Hydrology and Earth System Sciences Discussions, 2010
Understanding the role of ecosystems in modulating energy, water and carbon fluxes is critical to... more 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 HESSD
The timing, magnitude, and spatial distribution of snow cover and the resulting surface water inp... more The timing, magnitude, and spatial distribution of snow cover and the resulting surface water inputs (SWI) are simulated at a small catchment located in the rain-snow transition zone of southwest Idaho, USA. A physically based snow model is run on this 1.5 ha study catchment, which has an elevation range of 1600-1645 masl. The catchment is divided into relatively steep (mean slope angle of 21 degrees) northeast and southwest facing hill slopes by an ephemeral stream that drains to the southeast. SWI are fundamental controls on soil moisture, streamflow generation, groundwater recharge, and nutrient cycling. Although the timing of melt events is similar across the basin, southwest facing slopes receive smaller magnitude and more frequent SWI from mid winter snow melt, while the northeast facing slope receives greater SWI during the spring. Three spatial patterns are observed in the modeled SWI time series: (1) equal between slopes, (2) majority of SWI on southwest facing slopes, and (3) majority of SWI on northeast facing slopes. Although any of these three spatial patterns can occur during the snow season, four emergent SWI patterns emerge through the melt season: (1) near uniform, (2) controlled by topographic differences in energy fluxes, (3) transitional, and (4) controlled by snow distribution. Rain on snow (ROS) events produce similar SWI between the northeast and southwest facing slopes, with the difference being attributed primarily to snow distribution. Turbulent fluxes dominate the snowpack energetics in four of the five rain on snow events, and advective fluxes from precipitation are greater than 17% during the 2 rain on snow events in December and January. Net radiation fluxes dominate spring melt events. Variations in the method used to distribute precipitation may result in large differences in total precipitation to the basin.
Electronic sensors generate valuable streams of forcing and validation data for hydrologic models... more Electronic sensors generate valuable streams of forcing and validation data for hydrologic models but are often subject to noise which must be removed as part of model input and testing database development. We developed an automated precipitation correction program (APCP) for weighing bucket precipitation gauge records, which are subject to several types of mechanical and electronic noise and discontinuities, including gauge maintenance, missing data, wind vibration, and sensor drift. Corrected cumulative water year precipitation from APCP did not exhibit an error bias and matched measured water year total precipitation within 2.1% for 58 station years tested. Removal of low‐amplitude periodic noise was especially important for developing accurate instantaneous precipitation records at subdaily time steps. Model flexibility for use with other data types is demonstrated through application to time domain reflectometry soil moisture content data, which are also frequently subject to ...
The spatial variation of melt energy can influence snow cover depletion rates and in turn be infl... more The spatial variation of melt energy can influence snow cover depletion rates and in turn be influenced by the spatial variability of shortwave irradiance to snow. The spatial variability of shortwave irradiance during melt under uniform and discontinuous evergreen canopies at a U.S. Rocky Mountains site was measured, analyzed, and then compared to observations from mountain and boreal forests in Canada. All observations used arrays of pyranometers randomly spaced under evergreen canopies of varying structure and latitude. The spatial variability of irradiance for both overcast and clear conditions declined dramatically, as the sample averaging interval increased from minutes to 1 day. At daily averaging intervals, there was little influence of cloudiness on the variability of subcanopy irradiance; instead, it was dominated by stand structure. The spatial variability of irradiance on daily intervals was higher for the discontinuous canopies, but it did not scale reliably with canopy...
... Processes, 13:19351959. , Link, T., A. Winstral, and D. Garen, 2001: Simulating snowmelt p... more ... Processes, 13:19351959. , Link, T., A. Winstral, and D. Garen, 2001: Simulating snowmelt processes during rain-on-snow over a semi-arid mountain basin. Ann. Glaciol., in press. Marshall, SE and SG Warren, 1987: Parameterization of snow albedo for climate models. ...
