Scaling of surface soil moisture and solute movement from local- to field-scales in unsaturated soils

Richa Ojha, Purdue University

Abstract

Describing subsurface soil moisture and solute movement over a range of spatial scales is a major challenge because of multi-scale heterogeneity exhibited by many field soils. Soil moisture evolution is achieved by numerical solution of the Richards equation. Modeling solute movement requires the flow field from this numerical solution of the Richards equation as an input for yet another numerical solution of the advection-dispersion equation. These numerical solutions are computationally very intensive, and may not provide the insights that are possible from simpler analytical representations. Moreover, measurement techniques are not in accordance with the scale at which information on soil moisture or solute fluxes are desired. In situ point measurements are expensive and provide information at few selected points only. On the other hand, the spatial resolution of remote sensing data is too coarse for hydrologic applications. Consequently, new strategies are needed to span the intervening scales. The focus of this work is on utilizing physics-based approaches to span vertical solute movement and surface soil moisture from local (point)- to field-scale. This scaling needs to account for heterogeneity in soil properties, among which the spatial variability in soil saturated hydraulic conductivity typically has the most prominent role. This variability is modeled as a log-normal field, and forms the basis for the scaling hypotheses in this thesis. Analytical solutions for solute movement at the local-scale are developed to describe transport of solute particles along the main characteristic of the flow field. Local-scale model development is achieved by using a sharp-front approximation for water movement along with advective transport. Solutions from a local-scale model are first presented for pre- and post-ponding conditions associated with fields subjected to rainfall. These local solutions are upscaled to field-scale solute transport by appropriate integration methods. Expressions for travel time moments and mean field-scale behavior are developed. Theoretical results are corroborated with an extensive set of numerical Monte-Carlo simulations and 3-D numerical solutions for various forms of soil heterogeneity. Comparison with field-scale experimental results is provided for context. Model results describe both short term transient behavior of field-scale properties, as well as asymptotic behavior at a specified control plane. To further describe surface soil moisture evolution under rainfall at local- and field-scales, scaling curves are developed based on a sharp-front approximation. Apart from describing field-scale behavior, the analytical solutions assist in aggregation and disaggregation of surface soil moisture at different scales. The theoretical expressions developed for surface soil moisture and solute movement are compared with numerical and field experiments for corroboration. The study provides useful insight into the role of rainfall-controlled versus soil-controlled water and solute movement in heterogeneous unsaturated soils. Model results are expected to perform better for sandy loam and loamy sand soils where the sharp-front approximation for water movement is a good approximation and the solute transport is likely to be dominated by advection. Future work should strive for more generality in soil properties when describing solute movement. Extension to larger (watershed) scales will require understanding of heterogeneity not only in soil properties, but also in terrain geometry and stream networks.

Degree

Ph.D.

Advisors

Govindaraju, Purdue University.

Subject Area

Civil engineering

COinS