|Van Pelt, Robert - Scott|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2000
Publication Date: N/A
Interpretive Summary: Irrigated agriculture is a predominant user of water. Irrigation efficiencies range from 40% to 95%. Additionally, improper scheduling of irrigation water may create plant water stress and limit the benefit to the growing crop. Many methods are used to schedule irrigation including guessing, calendars, feeling the soil, climate driven models, measured soil lwater balance, and the use of tensiometers or other instruments that measure soil matric potential, the energy the plants must exert to extract water from the soil. Scheduling based upon the later method is generally considered to be the preferred method, but variability of soil physical factors makes the estimation of the field mean soil matric potential from a single randomly placed sensor uncertain. In the past, several sensors have been used so that the measurements might be averaged. This method is expensive and time consuming, however and has not been widely accepted by the agricultural community. This study investigated whether or not an idea location in a field could be located that would give a reliable estimate of the field mean soil matric potential. From the results of the study, we determined that such a location could be found and that in many cases, this location would suffice for estimation of the mean or a given extreme with a high level of certainty. This finding is in contrast with previous studies where determination of this phenomenon was not the primary objective. The determination that spatial patterns of soil matric potential do behave in a temporally stable manner has exciting implications for irrigation scheduling and environmental monitoring.
Technical Abstract: This study investigated the temporal stability of spatial patterns of soil matric potential both within & between sequential irrigation cycles. Sixty locations in a 1ha field were outfitted with a 1m neutron probe access tube & three tensiometers placed at 0.15, 0.3, & 0.5m depths. The observations obtained from 14 days of soil water content measurements & 46 days of soil matric potential measurements within eight irrigations cycles were analyze wit Spearman's rank correlation coefficients & a relative differencing technique. Soil cores ere collected at the end of the study for textural analysis. Soil water release curves were fitted for each depth of measurement at each field location. The results showed temporally stable soil water content spatial patterns & also indicated temporally stable soil matric potential spatial patterns if assumptions of full soil wetting at the beginning of the cycle & uniform evapotranspiration among locations were satisfied. Some time-associated drift was noted within ranges of soil matric potential, as well as drift associated with changes in field mean of soil matric potential. Although well correlated with predicted behavior based on analyses of soil water release characteristic curves, little covariance with texture could be found. Several locations in the field estimated the field mean soil matric potential to within 10% within a given range of potentials & a few estimated the field mean to within 20% across the entire range of potentials tested. Other locations estimated the lower & higher percentiles soil matric potential with similar accuracy. By utilizing one or two such locations in the field to measure soil matric potential to estimate either the mean or critical limits, this technique appears to be a powerful soil water management tool.