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In this research, study was made of representative soils of denton County (Texas) with the aim of correlating various soil-moisture factors through experimental analyses. Two accepted hypotheses of soil-moisture relations have been substantiated by experiment with Denton County soils.

Site-specific irrigation decisions require information about variations in soil moisture within the rooting depth actively being used by the crop. Producers have been using soil moisture sensors to make irrigation decisions, and it has been shown that soil moisture sensors can reduce water usage without reducing yields. There are still unanswered questions on improving efficiency with soil moisture sensors based on density and location of sensors within a field. This three-year study uses sensors to evaluate the spatio-temporal variability of soil moisture across an 18-ha production field in a corn/soybean rotation. The IDW results show that when uniform irrigation applications are made to the field, fewer sensors that are placed in better locations throughout the field can be as useful as a densely gridded array of sensors. Although, if variable rate irrigation (VRI) is being used, a dense array could be used the first season to fine tune management zones.

<p> Soil moisture is at the centre of the water balance and is of great concern with regards to crop growth and yield, irrigation planning, fertilization, climate change and non-point source pollution control. Information on soil moisture is not widely available, resulting in researchers relying on mathematical models to gain insight into soil moisture conditions. This thesis primarily focuses on long-term soil moisture characteristics, under given climate, soil and vegetation conditions. Long-term soil moisture characteristics are best described by statistics such as average soil moisture, and its standard deviation and frequency/probability distribution. After an extensive review of existing explicit or implicit soil moisture models, a deterministic water balance model was developed to simulate soil moisture at a point within the root-zone. The hydrological processes involved in the water balance are modelled using well-established methods. The continuous simulation model is unique from other leading deterministic models as it introduces the ecohydrological perspective by modelling actual evapotranspiration as a function of plant access to soil moisture. The validation of the model demonstrates that simplified soil moisture modelling is rational and practical.</p> <p> Soil moisture modelling is dependent on various input parameters related to the climate, soil and vegetation. Both local and global sensitivity analyses were carried out to investigate which input parameters influence the soil moisture regime the most. The analyses concluded that parameters representing soil texture are most important and thereby indicated that evapotranspiration is the most dominant process as it is significantly controlled by these parameters. Due to concerns of the impact of climate change and urban stormwater management, a better understanding of urban area soil moisture dynamics is required. The applicability of the continuous simulation model was demonstrated by investigating the influence of global warming on long-term soil moisture and evapotranspiration. Statistical analyses carried out on the post-simulated long-term soil moisture values clearly showed that even though temperatures are increasing, soil moisture and evapotranspiration have also increased because of the overall increase in precipitation. This phenomenon gives insight into the precipitation characteristics being strong enough to overpower the soil moisture loss process of evapotranspiration. As a part of the overall research, an analysis on antecedent soil moisture values for the purpose of urban stormwater management was performed. Empirical equations were derived to obtain antecedent soil moisture values from soil characteristics. Antecedent soil moisture information is essential in the application of the design storm approach while designing urban stormwater management infrastructure.</p> <p> The main purpose for the development and use of the deterministic model was to better understand the statistics and sensitivity of soil moisture and not as a predictive tool.</p> / Thesis / Doctor of Philosophy (PhD)

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