U53A-0701 1340h
Evaluation of the representativeness of automated snow water equivalent sensors in the Rio Grande headwaters using intensive field observations, remotely sensed snow cover data, and distributed snowmelt models
In spring 2001 and 2002 monthly snow surveys (i.e. April, May, and June) were undertaken to assess the spatial and temporal representativeness of snow water equivalent (SWE) values recorded at six snow telemetry (SNOTEL) stations in the Rio Grande headwaters. Snow depth data were interpolated using binary regression tree models and combined with snow density data and remotely sensed snow covered area to estimate the spatial distribution of SWE surrounding the SNOTEL sites. A physically based energy and mass balance snowmelt model was used to simulate the depletion of snow cover throughout the snowmelt season. Relative to the entire watershed, SNOTEL site locations are not representative of physiographic variables known to control snow distribution (i.e. elevation, slope, and incident solar radiation). At the watershed scale (3419 km$^{2}$) SNOTEL sites are located toward the western boundary of the watershed, an area of high snow cover persistence. Even relative to the 16, 4 and 1 km$^{2}$ areas that surround them, SNOTEL stations are not representative of the physiographic variables known to control snow distribution. These physiographic biases vary from site-to-site, with five of six sites located on relatively flat terrain and hence having a positive solar radiation bias. For the two water years studied, certain sites showed consistent overestimates of SWE relative to the surrounding 16, 4, and 1-km$^{2}$ areas. Other sites showed variability in SWE bias during the two years, as regression-tree model results suggested that different physiographic variables controlled snow distribution during the two water years. The results presented here will improve the ability to upscale SNOTEL data for evaluating and calibrating remote sensing algorithms and initializing, evaluating, and updating modeling efforts at the regional scale.
U53A-0702 1340h
Climate Mapping Challenges in Mountainous Regions
Mountainous regions encompass some of the most complex climates in the world. The presence of major topographic features, sometimes interacting with coastal effects, creates a myriad of spatially complex precipitation and temperature regimes. Typically, only a small number of these regimes are well-represented by surface observations. Therefore, producing accurate climate maps for these regions can be quite challenging. PRISM, a knowledge-based climate mapping system, was originally developed in 1991 to map precipitation in the mountainous western United States. Since then, it has been both generalized and refined to model more climate variables and address more climatological processes, and its use has expanded worldwide. Model improvements have come primarily as a result of lessons learned through repeated applications of the model and peer-review of the results. This paper will survey some of the major climatological processes driving temperature and precipitation patterns in mountainous regions, and how PRISM accommodates these processes. These include elevational gradients, rain shadows, coastal influences, temperature inversions, cold air drainage, and the varying orographic effectiveness of terrain features. Specific case studies will be drawn from Oregon, California, Colorado, and other locations.
<a href='http://www.ocs.oregonstate.edu/prism/' >http://www.ocs.oregonstate.edu/prism/
U53A-0703 1340h
Impact of Retreating Glaciers in an Intermontane Andean Watershed: Hydrochemical Analysis From the Callejon de Huaylas, Per\'{u}
The Callejon de Huaylas, Per\'{u}, is a well-populated 5000 km$^{2}$ watershed of the upper Rio Santa river draining the glacierized Cordillera Blanca. This tropical intermontane region features rich agricultural diversity and valuable natural resources, but currently receding glaciers are causing concerns for future water supply. A major question concerns the relative contribution of glacier meltwater to the regional stream discharge-from first order basins to the whole watershed. In July, 2004, we collected 37 water samples from streams, springs and precipitation over a 2000 m vertical range within the watershed and analyzed them for major dissolved ions and isotopic (\delta$^{18}$O) composition. The water chemistry is used to establish the extent of variability in the surface waters, and to identify different hydrologic end-member components. \delta$^{18}$O values for the waters range from -4.29\permil to -5.28\permil. There is a consistent trend towards lighter isotopes with greater percentage of glacier coverage in tributary stream catchments of the Rio Santa, with some exceptions due to evaporative enrichment in lakes. Samples taken along transects of these glacierized tributary streams become more isotopically enriched with lower elevation and greater distance from the glaciers. However, waters from the Rio Santa become less enriched with lower elevation. We hypothesize that the distribution of glacier mass in the mountain range causes a greater volume of glacial meltwater to join the Rio Santa at lower elevations. The water generally has a Ca-Mg-HCO$_{3}$ chemical signal. Samples along transects of tributary valleys show an increase in TDS and the Na:Mg concentration ratio with decreasing elevation. We see geochemical evidence for a small groundwater source in the tributaries and the Rio Santa. We propose that distinct chemical signatures of source water end-members may provide a means of quantifying the volumetric contribution of glacier meltwater over time.
