Scientific Investigations Report 2007–5205
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5205
The watershed models provide estimates of annual mean unit-area rates of precipitation, runoff, evapotranspiration (ET), and ground-water inflow, in inches per acre for water years 1990–2002. In this report, the annual mean rates are reported as water-equivalent heights, in inches. Mean annual volumes, in acre-feet, were computed for water years 1990–2002 by averaging the annual mean rates and multiplying by the drainage area of the watersheds, for comparison with the volumes estimated by Maurer and Berger (2007).
Simulated daily mean and annual mean runoff matches measured and reconstructed runoff reasonably well for most perennial watersheds, with the exceptions of Monument Creek, Sheridan Creek, and Jobs Canyon (table 4; figs. 11 and 12). The difference between simulated and measured or reconstructed mean annual runoff was 11 percent or less, also with the exceptions of Monument Creek, Sheridan Creek, and Jobs Canyon (table 4). The simulated mean annual runoff was 31 percent less than the reconstructed runoff for Monument Creek watershed (table 4). High amounts of runoff as a percentage of precipitation for Mott Canyon and Monument Creek watersheds were noted by Maurer and Berger (2007, p. 32; table 1) along with the observation that they are not incised as greatly into the mountain blocks as other watersheds. Both observations indicate that these watersheds may be underlain by less permeable and less fractured bedrock, which is consistent with no ground-water inflow simulated from the two watersheds (table 4, fig. 12).
Mean annual runoff was underestimated for Sheridan Creek watershed by 46 percent, no ground-water inflow from the watershed was simulated, and the rates of ground-water inflow simulated for Stutler Canyon watershed were considerably greater than any other watershed (table 4; fig. 12). These results are consistent with the observations of Maurer and Berger (2007, p. 30), who noted large differences in runoff as a percentage of precipitation from the two watersheds (table 1), and suggested that subsurface flow may be taking place from the Stutler Canyon watershed to the Sheridan Creek watershed.
The source of runoff in the Sheridan Creek watershed is a series of springs that issue from the base of a ridge separating the two watersheds, and the Stutler Canyon watershed lies at a higher altitude, to the west of the Sheridan Creek watershed (watersheds 8u and 9u, respectively, in fig. 3). For these reasons, estimates of ground-water inflow to Carson Valley were made from the combined areas of the Stutler Canyon and Sheridan Creek watersheds by Maurer and Berger (2007). Results of the watershed modeling suggest that the deficiency of simulated runoff from Sheridan Creek watershed, 600 acre-ft/yr, may be supplied from the 1,600 acre-ft/yr of subsurface flow simulated from Stutler Canyon watershed (table 4). The remaining 1,000 acre-ft/yr from Stutler Canyon watershed likely becomes ground-water inflow to basin-fill aquifers of Carson Valley.
Simulated daily mean runoff from Jobs Canyon watershed generally was overestimated from 1990 to 1995, but matches reconstructed runoff more closely from 1996 to 2002 (fig. 10). Annual mean runoff appears to match reconstructed runoff more closely than daily mean runoff from 1990 to 1995 (fig. 12), likely because of compensating differences from periods of under- and overestimation during the year. Mean annual runoff was overestimated by 18 percent for water years 1990–2002 (table 4). The difference between the simulated and estimated runoff volumes may not be meaningful because, as stated previously, the reconstructed daily mean flows have an uncertainty of as much as 30 percent. However, the reconstructed runoff represents the best available estimate of daily flows from the ungaged watersheds.
For all perennial watersheds, simulated precipitation was within 15 percent of that estimated by Maurer and Berger (2007), largely because effort was made during calibration to match the previous estimate of precipitation, as previously discussed. Simulated ET from the models generally was less than ET estimated by Maurer and Berger (2007) largely because of the greater volumes of simulated ground-water inflow, and also in part, because of differences between simulated and reconstructed runoff. The ET estimates made by Maurer and Berger (2007) were calculated as the difference between precipitation and the combined volumes of runoff and estimates of ground-water inflow. Their ET estimates therefore include the combined errors associated with ground-water inflow estimates and the reconstructed runoff estimates. ET estimates in the PRMS model are a summation of sublimation, soil water loss, canopy interception loss, and evaporation from impervious surfaces.
