12: Surface Energy Balance and Evapotranspiration
- Page ID
- 38743
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)In the previous two chapters we focused on evaporation and transpiration, two processes which are major energy sinks at the land surface. In this chapter, we will directly examine the energy balance for the land surface, represented on the right side of Fig. 12-1. We will consider the nature and fate of radiation received at the land surface, the exchange of sensible heat between the surface, the atmosphere, and the soil, and the consumption of energy (latent heat) during evapotranspiration. The surface energy balance is essentially a way of accounting for heat transfer between the land surface and its surroundings. This chapter's audio overview provides a starting point for understanding these topics [website].
Heat transfer is simply energy transfer that depends on the temperatures of the objects or systems involved. Temperature is a measure of the average kinetic energy of the random microscopic movements of the molecules or particles of a substance. Temperature is also the physical quantity which determines the direction of heat transfer between two substances in thermal contact with each other. Heat is transferred from regions of higher temperature to regions of lower temperature. Viewed in this way, temperature is somewhat analogous to water potential, the physical quantity which determines the direction of water flow.
- 12.1: Modes of heat transfer
- Convection is also an important mode of heat transfer at the land surface where wind enhances heat exchange between the surface and the atmosphere. In contrast, conduction is typically the dominant mode of energy transfer within the soil, where the soil constituents are in direct contact with each other. In the surface energy balance, the energy flux via radiation is often the largest single term, so we will consider this mode .
- 12.2: Radiation basics
- The Earth’s surface, which has an average temperature of approximately 287 K, emits radiation with a peak intensity of around 10 μm, which is in the infrared portion of the spectrum. Thus, in the context of the surface energy balance, the incoming solar radiation is called shortwave radiation
- 12.3: Climate Change
- hanges in the Earth’s radiation balance are a central issue in the phenomenon of climate change. Climate is the average weatherover a relatively long period of time, often defined as 30 years. Thus, climatechangerefers to long-term change in the average weather patterns for a location, a region, or the globe. One of the most well-known examples of climate change is the ongoing increase in the average surface air temperature and ocean surface temperature around the world, which is referred to asg
- 12.4: Net radiation
- We have seen previously that the land surface emits longwave radiation to the atmosphere. But we should not overlook the fact that the atmosphere emits longwave radiation both upward into space and downward toward the Earth’s surface. The magnitude of this downward longwave radiation depends on the temperature and emissivity of the atmosphere, according to Eq. 12-2. The atmospheric emissivity can range from 0.5 to nearly 1 [8]
- 12.5: Surface energy balance
- The latent heat flux (LE) is the energy that is absorbed by water at the Earth’s surface during evaporation or transpiration apart from any change in temperature. It is a flux or transfer of energy because we assume the resulting water vapor is transported away from the surface by diffusion and advection in the atmosphere. The latent heat flux is equal to the latent heat of vaporization for water multiplied by the evapotranspiration (ET) rate.
- 12.6: Modifying the surface energy balance
- Some land management practices are specifically designed to alter the surface energy balance. For example, people often cover some or all of the soil surface with materials intended to influence the surface energy balance and water balance and suppress the growth of undesired plants, and these materials are referred to as mulches .Common materials used for mulches include crop residues, leaves, wood chips, gravel, and plastic films....
- 12.7: Reference evapotranspiration
- One of the most widespread uses of the surface energy balance concept is for determining evapotranspiration. The actual evapotranspiration rate for a given time for a particular location on the land surface can be substantially influenced by site-specific management practices. Thus, often it is useful to estimate the potential or reference evapotranspiration
- 12.8: Problem set
- Use the Hargreaves method to estimate monthly ET0 for Arcadia, Oklahoma, USA in the year 2016, given the data below. Use a spreadsheet and report only the annual total (mm).Assuming a latent heat of vaporization of 2.465 MJ kg-1, one MJ m-2 is equivalent to 0.4057 mm of evaporated water.
- 12.9: References
- Ren, H., et al., Empirical algorithms to map global broadband emissivities over vegetated surfaces.IEEE Transactions on Geoscience and Remote Sensing, 2013. 51(5): p. 2619-2631. Sanchez, J.M., et al., Thermal infrared emissivity dependence on soil moisture in field conditions.IEEE Transactions on Geoscience and Remote Sensing, 2011. 49(11): p. 4652-4659.