Energy budget dynamics under forest canopies are strongly influenced by the large spatial variabi... more Energy budget dynamics under forest canopies are strongly influenced by the large spatial variability of radiative and turbulent transfers in this environment. Incoming solar radiation under canopies has a particularly high degree of spatial variability. Transmission of solar radiation through a forest canopy varies with the size and location of the canopy gaps, as well as canopy leaf area. Modeling this transmission has proven challenging owing to the highly variable nature of the gaps within and between tree crowns, particularly in discontinuous canopies. This study describes and simulates the solar variability incident on the snow surface beneath two conifer forests. Objectives of this work are (1) to evaluate the variability of incoming solar radiation data with respect to canopy structure and cloudiness, (2) to correlate measured solar radiation transmission with predicted solar transmission based on analysis of hemispherical photographs, and (3) to examine the impact of measured and predicted transmission factors on the seasonal net radiative exchanges and snow ablation, based on snow process modeling. Observations were made during the winters of 2002 and 2003 in two predominately lodgepole pine (Pinus contorta) stands, one discontinuous and one relatively uniform, at the Local Scale Observation Site (LSOS) in Fraser, CO, USA, as part of the Cold Land Processes Experiment (CLPX). The canopy structure of all trees in the 0.8 ha triangular plot was measured and mapped in detail. We measured incoming global solar radiation at the snow surface beneath the discontinuous and the uniform canopies using arrays of 10 upward looking pyranometers at each site. Incoming global solar radiation was also measured above the canopy. Hemispherical photographs were taken with a Nikon Coolpix 995 with a fisheye converter at each radiometer location in both canopies, and were analyzed with Gap Light Analyzer (GLA) software. We found good agreement between measured and GLA-predicted transmissivities (r 2 = 0.86) when all data from both years were considered. Transmission factors derived from hemispheric photos and GLA software can be used to specify the distribution of solar flux under a canopy, instead of direct solar flux measurements, without degradation in snow model melt predictions.
Precipitation variability and complex topography often create a mosaic of vegetation communities ... more Precipitation variability and complex topography often create a mosaic of vegetation communities in mountainous headwater catchments, creating a challenge for measuring and interpreting energy and mass fluxes. Understanding the role of these communities in modulating energy, water and carbon fluxes is critical to quantifying the variability in energy, carbon, and water balances across landscapes. The focus of this paper was: (1) to demonstrate the utility of eddy covariance (EC) systems in estimating the evapotranspiration component of the water balance of complex headwater mountain catchments; and (2) to compare and contrast the seasonal surface energy and carbon fluxes across a headwater catchment characterized by large variability in precipitation and vegetation cover. Eddy covariance systems were used to measure surface fluxes over sagebrush (Artemesia arbuscula and Artemesia tridentada vaseyana), aspen (Populus tremuloides) and the understory of grasses and forbs beneath the aspen canopy. Peak leaf area index of the sagebrush, aspen, and aspen 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. Missing periods of measured data were common and made it necessary to extrapolate measured fluxes to the missing periods using a combination of measured and simulated data. Estimated cumulative evapotranspiratation from the sagebrush, aspen trees, and aspen understory were 384 mm, 314 mm and 185 mm. A water balance of the catchment indicated that of the 699 mm of areal average precipitation, 421 mm was lost to evapotranspiration, and 254 mm of streamflow was measured from the catchment;
Estimates of bedrock infiltration from mountain catchments in the western U.S. are essential to w... more Estimates of bedrock infiltration from mountain catchments in the western U.S. are essential to water resource managers because they provide an estimate of mountain block recharge to regional aquifers. On smaller scales, bedrock infiltration is an important term in water mass balance studies, which attempt to estimate hydrologic states and fluxes in watersheds with fractured or transmissive bedrock. We estimate the a daily time series of bedrock infiltration in a small catchment in the rain snow transition zone in southwest Idaho, using the difference between measured stream discharge and modeled soil drainage. The accuracy of spatial patterns in soil water storage are optimized, rather than the more common approach of minimizing error in integrated quantities such as streamflow. Bedrock infiltration is estimated to be 289 mm ± 50 mm for the 2011 water year, which is 34% ± 12% of the precipitation (95% confidence). Soils on the southwest facing slope drain more often throughout the snow season, but the northeast facing slope contributes more total soil drainage for the water year. Peaks in catchment soil drainage and bedrock infiltration coincide with rain on snow events.
A detailed evaluation of climate conditions in a small alpine watershed, typical of much of the s... more A detailed evaluation of climate conditions 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 are used to characterize the climate at four locations in the watershed during that snow season. Data from these locations are then combined into two representative sites for the watershed. Measurement approaches and methodologies and the effectiveness of instrumentation used in the study are discussed, and an estimate of the uncertainty of the monitored meteorological parameters is made. The data are integrated into a continuous hourly time series of solar and thermal radiation, air, snow and soil temperature, humidity, and wind at the two representative sites in this remote alpine watershed for an entire snow season. Snow deposition and snow cover depth and density are measured manually at regular intervals throughout the snow season. While problems were encountered monitoring air and snow surface temperature, humidity, and wind, because of the extreme conditions which are likely to occur in an alpine environment, radiation is easily monitored, and the estimated uncertainty of all measured parameters was acceptably low. This effort was required to develop a high quality time series of integrated climate data to evaluate the components of the energy balance of the snow cover during both deposition and ablation conditions. measurement site [Davis and Marks, 1980; Davis et al., 1984]. Here we characterize climate conditions over the snow surface in a remote alpine watershed to evaluate the effect that these parameters have on snow distribution, metamorphism, and melt.
