|
WATER WITHDRAWAL INDEX |
|
|---|---|
|
Water Withdrawal Index. This slideshow requires the latest version of Adobe Flash Player.
|
What question does this index answer?How much groundwater and surface water is estimated to be withdrawn in each watershed for off-site use? Why is this important for hydrology?Groundwater and surface water naturally have seasonal fluctuations and annual variability. This variability is driven by climate and human activities, chiefly water withdrawals and land use change. Water withdrawals can reduce water storage in aquifers which in turn may reduce discharge to surface waters, or increase recharge from other sources to fill the depleted aquifer. The withdrawal of surface water can also directly reduce the flow in streams and the water levels in lakes and wetlands. (more detail below) |
    |
| CREATING THE INDEX | INDEX RESULTS | SUPPORTING SCIENCE | NEXT STEPS |
|---|---|---|---|
|
|
|
|
Ground water is connected to surface waterbodies, such as streams, lakes and wetlands, and the strength and speed of that connection depends on many factors. Ground water often sustains the flow in streams and the water in lakes and wetlands through discharge, especially during dry weather. The contribution of ground water to all stream flow in the U.S. has been estimated to be as large as 40% (Alley et al., 1999). This connection also allows ground water to be recharged from surface waters, as flows from wetlands, lakes, and rivers may be into adjacent underground reserves. Thus, ground water and surface water are more effectively managed as a single unit (Winter et al., 1998). Ground water and surface water have seasonal fluctuation and annual variability, which are driven by climate and respond to human activities (mainly water withdrawals and land use change).
Water withdrawals can lower water tables and reduce water storage in aquifers. In turn, withdrawals may: reduce discharge to or increase recharge from surface waters to depleted aquifers; increase recharge from other aquifers; increase the fluctuation in surface water levels; or cause permanent damage to an aquifer. Furthermore, the withdrawal of surface water can reduce the flow in streams and the water levels in lakes and wetlands, which in turn can reduce recharge to or increase discharge from ground water.
The index is based on the sum of permitted withdrawal from surface water and groundwater, plus an estimate of 260 gal/day from individual domestic wells for a family of four. The State Water Use Database (SWUD) was accessed, and the location and amount (gallons/year) recorded for each permit. Domestic well locations were obtained from the County Well Index. Total potential consumption was calculated by summing permitted use and estimated use for domestic wells, on an annual basis. Once through water for power generation was not included in the index.
The total permitted water withdrawal values ranged from 0 to 291,785 million gals per year (mg/y). Domestic well use ranged from 0 to 7,478 mg/y. These two values were combined to create a range of combined values of 0 to 291,785.7 mg/y.
Estimated total potential withdrawal data were scored for the relative withdrawal per watershed, scaling from 0 for the highest potential withdrawal to 100 for the lowest observed potential withdrawal. Scaling was linear across this range.
Water withdrawal is highest in the Twin Cities Metropolitan Area. High scores, indicating low water use, were found for the rest of the state of Minnesota.
Water withdrawals are concentrated with population, or in the case of the St. Louis River, an additional large permitted withdrawal. Ground and surface water permits are concentrated in the Twin Cities Metropolitan Area, and along the Mississippi River corridor to the north and west. Although domestic wells are found in all but the extreme northern portions of the state, their density is highest in the Metropolitan Area, thus these permits and withdrawals combine to create the largest demand. There are isolated large withdrawals in other parts of the state associated with large power plants and industrial installations.
Ground water withdrawal can cause the water quality to degrade by reducing cleaner subsurface flow relative to degraded surface flow. Overland flow, which may entrain chemical pollutants and sediment, is not diluted by naturally filtered groundwater.
Low water surface of streams and lakes can adversely affect the bank vegetation, causing habitat loss and a reduction in wildlife corridors.
Ground water withdrawal can result in land subsidence via three mechanisms: compaction of aquifer systems with clay and silt layers, dissolution and collapse of rocks that are relatively soluble in water, and dewatering of organic soils.