Simulated annual rates of ground-water inflow from the watersheds to the basin-fill deposits of Carson Valley generally are less than 5 in. for most watersheds, but are as great as 22 in. for Stutler Canyon, about 12 in. for Daggett Creek, and about 10 in. for Pine Nut and Buckeye Creeks (figs. 9 and 12). The high annual rates for Stutler Canyon watershed are explained by the likelihood of subsurface flow to Sheridan Creek (not simulated), which would reduce ground-water inflow contributions from Stutler Canyon watershed to the basin-fill deposits of Carson Valley. The high rates for Daggett Creek watershed are consistent with its low amount of runoff as a percentage of precipitation, 21 percent (table 1). The relatively low amount of runoff indicates that bedrock underlying the Daggett Creek watershed likely is more fractured and permeable than other watersheds, allowing greater rates of infiltration and ground-water inflow. The Daggett Creek watershed lies at the topographically lowest point along the crest of the Carson Range; further indication that bedrock underlying the watershed is more fractured, erodible, and permeable than other watersheds of the Carson Range. Bedrock underlying the Pine Nut and Buckeye Creek watersheds (watersheds 13g and 14g, respectively, in fig. 3) also may be more fractured than those of the Carson Range, as indicated by the number of mapped faults. Moore (1969, p. 18) describes the Pine Nut Mountains as being composed of several orographic blocks which have been tilted individually, in contrast to the Carson Range, which generally was uplifted along a single fault zone with a large displacement (fig. 3).
The simulated mean annual volume of ground-water inflow to the basin-fill aquifers of Carson Valley from the 14 perennial watersheds for 1990–2002 totaled 14,400 acre-ft, more than twice the 6,600 acre-ft estimated by Maurer and Berger (2007; table 4) using the water-yield method. In most cases, greater volumes of simulated ground-water inflow coincided with greater volumes estimated by Maurer and Berger (2007). The watershed models simulated ground-water inflow from Water Canyon and Fredericksburg Canyon watersheds, whereas Maurer and Berger (2007) estimated no ground-water inflow using the water-yield method.
Lacking measured streamflow data, calibration of the ephemeral watershed models was limited to the adjustment of precipitation volumes to match that estimated by Maurer and Halford (2004) because the amount of ephemeral runoff is uncertain. Simulated precipitation rates generally were less than those simulated for the perennial watersheds (tables 4 and 5), as would be expected because of the lower altitude of most of the ephemeral watersheds. Similarly, simulated ET rates generally were less than those for the perennial watersheds, primarily because of the lower rates of precipitation.
Simulated runoff and ground-water inflow for the ephemeral watersheds of the Carson Range depend greatly on the selected index model and precipitation inputs (table 5). For those watersheds that used the Daggett Creek index model, simulated runoff rates and volumes were similar in magnitude to simulated ground-water inflow rates and volumes. For those watersheds that used the Fredericksburg Canyon index model, simulated runoff rates and volumes were considerably greater than simulated ground-water inflow rates and volumes. The two ephemeral watersheds that have large volumes of runoff (watersheds 5e and 10e, table 5 and fig. 3) are large in area and, more importantly, have HRUs at altitudes greater than 8,000 ft, thus receiving proportionately more precipitation.
Simulated mean annual runoff rates ranged from about 5 to 7 in. for ephemeral watersheds using Daggett Creek watershed model as an index model, and from 9 to 13 in. for ephemeral watersheds using Fredericksburg Canyon as an index model (table 5). Mean annual ephemeral runoff from the Carson Range simulated from the models totaled 9,900 acre-ft with an overall runoff rate of 8.4 in., similar to the volume of 8,000 acre-ft and rate of 7 in. estimated by Maurer and others (2004, p. 14).
The large area of ephemeral runoff modeled near the Pine Nut Mountains (watershed 11e, fig. 3) had a mean annual simulated runoff volume of 800 acre-ft, for the area of 78,200 acres, or a rate of 0.1 in. The area of the watershed model near the Pine Nut Mountains was selected to include the general area of exposed semiconsolidated sediments (fig. 3). In this study, runoff from the eastern side of the valley floor underlain by alluvial fans was assumed to be negligible.
Mean annual ground-water inflow rates simulated for ephemeral watersheds of the Carson Range ranged from 0.07 to 0.7 in. for watersheds using the Fredericksburg Canyon index model, and from about 5 to 7 in. for watersheds using the Daggett Creek index model (table 5). The mean annual ground-water inflow simulated from ephemeral watersheds of the Carson Range totaled 3,500 acre-ft. Simulated mean annual ground-water inflow from the ephemeral watershed on the eastern side of the valley (watershed 11e) totaled 5,700 acre-ft, for an annual rate of 0.9 in. (table 5).
The simulated ground-water inflow from the perennial and ephemeral watersheds was combined with estimates of the infiltration of simulated ephemeral runoff to provide the total volumes of ground-water inflow to the basin-fill aquifers of Carson Valley from the Carson Range and Pine Nut Mountains (table 6). Application of the watershed models provides insight into the effect of climate variability on the water resources of Carson Valley. For this reason, ground-water inflow simulated from the watershed models was summarized for water years 1990–2002, for dry conditions during water years 1990–92, and for wet conditions during water years 1995–97 to show the variability in ground-water inflow.