U53A-0704 1340h
The relationship between snowpack and seasonal low flows in the Sierra Nevada: climate change and water availability in California
Seasonal low flows are important for sustaining aquatic ecosystems, and for supplying human needs during mid-summer. When the timing of water supply and demand do not coincide, humans rely on both natural and artificial storage. In California, the gap in timing between supply and demand is bridged primarily by the Sierra Nevada snowpack, which slowly melts throughout the spring and summer. However, most future climate scenarios suggest a decreased snowpack in the Sierra. Previous studies have investigated changes in snowmelt timing and spring snowmelt flood events. Here, by contrast, we explore how changes in the Sierra Nevada snowpack will affect annual low flows. We have identified all of the gauged catchments in the Sierra Nevada with unimpaired streamflow records and with at least ten years of overlapping snowpack and streamflow data. In each of these catchments, we have analyzed up to 40 years of historical snow and streamflow records. We find that annual minimum, mean, and maximum flows in these catchments all increase and decrease proportionally, or more-than-proportionally, as the annual peak snowpack water content changes from year to year. For every 10% decrease in snowpack, there is a 9-17% decrease in annual minimum flow. Minimum flows also occur earlier in years with smaller snowpacks; for every 10% decrease in snowpack, minimum flows occur 3-7 days earlier in the year. Finally, we find that in some catchments, annual low flows are significantly correlated not only with that year's snowpack, but with the previous year's snowpack as well. That is, seasonal low flows in some Sierra Nevada catchments exhibit a multi-year "memory" of snowmelt water inputs. We evaluate possible mechanisms that might underlie this observed memory effect. If these observed relationships between snow and flow hold in the future climate regime, the projected decrease in snowpack is likely to have a severe effect on seasonal low flows.
U53A-0705 1340h
Uncertainty in Projections of Impacts of Climate Change on Sierra Nevada mountain hydrology in California
Understanding the uncertainty in the projected impacts of climate change on California's Sierra Nevada hydrology will clarify where hydrologic impacts can be expected with higher confidence, and will help address scientific questions related to possible improvements in climate modeling. In this study, we focus on California, a region that is vulnerable to hydrologic impacts of climate change. We statistically bias correct and downscale the monthly temperature and precipitation projections from 10 global climate models (GCMs) from the Coupled Model Intercomparison Project. These GCM simulations include both a control period (with unchanging CO2 and other atmospheric forcing) and a perturbed period with a 1-percent per year increase in CO2 concentration. We force a distributed hydrologic model with bias-corrected and statistically downscaled GCM data, and generate streamflow at strategic points in the Sacramento-San Joaquin River basin. Among our findings are that inter-model variability does not prevent significant detection of decreases in summer low flows, increases in winter flows or the shifting of flow to earlier in the year. Uncertainty due to sampling of a 20-year period in an extended GCM simulation accounts for the majority of inter-model variability for summer and fall months, while varying GCM responses to temperature and precipitation forcing add to the variability in the winter. Inter-model variation in projected precipitation accounts for most of the uncertainty in winter and spring flow increases in both the North and South regions, with a greater influence in the North. The influence of inter-model precipitation variability on May-September streamflow decreases in later years, as higher temperatures dominate the hydrologic response, and melting snowpack has less influence.
