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 Resources: Water - General
July 11, 2013
USGS building national water census
Over 8,000 small watersheds targeted

 

The U.S. Geological Survey (USGS) is working toward meeting mandates written into law by Congress in 2009 to “provide a more accurate assessment of the status of the water resources of the United States.”  The most recent product of that work is USGS’s report, Progress toward Establishing a National Assessment of Water Availability and Use.  In basic terms, the USGS is endeavoring to build a national water census to determine how much water is nationally accessible and how much enters and is withdrawn from surface and ground-water sources. 

The USGS is the federal agency primarily responsible for the scien­tific evaluation of the natural resources of the United States, including its water and biological resources.  It is clear from the report that the USGS is just beginning to refine the many technical tools that will be needed to develop awater census that can “systematically provide information that will allow resource managers to assess the water availability of the Nation.”  The undertaking will also depend heavily on informational relationships with state, local, and regional water managers and many other stakeholders.  At present, USGS’s work is significantly constrained by underfunding. 

The current report outlines several approaches the USGS is taking toward developing the census.  The primary foundation for the work is based on the unifying theme of a water budget.  The USGS compares the budget to a checking account affected by deposits and withdrawals in the form of precipitation, evapotranspiration, surface-water and groundwa­ter flow into and out of a watershed, change in surface-water and groundwater storage, change in snow and ice storage, and human withdrawals and interbasin transfers.  It is the job of USGS and state, regional, and local water managers to estimate the amount of water that resides in or moves through each of these components and thus, provide information on the size of the water budget and how it has changed and can be expected to change into the future.
 
The water census is part of an overarching Department of the Interior (DOI) initiative called WaterSMART (Sustain and Manage America’s Resources for Tomorrow).  Under WaterSMART, USGS’s primary responsibility is the water census.  The U.S. Bureau of Reclamation is another lead agency, primarily covering water conservation. 

Gages and models

Calculating the water budget presents imposing challenges associated with the extraordinary diversity of the nation.  For example, the amount, timing, and hydrology of water deposits and withdrawals vary enormously by watershed and subwatersheds.  (A watershed is an area of land in which all water drains to the same place.)  The USGS has divided the nation into 21 watersheds or hydrologic units with a mean area of 164,300 square miles (mi2).  Each of these is then progressively subdivided into smaller and smaller units, with the smallest units–those with 12-digit hydrologic unit codes (HUCs)–having a mean area of 34 mi2.  There are 8,098 12-digit HUCs in the country.

The most reliable way to determine the water budgets of the largest watersheds is by aggregating the budgets of the 12-digit HUCs.  The USGS says it operates approximately 7,800 streamgages to provide information on floods, droughts, and current water availability across the U.S.  Because the streamgage network cannot provide direct observations of streamflow at every location of interest, infor­mation about ungaged locations is needed.  To meet this need, the USGS uses streamgage records to develop hydrologic models for specific basins. 

The models work quite well when the input data are well understood, for example, water amounts used in industrial activity.  But modeling evapotranspiration is still in its infancy.  The hydrology of underground aquifers also presents challenges to modeling.  The USGS estimates there are 30 to 40 principal aquifers that col­lectively account for more than 90 percent of the nation’s total groundwater withdrawals.  Aquifers cover large areas, commonly do not conform to surface water divides, and may obtain most of their recharge from locations that are far from those where the groundwa­ter is pumped from a well or flows to a surface water body.  Building the kind of data needed to power reliable groundwater models is an intensive effort involving a combination of information from the large-scale studies, data from observa­tion-well networks, analysis of streamflow records, and other available information. 

The quality and pace of USGS’s effort to create the census will, of course, depend on funding.  The 2009 SECURE Water Act authorized $20 million for each of fiscal years 2009 through 2023 for the assessment of national water availability and use.  But so far, the only appropriations that have occurred are $4 million in 2011 and $6 million in 2012–i.e., $10 million of the $80 million authorized for this period. 

The USGS has directed the available funds primarily into the start-up of three geo­graphic focus area studies.  According to the USGS, these studies will meet the congressional requirement that the water resource assessment consider areas of “significant water use conflicts or shortages that have occurred or are occurring.”  Progress on the three studies is summarized below.

Apalachicola-Chattahoochee-Flint (ACF) River Basin

This ACF river basin covers 19,800 mi2 in Georgia, south­eastern Alabama, and northwestern Florida.  Conflicts among the three states over water resources in the basin have resulted from increases in water use for public supply, industry, power generation, and agriculture as the region has grown and developed over the past 50 years. Conflicts over water are not limited to interstate issues; during drought conditions, compe­tition among all water users can become pronounced.

