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The success of time domain reflectometry (TDR) has !ed to the development of other instruments that use the soil dielectric constant as the basis for determining volumetric soil water content. An example is the Water Content Reflectometer (WCR; Campbell Scientific, Logan, UT), which is much less expensive than TDR and is used widely, although little has been published concerning its applicability to soil water content monitoring. The primary objectives of this study were to determine the WCR soil water calibration for different soils and to investigate how it is affected by changing temperature. We found the individual sensors to be very precise (CV [is less than or equal to] 0.05) under the controlled laboratory conditions of this study. Variability among sensors, determined in air and ethanol, indicated significant ([Alpha] = 0.05) sensor differences that were largely accounted for with a simple additive correction. Sensor soil water calibration was investigated in four soils under varying water contents across a 40 [degrees] C temperature range. We found that (i) soil water calibration was significantly ([Alpha] = 0.05) different for each soil tested, (ii) there was a significant ([Alpha] = 0.05) temperature response for all soils, and (iii) the effect of temperature varied with soil water content and soil type. Both the soil type and temperature sensitivities we observed were probably due to the relatively high electrical conductivity (EC) of the soils tested.

Understanding the role of ecosystems in modulating energy, water and carbon fluxes is critical to quantifying the variability in energy, carbon, and water balances across landscapes. This study compares and contrasts the seasonal surface fluxes of sensible heat, latent heat and carbon fluxes measured over different vegetation in a rangeland mountainous environment within the Reynolds Creek Experimental Watershed. Eddy covariance systems were used to measure surface fluxes over low sagebrush (Artemesia arbuscula), aspen (Populus tremuloides) and the understory of grasses and forbs beneath the aspen canopy. Peak leaf area index of the sagebrush, aspen, and as pen 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. Estimated cumulative evapotranspiratation from the sagebrush, aspen understory, and aspen trees were 399 mm, 205mm and 318 mm. A simple water balance of the catchment indicated that of the 700mm of areal average precipitation, 442mm was lost to evapotranspiration, and 254mm of streamflow was measured from the catchment; water balance closure for the catchment was within 7 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. Higher evapotranspiration from the sagebrush was 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 offset by evaporative cooling. Results from this study illustrate the influence of vegetation on the spatial variability of surface fluxes across mountainous rangeland landscapes. [ABSTRACT FROM AUTHOR]

Commercial soil water sensors may improve irrigation management but lose accuracy when low quality water is used. We evaluated the WET, 5TE, and ML2 sensors in different soils and water qualities. The ML2 accuracy was not affected by either while the other two sensors were strongly affected by both. A cost-effective calibration technique effectively adjusts for differences among soils. Commercial dielectric soil water sensors may improve irrigation management by providing continuous field soil water information. The accuracy of these sensors may be compromised, however, when low quality, high electrical conductivity irrigation water is used. The magnitude of this effect is expected to vary among sensors depending on the sensor measurement frequency (f), decreasing with increasing f, thus providing a potential criterion for sensor selection. We examined the effect of varying solution electrical conductivity (σw) on the sensor-measured dielectric permittivity (εs) of the following three soil water sensors: the WET (f = 20 MHz), 5TE (f = 70 MHz), and ML2 (f = 100 MHz). We also evaluated the efficacy of using a simple, cost effective, two-point calibration technique (CAL) to adjust for differences among soils. We found that εs varied dramatically, in some cases more than 15 dielectric units, among sensors in different soils under uniform soil water and σw conditions. The relative εs response of the WET and ML2 sensors was consistent with their respective f, and the 5TE response was not. Thus, the WET εs was almost always greater than the ML2, while the 5TE was generally less than the ML2. In all cases, the ML2 εs could be described with a single, commonly used calibration equation. We also found that varying σw from 1.2 to 6 dS m-1 had little effect on εs, indicating that a single calibration may work for that range of irrigation water quality. Finally, we found that the CAL procedure, as evaluated with the RMSE, was effective for all soils and sensors. [ABSTRACT FROM AUTHOR]

Several methods for quantifying ET for two rangeland vegetation types over an 8-yr study period agreed well with each other. Methods included eddy covariance systems, soil water storage loss measured by time domain reflectometry and neutron probe, and model simulation. Evapotranspiration (ET) was quantified for two rangeland vegetation types, aspen and sagebrush-grassland, over an 8-yr study period by comparing the following approaches for estimating ET: eddy covariance systems (EC, available for only 6 yr); soil water storage loss measured by time domain reflectometry (TDR) and neutron probe; and model simulation. The research site, the Upper Sheep Creek catchment in the Reynolds Creek Experimental Watershed, is part of a study on the effects of prescribed fire and vegetation removal. Estimates of seasonal ET for the aspen using EC with the turbulent fluxes adjusted to force energy balance closure, a 180-cm TDR soil profile, two 225-cm neutron access tubes, and the Simultaneous Heat and Water (SHAW) model agreed well with each other. If the two neutron probes were averaged, the RMSD of all approaches over the 6 yr was within 8% of the average. For the sagebrush-grassland, a 120-cm TDR profile underestimated seasonal ET for all years except the year immediately following the prescribed fire, when rooting depth likely had not recovered. A 195-cm neutron probe access tube located within 30 m of the stream underestimated ET for every year except when there was no streamflow, suggesting that lateral flow may have biased the results for this tube. A comparison of the other methods (EC flux adjusted to force energy balance closure, soil water loss measured from a 225-cm neutron access tube, and SHAW model simulation) agreed within 3% during the 6 yr with EC measurements at that site. [ABSTRACT FROM AUTHOR]

Hydrologists have long struggled with the problem of how to account for the effects of spatial variability in precipitation, vegetation and soils. This problem is particularly acute in snow-fed, semi-arid watersheds, which typically have considerable variability in snow distribution and vegetation communities on scales much smaller than that addressed by most hydrological modelling. In this study, two approaches were used to compute a water balance using two years of data collected at the Upper Sheep Creek Watershed, a 26-ha semi-arid mountainous sub-basin within the Reynolds Creek Experimental Watershed in south-west Idaho, USA. The first water balance approach (uniform approach)assumed that the entire watershed was homogeneous; the second approach computed a partial water balance for each of three landscape unitsand then computed an aggregated water balance for the watershed. Runoff and change in groundwater storage were not distinguishable between landscape units; thus, the only difference between the two approaches was in the estimation of the two major components, precipitation and evapotranspiration (ET). Precipitation, which occurs predominantlyas snow, was measured within each landscape unit directly and adjusted for drifting. ET was estimated using the simultaneous heat and water model (SHAW) and validated with measurements from Bowen ratio instruments. Precipitation input for two years was approximately 480 and 700 mm, respectively; ET was approximately 450 and 410 mm, respectively. The water balance for the aggregated approach had a discrepancy of -17 and 55 mm, respectively for the two years, while the uniform approach was within 42 and 86 mm, respectively. (Negative values indicate more estimated outflow than inflow.) The differences in precipitation estimates for the two approaches were greatest for the second year owing to more variability across the watershed, which the uniform approach did not adequately address. The largest difference between the a [ABSTRACT FROM AUTHOR]