U53A-0706 1340h
Stream Discharge and Sediment Load Variation During Three Dry Years at Kings River Experimental Watershed in the Southern Sierra Nevada in California
The Kings River Experimental Watershed (KREW) is now in its third year of data collection on eight small watersheds from 1600-2400 m in the Sierra Nevada. We are collecting meteorology, stream discharge, sediment load, water chemistry, stream microclimate, shallow soil water chemistry, vegetation, macro-invertebrate and air quality data. This paper examines connections between meteorology, stream discharge and sediment yield in water years 2003 and 2004, which were generally dry with below average snow packs. The current effort is to establish a baseline variation in these watershed characteristics prior to treatments. The Sierra National Forest will thin and/or prescribed burn six of these watersheds in 2006 and 2007 while two will remain as controls.
<a href='http://www.fs.fed.us/psw/programs/snrc/' >http://www.fs.fed.us/psw/programs/snrc/
U53A-0707 1340h
High-Elevation Response of Conifers to Climate Change in the Sierra Nevada and Western Great Basin, USA: Treeline Elevation is Not the Primary Effect
Traditionally change in alpine treeline elevation has been treated as the primary response by conifers to climate change, and considerable effort has gone into discriminating among complex climatic factors that determine a binary move (up or down slope). Four independent studies in the eastern Sierra Nevada (SN) and western Great Basin (GB), which assessed conifer response to historic climates at scales of decades to millennia, suggest that changes in slope, aspect, growth form, species composition, and forest structure are equally if not more important than changes in treeline elevation. (1) Over the past 3.5 millennia, limber pine (Pinus flexilis) alternately grew in sparse populations on highly restricted (NE-facing) sites in the Wassuk Range (GB) and sites in the SN during centuries of drought versus widespread on many slopes and aspects within similar elevation zones during favorable climate periods. (2) During the past millennium, the summit of Whitewing Mtn (near current local treeline, SN) has been alternately occupied by a mixed conifer forest (seven species), scattered, dwarfed pines (one species) and no trees, varying with distinct climate periods. (3) Treeline elevation of whitebark pine (P. albicaulis) in the Yosemite National Park region (SN) over the last millennium has remained stable, whereas climate periods are marked by changes in growth, growth form and forest density. (4) During the 20th century, type conversions from meadow to forest and from bare ground (former snowfields) to forest correspond to multi-decadal climate variability. These studies suggest that future global change may unfold as complex changes in slope, aspect, forest composition and structure rather than simple shifts in species and plant communities up or down slope.
U53A-0708 1340h
Reconstructing a Past Climate Using Current Multi-species' Climate Spaces
We present an analysis of a ghost forest on WhiteWing Mt at 3000 m in the eastern Sierra Nevada, southeast of Yosemite NP. Killed by a volcanic eruption about 650 years ago, the deadwood on WhiteWing dates by standard tree-ring analysis to 800-1330 CE, during the Medieval Warm Anomaly. Individual stems have been identified by wood anatomical characteristics as Pinus albicualis, P. monticola, P. jeffreyi, P. contorta, P. lambertiana, and Tsuga mertensiana. With the exception of P. albicualis, which is currently in krummholz form at this elevation, the other species are 200 m or more lower in elevation. One, P. lambertiana, is west of the Sierran crest and 600 m lower in elevation. Assuming that climatic conditions on Whitewing during this period were mutually compatible with all species, we reconstruct this climate by the intersection of the current climatic spaces of these species. We did this by first generating individual species' ranges in the Sierran ecoregions through selecting vegetation GIS polygons from the California Gap Analysis database (UCSB) that contain the individual species. Climatic spaces for each species were generated by the GIS intersection of its polygons with 4 km gridded polygons from PRISM climatic estimates (OSU); this was done for annual, January, and July maximum and minimum temperature, and precipitation, merged together for each species. Climatic intersections of the species were generated from the misclassified polygons of a discriminant analysis of species by the climatic data. The average data from these misclassified polygons suggest that the climate on WhiteWing during the existence of this forest community was 230 mm, 1oC, and 3oC greater than present in precipitation, and maximum and minimum temperature, respectively.
