Submitted to: Agricultural and Forest Meteorology Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/6/1998
Publication Date: N/A
Interpretive Summary: Water use by crops depends upon the amount of water in the soil, the type of crop, and the amount of energy available for the evaporation process. Agricultural crops vary in their water use, and producers will have to understand the effect of various management decisions on the water use patterns by different crops. One of the limitations to this understanding has been the availability of instruments that can be placed in fields to measure the water use rates. This type of instrumentation is important to measure the water use throughout an entire year and not only for short periods of time. There are units that can be placed in the field to provide reliable short term information, however, their performance over longer time periods has not been studied. We compared a simple measurement system with a more sophisticated unit over a 6 year period. The results reveal that we can reliably and accurately make measurements throughout a year. This provides confidence in the measurements of different managemen practices that producers use in growing crops and their effect on water use or potential water savings.
Technical Abstract: Water use by agricultural canopies represents the largest portion of the net radiation energy over a year. Estimates of water use in the Midwest range up to 75 percent of the energy used via the process of evaporation. However, there are variations in the partitioning of energy due to changes in soil surface management, crop rotation, and soil. These changes occur throughout the year and may not be detectable during short measurement intervals. The Bowen ratio system offers a method that is reliable and rugged for continuous use in the field environment while eddy correlation equipment provides a direct measure of the latent and sensible heat fluxes. The purpose of this study was to compare Bowen ratio measurements to eddy correlation measurements for a number of different crops and tillage practices. These data were collected in Walnut Creek watershed from 1991 to 1996 using Bowen ratio equipment positioned in the center of production sized fields. Eddy correlation equipment were located adjacent to the Bowen ratio equipment for a series of days that represented a range of environmental conditions. We were able to gather 171 days of comparison data over this time. The net radiation values ranged from 100 to 700 W m**-2, which represents the expected variation throughout a year in the Midwest. Eddy correlation measurements exhibited less variation than Bowen ratio measurements over the comparison days, however, there was no bias in either method. These units are consistent in their response, and when properly maintained produce quality data that represent the conditions of various crops and soil conditions. Measurements throughout the year provide an opportunity to determine the effect of cropping system on the hydrological resources.