While the Carson River basin has undergone a lot of changes over the quarter century between 1981 and 2006, those changes haven't been enough to show a big difference in the amount of water carried by the river, according to a study conducted by the U.S. Geological Survey.
The study showed that changes in the river's streamflow for the 25-year period of the study were either not great enough to be measured, masked by precipitation during periods of greatest change or offset by other land and water uses.
The study revealed that the amount of irrigated agricultural land in Carson Valley has remained stable. Estimates that 48,000 acres of land was being irrigated in the 1970s may have been exaggerated after looking at aerial photos from 1967, according to the study conducted by Douglas K. Maurer, Angela P. Paul, David L. Berger and C. Justin Mayers.
"An examination of aerial photographs taken in 1967 shows about 700 acres of irrigated fields had been replaced by homes by 1979, the report said. "The apparent decrease in irrigated acreage from the 1970s to 2004 is largely the result of inaccuracies in the acreage estimates for the early 1970s."
The report said land-use changes in Carson Valley included about 37,000 acres of irrigated land in the Nevada part. Another 4,000 acres of wetlands, open water and riparian vegetation may have been included in the earlier study.
Irrigating Carson Valley's fields is responsible for an annual decrease in flow of the river over the 67 years that records have been kept. An estimated 47,000 acre feet go to irrigate crops.
In Carson Valley, water levels in wells on the Western alluvial fans and in shallow wells on the Valley floor fluctuated in response to changes in annual precipitation, but overall showed no evidence of long term trends from 1981-2006. Water levels in artesian wells, deep irrigation wells and wells on the east side of the Valley show 10-15 foot decline from 1981-2006. Scientists said it was difficult to tell if the decline was caused by pumping or the low precipitation in 1987 to 1992 and from 1999-2004, the longest periods of extreme drought on record.
Beginning in the late 1980s about 6,300 acres of land in the eastern and southern parts of the Valley were irrigated by sprinklers using surface, ground water and treated effluent.
By 2005, 8000 acres of land was being irrigated by 9,500 acre-feet of treated effluent.
Maurer and Berger concluded that groundwater and surface water interactions take place throughout the Valley, but the volumes are likely minimal. Location and amount vary during the irrigations season and from wet to dry years.
Surface water is used largely for irrigation of pasture grasses and alfalfa in Carson, Dayton and Churchill Valleys. Municipal use mainly comes from ground water.
There are about 3,500 domestic wells in Carson Valley. Pumpage is figured at 1.12 acre feet per well. Irrigation pumping is 3,000 acre feet in wet years, 5,000 acre-feet in average years and 10,000 acre feet in dry years. In then 1980s and 1990s, pumping increased to more than 30,000 acre feet.