U53A-0709 1340h
High Elevation Monitoring in the North American Tropics: Ecosystem/Climate Relationships on Nevado de Colima, Mexico
High elevation monitoring in the tropics is uncommon. Presented here are the 2001-2004 results of an intensive field study from Nevado de Colima, Mexico. The site is at 3800 m at 19° 34' N, a few hundred meters below tree line. We have been co-monitoring weather and tree growth at half-hour intervals, as well as seasonally averaged stable isotopes throughout the hydrologic/biologic cycle. The site is under the influence of the North American monsoon, which determines a wet-summer, dry-winter climatic regime. Using point and band dendrometers, we have shown the response of high elevation Pinus hartwegii trees to changing weather patterns and attempted to pinpoint factors related to onset and cessation of growth in these high elevation tropical trees. Precipitation, temperature and relative humidity are shown to influence stem size at a range of timescales. Along with the stable isotope data collected to date, we hope to build a model of tree growth and stable isotope incorporation into tree-ring cellulose. This will allow a calibrated chemical reconstruction of seasonal growth response to fluctuations in the monsoon over the length of the tree ring record ($>$350yr). We also had the unfortunate experience of monitoring several of our instrumented trees during a round-headed pine beetle (Dendroctonus adjunctus) infestation following an exceptionally dry winter the year before. These data may provide additional insight into tree response to drought stress and physiological response to bark beetle attacks.
<a href='http://woods.geography.unr.edu' >http://woods.geography.unr.edu
U53A-0710 1340h
GLORIA Alpine Plant Monitoring in the White Mountains, Inyo County, California.
The GLORIA project (Global Observation Research Initiative in Alpine Environments: www.gloria.ac.at) is a worldwide effort coordinated by the University of Vienna Institute of Ecology and Conservation Biology, to monitor climate effects on alpine peaks around the world. In the summer of 2004 the University of California, White Mountain Research Station teamed up with the U.S. Forest Service to initiate GLORIA monitoring sites on 4 summits in the White Mountains. The lower three summits consist of granitic rock, and range from 3240m to 3975m in elevation, while the upper summit is on metavolcanic rock on the shoulder of White Mountain Peak at 4285m. For each summit we followed the rigorous GLORIA sampling design and recorded baseline data on plant species composition, cover, and frequency. Permanent monitoring plots were set up, and dataloggers installed to measure soil temperature. In addition, we are discussing ways to augment the standard GLORIA sampling protocol by setting up a White Mountain "GLORIA master site." This would involve (1) remeasurement of the GLORIA summits using alternative sampling procedures, for example random quadrat sampling, to facilitate cross-comparison with other monitoring efforts by agency and university scientists, (2) a parallel summit transect on a chemically contrasting bedrock lithology, formally known as the Reed Dolomite, which produces magnesium-rich carbonate soils, and is the principle host rock to the ancient Bristlecone forest, .and (3) expanding sampling to include animal taxa. We also plan to complete a detailed geomorphic and geologic description of each site to include in the monitoring database.
<a href='http://www.wmrs.edu/projects/gloria project/default.htm' >http://www.wmrs.edu/projects/gloria project/default.htm
U53A-0711 1340h
A New GLORIA Target Region in the Sierra Nevada, California, USA; Alpine Plant Monitoring For Global Climate Change
The Global Observation Research Initiative in Alpine Environments (GLORIA) is an international research project with the goal to assess climate change impacts on vegetation in alpine environments worldwide. Standardized protocols direct selection of each node in the network, called a target region, which consists of a set of four geographically proximal mountain summits at elevations extending from treeline to the nival zone. For each summit, GLORIA specifies a rigorous mapping and sampling design for data collection, with re-measurement intervals of five years. Whereas target regions have been installed in six continents, prior to 2004 none was completed in North America. In cooperation with the Consortium for Integrated Climate Research in Western Mountains (CIRMOUNT), three target regions were completed by September 2004, one in the Sierra Nevada, California, one in the White Mountains, California, and one in Glacier National Park, Montana. The SIERRA NEVADA (GLORIA code: SND) target region lies along the Sierra Nevada crest in the Yosemite National Park/Mono Lake region. The four summits well represent the GLORIA design standards, being little visited by climbers, outside domestic grazing allotments, relatively rounded in shape, situated within one climate region, related substrate types (metamorphic), and extending from treeline to the highest elevation zones in the area. The four summits include the subordinate peak of Mt Dunderberg (3744m), two lesser peaks of Mt Dunderberg (3570m and 3322m) and a summit along the Yosemite National Park boundary region south of Mt Conness (3425m). Preliminary data indicate that numbers of vascular plant species, from lowest to highest summit, were 40, 36, 12, 22 (total for SN, 67). Only 1 species (Elymus elymoides ssp. californicus) occurred on all four summits; 8 species occurred on three summits; no exotic species was detected. The most distant summit, also most distinct in substrate, had the largest number of unique species. The genus Carex (Cyperaceae) had the most species represented (five). Only one tree species (Pinus albicaulis) occurred within the summit areas. Data analysis of the baseline measurements has just begun; the standardized GLORIA protocols will enable direct comparisons among summits within the target region, across target regions in California, among the three target regions in North America, and with established GLORIA regions in other continents.
