tmin)RHmax

100% + e0(Tmax)RHmin100%

2(5)

where ea is the actual vapour pressure, e0(Tmin) is the saturation vapour pressure at the dailyminimum temperature, e0(Tmax) is the saturation vapour pressure at the daily maximum temperature,RHmax is the daily maximum relative humidity [%] and RHmin is the daily minimum relative humidity[%]. [1]

Figure 6: Pyranometer is used for measuring solar short-wave radiation (Rs) within a wavelengthrange from 300 nm to 3000 nm. It contains a sensor that measures the intensity of solar radiation ona horizontal plane and a glass dome that limits the spectral response from 300 nm to 3000 nm. [8]

2.1.4 Solar radiation

The last meteorological parameter required for the Penman-Monteith equation is incoming solarradiation (Rs). Solar radiation is measured with pyranometer as shown in Figure 6. The net radiationin (1) is given by the di�erence between Rns and Rnl :

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Rn = Rns −Rnl (6)

where Rns = (1 − α)Rs is the net short-wave radiation [MJ m-2 day-1], α is the albedo of thereference surface and Rs is the measured solar radiation [MJ m-2 day-1]. Rnl is the net long-waveradiation [MJ m-2 day-1]. It depends on the Tmax and the Tmin, actual vapour pressure ea from (5)and the ratio Rs/Rso which represents relative cloudiness. Rs is the incoming solar radiation and Rso

is the clear-sky solar radiation. On a cloudy day with dense clouds the ratio is close to 0.3 and whenthe sky is clear the ratio is close to 1.

Soil heat �ux density at the surface, G, from (1) is relatively small and can be ignored. Gday =0. [1]

2.2 Crop Evapotranspiration under standard conditions (ETc) - Potential Evap-otranspiration

Evapotranspiration can also be estimated for the speci�c crop and not only from the reference surface.ETc is calculated under the standard conditions, which refer to crops grown in large �elds underoptimal soil water and agronomic conditions:

ETc = KcETo (7)

where ETc is crop evapotranspiration under standard conditions, Kc is the crop coe�cient andETois the reference evapotranspiration from (1).

Crop Kc ini Kc mid Kc end

Broccoli 0.70 1.05 0.95

Garlic 0.70 1.00 0.70

Tomato 0.60 1.20 0.80

Strawberries 0.40 0.85 0.75

Spring Wheat 0.30 1.15 0.35

Apples 0.45 0.90 0.70

Olives 0.65 0.75 0.70

Rice 1.05 1.20 0.75

Banana 1.00 1.20 1.10

Kiwi 0.40 1.05 1.05

Table 1: Kc typical values during the growing period. Kc ini represents the initial stage, Kc mid

the mid-season stage and Kc end the end of the late season stage. [1]

Kc represents four characteristics that distinguish reference grass from crop:a) crop height (it a�ects the aerodynamic resistance of the crop)b) re�ectance of the soil surface (it in�uences the net radiation from (1))c) resistance of the crop to vapour transfer (it depends mostly on leaf age and condition)d) evaporation from soil (Kc integrates e�ects of evaporation and evapotranspiration)

Kc changes during the development of the crop. As shown in Table 1, we have three di�erentvalues for Kc during the growth. Values are higher at the middle of the vegetative season and lowerat the beginning and at the end. The Kc values larger than 1 (Kc > 1) mean that the ETc rate isfaster than the ETo. The Kc values smaller than 1 (Kc < 1) mean that the ETc rate is slower thanETo. [1]

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2.3 Crop Evapotranspiration under soil water stress conditions (ETc adj) - ActualEvapotranspiration

In wet soil, water is more or less free to move and is easily transferred up through the roots. Whenthe soil is dry, the water is strongly bound by capillary forces and less smoothly transported upwards.And when the level of the soil water drops below the threshold value, the crop is water stressed.The crop coe�cient, Kc and the reference evapotranspiration, ETo, are multiplied by the water stresscoe�cient Ks. The evapotranspiration under the soil water stress conditions is de�ned by:

ETc adj = KsKcETo (8)

where ETc adj is the evapotranspiration under the soil water stress conditions, Ks the water stresscoe�cient, Kc the crop coe�cient and ETo the evapotranspiration from (1).

