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The Evaporation Concept
In the first place, the evaporation concept will be explained. We will go through different types of evaporation, like the actual, potential and reference ones. We will then focus on the estimation of actual evaporation (ET) and potential evaporation (ETP) by discussing various methodologies to estimate them. Actual evaporation will be classified into fundamental equations, observation and estimation methods, and remote sensing methods. ET’s estimation through LSMs will also be discussed, introducing the Budyko scheme. This scheme will give way to the explanation of methods to estimate ETP. These will be grouped into physically and empirical-based methods.
It has to be noted that whenever we refer to potential evaporation, we will be dealing with estimates of it. The reason is that ETP is a conceptual flux. In addition, its estimates may differ according to the assumptions and methods used to approach it.
Introduction
If a wet surface and the air surrounding it are considered, two processes can be identified describing the exchange of water molecules between them. When molecules move from the air to the surface, condensation occurs. The opposite process, molecules moving from the surface towards the air, is called vaporisation (Shuttleworth, 1992), which is related to evaporation. The difference between condensation and vaporisation is known as the evaporation rate.
In the previous sections, it has been noted that water exchanges regarding processes from the hydrological cycle imply a change of phase. The concept of evaporation encompasses processes where liquid water is transferred to the atmosphere as water vapour through turbulence. In these, energy is absorbed and the surface is cooled. Shukla and Mintz (1982) estimated the impact of this cooling effect on the surface temperature and found that from 20º South northwards, the temperature would increase about 15ºC to 20ºC if no evaporation occurred. When the process of evaporation takes place in the leaves of a plant, i.e., through leaf stomata, it is called transpiration (Dingman, 1992). In fact, a distinction can be made between the estimation of this parameter and that of others related to meteorological processes, like precipitation for example. The reason is that plant physiology is involved in evapotranspiration, meaning that it is not only related to meteorology, but also to biology.
The term evapotranspiration appears when both evaporation and transpiration take place. In fact, evaporation and evapotranspiration are sometimes used indistinctly in the literature. We will use the latter one in those cases where the methodology adopts it, but whenever there is no clear allusion to it, we will talk about evaporation. It should be remarked that we will not deal with sublimation of snow or ice.
Various types of evaporation processes have been identified. The most relevant ones are defined below:
Actual evaporation (ET)
It is the quantity of water that is actually transferred as water vapour to the atmosphere from an evaporating surface (Wiesner, 1970). This surface can vary: ocean, rivers, lakes, soil, vegetation, etc. Actual evaporation can be decomposed into the evaporation of bare soil, the vegetation transpiration and the evaporation for rain interception.
There are five basic climatological parameters in its computation. The first two are the available radiative energy and the air temperature, which provide the energy needed to vaporise the molecules. The third one is the air humidity, which is key in the vapour pressure gradient between the surface and the atmosphere. This gradient is the driving force that removes water vapour from the surface. It has to be noted that the vapour pressure deficit is used instead of the gradient in some methodologies. The fourth one is the wind speed, which is in charge of generating the turbulence needed to transfer the saturated air to the atmosphere, replacing it by a drier one. This action allows the evaporation process to continue. Otherwise, the air above the evaporating surface would end up saturating, and the evaporation would eventually cease. Finally, there is the water availability to be evaporated. In addition to these parameters, other surface variables are also important, like the saturated specific humidity (to compute the above mentioned vapour pressure gradient), or the type of vegetation (which defines the roughness and albedo that have also an impact on evaporation).
Presentation of the study framework
Potential evaporation (ETP)
The literature provides different definitions for the concept of “potential evapotranspiration”. It was coined in 1948 by Thornthwaite, when he referred to it not as the actual transfer of water to the atmosphere but the one that would be “possible under ideal conditions of soil moisture and vegetation”. To do so, he put as an example that the desert’s vegetation is sparse due to the fact that water availability is deficient. However, if there was more water, vegetation would take profit of it, use it and increase its presence.
He highlighted some important facts, like that it has to be determined experimentally since it can not be measured directly, and that it depends only on climate conditions, because the availability of water is assumed to be complete. He also remarked that just as actual evaporation, potential evaporation is an important climatic parameter. Among other reasons, it allows to define the moisture availability factor (β).
