Characterization of hydrologic parameters and processes in shrink-swell clay soils Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z029p753b

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  • Vertisols and other vertic-intergrade soils are found all over the globe, including many agricultural and urban areas. These soils are characterized by their cyclical shrinking and swelling behaviors, where bulk density and porosity distribution both vary as functions of time and/or soil moisture. In turn, alterations in physical soil parameters become manifest as crack networks, which open during the shrinkage phase and re-seal as the soil swells. As a consequence, when present these soils can significantly impact agriculture and infrastructure, and can act as a dominant control on the local hydrology. Therefore, understanding how water moves through and interacts with these soils is of utmost importance to ensure their proper utilization and management. This study had three objectives: 1) to identify the role of initial soil moisture on infiltration in a rigid (non-swelling) soil; 2) to quantify variation in the soil's hydraulic properties, using single-ring infiltration measurements taken in a vertic soil over a 1.5 year period; and 3) to build on these concepts to examine data from a set of instrumented field plots in a vertic soil located in the Secano Interior region of Chile. Significant findings of the study include 1) the development of a new formulation to describe a soil's wetting front potential (a measure of capillary pull) in terms of initial degree of saturation; 2) the creation of a simple correction to approximate the decrease of wetting front potential in wetter soils, thereby improving the accuracy of the traditional Green and Ampt sorptivity model in such conditions; 3) an equation to accurately estimate hydraulic parameters from short-duration infiltration tests, when steady-state conditions have not been realized; 4) a new theoretical model to estimate crack porosity as a function of soil moisture, which was developed based on the soil shrinkage curve; and 5) modification of the traditional two-term Philip infiltration model for use in vertic soils. The field study also showed that crack networks cause highly complex and non-linear wetting of the soil profile, with water simultaneously infiltrating from the soil surface and from the soil-crack interfaces; that cracks can seal at the surface while remaining open and hydraulically active below the surface, which indicates that surface-based monitoring alone may not be sufficient to predict water movement and soil response in vertic soils; and that the transition between infiltration and runoff may be strongly correlated with the cumulative amount of net precipitation that had reached the soil surface, so that cumulative amount of precipitation has the potential to be a simple yet accurate metric to predict runoff in vertic soils. These findings offer improved understanding of soil-water interactions in vertic soils, and reveal that very simple concepts underlie seemingly complex systems. As a result, the concepts and formulations developed in this study should allow for straightforward integration into other studies and models.
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