Danny Marks

Climate and energy exchange at the snow surface in the Alpine Region of the Sierra Nevada: 2. Snow cover energy balance

A detailed evaluation of surface climate and energy exchange at the snow surface in a small alpine watershed, typical of much of the southern Sierra Nevada, is presented for the 1986 water year. Measurements of snowfall, meteorological and snow cover conditions, and snow cover ablation, described in part 1 of this paper (Marks et al., this issue), are used to characterize the climate. Each form of energy transfer, radiation, sensible and latent heat flux, soil heat flux, and heat flux by mass advection, is evaluated separately to determine how its magnitude changes during the snow season. These are then combined to approximate a snow cover energy balance and determine the relative importance of each form of energy transfer in the seasonal energy and mass balance of the snow cover. Radiation and sensible and latent heat flux dominate the snow cover energy balance throughout the snow season. During snowmelt, radiation accounts for between 66 and 90% of the energy available for melt. Sensible and latent heat transfer during this time are of approximately equal magnitude but are usually of opposite sign and therefore cancel. Calculated sublimation during the entire snow season accounted for the loss of about 20% (approximately 50 cm snow water equivalent) of the mass of the snow cover. This experiment shows that energy and mass transfer can be adequately monitored at a remote site using a combination of measured and modeled parameters and that the energy balance of the snow cover in the alpine zone of the Sierra Nevada is dominated by net radiation during snowmelt.

A comparison of geostatistical procedures for spatial analysis of precipitation in mountainous terrain

Abstract Spatially distributed measurements or estimates of precipitation over a region are required for modelling of hydrologic processes and soil moisture for agricultural and natural resource management. Simple interpolation methods fail to consider the effects of topography on precipitation and may be in considerable error in mountainous regions. The performance of three geostatistical methods for making mean annual precipitation estimates on a regular grid of points in mountainous terrain was evaluated. The methods were: (1) kriging; (2) kriging elevation-detrended data; and (3) cokriging with elevation as an auxiliary variable. The study area was the Willamette River basin, a 2.9 million hectare region spanning the area between the Coast Range and the Cascade Range in western Oregon. Compared with kriging, detrended kriging and cokriging both exhibited better precision (as indicated by estimation coefficients of variation of 16 and 17% vs. 21%; and average absolute errors of 19 and 20 cm vs. 26 cm) and accuracy (as indicated by average errors of −1.4 and −2.0 cm vs. −5.2 cm) in the estimation of mean annual precipitation. Contour diagrams for kriging and detrended kriging exhibited smooth zonation following general elevation trends, while cokriging showed a patchier pattern more closely corresponding to local topographic features. Detrended kriging and cokriging offer improved spatially distributed precipitation estimates in mountainous terrain on the scale of a few million hectares. Application of these methods for a larger region, the Columbia River drainage in the USA (57 million hectares), was unsuccessful due to the lack of a consistent precipitation-elevation relationship at this scale. Precipitation estimation incorporating the effects of topography at larger scales will require either piecewise estimation using the methods described here or development of a physically based orographic model.

The sensitivity of snowmelt processes to climate conditions and forest cover during rain-on-snow: a case study of the 1996 Pacific Northwest flood

A warm, very wet Pacific storm caused significant flooding in the Pacific Northwest during February 1996. Rapid melting of the mountain snow cover contributed to this flooding. An energy balance snowmelt model is used to simulate snowmelt processes during this event in the Central Cascade Mountains of Oregon. Data from paired open and forested experimental sites at locations at and just below the Pacific Crest were used to drive the model. The event was preceded by cold, stormy conditions that developed a significant snow cover down to elevations as low as 500 m in the Oregon Cascades. At the start of the storm, the depth of the snow cover at the high site (1142 m) was 1.97 m with a snow water equivalent (SWE) of 425 mm, while at the mid-site (968 m) the snow cover was 1.14 m with a SWE of 264 mm. During the 5‐6 day period of the storm the open high site received 349 mm of rain, lost 291 mm of SWE and generated 640 mm of runoA, leaving only 0.22 m of snow on the ground. The mid-site received 410 mm of rain, lost 264 mm of SWE to melt and generated 674 mm of runoA, completely depleting the snow cover. Simulations at adjacent forested sites showed significantly less snowmelt during the event. The snow cover under the mature forest at the high site lost only 44 mm of SWE during the event, generating 396 mm of runoA and leaving 0.69 m of snow. The model accurately simulated both snow cover depth and SWE during the development of the snow cover prior to the storm, and the depletion of the snow cover during the event. This analysis shows that because of the high temperature, humidity and relatively high winds in the open sites during the storm, 60‐90% of the energy for snowmelt came from sensible and latent heat exchanges. Because the antecedent conditions extended the snow cover to very low elevations in the basin, snowmelt generated by condensation during the event made a significant contribution to the flood. Lower wind speeds beneath the forest canopy during the storm reduced the magnitude of the turbulent exchanges at the snow surface, so the contribution of snowmelt to the runoA from forested areas was significantly less. This experiment shows the sensitivity of snowmelt processes to both climate and land cover, and illustrates how the forest canopy is coupled to the hydrological cycle in mountainous areas. #1998 John Wiley & Sons, Ltd.

