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In 1988, the US Geological Survey began a study of the effects of potential climate change on the water resources of the Gunnison River basin. The Gunnison River, in southwestern Colorado, is an important tributary of the Colorado River, contributing approximately 40% of the flow of the Colorado River at the Colorado/Utah stateline. As part of the study, the sensitivity of annual and seasonal runoff in the East River basin, a sub-basin of the Gunnison River basin, to changes in temperature and precipitation was examined. To perform the sensitivity analyses, hypothetical climate changes were used to alter current time series of temperature and precipitation. The altered time series were then used as inputs to a hydrological...
The flow regime is regarded by many aquatic ecologists to be the key driver of river and floodplain wetland ecosystems. We have focused this literature review around four key principles to highlight the important mechanisms that link hydrology and aquatic biodiversity and to illustrate the consequent impacts of altered flow regimes: Firstly, flow is a major determinant of physical habitat in streams, which in turn is a major determinant of biotic composition; Secondly, aquatic species have evolved life history strategies primarily in direct response to the natural flow regimes; Thirdly, maintenance of natural patterns of longitudinal and lateral connectivity is essential to the viability of populations of many riverine...
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Two distributed parameter models, a one-dimensional (1D) model and a two-dimensional (2D) model, are developed to simulate overland flow in two small semiarid shrubland watersheds in the Jornada basin, southern New Mexico. The models are event-based and represent each watershed by an array of 1-m2 cells, in which the cell size is approximately equal to the average area of the shrubs. Each model uses only six parameters, for which values are obtained from field surveys and rainfall simulation experiments. In the 1D model, flow volumes through a fixed network are computed by a simple finite-difference solution to the 1D kinematic wave equation. In the 2D model, flow directions and volumes are computed by a second-order...
Vegetation, particularly its dynamics, is the often-ignored linchpin of the land-surface hydrology. This work emphasizes the coupled nature of vegetation-water-energy dynamics by considering linkages at timescales that vary from hourly to interannual. A series of two papers is presented. A dynamic ecohydrological model [tRIBS + VEGGIE] is described in this paper. It reproduces essential water and energy processes over the complex topography of a river basin and links them to the basic plant life regulatory processes. The framework focuses on ecohydrology of semiarid environments exhibiting abundant input of solar energy but limiting soil water that correspondingly affects vegetation structure and organization. The...
The atmospheric water balance over the upper Colorado River is evaluated twice daily for the seven winter seasons 1957?1963. The atmospheric water balance yields the exchange of water and water vapor at the earth-atmosphere interface through the observation of the spatial and time distributions and fluxes of water vapor in the atmosphere over the basin. The quantity precipitation minus evaporation is determined as a residual of the computation and is accumulated for daily and seasonal values. In addition, a natural period analysis is performed; the natural periods are delineated by homogeneity in the parameter precipitation minus evaporation. The dry periods are shown to exhibit a seasonal trend in evaporation rate...
Recent research has suggested that changes in temperature and precipitation events due to climate change have had a significant impact on the availability and timing of streamflow. In this study, monthly temperature and precipitation data collected over 29 climate divisions covering the entire Colorado River basin and monthly natural flow data from 29 U.S. Geological Survey (USGS) gauge locations along the Colorado River are investigated for trend or step changes using parametric and nonparametric statistical tests. Temperature increases are persistent (at least 10 climate divisions over 6 months in trend analysis) throughout the year over the Colorado River basin, whereas precipitation only notably increased over...
We investigate herein the hypothesis that there is a significant relationship between bed particle mobility and benthic invertebrate abundance in the gravel-bed channel of the upper Colorado River in Rocky Mountain National Park. A large diversion channel called the Grand Ditch normally diverts a significant portion (?50%) of the annual snowmelt runoff from the watershed northward out of the basin. In May 2003, a ?30-m section of the ditch was breached, contributing substantially to the magnitude and duration of discharge in the Colorado River until the ditch breach was repaired in July of that year. As a result, all grain sizes in the river channel were mobilized, which contrasted sharply with the minimal gravel...