Snow is the dominant form of precipitation in the Reynolds Creek Experimental Watershed (RCEW). W... more Snow is the dominant form of precipitation in the Reynolds Creek Experimental Watershed (RCEW). Water from snowmelt is critical to the ecosystems and resources in RCEW because the water stored in the seasonal snow cover is the primary source of spring and summer soil moisture and streamflow. Snow water equivalent (SWE) has been measured at eight locations in RCEW every 2 weeks throughout the snow season (December 1 to June 1) for 35 water years (1962-1996). SWE was continuously monitored at one reference site for 14 water years (1983-1996). The measurement sites are described, the methods used are presented and discussed, these data are summarized, and examples of how they have been used are presented.
Archiv für Meteorologie, Geophysik und Bioklimatologie Serie B, 1979
A theoretically based model accurately calculates incoming longwave radiation under clear sky con... more A theoretically based model accurately calculates incoming longwave radiation under clear sky conditions in a remote alpine area. Data requirements are relatively simple and because the model is theoretically based it can be widely applied. The model calculates atmospheric radiation using a form of Brutsaert's method that is adjusted for variations in air pressure and radiation from adjacent slopes. Tests of both point and areal applications indicate that the method gives reliable results. When applied as a segment of an energy balance snowmelt model, the mode/provides information on energy exchange in an alpine environment and improved snowmelt runoff prediction. Radiation from cloud cover and canopy cover can be more easily isolated by using the model to account for clear sky radiation. It is possible that it can also calibrate atmospheric effects in thermal satellite data, thus enhancing the possibilities for using satellite radiometry in alpine areas. Zusammenfassung Ein Modell der langwelligen Strahlung far abgelegene alpine Gebiete Ein theoretisch fundiertes Modell berechnet zuverl~issig die einfallende langwellige Strahlung bei klarem Himmel in abgelegenen alpinen Gebieten. Die Datenansprtiche sind relativ einfach und auf Grund der theoretischen Fundierung kann das Modell unter vielfachen Bedingungen angewendet werden. Das Modell berechnet die atmosph~irische Rtickstrahlung unter Anwendung einer Art yon Brutsaert-Methode, welche auf Variation im Luftdruck und auf Strahlung yon benachbarten H~ingen abgestimmt wurde. Die Anwendung auf Met~punkte und Gebiete zeigt, daf~ die Methode zuverl~issige Resultate ergibt. Wird das Modell als Tefl eines Energiebilanzmodelles zur Berechnung der Schneeschmelze angewendet, so kann es Aussagen tiber den Energieaustausch in einer alpinen Umgebung sowie verbesserte Vorhersagen der Schneeschmelze liefern. Strahlung yon bedecktem Himmel oder yon einer Pflanzendecke kann leichter isoliert werden, denn aus Modellrechnungen kann die Strahlung des unbew61kten Himmels bestimmt werden. M6glicherweise kann das Modell zur Kalibrierung thermischer Satellitendaten bentitzt werden und dadurch die Anwendungsm6glichkeiten der Satellitenradiometrie in alpinen Gebieten verbessern.
This paper develops a multivariate mosaic subgrid approach to represent subgrid variability in la... more This paper develops a multivariate mosaic subgrid approach to represent subgrid variability in land surface models (LSMs). The k-means clustering is used to take an arbitrary number of input descriptors and objectively determine areas of similarity within a catchment or mesoscale model grid box. Two different classifications of hydrologic similarity are compared: an a priori classification, where clusters are based solely on known physiographic information, and an a posteriori classification, where clusters are defined based on high-resolution LSM simulations. Simulations from these clustering approaches are compared to high-resolution gridded simulations, as well as to three common mosaic approaches used in LSMs: the “lumped” approach (no subgrid variability), disaggregation by elevation bands, and disaggregation by vegetation types in two subcatchments. All watershed disaggregation methods are incorporated in the Noah Multi-Physics (Noah-MP) LSM and applied to snowmelt-dominated s...
Snow is an important natural reservoir that holds water on the landscape for release later in the... more Snow is an important natural reservoir that holds water on the landscape for release later in the season in western North America and other portions of the world. As air temperature increases with global climate change, the character of the generally established seasonal snowcover will be affected. To study the specific response to variable climate, a carefully collected and processed meteorological data set for the 1984-2008 water years (WYs) was assembled for a snow-dominated headwater mountain catchment. The data were used to force a physically based, distributed energy balance snow model to simulate patterns of snow deposition and melt over the catchment for the 25-year period. This period covers both the highest (1984) and lowest (1992) snow seasons on record and exhibits extreme inter-annual variability. This unique forcing data set captured meteorological conditions that resulted in the range of variability in snowcover accumulation, timing of ablation, and the timing and amount of surface water input (SWI), and discharge during the 25-year study period. SWI is the amount of liquid water delivered to the soil surface from melting snow or from rain that passes through the snowcover or falls directly on the soil. Warm winters, characterized by early-and midwinter rain, triggered earlier inputs from SWI and response in discharge than cool winters. Cool conditions prolonged the generation of SWI and streamflow out of the basin. Very wet conditions that were warm passed 50% of the SWI 27 days earlier and passed 50% of the discharge 15 days earlier, when compared to very wet conditions that were cool. Warmer conditions produced less snow water equivalent, shortened the melt season, and would be expected to extend the summer drought.
