Surface fluxes and water balance of spatially varying vegetation within a small mountainous headwater catchment

Abstract

Abstract. Precipitation variability and complex topography often create a mosaic of vegetation communities in mountainous headwater catchments, creating a challenge for measuring and interpreting energy and mass fluxes. Understanding the role of these communities in modulating energy, water and carbon fluxes is critical to quantifying the variability in energy, carbon, and water balances across landscapes. The focus of this paper was: (1) to demonstrate the utility of eddy covariance (EC) systems in estimating the evapotranspiration component of the water balance of complex headwater mountain catchments; and (2) to compare and contrast the seasonal surface energy and carbon fluxes across a headwater catchment characterized by large variability in precipitation and vegetation cover. Eddy covariance systems were used to measure surface fluxes over sagebrush (Artemesia arbuscula and Artemesia tridentada vaseyana), aspen (Populus tremuloides) and the understory of grasses and forbs beneath the aspen canopy. Peak leaf area index of the sagebrush, aspen, and aspen understory was 0.77, 1.35, and 1.20, respectively. The sagebrush and aspen canopies were subject to similar meteorological forces, while the understory of the aspen was sheltered from the wind. Missing periods of measured data were common and made it necessary to extrapolate measured fluxes to the missing periods using a combination of measured and simulated data. Estimated cumulative evapotranspiratation from the sagebrush, aspen trees, and aspen understory were 384 mm, 314 mm and 185 mm. A water balance of the catchment indicated that of the 699 mm of areal average precipitation, 421 mm was lost to evapotranspiration, and 254 mm of streamflow was measured from the catchment;Correspondence to: G. N. Flerchinger (gerald.flerchinger@ars.usda.gov) water balance closure for the catchment was within 22 mm. Fluxes of latent heat and carbon for all sites were minimal through the winter. Growing season fluxes of latent heat and carbon were consistently higher above the aspen canopy than from the other sites. While growing season carbon fluxes were very similar for the sagebrush and aspen understory, latent heat fluxes for the sagebrush were consistently higher, likely because it is more exposed to the wind. Sensible heat flux from the aspen tended to be slightly less than the sagebrush site during the growing season when the leaves were actively transpiring, but exceeded that from the sagebrush in May, September and October when the net radiation was not offset by evaporative cooling in the aspen. Results from this study demonstrate the utility of EC systems in closing the water balance of headwater mountain catchments and illustrate the influence of vegetation on the spatial variability of surface fluxes across mountainous rangeland landscapes.

Inference of Soil Hydrologic Parameters from Electronic Soil Moisture Records

“…In semiarid Mediterranean climates, seasonal patterns in soil moisture storage and streamflow follow seasonal changes in precipitation, temperature, and solar radiation (McNamara et al, 2005: Flerchinger et al, 2010Seyfried et al, 2011). In this system, the water year begins with dry soil following the annual summer drought.…”

“…These sites represent various controls of soil texture, soil depth, vegetation, and annual precipitation (Flerchinger et al, 2010) on (Figure 1). In each instance, a pit was hand-excavated to bedrock or saprolite and two parallel profiles of soil moisture instruments, separated by 2 m, were installed horizontally into the pit face.…”

A Process-Based Application of State-and-Transition Models: A Case Study of Western Juniper (Juniperus occidentalis) Encroachment

“…Plant community composition, structure, and productivity are all strongly coupled with the timing and amount of ecosystem water availability (Flerchinger et al, 1998(Flerchinger et al, , 2010Rodriguez-Iturbe, 2000;Ryel et al, 2010;Chauvin et al, 2011;Roundy et al, 2014b). Sagebrush and herbaceous productivity is strongly tied to soil water recharge deep in the soil profile (Cline et al, 1977;Sturges, 1977;Richards and Caldwell, 1987;Caldwell and Richards, 1989;Ryel et al, 2004;Schlaepfer et al, 2012).…”

Eleven years of mountain weather, snow, soil moisture and streamflow data from the rain–snow transition zone – the Johnston Draw catchment, Reynolds Creek Experimental Watershed and Critical Zone Observatory, USA

“…Water vapor pressure (e a ) and dewpoint temperature (T d ) were calculated using measured T a , RH, and software tools from the Image Processing Workbench (IPW) (Frew, 1990;Marks et al, 1999b). The IPW tools are optimized for temperatures near 0 • C, providing greater accuracy for e a and T d as T a approaches 0 • C. This accuracy is critical for the determination of precipitation phase in the rain-to-snow transition zone.…”