Key Points
Forest thinning treatments in mountain regions may substantially alter snowmelt
Changes in snowmelt timing from forest thinning depend on slope and aspect.
Changes in snowmelt timing are ...caused primarily by shifts in radiation to snow.
A physically based model built upon extensive field observations of radiation dynamics and snow processes in cold regions forest environments was used to investigate the impacts of prescribed forest gap‐thinning treatments on spring snowmelt in a small Saskatchewan River headwater basin of the Canadian Rocky Mountains. Both field observations and model simulations showed that snow accumulations in small clear‐cut gaps was roughly double those under intact forest cover due to sublimation losses from the canopy. Consequently, mountain forests thinned with small clear‐cut gaps resulted in a substantial increase in the magnitude of spring snowmelt. However, the impact of forest thinning on the timing of snowmelt was highly dependent on slope orientation; thinning accelerated snowmelt on south facing slopes primarily through enhanced shortwave radiation, but retarded snowmelt on north facing slopes primarily through reduced incoming longwave radiation. As a result, thinning treatments across opposing north facing and south facing mountain slopes acted to substantially expand the spring melt period in the basin, and illustrated the important hydrological control imparted by intact forest cover through its synchronization of snowmelt across complex terrain. A sensitivity analysis of snowmelt timing to varying spring meteorological conditions strongly suggests that shifts in spring snowmelt runoff from similar forest thinning treatments in mountain regions will depend on the slope and aspect at which they occur in combination with the seasonal timing of snowmelt resulting from latitude‐controlled solar elevation effects.
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•WEPP modeling was conducted on paired, nested watersheds with forest managements.•WEPP-simulated daily streamflow was in close agreement with observed values.•WEPP captured water and ...sediment yields and changes due to harvesting practices.•Results demonstrated WEPP’s potential as a decision-aid tool in forest management.
The Water Erosion Prediction Project (WEPP) model was applied to seven paired, nested watersheds within the Mica Creek Experimental Watershed located in northern Idaho, USA. The goal was to evaluate the ability of WEPP to simulate the direct and cumulative effects of clear-cutting and partial-cutting (50% canopy removal) on water and sediment yield. WEPP was modified to better represent changes in the Leaf Area Index during post-harvest forest vegetative recovery. Good agreement between simulated and observed streamflow was achieved with minimal to no calibration over a 16-year (1992–2007) period. For the seven watersheds and the entire study period, the overall Nash-Sutcliffe Efficiency (NSE), Kling-Gupta efficiency (KGE), and deviation of runoff volume (DV) between observed and simulated daily streamflow ranged 0.58–0.71, 0.67–0.81, and −4% to 9%, respectively. Good agreement between predicted and observed suspended sediment yield was achieved through the calibration of a single channel critical shear stress parameter. For sediment yield, NSE, KGE, and DV ranged 0.62–0.97, 0.43–0.97, and −2% to 2%, respectively, for the calibration period, and 0.61–0.93, 0.42–0.95, and −24% to 13%, respectively, for the period of model performance assessment. Regression analysis of observed- and WEPP-simulated increase in water and sediment yield following clear-cut treatment was similar; however, the WEPP-simulated increase was lower compared to observations particularly from the partial-cut watershed. The variability in the critical shear parameter for different stream channels in the study watersheds was directly related to the observed mean particle size on the stream bed and suggests that applications of the WEPP model in ungauged basins could potentially set the critical shear parameter based on particle size. Overall, the simulated results demonstrate the potential of WEPP as a modeling tool for forestland watershed management, particularly for estimating the effects of forest harvest on hydrograph fluctuations and consequently, stream sediment transport.
Previously, high resolution MRI to assess bone structure of deep-seated regions of the skeleton such as the proximal femur was substantially limited by signal-to-noise ratio (SNR). With the advent of ...new optimized pulse sequences in MRI at 1.5 T and 3 T, it may now be possible to depict and quantify the trabecular microarchitecture in the proximal femur. The purpose of this study was to investigate the feasibility of assessing trabecular microstructure of the human proximal femur in vivo with MR imaging at 1.5 T and 3 T. MR images of six young, healthy male and female subjects were acquired using standard clinical 1.5-T and high-field 3-T whole-body MR scanners. Using a T2/T1-weighted 3D FIESTA sequence (and a 3D FIESTA-C sequence at 3 T to avoid susceptibility artifacts) a resolution of 0.234 x 0.234 x 1.5 mm(3) was achieved in vivo. Structural parameters analogous to standard bone histomorphometry were determined in femoral head and trochanter regions of interest. Bone mineral density (BMD) measurements were also obtained using dual-energy X-ray absorptiometry (DXA) for the femoral trochanter in the same subjects. The bone structure of the proximal femur is substantially better depicted at 3 T than at 1.5 T. Correlation between the structural parameters obtained at both field strengths was up to R =0.86 for both the femoral head and the trochanteric region. However, the resolution of the images limits the application of 3D structural analysis, making the assessment more akin to 2D textural measures, which may be correlated to histomorphometric but are not identical measures. This feasibility study establishes the potential of MRI as a means of imaging proximal femur structure, and improvements in technique and resolution enhancements are warranted.