U53A-0712 1340h
The Glacier National Park GLORIA Project: A new US Target Region for Alpine Plant Monitoring Installed in the Northern Rocky Mountains, Montana
The Global Observation Research Initiative in Alpine Environments (GLORIA) is an international research network whose purpose is to assess climate change impacts on vegetation in alpine environments worldwide. A standard protocol was developed by the international office in Vienna, Austria, and has specific site requirements and techniques that allow sites to be compared worldwide. This protocol requires four summits to be selected within a target region, covering zonal differences of subalpine to nival, and on each of these summits intensive vegetation plots are set up and monitored on a five year interval. Only three target regions in North America have been completed to date, one in Glacier National Park, Montana, and the other two in the Sierra Nevada and White Mountains, California. The four GLORIA summit plots in Glacier National Park were completed over the summers of 2003 and 2004. Because the Continental Divide bisects Glacier National Park (north to south), we chose summits only East of the divide to stay within a similar climatic pattern. Establishing sites was difficult due to the steep and rocky glaciated terrain and the remoteness of suitable sites that required multi-day approaches. Our highest summit (Seward Mtn. 2717 m) is the northernmost and our lowest summit (Dancing Lady Mtn. 2245 m) is southernmost. Treeline is strongly influenced by terrain and is significantly more variable than in the central Rocky Mountains. This also was true of zonal differences of alpine vegetation. Subalpine and even grassland species were found on the same summits as upper alpine species and areas considered subnival. While different zonal areas often occurred on one summit, they were highly influenced by the aspect and slope of that summit area. Between 51 and 82 vascular plants were documented on each summit. There was a high degree of variability in species diversity and percent cover on each summit that was correlated to directional exposure. The summit morphology caused loose vegetative associations, or micro-communities, that varied with exposure, slope angle, and substrate character. Species that exhibited dominance within the target region were Smelowskia calycina var. americana, Polemonium viscosum, Achillea millefolium, Erigeron compositus var. glabratus, and Potentilla fruticosa L. These species reflected the same variability in percent cover on the four sides of the summit areas as did the vegetation as a whole, but were present on all sides.