Snowmelt is the principal source for soil moisture, groundwater recharge , and stream-¯ow in moun... more Snowmelt is the principal source for soil moisture, groundwater recharge , and stream-¯ow 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 runo 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.
The snowcover energy balance is typically dominated by net radiation and sensible and latent heat... more The snowcover energy balance is typically dominated by net radiation and sensible and latent heat fluxes. Validation of the two latter components is rare and often difficult to undertake at complex mountain sites. Latent heat flux, the focus of this paper, is the primary coupling mechanism between the snow surface and the atmosphere. It accounts for the critical exchange of mass (sublimation or condensation), along with the associated snowcover energy loss or gain. Measured and modelled latent heat fluxes at a wind-exposed and wind-sheltered site were compared to evaluate variability in model parameters. A well-tested and well-validated snowcover energy balance model, Snobal, was selected for this comparison because of previously successful applications of the model at these sites and because of the adjustability of the parameters specific to latent heat transfer within the model. Simulated latent heat flux and snow water equivalent (SWE) were not sensitive to different formulations of the stability profile functions associated with heat transfer calculations. The model parameters of snow surface roughness length and active snow layer thickness were used to improve latent heat flux simulations while retaining accuracy in the simulation of the SWE at an exposed and sheltered study site. Optimal parameters for simulated latent heat flux and SWE were found at the exposed site with a shorter roughness length and thicker active layer, and at the sheltered site with a longer roughness length and thinner active layer. These findings were linked to physical characteristics of the study sites and will allow for adoption into other snow models that use similar parameters. Physical characteristics of wind exposure and cover could also be used to distribute critical parameters in a spatially distributed modelling domain and aid in parameter selection for application to other watersheds where detailed information is not available.
This study assessed the spatial variability of downwelling longwave and shortwave radiation under... more This study assessed the spatial variability of downwelling longwave and shortwave radiation under dense and open evergreen canopies during the snowmelt period in the Rocky Mountains near Fraser, Colorado, USA. We compared their magnitudes and spatial distributions under cloudy and clear-sky conditions over a melting snowcover. Through the use of radiometer arrays, thermocouples, and a portable scanning thermal radiometer, a detailed picture of the spatial variability of radiant energy was gained. The variability of melt energy deriving from radiation on cloudy days was vastly reduced from that on sunny days. The sources of variability in melt energy were both long and shortwave but with fundamental differences.
Soil moisture in semi-arid regions dominated by winter precipitation is recharged as a bulk influ... more Soil moisture in semi-arid regions dominated by winter precipitation is recharged as a bulk influx during spring melt and dries down quickly during the ensuing months. This timing in combination with various spatially distributed landscape properties produces distinct spatial soil moisture patterns. The patterns must be recognized in a distributed manner to account for hydrologic processes of storage and redistribution.
Determining surface precipitation phase is required to properly correct precipitation gage data f... more Determining surface precipitation phase is required to properly correct precipitation gage data for wind effects, to determine the hydrologic response to a precipitation event, and for hydrologic modeling when rain will be treated differently from snow. In this paper we present a comparison of several methods for determining precipitation phase using 12 years of hourly precipitation, weather and snow data from a long-term measurement site at Reynolds Mountain East (RME), a headwater catchment within the Reynolds Creek Experimental Watershed (RCEW), in the Owyhee Mountains of Idaho, USA. Methods are based on thresholds of (1) air temperature (T a) at 0°C, (2) dual T a threshold, À1 to 3°C, (3) dewpoint temperature (T d) at 0°C, and (4) wet bulb temperature (T w) at 0°C. The comparison shows that at the RME Grove site, the dual threshold approach predicts too much snow, while T a , T d and T w are generally similar predicting equivalent snow volumes over the 12 year-period indicating that during storms the cloud level is at or close to the surface at this location. To scale up the evaluation of these methods we evaluate them across a 380 m elevation range in RCEW during a large mixed-phase storm event. The event began as snow at all elevations and over the course of 4 h transitioned to rain at the lowest through highest elevations. Using 15-minute measurements of precipitation, changes in snow depth (z s), T a , T d and T w , at seven sites through this elevation range, we found precipitation phase linked to the during-storm surface humidity. By measuring humidity along an elevation gradient during the storm we are able to track changes in T d to reliably estimate precipitation phase and effectively track the elevation of the rain/snow transition during the event.
