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CHAPTER 6.  LONG-TERM CHALLENGES TO TENNESSEE’S WATER SUPPLY

6.1 Baseline Issues Affecting Tennessee Water - Overview

6.2 The ‘Ambivalent Abundance’ of Tennessee’s Water

Tennessee’s surface and groundwater resources are abundant (see Figure 6.1).  This abundance is exemplified by, and documented in, many ways, including average annual precipitation¹ (parts of East Tennessee experience the heaviest precipitation in the continental U.S. outside of the Pacific Northwest), surface water storage capacity (man-made impoundments along the Tennessee and Cumberland Rivers alone store over 2.65 trillion gallons of water),² and groundwater storage capacity (an estimated 200 trillion gallons of water underlie the state in rock fractures and cavities formed by the dissolution of carbonate rock (Hutson, 1998; Hampson, 1995; Parks and Carmichael, 1990b, c, d; Broshears, 1986).³

6.2.1 Instream and Offstream Uses

Hydrologists and other water experts distinguish between instream and offstream water uses.  Instream uses are those that do not divert water from the stream channel, thus ensuring that all of it is continually available for further downstream use.  Major instream uses in Tennessee include hydroelectric power generation, navigation, recreation, aquatic habitat, and assimilation of wastewater discharge.  In 1995, hydroelectric plants used 122 billion gallons of water/day (Hutson, 1998).

Offstream uses remove water from streams or aquifers.  While much of this withdrawn water is eventually returned to streams or aquifers, some is permanently diverted into other drainages, incorporated in manufactured goods, or lost to evaporation/transpiration (also called “consumptive” use).  Major categories of consumptive use include thermoelectric power generation, industry and mining, domestic and commercial use, and agriculture.  In Tennessee in 1995, total offstream use averaged more than 10 billion gallons/ day, with 82% of this total accounted for by electric power generation alone - a figure that “dwarfs all other offstream categories “(Hutson, 1998).  Table 6.1 depicts Tennessee’s water uses. 

By “grand division,” the distribution of water use in the state offers an intriguing picture.  East Tennessee used about 2.6 times as much water in 1995 as Middle Tennessee and 24 times as much as West Tennessee.  If power generation is excluded, the comparison is 2.3 and 3 times as much, respectively.  This variation, in part, reflects physiographic as well as economic differences between regions.  In addition, while surface water use predominates in each “grand division,” (nearly 96% of total offstream water used in 1995 was surface water as opposed to 4% groundwater) groundwater is a significant source of supply in West Tennessee (see Figure 6.2). 

Groundwater constitutes 89% of the water used for non-power purposes in that region.  Moreover, approximately half of Tennessee’s population, residing in the western one-quarter of the state, relies on groundwater sources for drinking water.  Reasons for this high reliance include the presence of large quantities of good quality groundwater, the relative absence of suitable dam sites along streams in the region, and high sediment loads and poor water quality in streams draining to the Mississippi (Broshears, 1986; Hutson, 1998).

Source: Susan S. Hutson, Water Use in Tennessee, 1995.  U. S. Geological Survey and TDEC, 1998. 

6.2.2 User Trends and Their Significance for Future Conflicts

One major issue not revealed by Table 6.1 is trends in water use in Tennessee.  Studies of water use over five year intervals between 1975-1995 (by major categories depicted in Table 6.1) reveal “no consistent pattern of increase or decrease” (Hutson, 1995).  However, trends within various categories of water use, and the vulnerability of these trends to variations in precipitation, give rise to concern.  In general, water use for thermoelectric generation increased between 1975-1995, while agricultural water use increased from roughly 50 Mgal/d to over 85 Mgal/d between 1975-1990 and then declined (Hutson, 1998). 

A decrease in water use for electric power generation in 1985 has been attributed, at least in part, to an extended drought (see Hutson, 1998).  Meanwhile, projected population increases, new tourism, numerous proposals for new industry, and other sources of water demand are also important trends affecting water use. 