The debate over water availability has focused on the management of water in reservoirs that are operated by the U.S. Army Corps of Engineers.  These reservoirs typically have sufficient storage capacity for human uses and to maintain streamflows.  Information needed for reservoir management to maintain flows in the ACF mainstem rivers is largely avail­able.  However, information on the many water-availability influences that are distributed across the basin has received less- detailed study.  These influences include water withdrawals and wastewater returns, groundwater pumping for irrigation, interbasin transfers, storage in unman­aged reservoirs, and effects of increases in impervious surfaces.  Examination of these factors is the primary work of the ACF study.  The USGS says the study has three components:

  • Estimat­ing water use.  This involves development of a site-specific database of water use for the ACF river basin, developing improved methods for estimating agricultural withdrawals, and compil­ing available water-use projections.  Calculations of net water use will be improved by obtaining information on interbasin transfers, determining irrigation and septic-tank return flows, and estimating consumptive use by thermoelectric power plants.  
  • Simulating surface water and groundwa­ter flow.  Researchers are building a surface water model of the entire ACF river basin and a groundwater model of the lower ACF river basin.  These models will be linked where agricultural pumpage of ground­water is greatest to improve the simulation of groundwater/surface water interactions.  This part of the study involves coordinated field monitoring of streamflow, groundwater levels, and water quality during drought conditions in July 2011. 
  • Establishing ecological flow relations.  Ecological models will be used to predict changes in fish and mussel species occupancy due to variations in flow conditions associated with climate change, land-use change, and changes in water withdrawals or discharges.

Colorado River Basin

This basin covers about 246,000 mi2, including parts of Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming.  The river supplies water to more than 30 million people, irrigates nearly 4 million acres of cropland in the U.S. and Mexico, and supplies hydropower plants that generate more than 10 billion kilowatt-hours annu­ally.  The USGS is active in three areas:

  • Estimation of current water uses, particularly consumptive water use, and historical trends in water use.  More accurate quantification of these com­ponents of the basin water budget will expand the collection of water-use data to include information on some uses not currently estimated (for example, self-supplied commercial use).
  • Regional and field-scale assessments of evapotranspiration and the dynamic variation in snowpack water content.   This part of the study will couple several recently developed remote-sensing techniques with both new and existing ground-based measurements to extrapolate evapotranspiration across the entire Colorado River basin.  Recently devel­oped snow-water models will also be applied and calibrated.
  • Estimation of groundwater discharge to streams and rivers.  The groundwater contribution to streams in the Colorado River basin is one of the least understood components of the regional water budget.  Preliminary studies by the USGS indicate that between 20 percent and 60 percent of the surface water flow in the upper basin is derived from groundwater discharge.  Identification of stream reaches that receive large amounts of groundwater discharge in the upper Colorado River basin will be a major part of the study.  Information about the geologic con­trols on groundwater flow will be refined, and stream reaches likely to receive significant groundwater discharge will be identified.  Natural and anthropogenic chemical tracers will be used to confirm that groundwater discharge is occur­ring and to estimate its relative contribution.  This effort will culminate in a map indicating the location and amount of the groundwater contribution to the surface water flow system.

Delaware River Basin

This basin covers 13,500 mi2 in parts of New York, New Jersey, Penn­sylvania, and Delaware.  The basin has the largest interbasin withdrawal of water east of the Mississippi River and provides water to more than 15 million people.  Two decrees by the U.S. Supreme Court and coordina­tion by an interstate river basin commission are just part of the history of allocating water resources in the basin.  Concerns about the effects of new natural gas development and the freshwater requirements for a recently discovered endangered mussel species have added new complexities to water manage­ment in the upper part of the basin.  Products of this study will include:

  • A database of water withdrawal, water use, and return flow information for watersheds that will be accessible to water resource managers in the basin. 
  • A Web-based tool developed by using index streamgages to estimate baseline daily streamflow at ungaged streams in the basin from 1960 to 2010.
  • An evaluation of water needs for aquatic ecological systems within the basin, including an updated decision support system for sections of the river and development of flow/aquatic-assemblage response relations for tributaries. 
  • A hydrologic model of the nontidal portions of the watershed tributaries with an easy-to-use interface that will allow water resource managers to evaluate potential effects of future population, land use, or water demand scenarios.

As noted, funding for USGS’s water budget activities is currently 12 percent of the amount authorized in the SECURE Water Act.   In its 2013 budget request, the DOI is seeking an additional $13 million for USGS activities under the WaterSMART program.  That is still short of the $20 million per year authorized by Congress, but it should enable the USGS to substantially increase its water census activities.

Click here for the water census report.