K. R. Cooley

The Influence of the Spatial Distribution of Snow on Basin-Averaged Snowmelt

Spatial variability in snow accumulation and melt owing to topographic effects on solar radiation, snow drifting, air temperature and precipitation is important in determining the timing of snowmelt releases. Precipitation and temperature effects related to topography affect snowpack variability at large scales and are generally included in models of hydrology in mountainous terrain. The effects of spatial variability in drifting and solar input are generally included only in distributed models at small scales. Previous research has demonstrated that snowpack patterns are not well reproduced when topography and drifting are ignored, implying that larger scale representations that ignore drifting could be in error. Detailed measurements of the spatial distribution of snow water equivalence within a small, intensively studied, 26-ha watershed were used to validate a spatially distributed snowmelt model. These observations and model output were then compared to basin-averaged snowmelt rates from a single-point representation of the basin, a two-region representation that captures some of the variability in drifting and aspect and a model with distributed terrain but uniform drift. The model comparisons demonstrate that the lumped, single-point representation and distributed terrain with uniform drift both yielded poor simulations of the basin-averaged surface water input rate. The two-point representation was a slight improvement, but the late season melt required for the observed stream-flow was not simulated because the deepest drifts were not represented. These results imply that representing the effects of subgrid variability of snow drifting is equally or more important than representing subgrid variability in solar radiation.

Sub-grid parameterization of snow distribution for an energy and mass balance snow cover model

Representation of sub-element scale variability in snow accumulation and ablation is increasingly recognized as important in distributed hydrologic modelling, Representing sub-grid scale variability may be accomplished through numerical integration of a nested grid or through a lumped modelling approach. We present a physically based model of the lumped snowpack mass and energy balance applied to a 26-ha rangeland catchment with high spatial variability in snow accumulation and melt. Model state variables are snow-covered area average snow energy content (U), the basin-average snow water equivalence (W a ), and snow-covered area fraction (A f ). The energy state variable is evolved through an energy balance. The snow water equivalence state variable is evolved through a mass balance, and the area state variable is updated according to an empirically derived relationship, A f (W a ), that is similar in nature to depletion curves used in existing empirical basin snowmelt models. As snow accumulates, the snow covered area increases rapidly. As the snowpack ablates, A f decreases as W a decreases. This paper shows how the relationship A f (W a ) for the melt season can be estimated from the distribution of snow water equivalence at peak accumulation in the area being modelled. We show that the depletion curve estimated from the snow distribution of peak accumulation at the Upper Sheep Creek sub-basin of Reynolds Creek Experimental Watershed compares well against the observed depletion data as well as modelled depletion data from an explicit spatially distributed energy balance model. Comparisons of basin average snow water equivalence between the lumped model and spatially distributed model show good agreement, Comparisons to observed snow water equivalence show poorer but still reasonable agreement. The sub-grid parameterization is easily portable to other physically based point snowmelt models. It has potential application for use in hydrologic and climate models covering large areas with large model elements, where a computationally inexpensive parameterization of sub-grid snow processes may be important.

A ten-year water balance of a mountainous semi-arid watershed

Quantifying water balance components, which is particularly challenging in snow-fed, semi-arid regions, is crucial to understanding the basic hydrology of a watershed. In this study, a water balance was computed using 10 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 southwest Idaho, USA. The approach computed a partial water balance for each of three landscape units and then computed an aggregated water balance for the watershed. Runoff and change in ground water storage were not distinguishable between landscape units. Precipitation, which occurs predominantly as snow, was measured within each landscape unit directly and adjusted for drifting. Spatial variability of effective precipitation was shown to be greater during years with higher precipitation. Evapotranspiration, which accounted for nearly 90% of the effective precipitation, was estimated using the Simultaneous Heat and Water (SHAW) Model and validated with measurements from Bowen ratio instruments. Runoff from the watershed was correlated to precipitation above a critical threshold of approximately 450 mm of precipitation necessary to generate runoffOr 2 a 0:52U: The average water balance error was 46 mm, or approximately 10% of the estimated effective precipitation for the ten-year period. The error was largely attributed to deep percolation losses through fractures in the basalt underlying the watershed. Simulated percolation of the water beyond the root zone correlated extremely well with measured runoffOr 2 a 0:90U; which is derived almost entirely from subsurface flow. Above a threshold of 50 mm, approximately 67% of the water percolating beyond the root zone produces runoff. The remainder was assumed to be lost to deep percolation through the basalt. This can have important ramifications in addressing subsurface flow and losses when applying a snowmelt runoff model to simulate runoff and hydrologic processes in the watershed. q 2000 Published by Elsevier Science B.V.

Groundwater response to snowmelt in a mountainous watershed

Abstract Snowmelt recharge to shallow groundwater systems is the primary source of streamflow in many mountainous watersheds, but characteristics of these systems are not well understood, and their contribution to streamflow is often not appreciated. Data from a detailed study on the Upper Sheep Creek Watershed located within the Reynolds Creek Experimental Watershed in southwestern Idaho were analyzed to characterize the interactions between snowmelt, groundwater and streamflow. Response time between snowmelt, groundwater levels and streamflow was drastically different from year to year depending on the extent of the snowpack. Response time to snowmelt for piezometer and weirs located 135 m downslope from am isolated drift was 3–5 days during an average snow year and up to 70 days for a year with snow accumulation that was 40% of normal. The primary aquifer is believed to be unconfined during low snowmelt years and confined when normal or above-normal snowmelt causes high groundwater levels. Snowmelt from an isolated drift enters the primary aquifer upslope of the confining layer. Rapid response during years with normal snow accumulation is therefore primarily a pressure pulse through the confined aquifer. Recharge during years of low snow accumulation is insufficient to fill the primary aquifer to the confining layer,and response time is indicative of travel time through the aquifer.