A spatially distributed energy balance snowmelt model for application in mountain basins

Snowmelt is the principal source for soil moisture, ground-water re-charge, and stream-flow in mountainous regions of the western US, Canada, and other similar regions of the world. Information on the timing, magnitude, and contributing area of melt under variable or changing climate conditions is required for successful water and resource management. A coupled energy and mass-balance model ISNOBAL is used to simulate the development and melting of the seasonal snowcover in several mountain basins in California, Idaho, and Utah. Simulations are done over basins varying from 1 to 2500 km 2 , with simulation periods varying from a few days for the smallest basin, Emerald Lake watershed in California, to multiple snow seasons for the Park City area in Utah. The model is driven by topographically corrected estimates of radiation, temperature, humidity, wind, and precipitation. Simulation results in all basins closely match independently measured snow water equivalent, snow depth, or runoA during both the development and depletion of the snowcover. Spatially distributed estimates of snow deposition and melt allow us to better understand the interaction between topographic structure, climate, and moisture availability in mountain basins of the western US. Application of topographically distributed models such as this will lead to improved water resource and watershed management. Copyright # 1999 John Wiley & Sons, Ltd.

A global perspective of regional vegetation and hydrologic sensitivities from climatic change

. A biogeographic model, MAPSS (Mapped Atmosphere-Plant-Soil System), predicts changes in vegetation leaf area index (LAI), site water balance and runoff, as well as changes in biome boundaries. Potential scenarios of global and regional equilibrium changes in LAI and terrestrial water balance under 2 x CO2 climate from five different general circulation models (GCMs) are presented. Regional patterns of vegetation change and annual runoff are surprisingly consistent among the five GCM scenarios, given the general lack of consistency in predicted changes in regional precipitation patterns. Two factors contribute to the consistency among the GCMs of the regional ecological impacts of climatic change: (1) regional, temperature-induced increases in potential evapo-transpiration (PET) tend to more than offset regional increases in precipitation; and (2) the interplay between the general circulation and the continental margins and mountain ranges produces a fairly stable pattern of regionally specific sensitivity to climatic change. Two areas exhibiting among the greatest sensitivity to drought-induced forest decline are eastern North America and eastern Europe to western Russia. Regional runoff patterns exhibit much greater spatial variation in the sign of the response than do the LAI changes, even though they are deterministically linked in the model. Uncertainties with respect to PET or vegetation water use efficiency calculations can alter the simulated sign of regional responses, but the relative responses of adjacent regions appear to be largely a function of the background climate, rather than the vagaries of the GCMs, and are intrinsic to the landscape. Thus, spatial uncertainty maps can be drawn even under the current generation of GCMs.

A Sensitivity Study of Daytime Net Radiation during Snowmelt to Forest Canopy and Atmospheric Conditions

This study investigates the dependence of net radiation at snow surfaces under forest canopies on the overlying canopy density. The daily sum of positive values of net radiation is used as an index of the snowmelt rate. Canopy cover is represented in terms of shortwave transmissivity and sky-view factor. The cases studied are a spruce forest in the Wolf Creek basin, Yukon Territory, Canada, and a pine forest near Fraser, Colorado. Of particular interest are the atmospheric conditions that favor an offset between shortwave energy attenuation and longwave irradiance enhancement by the canopy, such that net radiation does not decrease with increasing forest density. Such an offset is favored in dry climates and at high altitudes, where atmospheric emissivities are low, and in early spring when snow albedos are high and solar elevations are low. For low snow albedos, a steady decrease in snowmelt energy with increasing canopy cover is found, up to a forest density close to the actual densities of mature spruce forests. Snowmelt rates for high albedos are either insensitive or increase with increasing canopy cover. At both sites, foliage area indices close to 2 are associated with a minimum in net radiation, independent of snow albedo or cloud cover. However, these results are more uncertain for open forests because solar heating of trees may invalidate the longwave assumptions, increasing the longwave irradiance.

