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A new method for determining profile-average and depth-wise hydraulic properties in heterogeneous mountain soils is presented using the GEOtop watershed model in 1-D vertical mode. Dry soil bulk density–converted volumetric soil water retention data are used to determine van Genuchten soil water retention parameters, and the Kozeny–Carman equation is used to determine saturated soil hydraulic conductivity. Optimum dry soil bulk densities are identified by minimizing the sum of squared error between measured and calculated soil water content time series. The new method was tested using soil moisture data from soil profiles at the Dry Creek Experimental Watershed, Boise, ID, and the Libby Creek Experimental Watershed, Laramie, WY. Results of different scenarios showed that the optimization of a single profile-average dry soil bulk density is a good option for describing soil water flow in the heterogeneous mountain soils. Soil water content modeling efficiency (ME) values of 0.084 ≤ ME ≤ 0.745 and –2.443 ≤ ME ≤ 0.373 were found for the Dry Creek and Libby Creek sites, respectively. Relatively low ME values for the deepest sensor depths for some scenarios were attributed to the overestimation of soil water freezing and uncertainty in the soil water retention function near saturation. The resulting calibration procedure is computationally efficient because only one parameter (dry soil bulk density) is optimized. [ABSTRACT FROM AUTHOR]

Prescribed fire can be used to return wild lands to their natural fire cycle, control invasive weeds, and reduce fuel loads, but there are gaps in the understanding of post-disturbance responses of vegetation and hydrology. The impact of a prescribed fire and subsequent aspen cutting on evapotranspiration (ET) and streamflow was assessed for the Upper Sheep Creek catchment, a 26-ha headwater catchment dominated by low sagebrush, mountain big sagebrush, and aspen within the Reynolds Creek Experimental Watershed. The 2007 prescribed fire consumed 100% of the mountain big sagebrush and approximately 21% of the low sagebrush. The aspen, which were mostly untouched by the fire, were cut in the fall of 2008. Post-disturbance ET and vegetation recovery were related to the loss of rooting depth. ET recovered within 2 years on the low sagebrush area with limited rooting depth, while that on the deeper-rooted mountain big sagebrush area took 4 years to recover. ET from the aspen trees, which can sprout from existing roots, recovered within 2 years. The influence of vegetation disturbance on streamflow was assessed using both empirical time trend analysis and process-based modelling. Although both approaches suggested approximately a 20% increase in streamflow during the 6 years post-disturbance, results from the empirical time trend analysis were marginally significant ( p = 0·055), while those from the process-based modelling were not statistically significant. Marginal streamflow response can be attributed to rapid post-disturbance recovery of the aspen where most of the streamflow originates. Published 2016. This article is a U.S. Government work and is in the public domain in the USA [ABSTRACT FROM AUTHOR]

The article discusses the effect of simulated soil water storage on streamflow generation in the mountainous snowmelt surroundings in Idaho. The author claims that soil processes affect the timing and amount of streamflow produced from snowmelt in the western U.S. Meanwhile, the article looked into the use of soil water balance modeling approach to integrate the impact of soil processes, particularly the soil water storage and the timing and amount of snowmelt generated streamflow. Moreover, the distributed simulations over the watershed for the same two years shows that streamflow movements are closely related to the overall watershed soil water storage capacity that serves as a threshold.

Water content reflectometers use time domain reflectometry (TDR) to estimate the apparent permittivity of soil, which in turn can be related to the soil water content. The objective of this study is to develop a physical model for water content reflectometers. The length of the sensor rods and the delay time of the circuitry in the probe head are the two unknown parameters. The two parameters are determined both analytically, using sensor readings in air and deionized water, and by optimization, using air and non-conductive fluids. The calibrated parameters are used to calculate the apparent permittivity as a function of water content for sensor readings in five soils, ranging from sand to silt loam. Calculated permittivity values are compared with Topp's permittivity-water content relationship. Results show that the calculated permittivity values for the sand compare reasonably well with Topp's equation. The permittivity in the sandy loam to silt loam soils is overestimated by as much as 104 dimensionless permittivity units. The overestimated permittivity values are due to dielectric dispersion and ionic conductivity, brought about by the low effective frequency in the electromagnetic pulse of the sensors as compared with standard TDR. The performance of the reflectometers may be improved by increasing the frequency of operation of the sensors from <175 MHz to >1 GHz. At higher frequencies, the sensors become less sensitive to ionic conductivity. Furthermore, dielectric dispersion becomes less of an issue at higher frequencies, thereby increasing the applicability of existing permitivity-water content relationships such as Topp's equation. [ABSTRACT FROM AUTHOR]