The Colorado River supplies water to 27 million users in 7 states and 2 countries and irrigates over 3 million acres of farmland. Global climate models almost unanimously project that human-induced climate change will reduce runoff in this region by 10-30%. This work explores whether currently scheduled future water deliveries from the Colorado River system are sustainable under different climate-change scenarios. If climate change reduces runoff by 10%, scheduled deliveries will be missed approximately 58% of the time by 2050. If runoff reduces 20%, they will be missed approximately 88% of the time. The mean shortfall when full deliveries cannot be met increases from approximately 0.5-0.7 billion cubic meters per...
The influence of woody vegetation and biological soil crusts on infiltration capacity is one of the several uncertainties associated with the ecohydrologic effects of woody plant encroachment into arid and semi-arid land systems. The objective of this study was to quantify the effects of Utah juniper (Juniperus osteosperma) and pinyon pine (Pinus edulis) on subcanopy and intercanopy ecohydrologic properties. We measured soil sorptivity, unsaturated hydraulic conductivity [K(h)], soil water content (SWC), and water repellency along radial line transects from under Juniperus and Pinus trees into the centre of the intercanopy space between trees. In the subcanopy, litter mounds, hydrophobic soils, and roots all appear...
The hydrology and geomorphology of large rivers in America reflect the pervasive influence of an extensive water control infrastructure including more than 75,000 dams. One hundred thirty-seven of the very large dams, each storing 1.2 km3 (106 acre feet) of water or more, alter the flows of every large river in the country. The hydrologic effects of these very large dams emerge from an analysis of the stream gage records of 72 river reaches organized into 36 pairs. One member of each pair is an unregulated reach above a dam, whereas the other is a regulated reach downstream from the same structure. Comparison of the regulated and unregulated reaches shows that very large dams, on average, reduce annual peak discharges...
Aim A regional analysis was used to explore the influence of river regulation on the dominance of non-native, invasive shrubs and trees. We addressed the following questions: (1) How do large dams affect hydrological parameters that influence riparian vegetation? (2) How do flow regimes affect the dominance of non-native woody species? (3) How do changes in flow regimes affect the dominance of non-native woody species? Location South-western USA. Methods We sampled the canopy cover of woody species on 179 point bars along seven non-dammed and thirteen dammed river segments. Wilcoxon rank sum tests were used to determine differences between flow parameters in dammed and non-dammed rivers. We used correlation analyses...
Ecological thresholds that lead to alternative community states can be exceeded through gradual perturbation or as a result of sudden disturbance. Many Great Plains streams have experienced dramatic changes in their hydrologic regime resulting from water and landuse changes that began as early as 1880. These changes, combined with the presence of many invasive species, have substantially altered the fish communities in this area. We quantified temporal changes in fish communities in 3 large river basins in relation to putative anthropogenic stressors, including increased sediment supply derived from row-crop agriculture (beginning in 1880), habitat fragmentation caused by reservoir construction (beginning in the...
We determined changes in willow (Salixspp.) cover in two valleys of the eastern slope of Rocky Mountain National Park,Colorado, USA, and related these changes to suspected causative factors. Changes in vegetation were inferred from digital maps generated from aerial photo-interpretation and field surveys conducted with a global positioning system. The decrease in riparian shrub cover was approximately 20% in both valleys over the period between 1937/46 and 1996, while the decline in tall willow (> 2 m tall) cover was estimated to be approximately 55%in both valleys. Suppressed willows (< 1.5 m tall) were predominantly located in areas affected by flooding and in areas where major river reductions were observed....
Riparian ecosystems in the south-western United States have undergone extensive physical and biological changes, due, in part, to alteration of natural flow regimes and suppression of fluvial processes. Many riparian ecosystem restoration projects are achieving success because they recognize the importance of restoring the hydrologic regime. In other words, these projects are restoring flows of water and sediment in sufficient quantities and with appropriate temporal and spatial patterns. Other projects have proceeded without recognition of the need to incorporate environmental stream flow requirements into management plans. To increase success rate of riparian ecosystem restoration, this paper describes some changes...