Of the 10 fastest growing states in the United States, in terms of population, 8 are in the southeast.  Florida is experiencing the second greatest population growth in the nation while Georgia is third and Tennessee is ninth (Solley, 1998).  Both Florida and Georgia have already had severe problems with water supply as a result of the growth the states have experienced.  While Tennessee has considerable water resources, both Florida and Georgia also have high precipitation and considerable surface water.  Nevertheless, the water resources in these states have not been adequate to meet the demands being made on them.

The implications of projected population growth and increased industrial and tourist related development for water management in Tennessee is considerable.  When growth puts strains on existing supplies, older uses, such as agriculture, may have to give way to allow water supplies to be diverted for new development.  This means lost value from riparian agricultural land.  Clearly, even without the added presence of external conflicts or outside water demands, such issues pose numerous challenges for Tennessee.  

If development outstrips supply, new sources of water must be found or development must be limited.  Tennessee already has numerous reservoirs to hold back water in wet months for use in dry months, but there is resistance on the part of recreation and environmental groups to new impoundments on the free flowing streams that the state still has.  New impoundments also have impacts on existing uses of downstream flow.  One apparent need is for accurate information on current and projected water withdrawals to help in long-range planning. 

6.3 Drought and Low Flow as Actual and Perceived Problems

Particular regional vulnerabilities directly play a part in the potential water conflicts which comprise the focus of this study.  While Tennessee is clearly blessed with a high average annual  precipitation rate, periodic drought, as well as periods of greater than normal precipitation, pose a major challenge to water supply and quality in the state.  As shown in Appendix A, much of the precipitation falling on Tennessee comes during the winter months.  Summer and early fall are dry seasons for most of Tennessee.  Yet, during this period, many uses, and especially irrigation, are at a maximum.  While agricultural irrigation constitutes a small percentage of overall use, together with climatic factors such as wind, humidity, and temperature (bringing the rate of evaporation to a peak during the summer) may produce local impacts to some farmers.

The period 1985-1988 was one of severe drought in Tennessee, particularly in the eastern part of the state.  During 1987-88 alone, precipitation statewide was about 75 percent of normal and even less during the summers.  Moreover, streamflow was about half of normal and during the summer, was dominated by groundwater discharge (Hoos, 1992: 500).  By contrast, precipitation in 1989 was about 130 percent of normal.  The definition of drought in this context is a period of at least 21 days with less than 0.25 inches of precipitation.  Tennessee has often had more than one drought per year.  During the period 1871-1953, there were a total of 112 droughts (1.3 per year).  Recurring dry spells are normal even during "wet" years (State of Tennessee Water Policy Commission, 1956: 34).

To underscore the importance of drought on streamflow, it is important to bear in mind that five major tributaries -- the Clinch (4,413 square miles), Holston (3,776 square miles), French Broad (5,124 square miles), Little Tennessee (2,627 square miles), and Hiwassee (2,700 square miles) -- account for about 86 percent of the annual mean discharge of 35,450 ft³ per second of the Tennessee River at Chattanooga.  Not only does this make total streamflow subject to variations in a few tributaries but, each of these basins exhibits distinctive climatic and runoff characteristics which make it hard to generalize about the impacts of “drought” or “above normal” precipitation on the Tennessee River (e.g., Hampson, 1995).

While below normal precipitation patterns can have dramatic and obvious adverse effects upon water supply including decreased hydropower generation, disruptions to navigation, degraded recreational opportunities, and decreased water availability for municipal supplies, precipitation extremes and variations between above- and below-normal flows can also affect water quality in streams and reservoirs (see Hoos, 1992: 500).  In 1991, for example, about 1 percent of the state’s stream miles were posted as being unusable or unsafe (by 1998, this figure had fallen to 0.2 percent).  This problem is attributed to excessive concentrations of fecal coliform in reaches downstream of municipal sewage treatment plants.  Moreover, other water quality problems in 1991, including nonpoint pollution from agriculture, were partly exacerbated by low-dissolved oxygen concentrations as a result of this drought (Hoos, 1992).  As of this writing, Tennessee is experiencing another sustained drought, underscoring the importance of this issue to the long-term security of water supply.  

