Surface soil water loss after summer rainfall in a semidesert grassland

Abstract

Surface soil water contents were analyzed for bare and litter‐ or gravel‐covered soils for 20 drying periods after summer rainfall in southeastern Arizona. Water contents were measured every minute by fiberglass cells calibrated for the sandy‐loam soil, and 30‐ to 60‐min averages were stored by microloggers. Water loss to equivalent matric potentials of ‐1.5 MPa for bare surface soils (1–3 cm) was fast (1.2 ± 0.5 days), moderate (2.6 ± 0.9 days), or slow (5.9 ± 1.4 days) when the upper 15 cm of soil at the start of the drying period had 13.3, 27.1, and 36.1 cm of water, respectively. Those water contents are near saturation and near field capacity for slow and moderate drying rates, respectively. For fast drying rates, water content was near field capacity at 1–3 cm but dry below 8 cm. Litter or gravel mulches increased the time of surface soil water availability by 0.7 to 1.9 days for fast and moderate drying periods, respectively. Drying from the surface down into the soil profile averaged 3.6 cm day‐1. Short summer rainstorms may germinate warm‐season grasses which will desiccate if subsequent rainfall is not persistent enough to maintain surface water availability and allow adventitious root development. Estimates of time that the drying front reaches the depth of seminal roots of warm‐season grasses were used to suggest rain‐rainless day patterns that might result in seedling mortality. Sowing during the summer rainy season when the probability of these patterns is minimal may increase revegetation success. High correlation of available water periods with meterological variables suggests that soil water dynamics could be accurately estimated using physically based models, and may permit more detailed modeling of seedling establishment.