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How will baseflow respond to climate change in the Upper Colorado River Basin?
Baseflow is critical to sustaining streamflow in the Upper Colorado River Basin. Therefore, effective water resources management requires estimates of baseflow response to climatic changes. This study provides the first estimates of projected baseflow changes from historical (1984 – 2012) to thirty-year periods centered around 2030, 2050, and 2080 under warm/wet, median, and hot/dry climatic conditions using a hybrid statistical-deterministic baseflow model. Total baseflow supplied to the Lower Colorado River Basin may decline by up to 33%, although this value may increase in the near future by 6% under warm/wet conditions. The percentage of baseflow lost during in-stream transport is projected to increase by 1 - 5% relative to historical conditions. Results highlight that climate driven changes in high elevation hydrology have impacts on basin-wide water availability. Study results have implications for human and ecological water availability in one of the most heavily managed watersheds in the world.
Response of lake chemistry to changes in atmospheric deposition and climate in three high-elevation wilderness areas of Colorado
Trends in precipitation chemistry and hydrologic and climatic data were examined as drivers of long-term changes in the chemical composition of high-elevation lakes in three wilderness areas in Colorado during 1985–2008. Sulfate concentrations in precipitation decreased at a rate of −0.15 to −0.55 μeq/l/year at 10 high-elevation National Atmospheric Deposition Program stations in the state during 1987–2008 reflecting regional reductions in SO 2 emissions. In lakes where sulfate is primarily derived from atmospheric inputs, sulfate concentrations also decreased although the rates generally were less, ranging from −0.12 to −0.27 μeq/l/year. The similarity in timing and sulfur isotopic data support the hypothesis that decreases in atmospheric deposition are driving the response of high-elevation lakes in some areas of the state. By contrast, in lakes where sulfate is derived primarily from watershed weathering sources, sulfate concentrations showed sharp increases during 1985–2008. Analysis of long-term climate records indicates that annual air temperatures have increased between 0.45 and 0.93°C per decade throughout most mountainous areas of Colorado, suggesting climate as a factor. Isotopic data reveal that sulfate in these lakes is largely derived from pyrite, which may indicate climate warming is preferentially affecting the rate of pyrite weathering.
Last interglacial plant macrofossils and climates from Ziegler Reservoir, Snowmass Village, Colorado, USA
Ninety plant macrofossil taxa from the Ziegler Reservoir fossil site near Snowmass Village, Colorado, record environmental changes at high elevation (2705 m asl) in the Rocky Mountains during the Last Interglacial Period. Present-day vegetation is aspen forest ( Populus tremuloides ) intermixed with species of higher ( Picea , Abies ) and lower ( Artemisia , Quercus ) elevations. Stratigraphic units 4–13 contain montane forest taxa found near the site today and several species that today generally live at lower elevations within ( Abies concolor , Lycopus americanus ) and outside Colorado ( Najas flexilis ). These data suggest near-modern climatic conditions, with slightly warmer summer and winter temperatures. This montane forest period was succeeded by a shorter treeless interval (Unit 14) representing colder and/or drier conditions. In units 15–16, conifer trees reoccur but deciduous and herb taxa are lacking, suggesting a return to warmer conditions, although cooler than during the earlier forest period. Comparison of these inferred paleoclimatic changes with the site's geochronologic framework indicates that the lower interval of sustained warmth correlates with late MIS 6–early 5b (~ 138–94 ka), the cold interval with MIS 5b (~ 94–87 ka), and the uppermost cool assemblages with MIS 5a (~ 87–77 ka).
Detecting Ecosystem Performance Anomalies for Land Management in the Upper Colorado River Basin Using Satellite Observations, Climate Data, and Ecosystem Models
This study identifies areas with ecosystem performance anomalies (EPA) within the Upper Colorado River Basin (UCRB) during 2005–2007 using satellite observations, climate data, and ecosystem models. The final EPA maps with 250-m spatial resolution were categorized as normal performance, underperformance, and overperformance (observed performance relative to weather-based predictions) at the 90% level of confidence. The EPA maps were validated using “percentage of bare soil” ground observations. The validation results at locations with comparable site potential showed that regions identified as persistently underperforming (overperforming) tended to have a higher (lower) percentage of bare soil, suggesting that our preliminary EPA maps are reliable and agree with ground-based observations. The 3-year (2005–2007) persistent EPA map from this study provides the first quantitative evaluation of ecosystem performance anomalies within the UCRB and will help the Bureau of Land Management (BLM) identify potentially degraded lands. Results from this study can be used as a prototype by BLM and other land managers for making optimal land management decisions.