Changes in temperature, oxygen and nutrients in streams and lakes caused by low flow can have many adverse effects on aquatic life. Long-term duration of low flow in streams and lakes can affect bank vegetation that provides important wildlife habitats and provides shade and organic materials to aquatic life.
There is a well-developed literature documenting the importance of groundwater for surface water quantity, quality, distribution, and flow, and how human withdrawals may affect these factors. However, this index only approximates the risk of excess withdrawals, due to lack of data on actual withdrawal rates, withdrawal points vs. precipitation and consumption locations, and measured variation in aquifer levels and recharge rates across Minnesota.
Groundwater plays an important role in the hydrological and nutrient budgets of lakes, especially when evaporation exceeds precipitation and there is no stream flow into or out of the lake. Peaks in water withdrawal occur from late March to early April and in late June to early July when groundwater is withdrawn for agricultural use from spring to summer. Groundwater levels often become lower than the lake level, resulting in lake water predominantly flowing out to the surrounding aquifer from spring to fall, but groundwater flows into the lake the remainder of the year. These flow variations may alter the nutrients and dissolved oxygen in the lake.
Water withdrawal for urban uses can also influence subsurface flow regimes. Increases in withdrawal and decreases in recharge of groundwater due to urbanization influence subsurface flow regimes, especially during low-flow conditions when the baseflow from groundwater could be the only source to a stream. Because of lag times in groundwater responses, withdrawal of ground water in the middle of an irrigation season can affect stream base-flow into late summer and early fall.
Ground water withdrawal may reduce summer flows in streams, with many adverse effects. A stream can rapidly warm in summer and cool in winter, which means unstable environments for the metabolism of aquatic life. Low flow combined with high temperature reduces dissolved oxygen in streams. Low flow also contains less particulate debris and dissolved constituents (Water on the Web, 2008). The hyporheic zone, the interface between stream water and ground water in the bed and banks of streams, is a focus of microbial activities and chemical transformations (Alley et al., 1999). The withdrawal of ground water or surface water can alter the flow direction and magnitude in the hyporheic zone, which affects those biological processes and may alter water quality. However, the effects of ground water withdrawal on hyporheic biota have not been well investigated (Alley et al., 1999).
Confidence in this metric is moderate. Although the best available data were used, they severely limit the kind and depth of analysis possible, and hence the metric employed. Given that metrics are expected to improve, both by improving data collection and by changes in specific calculations included in the metric. Although water use permits data are available statewide, there are likely significant numbers of missing domestic wells and for many wells only the permitted use is reported, not actual water used. In addition, withdrawals should be compared to some metric of availability, which could include total water available, but also incorporate minimum groundwater level and related flow requirements. Unfortunately, the current state of the data and knowledge does not support robust estimates of water supply.
An index should be developed which measures water withdrawals relative to water availability, specifically identifying the geography of supplies and uses, and is based on accurate measurements of both available and consumed water through space and time.
The current index only incorporates a measure of permitted consumption, and the measures of consumption are flawed. Many consumptive uses, particularly domestic wells, are not completely mapped. Water origin and available volumes should be mapped, such as watersheds above surface withdrawal sites, and aquifer volumes, recharge areas, and flow rates, because current aquifer recharge rates and natural and human transfers are poorly estimated. The boundaries of subsurface flow areas are not well known. Because there is insufficient data in the scientific literature to numerically rank this index and there are no known threshold values, the resulting values were ranked in equal intervals from the smallest to largest value.
These shortcomings may be removed, at substantial costs, by new and better data collection. Sub-surface flow directions may be collected and organized for all Minnesota regions. All wells and water access points could be mapped, and methods applied to measure and/or estimate actual, rather than permitted consumption.
We should also compare use with availability. This approach would require an analysis of: both consumptive and non-consumptive uses; geographic areas of each water withdrawal location; water supply and transport networks; the fate of water after use. Availability and consumption vary through time( e.g., due to droughts, annual climate cycles, and urban growth) so these factors should be incorporated in availability and use data - by estimating availability and consumption under drought conditions, for example.