Hydrology and Earth System Sciences Discussions, 2010
Understanding the role of ecosystems in modulating energy, water and carbon fluxes is critical to... more 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 HESSD
The timing, magnitude, and spatial distribution of snow cover and the resulting surface water inp... more The timing, magnitude, and spatial distribution of snow cover and the resulting surface water inputs (SWI) are simulated at a small catchment located in the rain-snow transition zone of southwest Idaho, USA. A physically based snow model is run on this 1.5 ha study catchment, which has an elevation range of 1600-1645 masl. The catchment is divided into relatively steep (mean slope angle of 21 degrees) northeast and southwest facing hill slopes by an ephemeral stream that drains to the southeast. SWI are fundamental controls on soil moisture, streamflow generation, groundwater recharge, and nutrient cycling. Although the timing of melt events is similar across the basin, southwest facing slopes receive smaller magnitude and more frequent SWI from mid winter snow melt, while the northeast facing slope receives greater SWI during the spring. Three spatial patterns are observed in the modeled SWI time series: (1) equal between slopes, (2) majority of SWI on southwest facing slopes, and (3) majority of SWI on northeast facing slopes. Although any of these three spatial patterns can occur during the snow season, four emergent SWI patterns emerge through the melt season: (1) near uniform, (2) controlled by topographic differences in energy fluxes, (3) transitional, and (4) controlled by snow distribution. Rain on snow (ROS) events produce similar SWI between the northeast and southwest facing slopes, with the difference being attributed primarily to snow distribution. Turbulent fluxes dominate the snowpack energetics in four of the five rain on snow events, and advective fluxes from precipitation are greater than 17% during the 2 rain on snow events in December and January. Net radiation fluxes dominate spring melt events. Variations in the method used to distribute precipitation may result in large differences in total precipitation to the basin.
Electronic sensors generate valuable streams of forcing and validation data for hydrologic models... more Electronic sensors generate valuable streams of forcing and validation data for hydrologic models but are often subject to noise which must be removed as part of model input and testing database development. We developed an automated precipitation correction program (APCP) for weighing bucket precipitation gauge records, which are subject to several types of mechanical and electronic noise and discontinuities, including gauge maintenance, missing data, wind vibration, and sensor drift. Corrected cumulative water year precipitation from APCP did not exhibit an error bias and matched measured water year total precipitation within 2.1% for 58 station years tested. Removal of low‐amplitude periodic noise was especially important for developing accurate instantaneous precipitation records at subdaily time steps. Model flexibility for use with other data types is demonstrated through application to time domain reflectometry soil moisture content data, which are also frequently subject to ...
The spatial variation of melt energy can influence snow cover depletion rates and in turn be infl... more The spatial variation of melt energy can influence snow cover depletion rates and in turn be influenced by the spatial variability of shortwave irradiance to snow. The spatial variability of shortwave irradiance during melt under uniform and discontinuous evergreen canopies at a U.S. Rocky Mountains site was measured, analyzed, and then compared to observations from mountain and boreal forests in Canada. All observations used arrays of pyranometers randomly spaced under evergreen canopies of varying structure and latitude. The spatial variability of irradiance for both overcast and clear conditions declined dramatically, as the sample averaging interval increased from minutes to 1 day. At daily averaging intervals, there was little influence of cloudiness on the variability of subcanopy irradiance; instead, it was dominated by stand structure. The spatial variability of irradiance on daily intervals was higher for the discontinuous canopies, but it did not scale reliably with canopy...
... Processes, 13:19351959. , Link, T., A. Winstral, and D. Garen, 2001: Simulating snowmelt p... more ... Processes, 13:19351959. , Link, T., A. Winstral, and D. Garen, 2001: Simulating snowmelt processes during rain-on-snow over a semi-arid mountain basin. Ann. Glaciol., in press. Marshall, SE and SG Warren, 1987: Parameterization of snow albedo for climate models. ...