Day ETo [mm day -1] Kc ETc [mm day -1] Ks ETc adj [mm day -1]

1 5.0 1.20 6.0 1.00 6.0

2 5.0 1.20 6.0 1.00 6.0

3 5.0 1.20 6.0 0.97 5.8

4 5.0 1.20 6.0 0.91 5.4

5 5.0 1.20 6.0 0.85 5.1

6 5.0 1.20 6.0 0.80 4.8

7 5.0 1.20 6.0 0.75 4.5

8 5.0 1.20 6.0 0.70 4.2

9 5.0 1.20 6.0 0.66 3.9

10 5.0 1.20 6.0 0.62 3.7

Table 2: Estimation of the evapotranspiration of the full grown tomato crop for the next 10 days.We can see that the estimate of Ks factor needs a daily water balance calculation. [1]

The ETc adj represents an actual evapotranspiration and is equal to the ETc, when water andagronomic conditions are optimal. The ETc adj is lower than ETc when the crop is water stressed.A practical example of water stress is shown in Table 2. We have a full grown tomato crop andfor the coming 10 days rain is not forecasted. The crop coe�cient for tomato is 1.2 (Table 1) andexpected ETo is 5 mm per day, which means that weather conditions remain approximately the samethroughout the period. Kc is above one, which means that the ETc is 6 mm per day. The �rst twodays of the period the tomato crop is not water stressed (Ks = 1), but in the next eight days Ks

gradually becomes smaller and so does the ETc adj. [1]The water stress coe�cient Ks depends on the soil moisture at �eld capacity and at permanent

wilting point. Field capacity is the amount of water that a well-drained soil can hold against gravityand a wilting point is the water content at which crop permanently wilts. After heavy rain �eldcapacity is reached. A few hours later the soil is well-drained and roots start to transport the watertowards the leaves. After a few days without precipitation the threshold value is reached as shownin Figure 7. Until now Ks has been equal to one, but then it drops below one and the crop is waterstressed. In the next few days Ks is dropping towards zero and when water content reaches the wiltingpoint, evapotranspiration stops. [1]

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Figure 7: A schematic demonstration of the water balance. In agriculture the water balanceis simply the di�erence between precipitation and evapotranspiration. When the water content isbetween the threshold value and the wilting point, equation (8) is multiplied by the water stresscoe�cient Ks and the evapotranspiration rate is slower than the potential one. [1]

3 The ECMWF water �uxes

The ECMWF (European Centre for Medium-Range Weather Forecasts) meteorological model uses acomplex parameterization to describe water �uxes from the surface into the atmosphere. The mainsurface scheme parameters are given by the TESSEL (Tiled ECMWF Scheme for Surface Exchangesover Land) scheme. [5]

3.1 Surface parameterization

At the contact between the surface and the atmosphere each grid-box is divided into fractions ortiles. Over the sea and freshwater two fractions are used, open and frozen water. Over the landTESSEL operates with up to six tiles: bare ground, low and high vegetation, shaded and exposedsnow and intercepted water. Surface �uxes for each tile are computed separately. In each grid-boxtwo vegetation types are present: low and high vegetation. Four layers with di�erent depths are usedto describe the soil.

The coverage Ci for the tile i depends on the presence of snow and intercepted water, and on thetype and relative area of high and low vegetation. As shown in Table 3 each vegetation type has itsown characteristics. [5]

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Index Vegetation type H/L rs, min [sm-1] cveg ar br

1 Crops, mixed farming L 100 0.90 5.6 2.6

2 Short grass L 100 0.85 10.7 2.6

3 Tall grass L 100 0.70 8.2 1.6

4 Evergreen trees H 250 0.90 6.7 2.2

5 Deciduous trees H 175 0.90 6.0 2.0

6 Desert / 250 0 4.4 1.0

7 Tundra L 80 0.50 9.0 9.0

8 Irrigated crops L 180 0.90 5.6 2.6

9 Shrubs L 225 0.50 6.3 1.6

10 Interrupted forest H 175 0.90 4.5 1.6

Table 3: Vegetation types used in TESSEL scheme. H/L refers to high or low vegetation. rs, min

is a minimum canopy or surface (for desert) resistance, cveg is a vegetation coverage. ar and brare attenuation coe�cients, which de�ne an exponential pro�le of the root distribution over the soillayers. [5]

3.2 ECMWF evapotranspiration equation

For the calculation of the water vapour �uxes resistance parameterization is used. As shown in Table3, the resistance is di�erent for each tile. Evapotranspiration for tile i is calculated as a water vapour�ux:

