Temporal and spatial assessment of evaporation, transpiration, and soil moisture redistribution

At a native stand of creosote bush (Larrea tridentata ) in North Las Vegas, a rainfall simulation study was conducted over a 12 month period from October 2005 to October 2006. Simulated rainfall occurred during the winter, spring, summer, and fall periods. Rainfall simulation systems were positioned on each of 12 plots, each containing a single creosote bush. Simulated rainfall events occurred at night with multiple short pulses designed to maximize infiltration while minimizing ponding. Yearly simulated rainfall amounts were set at 0, 15, 30 and 60 cm (replicated three times) and were approximately 0, 1.5, 3.0 and 6.0 times the natural rainfall. The cumulative reference evapotranspiration (ETref) was 156.7 cm and cumulative ambient precipitation was 7.9 cm. Soil and plant canopy surface to air temperature differentials (Ts-Ta and Tc-Ta) were assessed using an infrared thermometer (IRT). Significant differences were based on simulated rainfall treatment (P<0.001) and over time (P<0.001). Soil evaporation (E) measurements were obtained using a custom hemispherical chamber, results showed that 87% of the variability in chamber measurements could be explained by ETref, simulated rainfall amount, and the soil area (P<0.001). Transpiration measurements of individual plants were estimated using stem flow gauges and were normalized to a canopy leaf area basis. Soil surface volumetric water content was assessed using a hand held probe. Soil moisture with depth was assessed using both portable and permanently installed time domain reflectometry (TDR) sensors. TDR waveforms recorded were analyzed via custom post-processing algorithms written in the C++ programming language and based on the methods of Topp et al. (1980) and Herkelrath et al. (1991). Results showed that for the Mojave Desert with sparse, open vegetation of creosote bush under elevated precipitation, evaporation dominated, whereas transpiration was a minimal component of the soil-plant system. The level of soil moisture redistribution was greater under lower environmental demand, creating a seasonal change in water holding storage. Based on the water holding capacity of the soil profile, plant water uptake, and environmental demand, the recurrence interval for deep percolation can be predicted and used for long-term performance assessment studies of soil covers.