Long-term soil water dynamics across a desert basin: Broad-scale climatic drivers and local heterogeneity

In arid and semiarid ecosystems, plant community composition and production is primarily limited by soil water availability.  Soil profile development and soil-vegetation feedbacks can influence spatial and temporal patterns of plant available water (PAW).  In addition to affecting current plant communities, these relatively static properties contribute to ecosystem resilience to disturbance and climate.  We analyze a 20 year record of soil water content collected across varying soil types, landscape positions and plant communities in southern New Mexico USA to address some basic question regarding patterns of PAW in deep heterogeneous vadose zones common in arid landscapes.  Soil water was measured monthly to a maximum depth of 300 cm at ten locations within fifteen 70 by 70 m plots.  Soil profile data including texture, mineralogy, and moisture release curves was collected from soil pits adjacent to plots.  The 20 year record presented illustrates the importance of rare climatic events and soil property-plant community feedbacks for patterns of PAW in this system.  Broad scale deep recharge only occurred during three brief climatic events captured by this study.  Some of the most stable sources of available water in this landscape exist either deep in the soil profile (> 200 cm) or in well developed calcic or petrocalcic horizons (at ~100 cm).  Shallow water sources (< 50 cm) are typically very unpredictable in the summer growing season but are more predictable in the late winter.  There is no evidence for increased frequency of PAW under either shrub or perennial grass canopies.