The water stored in and flowing through the subsurface is fundamental for sustaining human activities and needs, feeding water and its constituents to surface water bodies and supporting the functioning of their ecosystems. Quantifying the changes that affect the subsurface water is crucial for our understanding of its dynamics and changes driven by climate change and other changes in the landscape, such as in land-use and water-use. It is inherently difficult to directly measure soil moisture and groundwater levels over large spatial scales and long times. Models are therefore needed to capture the soil moisture and groundwater level dynamics over such large spatiotemporal scales. This thesis develops a modeling framework that allows for long-term catchment-scale screening of soil moisture and groundwater level changes. The novelty in this development resides in an explicit link drawn between catchment-scale hydroclimatic and soil hydraulics conditions, using observed runoff data as an approximation of soil water flux and accounting for the effects of snow storage-melting dynamics on that flux. Both past and future relative changes can be assessed by use of this modeling framework, with future change projections based on common climate model outputs. By direct model-observation comparison, the thesis shows that the developed modeling framework can reproduce the temporal variability of large-scale changes in soil water storage, as obtained from the GRACE satellite product, for most of 25 large study catchments around the world. Also compared with locally measured soil water content and groundwater level in 10 U.S. catchments, the modeling approach can reasonably well reproduce relative seasonal fluctuations around long-term average values. The developed modeling framework is further used to project soil moisture changes due to expected future climate change for 81 catchments around the world. The future soil moisture changes depend on the considered radiative forcing scenario (RCP) but are overall large for the occurrence frequency of dry and wet events and the inter-annual variability of seasonal soil moisture. These changes tend to be higher for the dry events and the dry season, respectively, than for the corresponding wet quantities, indicating increased drought risk for some parts of the world.

The relevance, value and multi-dimensional application of soil moisture in many areas such as hydrological, meteorological and agricultural sciences have increased the focus on this important part of the ecosystem. However, due to its spatial and temporal variability, accurate soil moisture determination is an ongoing challenge. In the fall of 2013 and spring of 2014, the accuracy of five soil moisture instruments was tested in heavy clay soils and the Root Mean Squared Error (RMSE) values of the default calibration ranged from 0.027 and 0.129 m3 m-3. However, after calibration, the range was improved to 0.014 – 0.040 m3 m-3. The need for calibration has led to the development of generic calibration procedures such as soil texture-based calibrations. As a result of the differences in soil minerology, especially in clay soils, the texture-based calibrations often yield very high RMSE. A novel approach that uses the Cation Exchange Capacity (CEC) grouping was independently tested at three sites and out of seven different calibration equations tested; the CEC-based calibration was the second best behind in situ derived calibration. The high cost of installing and maintaining a network of soil moisture instruments to obtain measurements at limited points has influenced the development of models that can estimate soil moisture. The Versatile Soil Moisture Budget (VSMB) is one of such models and was used in this study. The comparison of the VSMB modelled output to the observed soil moisture data from a single, temporally continuous, in-field calibrated Hydra probe gave mean RMSE values of 0.052 m3 m-3 at the eight site-years in coarse textured soils and 0.059 m3 m-3 at the six site-years in fine textured soils. At field-scale level, the representativeness of an arbitrarily placed soil moisture station was compared to the mean of 48 data samples collected across the field. The single location underestimated soil moisture at 3 of 4 coarse textured fields with an average RMSE of 0.038 m3 m-3 and at only one of the four fine textured sites monitored with an average RMSE of 0.059 m3 m-3. / February 2017

Call number: LD2668 .T4 1958 H33 / Master of Science

Photocopy of typescript. / Digitized by Kansas Correctional Industries

Typewritten copy Includes bibliographical references The measurement of soil moisture in situ is called Part A; The study of soil moisture changes under a simple crop rotation is called Part B. Evaluates the Bouyoucos electrical resistance method of measuring soil moisture in situ; and studies the soil moisture under a simple crop rotation. Has been the subject of 2 papers entiled: Techniques associated with the use of gypsum block soil moisture meters / Aitchinson, Butler and Gurr; and, Gypsum block moisture meters as instruments for the measurement of tension in soil water / Aitchinson and Butler. The papers are attached. Part A includes a brief summary and discussion of these papers

