Drip irrigation in heterogeneous soils: steady-state field experiments for stochastic model evaluation.
Or D.
In many drip-irrigated fields, spatial variations in soil properties induce variations in wetting patterns about the drippers, resulting in uncertainty in the interpretation and use of sensor-based soil water information. In previous work, analytical tools to quantify the effects of spatial variation in soil hydraulic properties on wetting patterns were developed. The objective of this study was to validate some of these analytical results. Following the characterization of the soil hydraulic properties in the study site, water was supplied from arrays of surface and subsurface point sources at constant flow rates. About 160 tensiometers and 100 time domain reflectometry (TDR) probes were installed at fixed positions relative to the source (for each flow rate and source location) and were monitored daily. The resulting spatial moments of matric head and water content were compared with analytical model predictions. Results revealed a large discrepancy between measurements and predictions based on the field-estimated variability in alpha (the exponent of the unsaturated hydraulic conductivity function). Better agreement was obtained when a reduced variability in alpha was used. This suggests that soil hydraulic properties measured under ponding may be biased by the presence of large pores. Overall, the analytical model was capable of using information on the (permanent) spatial variability in soil hydraulic properties to predict the variability in flow attributes. However, even with better soil characterization and smaller soil variability, model predictions should be viewed as first approximations of the extent of variability in metric head and water content.
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