Ei =ρa

1ULCH,i

+ rc(qL − qsat(Tsk,i)) (9)

where ρa is the air density, UL is the wind speed of the lowest atmospheric model level (approx-imately 10 m above the ground), qL is speci�c humidity (mass ratio of water vapour content of themixture to the total air content [9]), qsat(Tsk,i) is saturated speci�c humidity at the skin temperaturefor each layer, CH,i (dimensionless) is the turbulent exchange coe�cient for high and low vegetationtiles due to di�erent atmospheric stabilities. [5]

Canopy resistance rc[s m-1] is given by:

rc =rs,min

LAIf1(Rs)f2(θ)f3(Da) (10)

where LAI (dimensionless) is leaf area index. LAI is produced daily for the land surface at 1km spatial resolution from Moderate Resolution Imaging Spectroradiometer (MODIS). MODIS is aninstrument on board of Terra satellite, which is approximately the size of a small school bus. [10]rs, min is a minimum canopy resistance [sm-1]. f1 is a function of short-wave radiation Rs. f2 is afunction of the soil moisture, which depends on attenuation coe�cients ar and br in Table 3. f3represents an atmospheric humidity de�cit, similar to the es - ea in the Penman-Monteith equation(1).

And �nally the grid box total water vapour �ux is expressed as:

E =8∑i=1

CiEi (11)

where Ci (dimensionless) is the coverage for the tile i and Ei is given by (9) and (10). [5]

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4 Conclusion

We conclude that due to methodological discrepancies between the Penman-Monteith method (1)and the ECMWF model calculation (9) a comparison is unviable. The Penman-Monteith Methoduses only four meteorological variables and the reference surface, while the ECMWF model forecast,besides the meteorological variables, also considers the appropriate crop type, its height and variabilityduring the growing season. Function f2 in (10) describes water stress and is one if the water contentis at the �eld capacity and zero if the water content is at the wilting point. Between the wiltingpoint and the �eld capacity function f2 is varying linearly. This is very similar to the coe�cient Ks

described in subsection 2.3. From the similarity of the function f2 in (10) and Ks in (8) we concludethat the ECMWF model forecasts an approximate actual evapotranspiration, which is comparable tothe crop evapotranspiration under soil water stress conditions (ETc adj) or actual evapotranspirationfrom subsection 2.3. Hence, from the ETo ETc should be calculated and then multiplied by the Ks

coe�cient to get the actual evapotranspiration (ETc adj). A comparison between the ETc adj and themodel calculation would be more accurate.

Entering the computed ECMWF model values for air humidity and temperature, wind speedand solar radiation into the Penman-Monteith equation (1) could be another way to compare theETo and the ECMWF calculation of evapotranspiration. In this case we get the model referenceevapotranspiration and not the actual one.

References

[1] R. G. Allen et al. 1998. Crop Evapotranspiration. Guidelines for computing crop water require-ments. FAO Irrigation and Drainage Paper 56. (http://www.kimberly.uidaho.edu/water/fao56/fao56.pdf)

[2] G. O`Hare, J. Sweeney. 1990. The Atmospheric Systems. An Introduction to Meteorology andClimatology. Oliver & Boyd. pp 74-80

[3] A. D. Ward, S. W. Trimble. 2004. Environmental Hydrology. Second edition. Florida. CLCPress LLC. pp 1-6

[4] L. Taiz. et al. 2015. Plant physiology and development. 6th edition. Sunderland, Mas-sachusetts. Sinauer Associates. pp 100-102

[5] European Centre for Medium-Range Weather Forecasts. 2015. IFS Documentation-Cy41r1.Part IV: Physical processes. pp 1-55, 113-155

[6] https://www.ethz.ch/content/dam/ethz/special-interest/usys/iac/iac-dam/images/group/landclim/rietholzbach/iac_landclim_Lysimeter_big.jpg (25.5.2016)

[7] P. Waller, M. Yitayew. 2016. Irrigation and Drainage Engineering. Springer InternationalPublishing. pp 67-81

[8] http://www.middletonsolar.com/products/product4.htm (30.5.2016)

[9] Z. Petkov²ek et. al. 1990. Meteorolo²ki terminolo²ki slovar. Ljubljana. SAZU and Dru²tvometeorologov Slovenije

[10] http://terra.nasa.gov/about (31.5.2016)

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