The concept has been redefined and modified, and various definitions can be found for it. For instance, Granger (1989) identifies five different types of potential evaporation. All of them share in common that it is the evaporation rate that would occur if the surface was saturated, but vary depending on the fact that i) the energy supply to the surface, ii) the atmospheric parameters, and / or iii) the surface temperature are held constant or not. Out of the five definitions, three of them are selected arguing that either the definition provides an easy methodology to estimate potential evaporation, or appropriate limits for evaporation from a non-stressed surface.
The above paragraph shows that there is not a unique stated ETP definition. In addition, since the definitions differ, the estimations are not likely to be equal. It should to be noted too that, for the same definition, implementations may adopt different assumptions. Therefore, all of these reasons evidence the complexity of the analysis of this variable.
In the study carried out for this thesis, the ETP is considered to be the amount of evaporation that would occur if enough water was available in the surface. No land surface process is considered to limit it. In other words, it is the atmospheric demand for water. Therefore, recalling the five key climatological parameters in the computation of the actual evaporation, the availability of water to evaporate is not considered in the estimation of potential evaporation. The other four parameters remain important ETP parameters.
Reference evapotranspiration (ETo)
Allen et al. (1998) define ETo as the evapotranspiration rate from a reference surface. The reference surface is a “hypothetical grass reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 sm-1 and an albedo of 0.23”. It describes it as an extensive surface of green well-watered grass of uniform height, actively growing and which completely covers the ground. Since the surface resistance is known, the only factors ETo depends on are climatological parameters. The four basic ETP parameters (available radiative energy, air temperature, humidity gradient / deficit and the wind speed) remain important ETo parameters. There is, however, an important difference between these two concepts, which is the land surface characterization. As detailed in the definition of the reference surface, there are parameters like the height, type and homogeneity of crop and albedo that are fixed.
The Food and Agriculture Organization (FAO) reference evapotranspiration equation provides a methodology that is recommended as the standard for estimating ETo. Its aim is to study the atmosphere’s evaporative demand independently of the type, development and management of the surface’s crop. Further on, crop evapotranspiration may be computed by multiplying ETo by the crop coefficients. These are computed taking into account the characteristics that differ the crop from the reference surface described above. There are two types of coefficients, single and dual (distinguishing crop transpiration and soil evaporation).
Pan evaporation (ETPpan)
ETPpan is the amount of water that evaporates from a pan. Allen et al. (1998) explain that pan evaporation shows the integrated effect of radiation, temperature, humidity and wind on evaporation from an open-water surface. Its measure can be converted to reference evaporation by applying empirical coefficients, called pan coefficients. However, it has to be noted that precautions must be taken. For example, the energy exchange between the borders and bottom of the pan must be considered regarding the energy balance. In addition, heat storage might be significant, causing evaporation during the night, opposite to most crops which transpire during daytime. Solar radiation reflection from water in the shallow pan and grass will differ, as well as the state of the air, regarding turbulence, temperature and humidity, above both surfaces. The pan must also be surrounded by a fence to prevent animals drinking the water. For Brutsaert and Parlange (1998), ETPpan can be considered as a good indicator of actual evaporation, but only when there is enough supply of soil moisture.
There are different types of pans. For example, if Class-A pans are to be compared with Colorado ones, it will be found that they differ in their shape (circular vs. squared), depth (25cm vs. 46cm), and material (galvanized iron vs. thick iron), among others. The setting also varies. While, A-pans are to be mounted on a wooden open frame, the Colorado ones are located in the soil. Due to these differences, the pan coefficients are pan specific. Their computation is approached in different ways: i) comparing pan evaporation with open-surface water estimates or by ii) empirically-derived relationships (Kohler et al., 1955) ; (Allen et al., 1998).
Pan evaporation is an important parameter to take into account. Among other reasons, this method provides a large temporal record of measured data corresponding to evaporation from an open water surface. In this thesis, we will not work with this concept. However, we wanted to draw the reader’s attention to the fact that there are further evaporation concepts, apart from the actual, potential and reference ones.