U53A-0713 1340h
Competing Interests and Concerns in the Rio Grande Basin: Mountain Hydrology, Desert Ecology, Climate Change, and Population Growth
In the mountainous American Southwest, the Rio Grande basin is a prime example of how conflicts, misconceptions, and competition regarding water can arise in arid and semi-arid catchments. Much of the Rio Grande runoff originates from snow fields in the San Juan Mountains of southern Colorado and the Sangre De Cristo Mountains of northern New Mexico, far from population centers. Large and rapidly growing cities, like Albuquerque, Las Cruces, El Paso, and Juarez, are located along the Rio Grande where it flows through the Chihuahuan Desert, the largest desert in North America(two NSF Long Term Ecological Research sites are located in the desert portion of the basin). As a result, the importance of snowmelt, which makes up 50-75% or more of the total streamflow in sub-basins above Elephant Butte Reservoir(in south central New Mexico) is hardly known to the general public. Streamflow below Elephant Butte Reservoir is rainfall driven and very limited, with the lower basin receiving only 170-380 mm of precipitation annually, most of it occurring during the months of July-September. Extreme events, such as drought and flooding, are not unusual in arid basins, and they are of increasing concern with regard to changes in frequency of such events under the impending conditions of climate change. Current water demands in the basin already exceed the water supply by 15% or more, so streamflow forecasts(especially from snowmelt runoff) are extremely valuable for efficient water management as well as for proper apportionment of water between Colorado, New Mexico, and Texas under the Rio Grande Compact of 1938 and between the U.S. and Mexico under the Treaty of 1906. Other demands on the water supply include Indian water rights, flood regulation, irrigated agriculture, municipal and industrial demands, water quality, riverine and riparian habitat protection, endangered and threatened species protection, recreation, and hydropower. To assess snow accumulation and cover and to produce streamflow forecasts, several techniques are being employed including manual snow surveys, automated SNOTEL measurements, satellite snow cover extent measurements, development of snow cover depletion curves, and input of these data to the Snowmelt Runoff Model(SRM) and other models for forecasting. Early season(November-January) SNOTEL measurements of snow water equivalent can be used in regression approaches to estimate streamflow volumes early enough to provide growing season planning for the types of crops to plant. Satellite snow cover is used directly in SRM for daily flow forecasts throughout the melt season starting as early as March. Additionally, SRM can automatically produce future hydrographs for climate change scenarios. For large river basins in arid and semi-arid areas, new technologies, like remote sensing, will be valuable in assisting water managers to make more efficient use of their limited water supply. Additionally, like meteorologists have done for the last 40 years, hydrologists need to make use of remote sensing data to communicate in real time with the public on the effects of snow accumulation, melt, and snowmelt runoff on human activities.
U53A-0714 1340h
Mountain Snowpack, Lowland Winter Precipitation, and Variability in Three Western US Mountain Ranges
Demands on mountain snowpack in the Western United States are increasing explosively and the margin between water supply and demand is disappearing. Since snowpack is the primary water source in most regions, this paper examines high elevation snowpack in localized regions of three mountain ranges of the western United States and contrasts this to cumulative winter precipitation in their abutting lowlands. The three regions are the Rockies, Cascades, and the Sierra Nevada. Through examination of inter-annual variability, extremes, and trends we found that snowpack variability is greatest in the Cascades and least in the Rockies. Analysis of extreme snowpack shows strong relationships with circulation in the Cascades and Sierra, but not in the Rockies. Extreme occurrences of widespread low snowpack (i.e. in all three ranges) are common, whereas those of widespread high snowpack are rare. Further, the relationship between high elevation snowpack and adjacent lowland precipitation (LWP) is de-coupled, suggesting that LWP should not be used as a proxy for upland snowpack conditions. At a finer spatial scale, two sites six km apart on the east slope of the Colorado Front Range show similarly significant contrasts in snowpack. Differences at this fine scale can also be resolved into different atmospheric circulation patterns. In conclusion, our analyses suggest that the snowpack in the Cascades may be the most sensitive to future climate change, while that in the Rockies the least.
U53A-0715 1340h
How the 2004 Onset of Snowmelt and Streamflow Varied with Elevation
In 2004, spring snowmelt began anomalously early across the Western United States. USGS-gaged streams draining the Sierra Nevada, Cascades, and Rocky Mountains all recorded a spring pulse of meltwater during the second week of March. However, data from streamgages monitoring nested streamgages along the Tuolumne and Merced Rivers in Yosemite National Park suggest that this early onset of melt did not occur uniformly at all basin scales and elevations. The Yosemite monitoring network has been operational since summer 2001, and gages are located at elevations from 1200 m to 3300 m in basins with various slopes and aspects. In 2002, spring melt began uniformly at all monitored elevations, and in 2003 spring melt began uniformly at all but the highest gage. However, in 2004, the onset of spring snowmelt varied widely. For example, streamflow on the South Fork of the Tuolumne River at 2040 m started flowing 7 March, and flows did not decline until the snowpack was depleted. In contrast, a gage monitoring inflow to Fletcher Lake at 3109 m recorded no flow prior to 30 April. Many gages at elevations between these extremes recorded a small flux of water in mid-March but no strong increase in streamflow until mid- to late-April. These differences in snowmelt onset dates were supported by not only in streamflow records but also by observation of vegetation, as botanists noticed flowers blooming anomalously early below 2800 m but at their average times in higher regions. This paper seeks to answer the following questions: What were the primary factors controlling the observed differences in snowmelt onset dates in 2004? How did solar radiation and temperature inputs differ between 2002, which had a uniform melt onset, and 2004? Was melt dominated by different processes at different locations? What are the implications for climate forecasts, which predict earlier spring onsets in a warmer climate? How can snowmelt and streamflow models better capture the different behavior observed at the highest altitudes?