How and why Upper Colorado River Basin land, water, and fire managers choose to use drought tools (or not)
On the Western Slope of Colorado, variable climate and precipitation conditions are typical. Periods of drought—which may be defined by lack of water, high temperatures, low soil moisture, or other indicators—cause a range of impacts across sectors, including water, land, and fire management. The Western Slope’s Upper Colorado River Basin (UCRB) was one of the first pilot areas in which the National Integrated Drought Information System (NIDIS) implemented a drought early warning system (DEWS) in 2009. NIDIS presently supports eight regional DEWS; as of 2016, the UCRB DEWS has been incorporated into an expanded Intermountain West (IMW) DEWS. The selection of the UCRB for an initial DEWS reflects the regional importance of drought information for managing water supply for agriculture and other uses, and the need for effective decision support related to drought. Additionally, new drought information products were developed specifically for the UCRB DEWS, and a number of others have been created since 2009, adding to the preexisting toolkit for drought decision making. The various elements of the UCRB drought early warning system can be expected to be more or less suitable for the needs of different decision makers. As a result, the UCRB makes an ideal case study to examine the use of scientific information products and tools in which the broad decision context (managing drought) is defined, but information needs of current and prospective users vary. Thus decision makers will make varied choices about which of the available tools to use or not use, depending on the particular management and institutional context in which they work. This report investigates the factors that affect the choices of decision makers about whether and how to use particular information sources, products, and tools. The investigation focused on the following research questions: What decisions do managers make related to drought in the Upper Colorado region and particularly the Western Slope of Colorado? About which impacts of drought are they most concerned? What indicators and information products do decision makers rely on to manage for the impacts of drought in this region? How do decision makers find out about and choose between available drought information sources, products, and tools? What gaps (if any) do they perceive in currently available drought information and tools? Studies of decision support tools or information sources often concentrate on the known users of a given tool(s). Such an approach can yield useful information; it provides rich insight into the experiences of users and can suggest design modifications to make existing tools more effective. Yet it is not an effective approach to capture the perspectives and needs of prospective tool users or to investigate the factors that affect whether or not someone chooses to use tools in the first place. To overcome this challenge, in this study the author instead used a geographically based sampling strategy in which a range of natural resource managers from preidentified Federal management units and selected State agencies on the Western Slope were considered prospective users of tools. Prospective users were then asked to describe in an open-ended fashion what information and tools they do or do not use and why. This approach allowed for respondents to report both use and nonuse of tools, and thus the ability to identify factors that influence information and tool use choices by managers.
Historical and prehistorical water levels of Mormon Lake, Arizona as a measure of climate change on the southwest Colorado Plateau, USA
Mormon Lake, elevation 2166 m with maximum historic surface area of 31.4 km 2 , lies in a forested endorheic basin covering 103 km 2 . It is the largest unaltered freshwater body on the 337,000 km 2 Colorado Plateau. Prehistorical (before AD 1878) highstands were ca. 9 and 24 m relative to depocenter datum. These levels likely occurred during four multidecadal episodes of cool, wet conditions between ca. 3.55 and 0.20 ka BP. Maximum historical levels (early 1900s) were up to 7.9 m, whereas modern (post-1941) levels were frequently zero or relatively low. Historical climate records indicate reconstructed lake levels correlate directly with annual precipitation and inversely with temperature. Early highstands were associated with above average precipitation and the lowest temperatures of the 116 yr record. The lake receded after 1941; thereafter, frequent drying and low-water levels resulted from recurrent drought and steadily increasing temperatures. Consequently, a wet episode from the 1970s to the 1990s had precipitation like the early 1900s, but highstands were only ca. 3.8 m. The historical lake-level chronology is consistent with changes of hydrologic balance predicted by climate models, that is, reduced effective precipitation (precipitation minus evaporation). These changes, particularly aridification, apparently began in the 1970s or earlier. Global oceanic and atmospheric climate modulate lake levels and regional hydroclimate.
