Patrick D. Royer

Density-Dependent Ecohydrological Effects of Piñon–Juniper Woody Canopy Cover on Soil Microclimate and Potential Soil Evaporation

Abstract Many rangeland processes are driven by microclimate and associated ecohydrological dynamics. Most rangelands occur in drylands where evapotranspiration normally dominates the water budget. In these water-limited environments plants can influence abiotic and biotic processes by modifying microclimate factors such as soil temperature and potential soil evaporation. Previous studies have assessed spatial variation in microclimate and associated ecohydrological attributes within an ecosystem (e.g., under vs. between woody canopies) or across ecosystems (e.g., with differing amounts of woody canopy cover), but generally lacking are assessments accounting systematically for both, particularly for evergreen woody plants. Building on recently quantified trends in near-ground solar radiation associated with a piñon–juniper gradient spanning 5% to 65% woody canopy cover, we evaluated trends in soil temperature and associated estimates of potential soil evaporation as a function of amount of woody canopy cover for sites overall and for associated canopy vs. intercanopy locations. Quantified soil temperature trends decreased linearly with increasing woody canopy cover for intercanopy as well as canopy patches, indicating the coalescing influence of individual canopies on their neighboring areas. Notably, intercanopy locations within high-density (65%) woody canopy cover could be as much as ∼10°C cooler than intercanopy locations within low-density (5%) cover. Corresponding potential soil evaporation rates in intercanopies within high-density woody canopy cover was less than half that for intercanopies within low density. Our results highlight ecohydrological consequences of density-dependent shading by evergreen woody plants on soil temperature and potential soil evaporation and enable managers to rapidly estimate and compare approximate site microclimates after assessing amounts of woody canopy cover. Such predictions of microclimate have general utility for improving management of rangelands because they are a fundamental driver of many key processes, whether related to understory forage and herbaceous species or to wildlife habitat quality for game or nongame species.

Spatial Estimation of Crop Evapotranspiration, Soil Properties, and Infiltrated Water for Scheduling Cotton Surface Irrigations

Estimates of spatially distributed crop evapotranspiration (ETc) over large fields could be particularly valuable for aiding irrigation management decisions in arid regions where surface irrigation systems are predominant. The objectives are to evaluate an irrigation scheduling approach that combines remote sensing inputs with field data to provide fine-scale, spatial monitoring of crop water use and soil water status within surface-irrigated fields. Remote sensing observations of vegetation index were used to spatially estimate basal crop coefficients within 4-m x 8-m zones within borders of a 4.9-ha cotton field. These data were used to compute ETc within zones using FAO-56 procedures. Spatial inputs of soil properties were estimated from a ground-based apparent soil electrical conductivity survey. Spatial distribution of infiltrated water along the furrow was estimated using hydraulic field measurements and irrigation simulation software. An existing daily time-step, soil water balance computer program was modified to incorporate the spatial information and provide simultaneous monitoring of crop and soil conditions in zones. Irrigation scheduling using the spatial monitoring approach compared favorably in yield to traditional cotton irrigation scheduling used in the area, but reduced water use by 7 to 9%, whereas it attained as much as 19% higher yield compared to scheduling based on assuming a uniform crop coefficient for all zones. Managing water for large surface-irrigated fields aided by decision support tools and approaches that allow spatial monitoring of crop water use and soil conditions could improve precision and timing of irrigation water scheduling.