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Texas Tech University (TTU) 2018

Evaluating different models for quantifying the hydraulic and thermal properties of pasture unsaturated soils

Kharel, Geeta

Titre : Evaluating different models for quantifying the hydraulic and thermal properties of pasture unsaturated soils

Auteur : Kharel, Geeta

Université de soutenance : Texas Tech University (TTU)

Grade : Master of Science (MS) 2018

Résumé partiel
A variety of empirical models have been developed to determine soil water retention and hydraulic and thermal properties of unsaturated soils. The evaluation of the applicability of these empirical models for a variety of soils, soil, water, and agricultural management systems under different climatic or environmental conditions continue to be an important concern. These fundamental soil properties are critical for many studies of vadose zone hydrologic processes especially in the semi-arid Southern High Plains (SHP), where the increasing pressure on the depleting Ogallala Aquifer has stressed to develop sustainable alternatives, for example, forage-livestock systems, to the irrigation-dependent crop monocultures for the SHP agriculture. The overall objective of this study was to evaluate : (1) four widely used closed-form unimodal soil water retention curve (SWRC) models, i.e., Brooks and Corey (the BC model), van Genuchten (the VG1 and VG2 models), Kosugi (the K model), and Fredlund and Xing (the FX model) models to describe and to combine with pore-size distribution models with which to predict the unsaturated hydraulic conductivity function, and (2) four soil thermal conductivity (λ) predictive models, i.e., Johansen, Côté and Konrad, Balland and Arp, and Lu et al. models for pasture unsaturated soils with. Soil samples (at soil depths of 0-5, 5-10, 10-15, and 15-20 cm) collected from two semi-arid pastures (referred to as the east and west pasture area) in the SHP region under a variety of long-term drip-irrigated and dryland forage management systems were used to quantitatively compare the performance of the selected SWRC and λ models for pasture soils. Model parameters were estimated by fitting these SWRC and λ models to the measured data. Measured SWRCs showed the enhanced soil water retention behaviors (i.e., saturated water content, field capacity water content, plant available water) in pasture unsaturated soils (0-20 cm) under different management practices. Differences in the measured SWRCs between the west and east pasture soils are primarily attributed to the higher clay content of the latter. Notably, the observed organic matter content at the east pasture areas was also relatively higher. Differences in mean water contents of the top 0-5 cm unsaturated soils between treatments for both pastures were significant at lower matric potentials, i.e., in the dry part of SWRCs. No effect of pasture management practices on the saturated water content (s) and field capacity water content (fc) was observed, while significant differences for r or wilting point water content (at -15000 cm H2O) values were found between treatments both in the east and west pasture areas. In contrast with the west pasture with mostly sandy clay loam soils, no significant differences in PAW (plant available water) between treatments were found in the clay loam east pasture likely due to its higher clay and organic matter contents as well as relatively lower soil bulk density

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Page publiée le 12 novembre 2018