Radiation is the main energy source for snowpack warming and melt in mountain needleleaf forests, and runoff from these forests is the main contributor to spring river flows in western North America. ...Utilizing extensive field observations, the effect of needleleaf forest cover on radiation and snowmelt timing was quantified at pine and spruce forest sites and nearby clearings of varying slope and aspect in an eastern Canadian Rocky Mountain headwater basin. Compared with open clearing sites, shortwave radiation was much reduced under forest cover, resulting in smaller differences in melt timing between forested slopes relative to open slopes with different aspects. In contrast, longwave radiation to snow was substantially enhanced under forest cover, especially at the dense spruce forest sites where longwave radiation dominated total energy for snowmelt. In both pine and spruce environments, forest cover acted to substantially reduce total radiation to snow and delay snowmelt timing on south-facing slopes while increasing total radiation and advancing snowmelt timing on north-facing slopes. Results strongly suggest that impacts on radiation to snow and snowmelt timing from changes in mountain forest cover will depend much on the slope and aspect at which changes occur.
In complex terrain, drifting snow contributes to ecohydrologic landscape heterogeneity and ecological refugia. In this study, we assessed the climate sensitivity of hydrological dynamics in a ...semiarid mountainous catchment in the snow‐to‐rain transition zone. This catchment includes a distinct snow drift‐subsidized refugium that comprises a small portion (14.5%) of the watershed but accounts for a disproportionate amount (modeled average 56%) of hydrological flux generation. We conducted climate sensitivity experiments using a physically based hydrologic model to assess responses of a suite of hydrologic metrics across the watershed. Experiments with an imposed 3.5 °C warming showed reductions in average maximum snow water equivalent of 58–68% and deep percolation by 72%. While relative decreases were similar across the watershed, much greater absolute decreases in snowpack occurred in the drift‐subsidized site than the surrounding landscape. In drift‐subsidized locations, warming caused a shift from a regime that included both energy‐ and water‐limited evapotranspiration conditions to exclusively water‐limited conditions. Warming also resulted in altered interannual variability of hydrologic metrics. The drift‐subsidized unit was more sensitive to warming than the surrounding landscape, with reduced potential for the effects of warming to be offset by increased precipitation. Despite spatially homogeneous changes in climate, the effects of climate change on the hydrological dynamics were spatially heterogeneous in this watershed due to the presence of lateral water transport in the form of drifting snow. These findings suggest an increase in hydrologic homogeneity across the landscape and relatively large changes in snow drift‐subsidized refugia.
Key Points
Snow drift‐subsidized hydrology‐based refugia diminished with simulated warming, creating greater hydrologic homogeneity
Drift‐subsidized sites historically varied between water and energy limitation, but became exclusively water‐limited in warmer scenarios
Modeled warming decreased interannual variability of peak SWE and increased variability of evapotranspiration and peak SWE timing
Immersion‐mode ice‐nucleating particle (INP) concentrations from an off‐road diesel engine were measured using a continuous‐flow diffusion chamber at −30°C. Both petrodiesel and biodiesel were ...utilized, and the exhaust was aged up to 1.5 photochemically equivalent days using an oxidative flow reactor. We found that aged and unaged diesel exhaust of both fuels is not likely to contribute to atmospheric INP concentrations at mixed‐phase cloud conditions. To explore this further, a new limit‐of‐detection parameterization for ice nucleation on diesel exhaust was developed. Using a global‐chemical transport model, potential black carbon INP (INPBC) concentrations were determined using a current literature INPBC parameterization and the limit‐of‐detection parameterization. Model outputs indicate that the current literature parameterization likely overemphasizes INPBC concentrations, especially in the Northern Hemisphere. These results highlight the need to integrate new INPBC parameterizations into global climate models as generalized INPBC parameterizations are not valid for diesel exhaust.
Key Points
Diesel and biodiesel exhaust do not produce significant ice‐nucleating particle concentrations
Photochemical aging does not increase ice‐nucleating particle concentrations in diesel exhaust
Current parameterizations may overemphasize black carbon ice‐nucleating particle concentrations globally
► A previous 10-year water balance of a mountainous catchment was extended to 24
years. ► Precipitation timing and soil water deficit explained annual streamflow variability. ► A conceptual model was ...developed from field data, simulations and previous studies. ► Merits of long-term catchment-scale research were demonstrated.
Long-term water balance investigations are needed to better understand hydrologic systems, especially semi-arid mountainous catchments. These systems exhibit considerable interannual variability in precipitation as well as spatial variation in snow accumulation, soils, and vegetation. This study extended a previous 10-year water balance based on measurements and model simulations to 24
years for the Upper Sheep Creek (USC) catchment, a 26
ha, snow-fed, semi-arid rangeland headwater drainage within the Reynolds Creek Experimental Watershed in southwestern Idaho, USA. Additional analyses afforded by the additional years of data demonstrated that the variability between streamflow and annual precipitation (
r
2
=
0.54) could be explained by the timing of precipitation and antecedent moisture conditions. Winter–spring precipitation and soil moisture deficit at the beginning of the water year accounted for 83% of the variability in streamflow, which was almost as accurate as applying the more complex physically-based Simultaneous Heat and Water (SHAW) numerical model (
r
2
=
0.85) over the three dominant land cover classes. A conceptual model was formulated based on field observations, numerical simulations and previous studies. Winter precipitation and spring snowmelt must first replenish the deficit within the soil water profile and ground water system before water is delivered to the stream. During this period, surface water and ground water are tightly coupled and their interaction is critical to streamflow generation. Shortly after snow ablation, however, water flux in the root zone becomes decoupled from the ground water system and subsequent precipitation does little to contribute to streamflow for the current year, but serves to offset ET and the soil moisture deficit at the beginning of the following year. This study demonstrates the merits of long-term catchment-scale research to improve our understanding of how climate and land cover interact to control hydrologic dynamics in complex mountainous terrain.
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