We must measure the impact of new water withdrawals at the lowest point of water availability.  In drought, existing uses are already straining the resources.  Nevertheless, every one of the municipal water suppliers that we contacted while doing the survey, reported that they had plans to increase capacity and withdrawals.  A perusal of the business sections of any of the newspapers in the state will find announcements of new development plans.  Almost all development, whether for tourism (motels, water parks, etc.), industry or residences, requires additional water withdrawals.  Even if water used for these new developments is cleansed and returned to the water source for use by others, some consumption will take place.  While matching opportunities for growth with available water resources would be an optimal solution, this is easier said than done. 

6.4 Climate Change and Tennessee’s Water

One possible trend which is beginning to attract the attention of water experts is the potential impact of global climate change on the state’s - and region’s - water supply.  In general terms, climate change constitutes a major departure in natural conditions which may affect water supply and quality.  Over the last century, the average temperature in Nashville, Tennessee, has increased nearly 1°F, and precipitation has increased by up to 10% in many parts of the state.  Experts concur that these past trends may or may not continue into the future.  Over the next century, Tennessee’s climate may change even more.⁴

Impacts of these temperature changes on water supply are, of course, highly uncertain.  EPA estimates that precipitation will increase slightly in winter (with a range of 0‑10%), by 20% in spring and fall (with a range of 10‑30%), and by 30% (with a range of 10‑50%) in summer.  Other possible changes in precipitation include a possible increase in the frequency and intensity of summer thunderstorms but overall decreased runoff due to a warmer climate.  Impacts could include declines in hydropower generation, disruptions to navigation, degraded recreational opportunities, and decreased municipal water supply along the reservoir systems of the Tennessee and Cumberland rivers.   Lower flows and higher water temperatures also could degrade water quality by lowering dissolved oxygen levels and concentrating pollutant  levels, especially in the state’s urban areas.  Finally, higher water temperatures could impair cold water fisheries below many dams, reduce the efficiency of industrial and power plant cooling systems, and make more difficult efforts to meet regulatory standards for acceptable downstream water temperatures (U.S. EPA, 1999). 

Conversely, if rainfall and runoff increase in the Tennessee region, then higher streamflows and lake levels could benefit hydropower production, enhance recreational opportunities, and improve water availability for water supplies.  Although higher flows would dilute pollutants, erosion and levels of pesticides and fertilizers in runoff from agricultural areas could increase, as could pollution from runoff from mining areas.  Many river basins in western Tennessee are susceptible to sedimentation and nutrient enrichment from farming activities.  Increased rainfall also could increase flooding, currently a problem in the steep terrain in eastern Tennessee, along the many unregulated streams throughout the state, and in growing urban areas.  Increased rainfall also could disrupt navigation during periods of high flow.  In short, the potential for climate change - like so many other factors beyond policy makers immediate control - aggravates existing water problems due both to periods of both low and high precipitation by compounding uncertainty in an already uncertain picture. 

Endnotes to Chapter 6

(1) Annual precipitation ranges from about 40 inches in some low‑lying, sheltered areas to more than 90  inches at elevations of more than 6,000 feet.  The ‘net’ average is about 50 inches/year, considerably higher than the U.S. average of 30 inches.  In East Tennessee, a “wet” region, average annual precipitation ranges from about 45 inches for the Holston River to almost 60 inches for the Little Tennessee River.  About 30 of this 50 inches evaporates or is otherwise lost for use within the state.  Annual runoff, then, is on average something like 20.2 inches per year. This is the average amount of water that is available for use or capture.

(2) The Tennessee and Cumberland rivers are major arteries in the eastern and central regions of the    state and constitute principal sources of water.  Both rivers contain well‑developed systems of impoundments that provide flood control, navigation, power generation, recreation, and minimum flows for water quality maintenance.  They also provide sources for municipal water supply.

(3) Most public and industrial water supplies in West Tennessee depend on groundwater sources.  These sources, which include the Memphis Sand Aquifer, Cockfield Formation, and Fort Pillow Aquifer, serve the western one-quarter of the state -an area comprising 51% of the state’s population.

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