Date of this Version
Lo, T. (2015). Quantification of Variable Rate Irrigation Benefits and Spatial Variability in Root Zone Water Holding Capacity. MS thesis. Lincoln, Neb.: University of Nebraska-Lincoln, Department of Biological Systems Engineering.
Variable rate irrigation (VRI) investment decisions require field-specific knowledge of benefits. The objective of this research was to help producers and consultants consider and quantify potential benefits of VRI. First, a conceptual model was developed for evaluating the public and/or private gain from adopting VRI where irrigation water supply is non-restrictive. Potential benefits were classified into three categories and were attributed to ten reasons. In the Central Plains at current prices, a small improvement in corn (maize) yield would make a large contribution to VRI profitability. Second, the potential irrigation withdrawal reduction from adapting VRI to spatial heterogeneity of root zone water holding capacity (R)—one particular benefit of VRI—was estimated for 49,224 center pivot irrigated fields in Nebraska. On each of these fields, the amount of R that is unutilized by conventional irrigation but can be mined annually by VRI was calculated from the statewide gridded Soil Survey Geographic database (gSSURGO). Over 51 mm of potential withdrawal reduction from this application of VRI was found on 2% of the analyzed fields. Third, based on field research, a method of conducting a field characterization of R was recommended for refining estimates of those withdrawal reductions and for informing VRI management. Field capacity (FC) was observationally determined by measuring in-situ soil water content after the wet soil has had time to drain following substantial precipitation, and R was spatially predicted by regression with a densely known auxiliary variable. As compared with FC values computed from gSSURGO and pedotransfer function outputs, FC values computed according to the observational method were more effective in accounting for observed soil moisture patterns at the study site. The field characterization of R, therefore, may be advantageous on fields where the expected profit from mining unutilized R with VRI exceeds the cost of characterization. Future research should present field demonstrations of VRI profitability, provide guidance on VRI management, and produce transferable methods for and field-specific results of quantifying VRI benefits.
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