Evaluation of techniques for on-farm monitoring of percolation and irrigation system performance

approved: John S. Selker Abstract approved: Jonathan D. Istokapproved: Jonathan D. Istok Efficient use of water in agricultural production involves accurate assessment and control of the quantity and spatial uniformity of excess percolation. Passive Capillary Samplers (PCAPS), which sample water from the vadose zone have shown potential to provide superior estimates of soil water flux compared to alternative methods. In a four-year study, 42 PCAPS installed in 21 commercial agricultural fields in Lane County, OR, USA were monitored monthly to evaluate their operational characteristics and ability to estimate soil solution flux. The PCAPS showed little evidence of technical failure, with only two of the 42 installed samplers found to operate inefficiently. Installation of 10 of the 42 samplers in locations susceptible to high or perched water tables resulted in submersion of the samplers, rendering them inoperable. On average, the PCAPS measured soil water flux 25% greater than that obtained from a water balance estimate. This discrepancy was attributed to a possible inaccuracy in water balance evapotranspiration estimates, along with a violation of the PCAPS design assumptions which suggests over-sampling would occur in the presence of high water tables. Analysis of the PCAPS collection ability indicates that Redacted for Privacy Redacted for Privacy to estimate the mean yearly recharge at each site with a 30% bound on the mean at the 0.05 confidence level, eight PCAPS are required. This number corresponds closely to the results of Brandi-Dohrn et al. (1996a) and is thought to be due to intrinsic variability of percolation. Spatial uniformity in irrigation water application is essential to reducing excess percolation. Twelve sprinkler irrigation systems used under commercial crop production in Lane County, OR were evaluated for equipment wear and performance. Field measurements of sprinkler nozzle size and discharge rate were recorded for each system and used to estimate water application patterns. New sprinkler nozzles were installed on six of the 12 irrigation systems to compare potential application rate and uniformity with existing system performance. Despite reducing the coefficient of variation in discharge between sprinklers from 10% to 2%, little increase in water application uniformity was attained by replacing the nozzles. A 13% decrease in mean water application rate was documented when new nozzles replaced worn parts. The over-application due to worn or mismatched nozzles gives rise to the potential for increased surface redistribution and deep percolation, resulting in water and nutrient

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