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Climate
Climate factors that influence water resources the most are annual precipitation and evaporation. Statewide, total precipitation averages approximately nine inches per year, making Nevada the most arid state. Although the climate is generally characterized as semi-arid to arid, actually precipitation, evaporation, and other climate factors vary greatly. Figure 2-4 shows the large regional variation in average annual precipitation.
Annual average precipitation ranges from three inches in the Mojave Desert region of southern Nevada to more than 40 inches (over 300 inches of snowfall) on Mount Rose in the Carson Range, near Lake Tahoe. Both elevation and latitudinal differences are causes for these extremes. Year to year and month to month, the amount of precipitation can fluctuate greatly. This variability creates uncertainty for irrigators, water suppliers, fish and wildlife managers, and stream flow forecasters. Factors contributing to unpredictable snow and rainfall patterns are seasonal variability in the approach of moisture-bearing storm fronts from the Pacific Ocean, and the rain shadow effect created by the Sierra Nevada Range along the state's western border as well as dozens of other high elevation mountain ranges.
Of the total annual precipitation falling in Nevada, on average less than 10 percent produces stream runoff or percolates downward to recharge aquifers. Nevada is desert-like, because on average, 90 percent of the moisture is returned to the atmosphere by evaporation and plant transpiration.
Similar to the state's precipitation pattern, the rate of evaporation varies tremendously in time and space. Key factors are elevation, latitude, and the type and density of vegetative cover. Average lake surface evaporation rates range from less than 36 inches per year in the west to over 80 inches per year in the south (Figure 2-5). Droughts and floods are relatively common in Nevada's highly variable climate. Years of average stream flow occur rarely. Alternating periods of high and low flows are the norm in Nevada. Many water users cope with low stream flow in summer and autumn with supplemental sources, such as reservoirs and groundwater.
For most water users that rely principally upon surface water, problems can begin when below average flows are experienced for two or more consecutive years. Dry soil and hot weather conditions during a drought lead to higher watering requirements, especially on farmland, parks and golf courses, and urban landscaping. Increasing withdrawals from reservoirs and wells can result in depletion of the supplemental water sources. In water basins where surface and groundwater resources are fully committed, extended recovery periods for depleted supplemental sources may raise uncertainty in the short and long term water supply picture for some (junior) water rights holders as well as aquatic ecosystems.
Periods of drought (i.e., consecutive years with stream flow less than 80 percent of the annual average) are frequent in Nevada. In many cases, Nevada's river systems experience more "below average water years" than "above average water years". Five serious drought periods occurred during the Twentieth Century. The periods were 1928-37, 1953-55, 1959-62, 1976-77 and 1987-94. The 1928-37 period possibly was the most severe and longest in northern Nevada.
The most recent drought was severe enough to effectively remind public water suppliers and agricultural operators of the limited nature of Nevada's water, as well as the environmental impacts of dry lakebeds and  streams to fisheries, wildlife habitat, and air quality. Droughts can also create or aggravate water quality problems for both surface water and groundwater sources. Over time, lower flow and less groundwater recharge tends to diminish quality of the remaining water.
Even though the average annual precipitation is only nine inches, floods are common and have occurred in all parts of the state. The intensity and damaging effects of floods in urban communities have increased steadily with population and development since the mid-1900s. Land development has encroached onto riverine and alluvial fan floodplains, decreasing floodwater storage capacity and increasing flood damage risk.
The most severe floods occur on the Truckee, Carson, Humboldt, and Walker rivers when warm winter rain falls on snow in the higher mountain ranges. Flash flooding from intense rainfall over relatively small areas is common in the larger, more sparsely vegetated watersheds of southern Nevada and on alluvial fans of smaller drainages throughout the state. Flooding from summer storms is typically sudden, and often life threatening. Rain-on-snow flooding along the major rivers usually takes many hours or days to develop, so time to prepare for flooding is available. However, peak flows and inundation extends over a longer period of time. In the Clark and Washoe county metropolitan areas where recent floods have seriously damaged lives and property, local government have developed regional flood control plans and programs and are actively working on controls to additional runoff generated by new development.
Interest is growing in retention and restoration of natural floodplain features and functions. An example is the development of the Truckee River Flood Management Plan, which started shortly after the devastating 1997 New Year's Day flood. In 1999, the Washoe County Board of Commissioners, with support of the cities of Reno and Sparks, the State Legislature, and local organizations, approved a 1/8-cent sales tax to be used for public safety and flood management in the Truckee Meadows region. The Community Coalition for Truckee River Flood Management was formed to coordinates with the U.S. Army Corps of Engineers. The Coalition includes about 25 local stakeholder organizations, 15 agencies, and interested individuals. In addition to flood protection, this plan addresses restoration and preservation of the River's natural habitat, scenic beauty, recreational amenities, and other environmental resources. Flood management concepts will be based on the natural processes and characteristics of the river.
 Winter Photos -- The Sierra Nevada Mountains
 Next Page -- Groundwater
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