Energy budget dynamics under forest canopies are strongly influenced by the large spatial variabi... more Energy budget dynamics under forest canopies are strongly influenced by the large spatial variability of radiative and turbulent transfers in this environment. Incoming solar radiation under canopies has a particularly high degree of spatial variability. Transmission of solar radiation through a forest canopy varies with the size and location of the canopy gaps, as well as canopy leaf area. Modeling this transmission has proven challenging owing to the highly variable nature of the gaps within and between tree crowns, particularly in discontinuous canopies. This study describes and simulates the solar variability incident on the snow surface beneath two conifer forests. Objectives of this work are (1) to evaluate the variability of incoming solar radiation data with respect to canopy structure and cloudiness, (2) to correlate measured solar radiation transmission with predicted solar transmission based on analysis of hemispherical photographs, and (3) to examine the impact of measured and predicted transmission factors on the seasonal net radiative exchanges and snow ablation, based on snow process modeling. Observations were made during the winters of 2002 and 2003 in two predominately lodgepole pine (Pinus contorta) stands, one discontinuous and one relatively uniform, at the Local Scale Observation Site (LSOS) in Fraser, CO, USA, as part of the Cold Land Processes Experiment (CLPX). The canopy structure of all trees in the 0.8 ha triangular plot was measured and mapped in detail. We measured incoming global solar radiation at the snow surface beneath the discontinuous and the uniform canopies using arrays of 10 upward looking pyranometers at each site. Incoming global solar radiation was also measured above the canopy. Hemispherical photographs were taken with a Nikon Coolpix 995 with a fisheye converter at each radiometer location in both canopies, and were analyzed with Gap Light Analyzer (GLA) software. We found good agreement between measured and GLA-predicted transmissivities (r 2 = 0.86) when all data from both years were considered. Transmission factors derived from hemispheric photos and GLA software can be used to specify the distribution of solar flux under a canopy, instead of direct solar flux measurements, without degradation in snow model melt predictions.
Precipitation variability and complex topography often create a mosaic of vegetation communities ... more Precipitation variability and complex topography often create a mosaic of vegetation communities in mountainous headwater catchments, creating a challenge for measuring and interpreting energy and mass fluxes. Understanding the role of these communities in modulating energy, water and carbon fluxes is critical to quantifying the variability in energy, carbon, and water balances across landscapes. The focus of this paper was: (1) to demonstrate the utility of eddy covariance (EC) systems in estimating the evapotranspiration component of the water balance of complex headwater mountain catchments; and (2) to compare and contrast the seasonal surface energy and carbon fluxes across a headwater catchment characterized by large variability in precipitation and vegetation cover. Eddy covariance systems were used to measure surface fluxes over sagebrush (Artemesia arbuscula and Artemesia tridentada vaseyana), aspen (Populus tremuloides) and the understory of grasses and forbs beneath the aspen canopy. Peak leaf area index of the sagebrush, aspen, and aspen 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. Missing periods of measured data were common and made it necessary to extrapolate measured fluxes to the missing periods using a combination of measured and simulated data. Estimated cumulative evapotranspiratation from the sagebrush, aspen trees, and aspen understory were 384 mm, 314 mm and 185 mm. A water balance of the catchment indicated that of the 699 mm of areal average precipitation, 421 mm was lost to evapotranspiration, and 254 mm of streamflow was measured from the catchment;
Estimates of bedrock infiltration from mountain catchments in the western U.S. are essential to w... more Estimates of bedrock infiltration from mountain catchments in the western U.S. are essential to water resource managers because they provide an estimate of mountain block recharge to regional aquifers. On smaller scales, bedrock infiltration is an important term in water mass balance studies, which attempt to estimate hydrologic states and fluxes in watersheds with fractured or transmissive bedrock. We estimate the a daily time series of bedrock infiltration in a small catchment in the rain snow transition zone in southwest Idaho, using the difference between measured stream discharge and modeled soil drainage. The accuracy of spatial patterns in soil water storage are optimized, rather than the more common approach of minimizing error in integrated quantities such as streamflow. Bedrock infiltration is estimated to be 289 mm ± 50 mm for the 2011 water year, which is 34% ± 12% of the precipitation (95% confidence). Soils on the southwest facing slope drain more often throughout the snow season, but the northeast facing slope contributes more total soil drainage for the water year. Peaks in catchment soil drainage and bedrock infiltration coincide with rain on snow events.
A detailed evaluation of climate conditions in a small alpine watershed, typical of much of the s... more A detailed evaluation of climate conditions 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 are used to characterize the climate at four locations in the watershed during that snow season. Data from these locations are then combined into two representative sites for the watershed. Measurement approaches and methodologies and the effectiveness of instrumentation used in the study are discussed, and an estimate of the uncertainty of the monitored meteorological parameters is made. The data are integrated into a continuous hourly time series of solar and thermal radiation, air, snow and soil temperature, humidity, and wind at the two representative sites in this remote alpine watershed for an entire snow season. Snow deposition and snow cover depth and density are measured manually at regular intervals throughout the snow season. While problems were encountered monitoring air and snow surface temperature, humidity, and wind, because of the extreme conditions which are likely to occur in an alpine environment, radiation is easily monitored, and the estimated uncertainty of all measured parameters was acceptably low. This effort was required to develop a high quality time series of integrated climate data to evaluate the components of the energy balance of the snow cover during both deposition and ablation conditions. measurement site [Davis and Marks, 1980; Davis et al., 1984]. Here we characterize climate conditions over the snow surface in a remote alpine watershed to evaluate the effect that these parameters have on snow distribution, metamorphism, and melt.