The amount and spatial distribution of deep drainage (downward movement of water across the bottom of the root zone) and groundwater recharge affect the quantity and quality of increasingly limited groundwater in arid and semiarid regions. We synthesize research from the fields of ecology and hydrology to address the issue of deep drainage in arid and semiarid regions. We start with a recently developed hydrological model that accurately simulates soil water potential and geochemical profiles measured in thick (>50 m), unconsolidated vadose zones. Model results indicate that, since the climate change that marked the onset of the Holocene period 10000-15 000 years ago, there has been no deep drainage in vegetated interdrainage areas and that continuous, relatively low (<-1 MPa) soil water potentials have been maintained at depths of 2-3 m. A conceptual model consistent with these results proposes that the native, xeric-shrub-dominated, plant communities that gained dominance during the Holocene generated and maintained these conditions. We present three lines of ecological evidence that support the conceptual model. First, xeric shrubs have sufficiently deep rooting systems with low extraction limits to generate the modeled conditions. Second, the characteristic deep-rooted soil-plant systems store sufficient water to effectively buffer deep soil from climatic fluctuations in these dry environments, allowing stable conditions to persist for long periods of time. And third, adaptations resulting in deep, low-extraction-limit rooting systems confer significant advantages to xeric shrubs in arid and semiarid environments. We then consider conditions in arid and semiarid regions in which the conceptual model may not apply, leading to the expectation that portions of many arid and semiarid watersheds supply some deep drainage. Further ecohydrologic research is required to elucidate critical climatic and edaphic thresholds, evaluate the role of important physiological processes (such as hydraulic redistribution), and evaluate the role of deep roots in terms of carbon costs, nutrient uptake, and whole-plant development. [ABSTRACT FROM AUTHOR]

We describe long-term lysimeter data collected at the Reynolds Creek Experimental Watershed. The length of record is >15 years for lysimeters at two sites representing different soil and climatic conditions. These data describe changes in soil water during the snow-free season. In addition to measuring changes in total soil water, soil water content profiles and soil temperature profiles were measured within or adjacent to the lysimeters and are reported in accompanying reports. These data are available to the public via the U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center anonymous ftp site ftp.nwrc.ars.usda.gov. [ABSTRACT FROM AUTHOR]

We describe long-term soil water data collected at the Reynolds Creek Experimental Watershed (RCEW). Data were collected for 10-25 years at 18 sites representing different climatic regimes and soils in the RCEW. Soil profile data are also available. High correlation between neutron probe and lysimeter measurements are the basis for assessing the accuracy of neutron probe-measured changes in soil water content. These data are available to the public via the U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center anonymous ftp site ftp.nwrc.ars.usda.gov. [ABSTRACT FROM AUTHOR]

We describe long-term soil temperature data collected at the Reynolds Creek Experimental Watershed (RCEW). These data were collected for over 15 years at five locations representing different climatic regimes and soils in the RCEW. Measurements were made at several depths at each site to a depth of 180 cm in most cases. Each site is located in close proximity to a climate station. Descriptive soil profile information is also available for each site. We illustrate two aspects of the data set, the impact of snow depth on soil temperature and the interannual variability of soil temperature at a site. These data are available to the public via the U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center anonymous ftp site ftp.nwrc.ars.usda.gov. [ABSTRACT FROM AUTHOR]

Examines the response of the Water Content Reflectiometer (WCR) to different soils, water content and temperature changes. Success of time domain reflectometry; WCR soil water calibration for different soils; Air, ethanol and oven-dry soil measurements; Electrical conductivity effects; Temperature effects.