Fragmentation and thermal risks from climate change interact to affect persistence of native trout in the Colorado River basin
Impending changes in climate will interact with other stressors to threaten aquatic ecosystems and their biota. Native Colorado River cutthroat trout (CRCT; Oncorhynchus clarkii pleuriticus ) are now relegated to 309 isolated high-elevation ( > 1700 m) headwater stream fragments in the Upper Colorado River Basin, owing to past nonnative trout invasions and habitat loss. Predicted changes in climate (i.e., temperature and precipitation) and resulting changes in stochastic physical disturbances (i.e., wildfire, debris flow, and channel drying and freezing) could further threaten the remaining CRCT populations. We developed an empirical model to predict stream temperatures at the fragment scale from downscaled climate projections along with geomorphic and landscape variables. We coupled these spatially explicit predictions of stream temperature with a Bayesian Network (BN) model that integrates stochastic risks from fragmentation to project persistence of CRCT populations across the upper Colorado River basin to 2040 and 2080. Overall, none of the populations are at risk from acute mortality resulting from high temperatures during the warmest summer period. In contrast, only 37% of populations have a greater than or equal to 90% chance of persistence for 70 years (similar to the typical benchmark for conservation), primarily owing to fragmentation. Populations in short stream fragments < 7 km long, and those at the lowest elevations, are at the highest risk of extirpation. Therefore, interactions of stochastic disturbances with fragmentation are projected to be greater threats than warming for CRCT populations. The reason for this paradox is that past nonnative trout invasions and habitat loss have restricted most CRCT populations to high-elevation stream fragments that are buffered from the potential consequences of warming, but at risk of extirpation from stochastic events. The greatest conservation need is for management to increase fragment lengths to forestall these risks.
What records have we been breaking?
"Today was another record-breaking day," the evening radio or television declares. High temperatures, low temperatures, floods, drought - take your choice. But how can we put these pronouncements in perspective? What do they really mean? We present two types of information in this article: 1) an analysis of daily air temperature and precipitation for Fort Collins and 2) an analysis of annual precipitation for Fort Collins. Each analysis provides a different meaning to the statement about a record-breaking day or year.
Summer runoff generation in foothill catchments of the Colorado Front Range
Climatic shifts, disturbances, and land-use change can alter hydrologic flowpaths, water quality, and water supply to downstream communities. Prior research investigating streamflow generation processes in mountainous areas has largely focused on high-elevation alpine and subalpine catchments; less is known about these processes in lower-elevation foothills and montane catchments. In these lower-elevation ecoregions, precipitation shifts seasonally from snow to rain, which can result in differing seasonal flowpaths. We analyzed stream water for electrical conductivity, SiO 2 , Ca, Mg, Na, Cl, SO 4 , K, and dissolved organic carbon on both a weekly and storm event basis from April to August 2018 in three small (<10 km 2 ) foothill catchments, and one larger (63.2 km 2 ) catchment extending from the foothills to the subalpine ecoregions, in the Colorado Front Range. Using two end-member hydrograph separations and concentration-runoff relationships, we inferred the dominant catchment-scale flowpaths of precipitation to the streams. We selected catchments with varying land use to investigate the relationship between these characteristics and hydrologic flowpaths. We observed that concentrations of lithogenic constituents generally increased and dissolved organic carbon decreased as seasonal runoff decreased in the three foothill catchments, reflecting a transition from shallow subsurface flowpaths to deeper subsurface flowpaths. Elevated SO 4 and Cl concentrations during low-flow periods in two of our catchments suggest that historical or current anthropogenic activities, such as mining, application of road salt, and/or near-stream septic systems, affect local stream and groundwater chemistry. In a foothill catchment with anthropogenic and geologic impervious surfaces, streamflow during storm responses was sourced from faster, surficial flowpaths compared to a less disturbed neighboring catchment, highlighting the influence of anthropogenic land-use on runoff generation. This study provides insight into the fundamental hydrology of foothill catchments and how they may function in the future with human development, precipitation shifts and disturbances.