Snow is the dominant form of precipitation in the Reynolds Creek Experimental Watershed (RCEW). W... more Snow is the dominant form of precipitation in the Reynolds Creek Experimental Watershed (RCEW). Water from snowmelt is critical to the ecosystems and resources in RCEW because the water stored in the seasonal snow cover is the primary source of spring and summer soil moisture and streamflow. Snow water equivalent (SWE) has been measured at eight locations in RCEW every 2 weeks throughout the snow season (December 1 to June 1) for 35 water years (1962-1996). SWE was continuously monitored at one reference site for 14 water years (1983-1996). The measurement sites are described, the methods used are presented and discussed, these data are summarized, and examples of how they have been used are presented.
Archiv für Meteorologie, Geophysik und Bioklimatologie Serie B, 1979
A theoretically based model accurately calculates incoming longwave radiation under clear sky con... more A theoretically based model accurately calculates incoming longwave radiation under clear sky conditions in a remote alpine area. Data requirements are relatively simple and because the model is theoretically based it can be widely applied. The model calculates atmospheric radiation using a form of Brutsaert's method that is adjusted for variations in air pressure and radiation from adjacent slopes. Tests of both point and areal applications indicate that the method gives reliable results. When applied as a segment of an energy balance snowmelt model, the mode/provides information on energy exchange in an alpine environment and improved snowmelt runoff prediction. Radiation from cloud cover and canopy cover can be more easily isolated by using the model to account for clear sky radiation. It is possible that it can also calibrate atmospheric effects in thermal satellite data, thus enhancing the possibilities for using satellite radiometry in alpine areas. Zusammenfassung Ein Modell der langwelligen Strahlung far abgelegene alpine Gebiete Ein theoretisch fundiertes Modell berechnet zuverl~issig die einfallende langwellige Strahlung bei klarem Himmel in abgelegenen alpinen Gebieten. Die Datenansprtiche sind relativ einfach und auf Grund der theoretischen Fundierung kann das Modell unter vielfachen Bedingungen angewendet werden. Das Modell berechnet die atmosph~irische Rtickstrahlung unter Anwendung einer Art yon Brutsaert-Methode, welche auf Variation im Luftdruck und auf Strahlung yon benachbarten H~ingen abgestimmt wurde. Die Anwendung auf Met~punkte und Gebiete zeigt, daf~ die Methode zuverl~issige Resultate ergibt. Wird das Modell als Tefl eines Energiebilanzmodelles zur Berechnung der Schneeschmelze angewendet, so kann es Aussagen tiber den Energieaustausch in einer alpinen Umgebung sowie verbesserte Vorhersagen der Schneeschmelze liefern. Strahlung yon bedecktem Himmel oder yon einer Pflanzendecke kann leichter isoliert werden, denn aus Modellrechnungen kann die Strahlung des unbew61kten Himmels bestimmt werden. M6glicherweise kann das Modell zur Kalibrierung thermischer Satellitendaten bentitzt werden und dadurch die Anwendungsm6glichkeiten der Satellitenradiometrie in alpinen Gebieten verbessern.
This paper develops a multivariate mosaic subgrid approach to represent subgrid variability in la... more This paper develops a multivariate mosaic subgrid approach to represent subgrid variability in land surface models (LSMs). The k-means clustering is used to take an arbitrary number of input descriptors and objectively determine areas of similarity within a catchment or mesoscale model grid box. Two different classifications of hydrologic similarity are compared: an a priori classification, where clusters are based solely on known physiographic information, and an a posteriori classification, where clusters are defined based on high-resolution LSM simulations. Simulations from these clustering approaches are compared to high-resolution gridded simulations, as well as to three common mosaic approaches used in LSMs: the “lumped” approach (no subgrid variability), disaggregation by elevation bands, and disaggregation by vegetation types in two subcatchments. All watershed disaggregation methods are incorporated in the Noah Multi-Physics (Noah-MP) LSM and applied to snowmelt-dominated s...
Snow is an important natural reservoir that holds water on the landscape for release later in the... more Snow is an important natural reservoir that holds water on the landscape for release later in the season in western North America and other portions of the world. As air temperature increases with global climate change, the character of the generally established seasonal snowcover will be affected. To study the specific response to variable climate, a carefully collected and processed meteorological data set for the 1984-2008 water years (WYs) was assembled for a snow-dominated headwater mountain catchment. The data were used to force a physically based, distributed energy balance snow model to simulate patterns of snow deposition and melt over the catchment for the 25-year period. This period covers both the highest (1984) and lowest (1992) snow seasons on record and exhibits extreme inter-annual variability. This unique forcing data set captured meteorological conditions that resulted in the range of variability in snowcover accumulation, timing of ablation, and the timing and amount of surface water input (SWI), and discharge during the 25-year study period. SWI is the amount of liquid water delivered to the soil surface from melting snow or from rain that passes through the snowcover or falls directly on the soil. Warm winters, characterized by early-and midwinter rain, triggered earlier inputs from SWI and response in discharge than cool winters. Cool conditions prolonged the generation of SWI and streamflow out of the basin. Very wet conditions that were warm passed 50% of the SWI 27 days earlier and passed 50% of the discharge 15 days earlier, when compared to very wet conditions that were cool. Warmer conditions produced less snow water equivalent, shortened the melt season, and would be expected to extend the summer drought.