Maurer and Berger (2007, p. 36) assumed all ephemeral runoff infiltrates the western alluvial fans and becomes ground-water inflow, supported by evidence that little ephemeral runoff from the Carson Range reaches the valley floor. Studies by Constantz and others (1994, p. 3261) in New Mexico, and Ronan and others (1998, p. 2142) in Eagle Valley, Nev., show that 4 to 6 percent of runoff from similar ephemeral streams is lost to evaporation and near-channel evapotranspiration. For this reason, the volumes of simulated ephemeral runoff that were assumed to infiltrate and become ground-water inflow were decreased by 5 percent. During extreme runoff events, some runoff likely reaches the valley floor. However, such events are rare and runoff reaching the valley floor is assumed to be a small percentage of mean annual runoff, and can be neglected.
Variability in ground-water recharge to basin-fill aquifers of Carson Valley reflects the year-to-year differences in climatic conditions. Quantifying potential changes in ground-water recharge during wet and dry hydrologic conditions is useful for estimating the ground-water and surface-water response to changing conditions, as well as for evaluating the hypothetical effects of long-term climate change on ground-water inflow to Carson Valley if annual precipitation is reduced. The mean annual ground-water inflow to basin-fill aquifers of Carson Valley simulated by the watersheds models for water years 1990–2002 was 35,000 acre-ft, with 19,000 acre-ft from the Carson Range and 16,000 acre-ft from the Pine Nut Mountains (table 6). Simulated mean annual ground-water inflow varied an order of magnitude, from 7,800 acre-ft during dry conditions (water years 1990–92), to 76,000 acre-ft during wet conditions (1995–97). The variation in ground-water inflow from dry to wet conditions is less in the Carson Range than the Pine Nut Mountains.
The large variability in ground-water inflow indicates that the annual volume of this source of recharge to basin-fill aquifers of Carson Valley depends greatly on climate. Knowledge of the potential range in ground-water inflow from wet to dry conditions is useful in developing drought-mitigation plans by water managers. Long-term climate changes that reduce the amount of annual precipitation have the potential to greatly affect ground-water inflow to the basin-fill aquifers of Carson Valley.
A major source of uncertainty in the estimates of ground-water inflow simulated by the watershed models was the selection of index models for the ephemeral watersheds of the Carson Range. For the ephemeral watersheds where runoff has not been measured, bedrock type was the only basis for index model selection. Therefore, simulated runoff could not be verified. The uncertainty in ground-water inflow from ungaged perennial watersheds from index model selection was considered much less because model parameters were adjusted to match the reconstructed runoff from each watershed. The uncertainty in ground-water inflow and runoff from the area of ephemeral runoff on the eastern side of the valley (watershed 11e, fig. 3) also is considered small because the Buckeye Creek index model represents an area with surficial geology similar to that of the area of ephemeral runoff.
The uncertainty involved with selection of the appropriate index model for ephemeral watersheds was evaluated by applying the mean annual unit-area rates obtained for watersheds modeled with each index model type to the entire area of ephemeral runoff from the Carson Range (table 7). The index models developed for the Daggett Creek and Fredericksburg Canyon watersheds may be viewed as end-members for estimates of ground-water inflow and ephemeral runoff from the Carson Range.
The mean annual unit-area values for runoff and ground-water inflow from the combined area of ephemeral watersheds of the Carson Range were 10.5 and 0.3 in., respectively for watersheds using the Fredericksburg Canyon index model, and 5.8 and 6.2 in., respectively for watersheds using the Daggett Creek index model (table 7). Applying these unit-area values to the entire area of all ephemeral watersheds, 14,300 acres, results in volumes of mean annual runoff ranging from 6,900 to 12,500 acre-ft, and volumes of mean annual ground-water inflow ranging from 400 to 7,400 acre-ft.
The potential range of ground-water inflow from the Carson Range can be assessed by substituting the low- and high-range values from the uncertainty analysis for ephemeral watersheds of the Carson Range in table 6. The resulting totals for annual ground-water inflow, after reducing the range in ephemeral runoff by 5 percent for evapotranspiration losses, range from 13,000 to 26,000 acre-ft. This range is considerably less than the range in annual ground-water inflow from dry to wet conditions, 7,200 to 32,000 acre-ft.
Watersheds of the Carson Range underlain by metamorphic rocks had runoff efficiencies of 38 to 45 percent, with the exception of Sierra Canyon (table 1). With the exceptions of the anomalous high-runoff-efficiency watersheds of Mott Canyon, Monument Creek, and Sheridan Creek, and the anomalous low-runoff efficiency watershed of Stutler Canyon, watersheds underlain by granitic rocks had runoff efficiencies of 21 to 31 percent. Thus, bedrock type appears to affect hydrologic processes controlling runoff and ground-water inflow to the basin-fill deposits of Carson Valley. For this reason, the volumes simulated for ephemeral watersheds of the Carson Range using the index model selected on the basis of bedrock type is considered to be the best and most reasonable estimate.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5205
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