Weathering the ride: Associations of heat, smoke, precipitation, and ozone with bus ridership in Colorado
Weather and climate variability increasingly shape urban travel behavior, yet the short-term temporal dynamics and contextual modifiers of weather-transit relationships remain poorly understood. We analyzed system-wide hourly bus ridership in the Denver metropolitan area from June 2022 through September 2023 using fixed-effects negative binomial distributed lag nonlinear models with lags up to 24 h. Nonlinear exposure-response and lag-response functions were specified for hourly Universal Thermal Climate Index (UTCI) and precipitation, with additional indicators for daily ozone exceedance and wildfire smoke. Models adjust for hour-of-day, day-of-week, and month-year fixed effects, with standard errors clustered by day. Stratified analyses assess heterogeneity by time period, season, fare policy, and shelter availability, with interaction evaluated using Wald tests. Cold thermal stress was associated with the largest, most persistent reductions in ridership, with cumulative declines of −15.7% (95% CI: −26.0%, −4.0%) over 0-24 h. Precipitation was associated with sharp but transient reductions concentrated within 3-6 h (−8.4% to −9.8%), with little evidence of longer-term displacement. Heat associations were weaker and context-dependent, with modest short-run increases, but net same-day declines. Associations varied by time of day and season, were attenuated during the free-fare period, and differed by shelter availability. In contrast, ozone exceedance and wildfire smoke exhibited limited and inconsistent associations, with measurable reductions primarily during morning commute during heavier smoke conditions. Overall, transit ridership is more strongly associated with short-term weather exposure, than with ambient air quality. Fare policy and stop-level infrastructure modify these associations, highlighting actionable strategies to enhance transit resilience under increasing climate variability.
Natural resource management decision-making under climate uncertainty: Building social-ecological resilience in southwestern Colorado
The goal of this project was to facilitate climate change adaptation that contributes to social-ecological resilience, ecosystem and species conservation, and sustainable human communities in southwestern Colorado. The team developed and piloted integrated adaptation planning tools and principles that merge the strengths of the iterative scenario process, the Adaptation for Conservation Targets (ACT) planning framework, institutional analysis, and climate modeling. These tools and principles generated practical strategies and scientific knowledge to advance climate change adaptation in both the Gunnison River Basin and San Juan River Basin and, potentially, other landscapes. A key objective of this project was to work with decision-makers to develop social-ecological adaptation strategies and coordinate actions to reduce the impacts of a changing climate on nature and people. To accomplish this, the project blended science (biophysical and social) and participatory approaches to integrate expert knowledge, land management decision making, and local needs. These adaptation strategies and their creation process were documented by the project team to assist communities and scientists elsewhere who are grappling with the challenges posed by climate change.
Preliminary geologic studies of cold climate dunes, North Park, Jackson County, Colorado
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Hydrological regime and climate interactively shape riparian vegetation composition along the Colorado River, Grand Canyon
Question How closely do riparian plant communities track hydrological and climatic variation in space, and how do interactions among hydrological and climatic filters influence success of flow management strategies? Location Grand Canyon, Arizona, USA. Methods Multi‐year vegetation surveys were conducted across three hydrological zones – active channel, active floodplain and inactive floodplain – within each of 42 sandbars that vary geographically in temperature and precipitation along a 400‐km river segment. Ecological niche models were used to estimate locally optimal conditions of maximum inundation duration, elevation above daily peak flow, mean annual precipitation, and mean maximum and minimum temperature for 16 of the most abundant woody and 58 most abundant herbaceous plant species. These estimates were used to calculate community‐weighted mean (CWM) environmental preferences, which were used to determine how closely vegetation preferences tracked local variation in environmental factors, and to assess interactive responses of species and communities to variation in hydrology and climate. Results Communities closely tracked hydrological variation across zones, but less so within zones. Communities tracked variation in minimum temperature more closely than maximum temperature or precipitation. At the species level, woody plants that were more abundant in wetter hydrological conditions were also more abundant in wetter climatic conditions, and vice versa. This relationship was even stronger at the community level, where there were significant negative relationships between CWM preferences of inundation duration and temperature for both woody and herbaceous vegetation. Conclusions The climate‐hydrology linkages found in this system suggest that increasing temperatures and drought are likely to reduce the inundation tolerance of riparian vegetation within the Grand Canyon. Increasing the duration of high flow events would likely reduce the abundance of encroaching woody vegetation, but could also reduce the resilience of remaining vegetation to heat waves and drought. The reinforcing effects of climatic and hydrological filters are likely to generally result in greater sensitivity of species composition to environmental change than if those environmental filters acted independently. These results have implications for predicting resource responses to environmental change, as well as prescriptions for direct vegetation management to enhance resilience.
