Lower Colorado River Flow System

Recent plans to develop areas within the Colorado regional flow system (CRFS) in eastern and southern Nevada have raised concerns that the development of new water supplies may detrimentally impact local ecosystems or regional ground-water flow and associated spring discharge. These concerns have renewed interest in obtaining more accurate geohydrologic information on ground-water aquifers and water budget estimates for the CRFS. Current estimates of basin and regional water budgets primarily are based on Nevada Division of Water Resources (NDWR) reconnaissance-level studies completed in the 1960s and early 1970s. The Reconnaissance Series reports assumed a balance between ground-water recharge and discharge (that is, steady-state ground-water flow conditions) in undeveloped hydrographic areas. Many of the reconnaissance studies assumed that total ground-water discharge is equivalent to evapotranspiration (ET) from major areas of phreatophytes on the valley floors of hydrographic basins. Rates of ET were estimated using aerial photographs and field mapping of vegetation and reported ET rates were for similar plant assemblages found throughout the western United States. Early reported ET rates were often based on tank experiments where soil and atmospheric conditions were quite different from those in the reconnaissance area. ET rates based on local conditions within a study area generally were not available. Advancements in technology now allow ET to be estimated using direct field measurements and multispectral satellite imagery data. Better estimates of ET will improve water budgets for hydrographic areas which will allow water managers to make more informed decisions about allocating water rights for future demands

Objectives

Estimate the volume of water discharged from the Colorado River flow system in southeastern Nevada. Specific tasks to accomplish the objective are to 1) delineate areas of phreatophytic vegetation using existing multispectral satellite imager data; 2) compute a representative ET rate for a major vegetation group with micrometeorologic data continuously collected at an instrumented station in the study area; and 3) estimate the annual volume of water discharged by ET by summing the product of acreage for each vegetation group by a representative ET rate.

Strategy and Approach

A preliminary delineation of principal vegetation groups and vegetation densities in the LCFS will be completed. Available satellite imagery will be used to delineate vegetation groups and densities and this initial delineation will be verified by field surveys. Estimates of ET rates from published reports and ongoing USGS studies will be compiled and used to improve data coverage and develop representative ET rates for similar vegetation groups in the LCFS.

One instrumented ET station will be placed within a selected vegetation group within the LCFS to collect meteorological data (air temperature, windspeed, relative humidity, surface temperature, net radiation, and precipitation), soil data (moisture, temperature, and soil heat flux), and ground-water data (water level and quality). The location of the ET station will be based primarily on the preliminary ET delineation of principal vegetation groups and associated densities. A higher priority will be placed on a vegetation group that is more abundant (greater areal coverage), has sparse ET data for the LCFS, and is accessible. The station will be instrumented for the Bowen ratio method, and may also be instrumented for the Eddy correlation method. In addition, evaporation of surface water following precipitation events and runoff from regional storms may be measured periodically. If possible, ET rates will be corrected for rainfall events by subtracting measured precipitation from ET. Also, if possible, water-level data will be collected near each ET station by installing one or more shallow piezometers. Lithologic samples collected during installation of the piezometers, and water samples collected from the completed piezometers, will be analyzed for major ions and total dissolved solids by the USGS National Water Quality Laboratory in Denver, Colorado.

The type, density, and acreages of major stands of phreatophytic vegetation will be estimated. Estimated ET rates in the LCFS will be evaluated to develop a method to perform first-order estimates of ET rates for different vegetation groups and densities. Final estimates of annual ET for each basin and the LCFS will be determined by applying representative ET rates to vegetation classes and summing the product of ET rate and corresponding acreage for each delineated vegetation class or index area.

Relevance and Benefits

Results of this work will provide resource managers and regulatory agencies with better hydrologic information about the quantity of ground water in the Colorado River flow system available for future withdrawals. Such information will allow more informed decisions to be made about allocating existing ground-water resources.