Water availability is crucial for organismal survival and growth in dryland environments, affecting both ecological interactions and carbon dynamics. The goal of this thesis is to develop soil water release curves (SWRCs) that link soil water potentials (Ψ) to soil water content (θ). Using the SWRCs, temporal soil water sufficiency curves are developed, which quantify the amount of time that dryland critical zones have enough water to sustain the physiology of organisms. These curves allow for effectively indicating water availability across different species, coverage types, and soil conditions, enhancing our understanding of water dynamics in drylands and contributing important parameters for a variety of studies. I examine the interaction between water, soil, and plant dynamics at two sites: the Ivey pecan farm in Tornillo, Texas and the Jornada Experimental Range in Las Cruces, New Mexico. I assess physical soil properties, including depth, texture, and ground cover types such as bare ground, creosote, mesquite, and grass. At the Ivey Pecan Orchard, fine and coarse sites were sampled to analyze variations in soil texture. Data from moisture sensors for the period of 2011-2021 were cross-verified with direct soil gravimetric measurements and SWRCs at corresponding depths. A corresponding adjustment in data allowed for accurate quantifications of soil moisture and subsequently conversions of these measurements into water potentials using the Fredlund-Xing (1994) model, thus providing a detailed view of moisture trends across different soil coverages and textures. At the Jornada Experimental Range, it was found that shallow soils at depths of 5 and 10 cm experienced significant increases in water loss (retained water less well), whereas deeper soils exhibited more water retention stability. Our
refined data showed that the upper 30 cm of soils under creosote and mesquite shrubs typically maintained water availability above the wilting point of creosote (-6 MPa) only slightly more than 50% of the time. Thus, we conclude that shallow (0-30 cm) soils in the shrubland has insufficient water availability for sustained plant health year-round, which is consistent with seasonal grass
dieback at the site. Shrub species, such as creosote and mesquite, likely compensate with access to deeper water sources via their rooting structures. Preliminary correlations of soil moisture data with carbon exchange measured via eddy flux tower were inconclusive, but further modeling could reveal important connections between water sufficiency and net carbon balance. The development
of temporal soil water sufficiency curves and their ability to predict water availability for organisms contribute to a broader understanding of organism water availability in drylands. This tool provides a solid foundation for future studies in drylands and works to advance the understanding of soil-plant-atmosphere relations in dryland critical zones.