Long-term snow database, Reynolds Creek Experimental Watershed, Idaho, United States

An extensive precipitation database has been developed over the past 35 years with the first records starting in January 1962 and going through September 1996 from the Reynolds Creek Experimental Watershed located near the north end of the Owyhee Mountains in southwest Idaho. Precipitation ranges from 236 mm on the lowest elevations at the north end of the watershed to 1123 mm at the southwest corner of the watershed. There are continuous 35 year records available for 12 sites, 20–32 year records available for 8 sites, 10–19 year records available for 25 sites, and 4–9 year records for 8 sites for a total of 53 sites. All of these data have been stored as breakpoint and hourly records in the U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center database. These breakpoint and hourly data are available from the anonymous ftp site: ftp.nwrc.ars.usda.gov.

SNOW MAPPING AND CLASSIFICATION FROM LANDSAT THEMATIC MAPPER DATA

Use of satellite multi-spectral remote-sensing data to map snow and estimate snow characteristics over remote and inaccessible areas requires that we distinguish snow from other surface cover and from clouds, and compensate for the effects of the atmosphere and rugged terrain. Because our space- borne radiometers typically measure reflectance in a few wavelength bands, for climate modeling we must use inferences of snow grain-size and contaminant amount to estimate snow albedo throughout the solar spectrum. Although digital elevation data may be used to simulate typical conditions for a satellite image, precise registration of an elevation data set with satellite data is usually impossible. Instead, an atmospheric model simulates combinations of Thematic Mapper (TM) band radiances for snow of various grain-sizes and contaminant amounts. These can be recognized in TM images and snow can automatically be distinguished from other surfaces and classified into clean new snow, older metamorphosed snow, or snow mixed with vegetation.

Point simulation of seasonal snow cover dynamics beneath boreal forest canopies

The accurate simulation of snowpack deposition and ablation beneath forested areas is complicated by the fact that the vegetation canopy strongly affects the snow surface energy balance. Data collected as part of the Boreal Ecosystem-Atmosphere Study are used to derive a series of simple canopy adjustments and drive a two-layer coupled energy- and mass-balance snowmelt model to simulate the deposition and ablation of the seasonal snowpack at six sites within the boreal forest for the 1994-1995 snow season. Snow cover energy gain in the spring is strongly controlled by canopy cover and is dominated by net radiation fluxes which contribute from 66% to 92% of the snow cover energy balance. Turbulent fluxes comprise 11% of the net energy balance on average, with minor contributions from soil and advected energy fluxes. Simulated depths at the forested sites generally show good agreement with measured snow depths, indicated by model efficiencies ranging from 0.90 to 0.94, with root-mean-square differences less than 5 cm. Seasonal snow covers in the boreal environment may be more sensitive to land use transitions, rather than climate shifts, due to the strong control exerted by vegetation canopies on radiation transfer processes.

Comparison of Snow Deposition, the Snow Cover Energy Balance, and Snowmelt at Two Sites in a Semiarid Mountain Basin

Abstract Significant differences in snow deposition, development of the seasonal snow cover, and the timing of melt can occur over small spatial distances because of differences in topographically controlled wind exposure and canopy cover. To capture important intrabasin hydrological processes related to heterogeneous snow cover and energy inputs, models must explicitly account for these differences. The “SNOBAL” point snow cover energy and mass balance model is used to evaluate differences in snow cover energy and mass balance at two sites in a small headwater drainage of the Reynolds Creek Experimental Watershed (RCEW) in the Owyhee Mountains of southwestern Idaho. Though these sites are separated by only 350 m, they are located in distinctly different snow cover regimes. The “ridge” site (elevation 2097 m) is located on a broad shelf on the southern ridge of RCEW, and the “grove” site (elevation 2061 m) is sheltered by topography and forest canopy in a grove of aspen and fir trees just in the lee of th...