Knowledge of the stress-strain relationship of the compacted soils in tension is of importance for understanding of cracking that occurs in earth structures, in particular embankment dams and landfill barriers. Understanding the correlation between tensile properties and traditional soil parameters and soil suction is essential in identifying problems associated with desiccation induced cracking. A series of extensive laboratory experiments were performed on three different soils from existing embankment dams. This thesis concentrated on the investigation of tensile strength in relation to the type of soil, compaction water content, compaction density ratio, rate of loading, soil suction, moisture retention characteristics and the effect of drying and wetting. Stress-strain behaviour and tensile properties indicated a dependence on soil type and compaction criteria. The plasticity index, clay content and type of mineral has shown a significant influence on tensile strength. Compaction dry of optimum resulted in an increase in strength. Compaction wet of optimum showed a decrease in strength and small increase in strain at failure. Higher compaction effort resulted in higher tensile strength, tensile stiffness and brittle stress-strain behaviour. Difference in loading rate revealed response time for initial tensile deformation as well as sustainable duration up to failure point. The effect of soil suction plays an important role in drying during which specimens exhibited a considerable strength increase. The magnitude of strength increase may have been contributed by a combination of suction, air entry value and compaction density. The effect of wetting could cause decreasing in suction and thus a reduction in strength. Based on the findings, it was concluded that the desiccation-induced may not necessarily occur due to an associated increase in tensile strength. However, an increase in tensile strength is likely to be accompanied by an increase in shrinkage. Therefore, desiccation-induced cracking is related to the interaction between moisture loss, change in soil suction, tensile stress and shrinkage.

The problem of cavity expansion in unsaturated soils is investigated. A unified constitutive model for unsaturated soils is presented in a critical state framework using the concepts of effective stress and bounding surface plasticity theory. Consideration is given to the effects of suction and particle crushing in the definition of the critical state. A simple isotropic elastic rule is adopted. A loading surface and bounding surface of the same shape are defined using simple and versatile functions. A limiting isotropic compression line exists, towards which the stress trajectories of all isotropic compression load paths approach. A non-associated flow rule is assumed for all soil types. Isotropic hardening/softening occurs due to changes in plastic volumetric strains as well as suction for some unsaturated soils, enabling account of the phenomenon of volumetric collapse upon wetting. Results of isotropic compression tests, oedometric compression tests and drained and undrained triaxial compression tests performed on Kurnell (quartz) sand in saturated and unsaturated states and subjected to stresses sufficient to cause particle crushing are presented and used to calibrate the model. The model is also calibrated using results reported in the literature for triaxial tests performed on saturated and unsaturated speswhite kaolin and three load paths. For both soils the model leads to a much improved fit between simulation and experiment compared to that for models based on conventional plasticity theory. The model is implemented into a cavity expansion analysis using the similarity technique, extended for application to unsaturated soils. Cylindrical and spherical cavities are considered, as are drained and undrained conditions. Cavity expansion results for the bounding surface model and conventional plasticity models are compared for saturated conditions. Substantial differences highlight the importance of adopting a model that accurately describes stress-strain behaviour. Cavity expansion results for the bounding surface model and saturated and unsaturated conditions are also compared. Substantial differences, particularly in the limit pressure, highlight the major influence of suction and the importance of accounting for this when using cavity expansion theory to interpret results of the cone penetration and pressuremeter tests.

Thesis (Ph. D.)--Oregon State University, 1964. / Typescript. Mounted photographs. Includes bibliographical references (leaves 99-109). Also available on the World Wide Web.

Graduation date: 1996

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references. Also available in electronic version.