Estimation of actual and potential evaporation
The aim of this section is to provide the reader with a general background of the main methodologies that exist to measure actual evaporation. These will be divided into fundamental equations, observation and estimation methods, as well as remote sensing and LSM methods.
ET from fundamental equations
The computation of evaporation can be approached through the use of two fundamental equations: the energy and water balances. In fact, throughout this section, we will deal with methods which are based on them.
It has to be noted that LET is the evaporation rate. This method is suitable when data is available for the rest of the variables. For example, some of the methods we will see, like the use of scillomenters, provide measurements of the H. Rn is given by the sum of the incident downward and upward shortwave and longwave radiations. It can be obtained through the use of remote sensing techniques or LSMs. Methods to estimate G range from simple ones (like approaching it through the Rn ) to more complex ones (like the use of specific sensors or through numerical models) using the canonical one-dimensional heat diffusion equation (Carslaw and Jaeger, 1959).
Water balance.
One of the main advantages of using this method is that the estimation can be performed at different spatial scales. There is even a methodology developed to estimate evaporation through the atmospheric water balance.
Presentation of the study framework
Precipitation can be obtained from measurements with surface rain gauges or through remote sensing techniques. An example is the Meteosat Second Generation, MSG’s Multi-sensor Precipitation Estimate (MPE), that approaches rainfall intensity under the assumption that it is more likely that cold clouds produce precipitation than warmer ones. While rain gauges provide data dating back more than a century ago, its spatial coverage is far too limited, since there is not much data available in certain regions from the high latitudes, tropics or arid areas. Satellite retrievals have the opposite problem, their main disadvantage is that data started to be collected some decades ago, and we do not have long time series of precipitation.
Data sets containing global runoff are scarce. In addition, groundwater fluxes may not have been properly measured or included in them. Therefore, runoff data sets may not be as reliable as desired. For instance, Peel and McMahon (2006) wonder if the continental runoff data set from Gedney et al. (2006) is representative of the observed runoff conditions and thus if it is reliable enough to be used.
dw/dt represents the amount of water removed or added from the stored water. In most cases it is neglected when computing ET on annual scales. However, it has to be taken into account that it is not always negligible. For example, if the impact of human use of water is considered. Regional and global estimates can be obtained by the Gravity Recovery and Climate Experiment (GRACE) satellite.
Keeping in mind that the aim of our study is to analyse the sensitivity of methods to estimate ETP to climate change, we would like to draw the reader’s attention to the fact that the trends from some of the variables (for example P and Q from the water balance) may be impacted by climate change and thus affect their sensitivity and that of ET.
Presentation of the study framework
that the aerodynamic resistance to heat is equal to that of water vapour in the constant flux layer, the ratio may be approached through vertical gradients of air temperature (Ta) and humidity (q). Therefore, evaporation may be obtained by means of these gradients.
This methodology is based on the turbulent transport theory. H and LET are obtained by calculating the covariance of measured heat and moisture fluxes with the vertical wind speed. An example is given in Wilson et al. (2002).
Table of contents :
1 Introduction
1.1 General background
1.2 Motivation
1.3 General methodology
1.4 Document structure
2 Presentation of the study framework
2.1 The hydrological cycle
2.2 The climate change
2.3 The Evaporation Concept
2.4 Soil Moisture
2.5 Land Surface Models
2.6 Remote Sensing: SMOS
3 Methodology
3.1 Introduction
3.2 Simulations performed with ORCHIDEE
3.3 Potential Evaporation’s sensitivity to climate change
3.4 Brightness Temperatures comparison between SMOS’s observations and a radiative transfer model output
4 Potential evaporation sensitivity to climate change
4.1 Introduction
4.2 Potential evaporation estimation through an unstressed surface-energy balance and its sensitivity to climate change
4.3 Further analysis performed
4.4 Partial conclusion
5 Comparison of measured and modelled brightness temperatures
5.1 Introduction
5.2 Soil moisture comparison
5.3 Brightness temperature comparison
5.4 Partial conclusion and discussion
6 Synthesis and perspectives
6.1 Synthesis
6.2 Perspectives
7 Appendices
7.1 List of acronyms
7.2 List of constants and variables
8 Bibliography
