Thesis Defense: Erin Toulou

Title: Understanding Soil Hydraulic Properties Across a Rain-Snow Transition Zone: Implications for Plant Water Uptake

Program: Hydrologic Sciences MS

Committee Chair: Jim McNamara

Committee: Jim McNamara, Qifei Niu, Alejandro Flores

Abstract: Semi-arid, water-limited watersheds can make plant water availability scarce, creating environments causing plant stress. The western region of the USA has been experiencing a sudden decline in snow-fractions, shifting snow-dominated regimes to more rain-dominated. In this research, we are quantifying soil hydraulic properties to better understand how it supports plant-water availability. To accomplish this, site-specific soil hydraulic properties were obtained using laboratory methods. Due to the coarse nature of DCEW soils, constructing soil-water retention curves was difficult due to instrument limitations and fitting constraints using the van Genuchten function. Soil hydraulic parameters from the van Genuchten equation were obtained and compared between elevation, soil depth, and ecosystem type (riparian vs. hillslope). Our results indicate that physical soil properties such as bulk density, soil texture, and organic content have a larger impact on water storage compared to fitted retention characteristics. Field capacity and wilting points were evaluated between field-based estimates from soil moisture field sensors based on the premise of Chandler et al., 2017’s methods and compared to calculated values using lab-derived soil hydraulic properties, and permanent field capacity (340 cm) and wilting point (15927cm). Our results showed field-estimated values were overestimated in comparison with lab-derived values and differed substantially, with a p-value < 0.05. We conclude that physical soil properties are a dominant control relating to plant-water availability.