Water Resources Sustainability and the Hydrologic Cycle
The hydrologic cycle provides the basis of water resources sustainability. The hydrologic cycle is the continuous movement of water on, above, and below the surface of the earth, generally with a minimal overall fluctuation of water (near equilibrium state). Water resources sustainability is ensuring that this overall fluctuation of water within the hydrologic cycle remains near equilibrium.
The hydrologic cycle explains why the depletion of ground water affects surface water. Surface and ground water systems are linked components of the hydrologic continuum and it is imperative to characterize them together in order to address the complex issue of water resources sustainability. For example, if water will be withdrawn from the groundwater system at a rate that will decline or deplete that system, less water will be discharged back into the surrounding rivers/streams, lowering water levels and potentially affecting stream flow or drying up wetlands and water bodies.
To understand water resources sustainability, it is necessary to understand the relationship within the hydrologic cycle between the atmosphere, hydrosphere, lithosphere, pedosphere, biosphere and anthroposphere.
Components of Water Resources Sustainability
Human activities that impact a ground water system ultimately impact the discharge; therefore, when researching hydrology, more influence should be on the discharge (Bredehoft, 2007).
Safe-yield (Water-Budget Myth)
is commonly used as the basis of state and local water-management policies. Safe-yield
limits ground water pumping to the amount that is naturally replenished and does not take into account discharge from the system. If pumping equals recharge, surface water will be consumed, eventually leading to aquifer depletion. The safe-yield concept ignores the discharge to streams, springs and seeps; safe-yield is maximized by drying up streams (Sophocleous, 1997). In most cases, sustainability will be less than the average annual recharge, suggesting that the generalization that sustainability equals recharge is incorrect (Seward et.al., 2006).
Recharge and discharge rates are key components of quantifying the amount of sustainable freshwater. Recharge rates quantify the sustainable use of ground water by humans and discharge rates quantify the sustainable use of surface water by humans. The key indicator in freshwater sustainability is the ratio of renewable water supply to water use by humans and the environment (Kanivetsky and Scmagin, 2005).
Factors Affecting Water Resources Sustainability
The change toward a sustainable culture must be addressed at the university level (Berry, 2000). Students need to understand how sustainability fits into their professions and how upon graduation their lifestyles could be adjusted to include sustainability. Currently, curriculum focuses on bigger, better, and faster scientific and technological advances. It needs to include "sustainable". Sustainable science and technology should focus on the dynamic interactions between nature and society. Research needs to focus on how to integrate institutional analysis models with emerging frameworks of vulnerability that acknowledge the human-environment systems and identify the system components for resilience and vulnerability.
Ground water discharge to rivers and streams can be influenced by ground water extraction, urbanization of river systems, and additional land-use/cover changes (Sophocleous, 2007). Robert Glennon includes a Minnesota example of how ground water extraction for agricultural irrigation can influence water resources in Water Follies (2002). Between the 1940s and 1990s over 70 center-pivot irrigation systems were installed within two miles of the Straight River, pumping approximately 3 billion gallons of groundwater each growing season. Pumping fromt eh deep aquifer increased recharge from the shallow aquifer to the deep aquifer and reduces discharge from the shallow aquifer to the Straight River. The River's water level is dropping and water temperature and nitrate levels are increasing. The River is inhabited by brown trout, which prefer cold water temperatures. Although the trout population is not currently at risk, the Minnesota DNR concluded in 1999: "Potential expansion in potato farming and irrigation could put the Straight River trout population at further risk of thermal impact and eventually raise water temperatures beyond their threshold of survival."
Climate Change or Drought
Droughts can have limited or substantial effects on long-term sustainability. If ground water storage is large, then the effects on the existing ground water develpment will be minimal. However, if there has already been a sustantial reduction of ground water storage due to over-development, then additional withdrawals can impact water levels in surrounding water bodies.
Ground water systems typically respond slower to short-term climate variabilities than do surface water systems. Ground water recharge can be altered by increases or decreases in average precipitation and temperature. Recharge can also be altered by a change in the seasonal distribution of precipitation. When ground water recharge is altered, severe and longer lasting droughts can occur. Climate variabilities can result in vegetation changes, which in turn, affects evapotranspiration rates. Each of these climatic issues may result in an increased demand on the ground water system as a backup to the source water supply.
Ground Water Development
Several negative effects can result from widespread pumping (Alley et al., 1999)circ1186.pdf
. These include:
- regional decreases in aquifer storage, particularly in unconfined aquifers
- wells may become dry due to lower ground water head
- increased pumping costs due to increased vertical pumping distance
- increased rate of movement of contaminated ground water
- land subsidence, and
- intrusion of saline water into ground water system.
In the News
With ethanol perceived as a renewable energy source, the development of ethanol manufacturing plants within Minnesota is on the rise. According to the Minnesota Department of Agriculture,
there are currently 19 facilities within the State of Minnesota, producing approximately 850 million gallons of ethanol per year. This contributes to the water sustainability concern because it takes 3 gallons of water to produce 1 gallon of ethanol. These extraction rates could significantly effect Minnesota's hydrologic system, including the reduction of aquifer storage volumes, lowering surface water levels and depleting wetland habitats.