In this paper we examine how the nature of spatial variability affects hydrologic response over a range of scales using five field studies as examples. The nature of variability was characterized as either stochastic, when random, or deterministic, when due to known, nonrandom sources. We have emphasized how that characterization may change with the scale of hydrologic model. The five field examples, along with corresponding sources of variability, were (1) infiltration and surface runoff affected by shrub canopy, (2) groundwater recharge affected by soil depth, (3) groundwater recharge and streamflow affected by small-scale topography, (4) frozen soil runoff affected by elevation, and (5) snowfall distribution affected by large-scale topography. In each example there was a scale, the deterministic length scale, over which the hydrologic response was strongly dependent upon the specific, location-dependent ecosystem properties. Smaller-scale variability may be represented as either stochastic or homogeneous with nonspatial data. In addition, changes in scale or location sometimes resulted in the introduction of larger-scale sources of variability that subsume smaller-scale sources. Thus recognition of the nature and sources of variability can reduce data requirements by focusing on important sources of variability and using nonspatial data to characterize variability at scales smaller than the deterministic length scale. All the sources of variability described are present in the same watershed and affect hydrologic response simultaneously. Physically based models should therefore utilize both spatial and stochastic data where scale appropriate. Other implications for physically based modeling are that modeling algorithms should reflect larger-scale variability which generally has greater impact and that model and measurement grids should be consistent with the nature of variability. [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]

Snowmelt in mountainous areas is an important contributor to river water flows in the western United States. We developed a distributed model that calculates solar radiation, canopy energy balance, surface energy balance, snow pack dynamics, soil water flow, snow-soil-bedrock heat exchange, soil water freezing, and lateral surface and subsurface water flow. The model was applied to describe runoff generation in a subcatchment of the Dry Creek Experimental Watershed near Boise, ID. Calibration was achieved by optimizing the soil water field capacity (a trigger for lateral subsurface flow), lateral saturated soil hydraulic conductivity, and vertical saturated hydraulic conductivity of the bedrock. Validation results show that the model can successfully calculate snow dynamics, soil water content, and soil temperature. Modeled streamflow for the validation period was underestimated by 53%. The timing of the streamflow was captured reasonably well (modeling efficiency was 0.48 for the validation period). The model calculations suggest that 50 to 53% of the yearly incoming precipitation in the subcatchment is consumed by evapotranspiration. The model results further suggest that 34 to 36% of the incoming precipitation is transformed into deep percolation into the bedrock, while only 11 to 16% is transformed into streamflow. [ABSTRACT FROM AUTHOR]

The ability to quantify soil-atmosphere water and energy exchange is important in understanding agricultural and natural ecosystems, as well as the earth's climate. We developed a one-dimensional vertical model that calculates solar radiation, canopy energy balance, surface energy balance, snowpack dynamics, soil water flow, and snow-soil-bedrock heat exchange, including soil water freezing. The processes are loosely coupled (solved sequentially) to limit the computational burden. The model was applied to describe water and energy dynamics for a northeast-facing mountain slope in the Dry Creek Experimental Watershed near Boise, ID. Calibration was achieved by optimizing the saturated soil hydraulic conductivity. Validation results showed that the model can successfully calculate seasonal dynamics in snow height, soil water content, and soil temperature. Both the calibration and validation years confirmed earlier results that evapotranspiration on the northeast-facing slope consumes approximately 60% of yearly precipitation, while deep percolation from the soil profile constitutes about 40% of yearly precipitation. [ABSTRACT FROM AUTHOR]

The soil moisture characteristic (SMC) is a fundamental soil property for simulating soil water dynamics but is difficult and time-consuming to measure. When the soil is frozen, the soil water potential and liquid water content are strongly dependent on temperature. The relation between soil freezing temperatures and liquid water content, termed the soil freezing characteristic (SFC), is related to the SMC. With the widespread use of time-domain reflectometry (TDR) to measure liquid water in frozen soil, simultaneous measurement of soil temperature and liquid water content under frozen conditions enables in situ estimation of the SMC curve through its similarity to the SFC. We investigated the applicability of deducing the SMC curve from in situ measurements of the SFC for simulation of both frozen and unfrozen soil water dynamics. Results suggest that SMC parameters deduced from the SFC may be used for model simulations without a significant loss of accuracy, as compared with model simulations based on pressure plate analyses. [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]