Closing the water balance with cosmic-ray soil moisture measurements and assessing their relation to evapotranspiration in two semiarid watersheds

Abstract

Abstract. Soil moisture dynamics reflect the complex interactions of meteorological conditions with soil, vegetation and terrain properties. In this study, intermediate-scale soil moisture estimates from the cosmic-ray neutron sensing (CRNS) method are evaluated for two semiarid ecosystems in the southwestern United States: a mesquite savanna at the Santa Rita Experimental Range (SRER) and a mixed shrubland at the Jornada Experimental Range (JER). Evaluations of the CRNS method are performed for small watersheds instrumented with a distributed sensor network consisting of soil moisture sensor profiles, an eddy covariance tower, and runoff flumes used to close the water balance. We found a very good agreement between the CRNS method and the distributed sensor network (root mean square error (RMSE) of 0.009 and 0.013 m 3 m −3 at SRER and JER, respectively) at the hourly timescale over the 19-month study period, primarily due to the inclusion of 5 cm observations of shallow soil moisture. Good agreement was also obtained in soil moisture changes estimated from the CRNS and watershed water balance methods (RMSE of 0.001 and 0.082 m 3 m −3 at SRER and JER, respectively), with deviations due to bypassing of the CRNS measurement depth during large rainfall events. Once validated, the CRNS soil moisture estimates were used to investigate hydrological processes at the footprint scale at each site. Through the computation of the water balance, we showed that drier-than-average conditions at SRER promoted plant water uptake from deeper soil layers, while the wetter-than-average period at JER resulted in percolation towards deeper soils. The CRNS measurements were then used to quantify the link between evapotranspiration and soil moisture at a commensurate scale, finding similar predictive relations at both sites that are applicable to other semiarid ecosystems in the southwestern US.

Percolation observations in an arid piedmont watershed and linkages to historical conditions in the Chihuahuan Desert

“…Summer periods in 2013 and 2014 had particularly large rainfall amounts, exhibiting high soil moisture values that matched well in the θ SN and θ CRNS methods (Schreiner‐McGraw et al. ). Fig.…”

“…() and Schreiner‐McGraw et al. () describe monitoring efforts that began in 2010 at the watershed with the establishment of a dense network of rainfall, runoff, soil moisture, and soil temperature sensors, as well as meteorological, radiation, and energy flux measurements at an eddy covariance (EC) tower (Fig. ).…”

“…Data from the soil moisture probes were averaged by depth and location to obtain a spatially averaged quantity (θ SN ) consistent with the CRNS measurement (Schreiner‐McGraw et al. ). CRNS data were used to gap‐fill the soil moisture observations when necessary.…”

Elevated CO2 did not affect the hydrological balance of a mature native Eucalyptus woodland

“…Hourly E floor was calculated as: where z is the depth of the theta probes (5 cm) and is the difference in θ v between consecutive hourly averages ( t 1 – t 2 ). A decrease in θ v from 0 to 5 cm results from evapotranspiration, but also from water infiltration (Schreiner‐Mcgraw, Vivoni, Mascaro, & Franz, ); thus, to avoid overestimation of E floor , we only calculated E floor for days with P = 0 mm per day and preceded by a day with P < 2 mm per day (454 of 730 days passed these criteria). We validated our approach with E floor measurements made at one location adjacent to ring 1 (Figure S1).…”

“…We calculated the terms of the water balance for each season (summer, DJF; autumn, MAM; winter, JJA; and spring, SON). We considered that we had closed the water balance when the sum of the water balance components (Equation ) was at least 75% of precipitation for that period, that is |( P ‐ R + D g + Δ S + E t )/ P | < 0.25 (Schreiner‐Mcgraw et al., ).…”