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United States Department of Agriculture

Agricultural Research Service

Title: Energy and Water Balances at Soil-Plant-Atmosphere Interfaces

Author
item Evett, Steven

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: February 19, 1998
Publication Date: N/A

Interpretive Summary: The energy balance of the earth's surface is the sum of all energy flows to and from the surface. These include sunshine and light reflected back into space, heating of the atmosphere by the relatively warmer surface, heating and cooling of soil or water bodies, and heating or cooling of the surface by relatively warmer or colder winds blowing across it. Understanding the energy balance and being able to measure or estimate its component parts are key to understanding and predicting many processes in natural and manmade environments. These include timing of seed germination, rapidity and depth of root growth, growth and maturation of plants, plant water use and yield, and decomposition of plant residues, just to name a few in the agricultural scene. The energy balance is as important as the soil water balance. Soil water balance is the result of precipitation and irrigation, runoff or runon, infiltration into the soil, evaporation from the soil and plant water use, and losses to deep percolation below the root zone or movement of water from shallow water tables upward into the root zone. The ability to measure and predict these is important for management of agricultural production schemes and environmental protection and remediation. This chapter in the CRC Handbook of Soil Science was written to provide scientists and engineers in other fields with a resource for use in their work and a guide for further study.

Technical Abstract: This chapter in the CRC Handbook of Soil Science was written to provide scientists and engineers in other fields a resource for methods of estimation and measurement of the components of the surface energy balance and soil water balance and as a guide to further study. The surface energy balance is described as 0 = Rn + G + H + LE, where Rn is net radiation, G is soil heat flux, H is sensible heat flux, and LE is latent heat flux. Examples of values of these components are discussed for cropped and bare soil surfaces and for diurnal and yearly cycles. Net radiation is described in terms of its component fluxes in the short wave and long wave, and methods of measurement of the components or the whole are presented. Estimation of solar radiation is described in detail followed by estimation and measurement of the long wave components of Rn. Examples of estimation of Rn for alfalfa and grass are presented. Soil heat flux is described theoretically, and measurement with soil heat flux plates is discussed as is estimation of G from diurnal variations of temperature and water content. Attention is focused on estimation of surface heat flux from subsurface measurements made with plates, combined with measurements of soil water content, and temperature above the plates. Bowen ratio and eddy correlation techniques for measurement of latent and sensible heat fluxes are discussed as are weighing lysimeter methods of measuring latent heat flux. Lysimeter methods are discussed in the context of the soil water balance. Other methods of measuring the change in storage component of the water balance, such as neutron scattering and time domain reflectometry, are discussed as well. Runoff and precipitation measurement are discussed briefly with references for further study cited.

Last Modified: 11/27/2014
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