Climate change impacts on native cutthroat trout habitat in Colorado streams
Headwater streams support vital aquatic habitat yet are vulnerable to changing climate due to their high elevation and small size. Coldwater fish are especially sensitive to the altered streamflow and water temperature regimes during summer low flow periods. Though previous studies have provided insights on how changes in climate and alterations in stream discharge may affect habitat availability for various native cutthroat trout species, suitable physical habitats have not been evaluated under future climate projections for the threatened Greenback Cutthroat Trout (GBCT) native to headwater regions of Colorado, USA. Thus, this study used field data collected from selected headwater streams across the current distribution of GBCT to construct one-dimensional hydraulic models to evaluate streamflow and physical habitat under four future climate projections. Results illustrate reductions in both predicted streamflow and physical habitat for all future climate projections across study sites. The projected mean summer streamflow shows greater decline (−52% on average) compared to the projected decline in mean August flow (−21% on average). Moreover, sites located at a relative higher elevation with larger substrate and steeper slope were projected to experience more reductions in physical habitat due to streamflow reductions. Specifically, streams with step-pool morphologies may experience grater changes in available habitat compared to pool-riffle streams. Future climate change studies related to coldwater fish that examine spatial variation in flow alteration could provide novel data to complement the existing literature on the thermal characteristics. Tailoring reintroduction and management efforts for GBCT to the individual headwater stream with adequate on-site monitoring could provide a more holistic conservation approach.
A climatic record from 14C-dated wood fragments from southwestern Colorado
Deuterium concentrations in trees are related to the climatic temperature at which the trees grew. Deuterium analyses were made on all available (39) 14C-dated (all 14C dates cited are uncorrected) wood fragments collected from Lake Emma sediments. The 14C dates range from 9600 to 5400 'B.P.'. Tree line was above Lake Emma at 9600 'B.P.', was at Lake Emma at about 5000 'B.P.', and is 80 m below Lake Emma at the present time. The isotopic records at the various intervals of time coincide very well with this history. The range of ??D values is maximum at 9600 'B.P.' and is minimum at about 5400 'B.P.'. These data allow us to estimate the temperature range for the area between tree line and Lake Emma between these times. These results confirm previously observed cooling trends from several sources in the Western Hemisphere.
Climatic influences on the growth of subalpine trees in the Colorado Front Range
We examined variations in tree growth responses to climatic variations among different tree species and habitat types in the subalpine zone of the Colorado Front Range. We constructed 25 tree ring site chronologies (11 of Picea engelmannii , 9 of Abies lasiocarpa , 4 of Pinus contorta var. latifolia , and 1 of Pinus flexilis ) from a series of subalpine habitats ranging from xeric to wet. To establish tree growth responses to climatic variation, we used correlation and response function analyses to compare variations in ring widths with monthly temperature and precipitation records. At the driest sites, growth of Picea and Abies tracked climatic variation similarly. At mesic and wet sites, however, these species differed in their responses to climatic variation. The responses of Pinus contorta , sampled over a narrower range of habitat types, differed from those of Picea and Abies but did not differ among sites. Steep environmental gradients in the subalpine zone of the Front Range accounted for most of the observed differences in growth responses to climatic variation. Even at adjacent sites that differ only slightly in topographic position, tree growth responses to climatic variation were distinct. Interspecific differences in response to climatic variations generally were less important than site differences. Intersite differences in tree growth responses to climatic variation can be used as indicators of environmental differences among subalpine habitats.
High resolution SnowModel simulations reveal future elevation-dependent snow loss and earlier, flashier surface water input for the Upper Colorado River Basin
Continued climate warming is reducing seasonal snowpacks in the western United States, where >50% of historical water supplies were snowmelt-derived. In the Upper Colorado River Basin, declining snow water equivalent (SWE) and altered surface water input (SWI, rainfall and snowmelt available to enter the soil) timing and magnitude affect streamflow generation and water availability. To adapt effectively to future conditions, we need to understand current spatiotemporal distributions of SWE and SWI and how they may change in future decades. We developed 100-m SnowModel simulations for water years 2001–2013 and two scenarios: control (CTL) and pseudo-global-warming (PGW). The PGW fraction of precipitation falling as snow was lower relative to CTL, except for November–April at high elevations. PGW peak SWE was lower for low (−45%) and mid elevations (−14%), while the date of peak SWE was uniformly earlier in the year for all elevations (17–23 days). Currently unmonitored high elevation snow represented a greater fraction of total PGW SWE. PGW peak daily SWI was higher for all elevations (30%–42%), while the dates of SWI peaks and centroids were earlier in the year for all elevations under PGW. PGW displayed elevated winter SWI, lower summer SWI, and changes in spring SWI timing were elevation-dependent. Although PGW peak SWI was elevated and earlier compared to CTL, SWI was more evenly distributed throughout the year for PGW. These simulated shifts in the timing and magnitude of SWE and SWI have broad implications for water management in dry, snow-dominated regions.