<a href='http://tenaya.ucsd.edu/{\sim}jessica/' >http://tenaya.ucsd.edu/{\sim}jessica/
U53A-0716 1340h
The hydrologic and biogeochemical response of undisturbed mountain ecosystems in the Western United States to multiple stressors: Interactions between climate variability and atmospheric deposition of contaminants
Wilderness areas and national parks of the West are largely protected from acute changes in land use such as urbanization and natural resource development. However, the ecosystems in these areas are sensitive to both climate variability and atmospheric deposition of acids, nitrogen (N), and toxic contaminants, and these stressors interact in ways that we are just beginning to understand. Here we examine some examples of the interactions between climate variability and nitrogen and mercury cycling in high elevation watersheds. During the recent drought, which began in 2000, streamwater nitrate concentrations nearly doubled in the Loch Vale watershed in Rocky Mountain National Park, exceeding 60 $\mu$M during early snowmelt. Much of the elevated nitrate resulted from an increased percentage contribution to streamwater of nitrate-rich shallow groundwater. In a nearby pond used for breeding by a threatened amphibian species, nitrate concentrations were negligible but ammonium concentrations were extremely high (850 $\mu$M) during the drought. In this case, organic N in pond sediments was likely mineralized and released during cycles of drying and rewetting of pond sediments. Even after 2 years of near-average precipitation, water levels remained below normal and ammonium concentrations remained elevated, indicating that the hydrologic response of this small system has a timescale of many years. Mercury (Hg) deposition at high elevations of the Rocky Mountains is comparable to that of the Midwest and Northeast, but the processes that control Hg cycling in alpine/subalpine ecosystems are not well understood. Methylation and bioaccumulation of Hg must occur before Hg reaches levels harmful to the ecosystem or human health, and both climate and nutrient cycling affect these processes. Fluctuating water levels caused by climate variability can mobilize Hg from lake and pond sediments, increasing reactivity and bioavailability of Hg in the ecosystem. Increased nutrient release from the terrestrial ecosystem (eg. from N saturation) may increase productivity and accumulation of organic matter, altering Hg cycling in the aquatic system. Long durations of ice cover and thick snowpacks are likely to cause elevated methyl Hg in aquatic ecosystems. Snow and ice cover on lakes promotes hypoxia in lake water, favoring production and accumulation of methyl Hg- the percentage of methyl-Hg in lake water under snow and ice was as much as 6 times greater than the percentage measured during late summer in a northwestern Colorado lake. Analysis of long-term trends indicates that climate variability is increasing in the Mountain West. Climatic extremes appear to exacerbate adverse impacts of atmospheric deposition, as well as stressing ecosystems directly. A better understanding of these interactions is needed in order to predict the response of mountain ecosystems to future changes in climate and atmospheric deposition.
U53A-0717 1340h
The Airborne Carbon in the Mountains Experiment
Mountain landscapes of the Western US contain a significant portion of the North American carbon sink. This results from the land use history of the region, which has a preponderance of potentially aggrading mid-aged stands. The issue is significant not only because of the significant sink but because of the vulnerability of that sink to drought, insects, wildfire and other ecological changes occurring rapidly in the West. Quantification of the carbon budgets of western forests have received relatively limited attention, in part because direct carbon flux measurements are believed to be difficult to apply in complex landscapes. New techniques that take advantage of organized nighttime drainage flows may allow quantification of respiration on scales inaccessible in level landscapes, while Lagrangian airborne measurements may allow daytime fluxes to be quantified. Airborne and ground-based measurements during the summer of 2004 in Colorado show substantial drawdown of atmospheric carbon dioxide during the day and strong enrichment of the nocturnal boundary layer from nighttime respiration. We present a strategy whereby in situ measurements at multiple scales, remote sensing and data assimilation may be used to quantify carbon dynamics in mountain landscapes. Larger scales of integration may be possible in mountainous than level landscapes because of the integrative flow of air and water, while because of high heterogeneity, scaling from detailed local process studies remains difficult.
