Soil spatial variability

Publisher Summary The chapter identifies phosphorus transfer (PT) as a multidisciplinary issue and focuses areas where future research is required. Potentially mobile P (PMP) is a useful concept because it attempts to weight soil P status in terms of environmental significance rather than by means of conventional soil measurement. The chapter describes agronomic management strategies, but the approaches are orientated towards agronomic rather than environmental concerns. The most significant area for further developing understanding of P transfer is hydrology, with its complexities of spatial and temporal variability. The chapter also describes new model approach that allows improved integration, which is of particular relevance when assessments of management effects and options for mitigation strategies are required. The new model structure provides a simple means for classifying and rationalizing P transfer and a basis for a spatial land use model. Moreover, it provides a basis for discussion and a means for defining future research needs on diffuse P transfer and its effects.

Transfer of phosphorus from agricultural soils

The one-dimensional convection–dispersion equation is often used to estimate the risk of nonpoint source groundwater contamination and the dispersivity in this equation is known to be a sensitive parameter for predicting the mass that leaches through the vadose zone to the groundwater. We derived a database of dispersivities from leaching studies in soils. Besides dispersivities, the database contains information about experimental parameters: transport distance, scale of the experiment, flow rate, boundary conditions, soil texture, pore water velocity, transport velocity, and measurement method. Dispersivities were found to increase with increasing transport distance and scale of the experiment. Considerably larger dispersivities were observed for saturated than for unsaturated flow conditions. No significant effect of soil texture on dispersivity was observed, but the interactive effects of soil texture, lateral scale of the experiment, and flow rate on dispersivity were significant. In coarse-textured soils, lateral water redistribution may take place across relatively larger distances, which explains the larger dependency of dispersivity on lateral scale of the experiment in coarse- than in fine-textured soils. The activation of large interaggregate pores may explain the increase in dispersivity with increasing flow rate in fine-textured soils, which was not observed in soils with a coarser texture. The distribution of dispersivities was positively skewed and better described with a lognormal than a normal distribution. Different experimental factors explained 25% of the total variability of log e -transformed dispersivities. The unexplained variance of the dispersivity was large and its coefficient of variation was 100%.

Review of Dispersivities for Transport Modeling in Soils

The increased use of trickle or drip irrigation is seen as one way of helping to improve the sustainability of irrigation systems around the world. However, soil water and solute transport properties and soil profile characteristics are often not adequately incorporated in the design and management of trickle systems. In this paper, we describe results of a simulation study designed to highlight the impacts of soil properties on water and solute transport from buried trickle emitters. The analysis addresses the influence of soil hydraulic properties, soil layering, trickle discharge rate, irrigation frequency, and timing of nutrient application on wetting patterns and solute distribution. We show that (1) trickle irrigation can improve plant water availability in medium and low permeability fine-textured soils, providing that design and management are adapted to account for their soil hydraulic properties, (2) in highly permeable coarse-textured soils, water and nutrients move quickly downwards from the emitter, making it difficult to wet the near surface zone if emitters are buried too deep, and (3) changing the fertigation strategy for highly permeable coarse-textured soils to apply nutrients at the beginning of an irrigation cycle can maintain larger amounts of nutrient near to and above the emitter, thereby making them less susceptible to leaching losses. The results demonstrate the need to account for differences in soil hydraulic properties and solute transport when designing irrigation and fertigation management strategies. Failure to do this will result in inefficient systems and lost opportunities for reducing the negative environmental impacts of irrigation.

Analysis of soil wetting and solute transport in subsurface trickle irrigation

A large-scale natural-gradient tracer test in a highly heterogeneous aquifer at the Macrodispersion Experiment (MADE) site on the Columbus Air Force Base in Mississippi is simulated using three-dimensional hydraulic conductivity distributions derived from borehole flowmeter test data. Two methods of hydraulic conduct(fBm), are used to construct the hydraulic conductivity distributions needed by the numerical model. Calculated and observed mass distributions are compared to evaluate the effectiveness of the dual-domain mass transfer approach relative to the single-domain advection-dispersion approach. The results show that the classical Fickian advection-dispersion model can reproduce reasonably well the observed tritium plume above a certain concentration limit but fails to reproduce the extensive spreading of the tracer at diluted concentrations as observed in the field. The alternative dual-domain mass transfer model is able to represent the rapid, anomalous spreading significantly better while retaining high concentrations near the injection point. This study demonstrates that the dual-domain mass transfer approach may offer a practical solution to modeling solute transport in highly heterogeneous aquifers where small-scale preferential flow pathways cannot be fully and explicitly represented by the spatial discretization of the numerical model.

/pdf/a-dual-domain-mass-transfer-approach-for-modeling-solute-32vxyuvezj.pdf

A dual-domain mass transfer approach for modeling solute transport in heterogeneous aquifers: application to the macrodispersion experiment (MADE) site

D. Ed Henderson

Papers

Dispersion parameters for undisturbed partially saturated soil.