Submitted to: Handbook of Soil Science
Publication Type: Book / Chapter
Publication Acceptance Date: July 30, 2010
Publication Date: November 17, 2011
Citation: Evett, S.R., Prueger, J.H., Tolk, J.A. 2011. Water and energy balances in the soil-plant atmosphere continuum. In: Huang,P.M., Li,Y., Sumner,M.E.,editors. Handbook of Soil Sciences: Properties and Processes, 2nd Edition. Boca Raton, Florida: CRC Press. p. 6-1 to 6-44. Interpretive Summary: Light from the sun provides not only all the energy for photosynthesis, which leads to food and fiber production, but it is the energy source that drives the water cycle on earth, including evaporation from soils and plants. Water resource constraints are becoming more severe in the U.S. at the same time that pressure to produce more crops for biofuels, food, and fiber is increasing. So, understanding of the energy and water balances is key to solving many problems related to agricultural production. Scientists at the USDA-ARS Conservation & Production Research Laboratory describe in this chapter both the theoretical and practical aspects of land surface energy and water balances. The water balance is composed of infiltration of water into the soil from irrigation or precipitation, evaporative losses from the soil surface (some avoidable), uptake of water from the soil by plants that is key for crop production, and losses of water to runoff and deep percolation. The energy balance depends on the strength of sunlight, the reflection of light by crop and soil surfaces, and the absorption of light by crop and soil surfaces that leads to heating and which drives evaporation. Understanding the energy balance means understanding how much energy is available to cause losses of water to evaporation and to transform light into food – two competing but essential processes in the energy and water balances. This understanding helps scientists and engineers design new agricultural practices to produce more food with less water.
Technical Abstract: Energy fluxes at soil-atmosphere and plant-atmosphere interfaces can be summed to zero because the surfaces have no capacity for energy storage. The resulting energy balance equations may be written in terms of physical descriptions of these fluxes; and have been the basis for problem casting and solving in diverse fields of environmental and agricultural science such as estimation of evapotranspiration (ET) from vegetated surfaces, estimation of evaporation from bare soil, rate of soil heating in spring (important for timing of seed germination), rate of residue decomposition (dependent on temperature and water content at the soil surface), and many other problems. The water balances at these surfaces are implicit in the energy balance equations. The soil water balance equation is different from, but linked to, the surface energy balances, a fact that has often been ignored in practical problem solving. Components of the energy and water balances are discussed in detail, with theory, methods of estimation, and methods of measurement discussed and compared for efficacy.