<a href='http://swiki.ucar.edu/acme' >http://swiki.ucar.edu/acme
U53A-0718 1340h
Trends in Snowfall Versus Rainfall for the Western United States
The western U.S. depends heavily on snowpack to help retain its wintertime freshwater endowment into the drier spring and summer months. A well-documented shift towards earlier runoff can be attributed to 1) more precipitation falling as rain instead of snow, and 2) earlier snowmelt. The present study shows a regional trend toward a decreasing ratio of winter snow water equivalent (SWE) to total precipitation since 1948. This trend is attributable to a shift toward more rainfall rather than a decrease in overall precipitation. At the monthly scale, the trend is most pronounced in January (coastally) and March (west-wide), corresponding to warming trends in those months for average wet-day maximum temperatures. Implications for historical and projected runoff timing shifts are discussed.
U53A-0719 1340h
Meteorological Drought in the Mountainous West
We examine meteorological drought in western North America in the climatic context of the entire continent and adjacent ocean basins. Indices that describe the intensity and spatial extent of dry and wet conditions are computed from station precipitation data at thousands of stations with daily precipitation and temperature observations across North America as well as for geographically specific subsets focusing on the Mountainous West. The evolution of regional drought histories will then be examined in the context of local temperatures over land as well as climatic variability of the Pacific and Atlantic basins as described by dominant sea surface temperature and atmospheric pressure patterns. The strong interdecadal modulation of interannual extremes will be discussed. The relationships between dry, hot, wet and cold conditions will also be described on a regional and seasonal basis. Additionally, we will examine the contribution of daily precipitation extremes to wet and dry year totals as a function of location and season. This will be done for specific extreme wet and dry episodes. By documenting and understanding regional drought relationships with local weather and large-scale climate, we seek to gain insight into the mechanisms that cause drought in the West. We aim especially to understand the causes of the current persistent dry condition and in so doing project the likely future evolution of western North American hydrology.
U53A-0720 1340h
Runoff Simulation over the Sierra Nevada Region Using a Coupled Regional Climate Model
The land surface scheme (NOAH) in the current version of the Penn State-National Center for Atmospheric Research (NCAR) fifth generation Mesoscale Model (MM5) insufficiently treats snowmelt runoff over the Sierra Nevada region due to an insufficient treatment of the snow processes. To improve snowmelt runoff simulation, we have coupled the newly released NCAR Community Land Model version 3 (CLM3) to MM5. CLM3 physically describes the mass and heat transfer within the snowpack using 5 snow layers that include liquid water and solid ice. Interactions among the snow, soil, and vegetation are a function of the CLM3 mass and energy equations. Additionally, a river routing scheme has been adopted in CLM3 to better describe the runoff hydrograph. Several observed datasets from different sources were used to evaluate the model output, including snow depth, temperature, and precipitation from the automated Snowpack Telemetry system, snow cover and vegetation indices from the MODIS satellite data, and streamflow data from the U.S. Geological Survey. In this presentation, we describe the results from an MM5-CLM3 integration from April 1 to June 30, 1998 with 60 km and 20 km nested domains. The results indicate that the coupled model significantly improves the simulation of the snow mass, resulting in a better description of the runoff in the Sierra Nevada region. The application of the river routing scheme further improves the runoff hydrograph simulation. Meanwhile, MM5-CLM3 produces better simulations for the surface air temperature and precipitation as it has more realistic descriptions of the surface energy balance and hydrological cycle when compared to the original version of MM5 with the NOAH land surface model. The coupling of the advanced CLM3 with MM5 significantly improves the regional hydroclimate and water resources predictability.