Measuring and modeling soil salinity under flood-irrigation with dairy wastewater
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Measuring and modeling soil salinity under flood-irrigation with dairy wastewater

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The objective of this study was to evaluate the effectiveness of commercial environmental sensors and public domain modeling applications in observing soil salinity levels and salt fluxes in dairy waste lagoon irrigation operations in the San Joaquin Valley. The physical sensors employed recorded temperature, soil moisture, and electrical conductivity at multiple depths at two dairy sites, one characterized by clayey soils (Clay Dairy) and the other by sandy soils (Sandy Dairy). The sensors performed reliably with negligible temperature dependency. An exception was that high salinity in the Clay Dairy soil led to water content overestimation that required correction. The HYDRUS 1-D software package successfully modeled field measurements of water content at the Sandy and Clay Dairies (R2 = 0.78, and R2 = 0.75 respectively), and overall salinity (R2 = 0.69 and R2 = 0.72 respectively). Estimated hydraulic parameters at both sites varied with depth but were consistent with values gleaned from the literature. Model fits of the solute transport at the sandy site resulted in large ranges in dispersivity values due to the scale of the soil domain. While the Clay Dairy hydraulic characteristics were reasonably well described by the model, the model was incapable of describing solute transport effectively, especially at the shallower depths. This result may have been due to high dispersion, and lateral movement of water and salts not accounted for by the 1-dimensional form of the convection-dispersion equation. Long-term model projections showed distinct peaks in water and salt during irrigation events and subsequent drainage at both sites. The less permeable soil at the Clay Dairy facilitated greater water and nutrient retention in the root zone, while continued irrigation at the Sandy Dairy showed movement of salt to speculated depths of groundwater (20 to 40 meters) on a time scale of weeks, suggesting substantial salt fluxes into deep groundwater over time. Coupling detailed monitoring with more advanced models (2D/3D, groundwater-surface water) would prove beneficial in describing water and salt movement and accumulation in the vadose zone and the water table to better understand the risks salinization poses to groundwater and crop yield.

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