Aim: To test the hypothesis that anthropogenic alteration of stream-flow regimes is a key driver of compositional shifts from native to introduced riparian plant species. Location: The arid south-western United States; 24 river reaches in the Gila and Lower Colorado drainage basins of Arizona. Methods: We compared the abundance of three dominant woody riparian taxa (native Populus fremontii and Salix gooddingii, and introduced Tamarix) between river reaches that varied in stream-flow permanence (perennial vs. intermittent), presence or absence of an upstream flow-regulating dam, and presence or absence of municipal effluent as a stream water source. Results: Populus and Salix were the dominant pioneer trees along...
We use a spatially explicit landscape model to investigate the potential role of rainfall on shrub?grass transitions in the Jornada Basin of southern New Mexico during the past century. In long-term simulations (1915?1998) along a 2700 m transect running from a dry lake bed to the foothills of a small mountain, we test two hypotheses: (i) that wetter winters and drier summers may have facilitated shrub encroachment in grasslands, and (ii) that increases in large precipitation events may have increased soil water recharge at deeper layers, thus favoring shrub establishment and growth. Our model simulations generally support the hypothesis that wetter winters and drier summers may have played a key role, but we are...
Although our current (1990) knowledge of hydrologic and hydraulic processes is based on many years of study, there are river environments where these processes are complex and poorly understood. One of these environments is in mountainous areas, which cover about 25 percent of the United States. Use of conventional hydrologic and hydraulic techniques in mountain-river environments may produce erroneous results and interpretations in a wide spectrum of water-resources investigations. An ongoing U.S. Geological Survey research project is being conducted to improve the understanding of hydrologic and hydraulic processes of mountainous areas and to improve the results of subsequent hydrologic investigations. Future...
Slope wetlands generally occur at breaks in slope where discharging ground water maintains moist soil conditions. They often are found on the perimeter of highly permeable alluvial fans, but there have been no detailed hydrologic studies of these particular wetlands. We combined stream and spring flow measurements with five years of water-level and piezometric data to understand the hydrology of a 1.6 ha slope wetland at the base of a 5.2 ha alluvial fan in the central Rocky Mountains of Colorado. Step changes in streamflow inputs resulting from an upstream water diversion helped confirm the linkages inferred from the hydrometric data. Nearly 30% of the streamflow along a 180-m reach on the alluvial fan was lost...
The Green River is a major tributary of the Colorado River with a drainage area of 115 770 km2 in Colorado, Utah and Wyoming. The influence of Flaming Gorge Dam on sediment transport and the potential for future channel change were studied using comparative analysis of historical aerial photographs from 1952 to 1987 and geographical information systems, published sediment (1951-86) and discharge (1965-87) records, and sediment data collected during 1986-8. Since the closure of the dam in 1964, new equilibrium channel widths were apparently achieved by 1974 in the reach 161-279 km below Flaming Gorge Reservoir and by 1981 in the reach 465-509 km below the reservoir. Recent high flows have resulted in an increase...
This paper considers the risk of drought and develops drought scenarios for use in the study of severe sustained drought in the Southwestern United States. The focus is on the Colorado River Basin and regions to which Colorado River water is exported, especially southern California, which depends on water from the Colorado River. Drought scenarios are developed using estimates of unimpaired historic streamflow as well as reconstructions of streamflow based on tree ring widths. Drought scenarios in the Colorado River Basin are defined on the basis of annual flow at Lees Ferry. The risk, in terms of return period, of the drought scenarios developed, is assessed using stochastic models. Published in Journal of the...
map background search result map search result map One- and two-dimensional modelling of overland flow in semiarid shrubland, Jornada basin, New Mexico One- and two-dimensional modelling of overland flow in semiarid shrubland, Jornada basin, New Mexico