Impacts of spatially and temporally varying snowmelt on subsurface flow in a mountainous watershed: 2. Subsurface processes

Abstract The impacts of spatial and temporal variations of snowmelt recharge on subsurface flow in a small mountainous watershed were investigated using field measurements and numerical simulations. A two-dimensional, variably saturated flow model (VAM2D) was used to characterize the hillslope aquifer and to delineate subsurface flow mechanisms. Spatially varying snowmelt along a hillslope transect described in the preceding paper were used as input for the subsurface flow analyses. Simulations indicated that the heterogeneous hillslope aquifer provides hydrogeological conditions for confined and unconfined groundwater flow depending on the extent of snowmelt recharge. The spatial and temporal distribution and amount of snowmelt recharge play important roles in determining when flow is governed by confined and unconfined flow. Results showed that the VAM2D model was able to simulate piezometric measurements reasonably using calibrated hydraulic parameters. Sensitivity analyses showed that the flow regime ...

A uniform versus an aggregated water balance of a semi-arid watershed

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 units and 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 predominantly as 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 the 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 aggregated and uniform approach for both years was the estimated ET. This was attributed to the inability of the uniform approach to associate areas of the watershed having more vegetation and leaf area with areas having soil water available for transpiration. Differences in ET estimates for the two approaches were least during the second year when water was less limiting and potential ET was less. This suggests that ET can be aggregated more easily when water is not limiting using the average leaf area index independently of the spatial variability in leaf area.

Precipitation Erosivity Index Estimates in Cold Climates

ABSTRACT VARIATIONS in precipitation in mountainous regions increase the difficulty of estimating precipitation based parameters. Estimates of the USLE erosivity index (EI) are further complicated by snowfall in these and other cold climate areas. This report describes some of the variations in EI relationships observed at the Reynolds Creek Experimental Watershed in southwestern Idaho. Snowfall accounted for up to 71% of EI based on annual precipitation. Therefore, only summer EI values are considered valid for establishing the erosivity index. Summer EI values increased 28 to 59% when all storms were considered rather than only storms over 13 mm in volume..

Predicting Snowmelt Runoff on Sagebrush Rangeland Using a Calibrated Spur Hydrology Model

ABSTRACT This study evaluates the ability of the SPUR (Simulation of Production and Utilization of Rangelands) model to predict monthly runoff on sagebrush watersheds where runoff is generated by spring snowmelt. Four watersheds in southwest Idaho were used to evaluate the model. Predicted monthly runoff was compared to actual runoff using a different part of the runoff record than what was used to calibrate the model. Results indicate that the calibrated SPUR hydrology model adequately simulates the volume and timing of monthly runoff for watersheds that have a relatively uniform snow cover. Transferability of the model for uncalibrated watersheds with uniform snow cover was also demonstrated. For most rangeland watersheds where snowmelt runoff is important, however, the spatial variability of snow cover is high; runoff is supplied mainly by one or more isolated drifts. The model works poorly under these kinds of conditions.

Groundwater response to snowmelt in a mountainous watershed: testing of a conceptual model

Abstract Snowmelt recharge to shallow (less than 25 m) groundwater systems is the primary source of streamflow in many mountainous watersheds, but characteristics of these systems are not well understood. Response time between snowmelt, piezometers and streamflow in the Upper Sheep Creek Watershed within the Reynolds Creek Experimental Watershed differs drastically from year to year depending on the extent of recharge from snowmelt. This is believed to be caused by groundwater flow becoming confined during years with normal or above normal snowmelt recharge. A two-dimensional, variably saturated, groundwater model was applied to 3 years of data to validate the computer model and to test the conceptual model of the basin. Groundwater response measured in piezometers was simulated quite accurately when a confining layer (observed in drilling logs) was included, but not when the confining layer was omitted. Simulation results give credence to the variably saturated groundwater model, support the conceptual model of the basin, and improve our understanding of the shallow groundwater system in this mountainous watershed.

Evaluating climate change impacts in snowmelt basins

The implications of global climate change for hydrology and water resources are likely to be complex, widespread, and significant for both natural ecosystems and society. Yet our understanding of these implications remains rudimentary despite considerable effort and research over the last decade. One of the most difficult hydrologic problems in this area is evaluating the impacts of climate change in hydrologic basins affected by snowfall and snowmelt, especially high-latitude and high-altitude watersheds. Many of these watersheds are the headwaters for major rivers and they often provide substantial amounts of water for human and ecosystem use. Evaluating the impacts of climate change in these basins will help us better understand how to improve the management and protection of our water resources systems. In April 1993, a roundtable workshop was held in Santa Fe, N. Mex., to discuss hydrologic models for evaluating the impacts of climate change in snowmelt basins.