Scientific Investigations Report 2006–5305

Scientific Investigations Report 2006–5305

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Potential Effects of Land- and Water-Use Changes

The water-budget components of Carson Valley will be most greatly affected by: changes in land use from agricultural areas or areas of phreatophytic vegetation, to residential or commercial use; or changes in water use that include the increased application of effluent for irrigation, increased ground-water pumping, and changes in the configuration of the surface-water irrigation distribution system. With the exception of increased ground-water pumping, these changes will tend to decrease the volume of water lost to ET from streamflow of the Carson River and streams tributary to the valley floor and increase the volume of Carson River outflow from the valley. A numerical ground-water model would most accurately determine the net effect of these changes in land and water use.

Changes in water levels in wells on the eastern side of Carson Valley provide evidence of the effects of changes in the configuration of the irrigation distribution system. Water levels at wells 6, 8, and 9 (figs. 7 and 19) have declined about 20 ft from those in the early 1990s. The declines likely were caused by the discontinued use in 1997 of reservoirs about 1 mi from the wells which had been in use since the early 1900s (fig. 7). Infiltration from the reservoir maintained relatively high water levels in the surrounding area, which are continuing to decline in 2006. Conversely, water levels in a well near the mouth of Buckeye Creek on the eastern side of Carson Valley (well 7) have risen about 20 ft from those in the early 1990s, likely caused by infiltration losses from an effluent reservoir also about 1 mi to the northeast (figs. 7 and 19). Installation of a new ditch in the late 1990s near well 30 caused water levels to decline about 6 ft, and water levels at well 22 declined about 2 ft after flow from a nearby irrigation ditch was placed into an underground pipe. A change from agricultural use to residential use with some continued irrigation near well 23 (fig. 19) may be in part responsible for declines of about 10 ft, however, the well also is within 1 mi of a reservoir with discontinued use (fig. 7).

Discontinued use of the remaining reservoir on the eastern side of Carson Valley likely would cause water-level declines of a similar magnitude and over a similar area. Installation of deep ditches that act as drains, and lining or piping of small ditches also would likely cause water-level declines of 2 to 5 ft in areas relatively near the change.

Currently planned land-use changes in Carson Valley include as much as 350 acres where use would change from agricultural to residential or commercial use (Mimi Moss, Douglas County Planning, oral commun., 2005). The land-use change also is adjacent to a major irrigation ditch that currently is not planned to be piped but could be in the future. The decrease in irrigated land would result in a decrease in ET of about 1,000 acre-ft/yr, assuming an ET rate of 3.0 ft/yr. The canal is on the southeastern part of the valley where depth to water is greater than about 20 ft. The potential reduction in infiltration losses from the canal, assuming a loss rate of 2 ft/d and a canal width of about 10 ft, would be about 900 acre-ft/yr. The planned land-use changes likely would cause an increase in streamflow of the Carson River of about 2,000 acre-ft/yr from the decrease in ET and infiltration losses. The change in land use also is likely to increase the runoff of precipitation from impervious surfaces, however, the effect of this change depends on the use of the storm-water drainage. Along with the increase in flow of the Carson River, ground-water levels in the areas of land-use change likely would decline somewhat, however, it is difficult to predict the magnitude of the water-level declines and the area which may be affected without a detailed study of the area undergoing change.

Another potential land-use change is the development of 19-acre parcels on agricultural land (Mimi Moss, Douglas County Planning, oral commun., 2005). Irrigation ditches on these parcels would be maintained if downstream users are present, however, it is not certain that irrigation would continue. The effect of this type of development on the water budget depends on the location where irrigation is discontinued and the resulting vegetation that may replace the irrigated crops. If irrigation is discontinued on the valley floor where irrigated pasture grasses are present, the decrease in ET can be estimated from the area of discontinued irrigation, multiplied by 1.3 ft/yr, the difference between the ET rate of 3.0 ft/yr for irrigated pasture grasses and 1.7 ft/yr for non-irrigated pasture grasses. The decrease in ET caused by development in areas of native phreatophytes could be estimated in a similar manner, using an ET rate of about 1.9 ft/yr for the acreage removed. Land-use changes in areas currently without irrigated crops or pasture grasses, or without native phreatophytes, likely would have little effect on the overall water budget of Carson Valley.

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