Snowmelt is the principal source for soil moisture, groundwater recharge , and stream-¯ow in moun... more Snowmelt is the principal source for soil moisture, groundwater recharge , and stream-¯ow 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 runo 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.
The snowcover energy balance is typically dominated by net radiation and sensible and latent heat... more The snowcover energy balance is typically dominated by net radiation and sensible and latent heat fluxes. Validation of the two latter components is rare and often difficult to undertake at complex mountain sites. Latent heat flux, the focus of this paper, is the primary coupling mechanism between the snow surface and the atmosphere. It accounts for the critical exchange of mass (sublimation or condensation), along with the associated snowcover energy loss or gain. Measured and modelled latent heat fluxes at a wind-exposed and wind-sheltered site were compared to evaluate variability in model parameters. A well-tested and well-validated snowcover energy balance model, Snobal, was selected for this comparison because of previously successful applications of the model at these sites and because of the adjustability of the parameters specific to latent heat transfer within the model. Simulated latent heat flux and snow water equivalent (SWE) were not sensitive to different formulations of the stability profile functions associated with heat transfer calculations. The model parameters of snow surface roughness length and active snow layer thickness were used to improve latent heat flux simulations while retaining accuracy in the simulation of the SWE at an exposed and sheltered study site. Optimal parameters for simulated latent heat flux and SWE were found at the exposed site with a shorter roughness length and thicker active layer, and at the sheltered site with a longer roughness length and thinner active layer. These findings were linked to physical characteristics of the study sites and will allow for adoption into other snow models that use similar parameters. Physical characteristics of wind exposure and cover could also be used to distribute critical parameters in a spatially distributed modelling domain and aid in parameter selection for application to other watersheds where detailed information is not available.
This study assessed the spatial variability of downwelling longwave and shortwave radiation under... more This study assessed the spatial variability of downwelling longwave and shortwave radiation under dense and open evergreen canopies during the snowmelt period in the Rocky Mountains near Fraser, Colorado, USA. We compared their magnitudes and spatial distributions under cloudy and clear-sky conditions over a melting snowcover. Through the use of radiometer arrays, thermocouples, and a portable scanning thermal radiometer, a detailed picture of the spatial variability of radiant energy was gained. The variability of melt energy deriving from radiation on cloudy days was vastly reduced from that on sunny days. The sources of variability in melt energy were both long and shortwave but with fundamental differences.
Soil moisture in semi-arid regions dominated by winter precipitation is recharged as a bulk influ... more Soil moisture in semi-arid regions dominated by winter precipitation is recharged as a bulk influx during spring melt and dries down quickly during the ensuing months. This timing in combination with various spatially distributed landscape properties produces distinct spatial soil moisture patterns. The patterns must be recognized in a distributed manner to account for hydrologic processes of storage and redistribution.
Determining surface precipitation phase is required to properly correct precipitation gage data f... more Determining surface precipitation phase is required to properly correct precipitation gage data for wind effects, to determine the hydrologic response to a precipitation event, and for hydrologic modeling when rain will be treated differently from snow. In this paper we present a comparison of several methods for determining precipitation phase using 12 years of hourly precipitation, weather and snow data from a long-term measurement site at Reynolds Mountain East (RME), a headwater catchment within the Reynolds Creek Experimental Watershed (RCEW), in the Owyhee Mountains of Idaho, USA. Methods are based on thresholds of (1) air temperature (T a) at 0°C, (2) dual T a threshold, À1 to 3°C, (3) dewpoint temperature (T d) at 0°C, and (4) wet bulb temperature (T w) at 0°C. The comparison shows that at the RME Grove site, the dual threshold approach predicts too much snow, while T a , T d and T w are generally similar predicting equivalent snow volumes over the 12 year-period indicating that during storms the cloud level is at or close to the surface at this location. To scale up the evaluation of these methods we evaluate them across a 380 m elevation range in RCEW during a large mixed-phase storm event. The event began as snow at all elevations and over the course of 4 h transitioned to rain at the lowest through highest elevations. Using 15-minute measurements of precipitation, changes in snow depth (z s), T a , T d and T w , at seven sites through this elevation range, we found precipitation phase linked to the during-storm surface humidity. By measuring humidity along an elevation gradient during the storm we are able to track changes in T d to reliably estimate precipitation phase and effectively track the elevation of the rain/snow transition during the event.
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Papers by D. Marks