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United States Department of Agriculture

Agricultural Research Service

Title: Energy Balance Comparisons over Multiple Surfaces

Authors
item Hatfield, Jerry
item Prueger, John
item Kustas, William
item Hipps, Lawrence - UTAH STATE UNIVERSITY

Submitted to: Agricultural and Forest Meteorology Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 6, 1998
Publication Date: N/A

Interpretive Summary: Water use by different vegetative surfaces varies because of the overall climate and the type of crop. There have been a number of assumptions about the water use rates of different crops within a landscape, however, there have been relatively few measurements using common measurement systems over a number of different crops. These data are important to understanding the effect of different plants on their water use and the potential response to global climate change. We compared the water use rates of plants in Iowa, Oklahoma, and New Mexico, which represented a variety of plants from summer annuals (corn and soybean) to desert perennial species (mesquite bushes). These measurements revealed that water use patterns are dependent upon the crop canopy and soil water availability as the primary factors. Using these results helps to develop better descriptions of how farmers could manage different plants to improve ethe efficiency of water use throughout the United States.

Technical Abstract: Energy exchanges over surfaces are dependent upon the net radiation, characteristics of the canopy or surface, and the available water for evaporation. Most studies have been conducted within the confines of a single surface in terms of examining the energy balance and the partitioning of energy. Over the past 5 years we have measured the energy balance over a range of different surfaces. These surfaces include corn, soybean, oats, rye, warm season grasses, cool season grasses, and mesquite in Iowa, Oklahoma, and New Mexico. A Bowen ratio system was placed over each surface in a location that provided adequate fetch for the instruments. Eddy correlation equipment was used to provide direct measurements of latent and sensible heat fluxes for these conditions. These surfaces represented a range of net radiation and soil heat fluxes, and the latent heat fluxes ranged from nearly 0 to over 100 percent of the net radiation (advection). Evaporative fractions calculated as the ratio of latent heat fluxes to net radiation ranged from nearly 0 to over 1. Corn and soybean had the largest evaporative fractions while the warm season grasses had the lowest fraction. We calculated the alpha values to represent the variations among the surfaces and these ranged from 1 to 1.5. The alpha term varied with soil water availability within a surface. These tools represent a method to compare among surfaces that permits a direct comparison of the partitioning of energy.

Last Modified: 11/23/2014
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