What percentage of the estimated annual runoff from each catchment watershed is consumed, based on reported water use?
Why is this important for hydrology?
Managing water use is a primary component of sustainable water resource management. Careful management of water withdrawals ensures that water is available for human use as well as meeting environmental needs, such as maintaining freshwater ecosystems and water quality. Sustainable management also considers the need for adequate water resources into the future.
The amount of available groundwater and surface water has seasonal fluctuations and annual variability. The natural variability is driven by climate. Additional variability is introduced by human activities, chiefly water withdrawals and land use. In wet years, the demand for water use is low, while in dry years the demand for water increases creating greater risk of impacts from permitted water withdrawal.
Groundwater and surface water withdrawals are managed through a statewide permitting process. A water use permit is required for non-domestic use of groundwater and surface water when the use will exceed 10,000 gallons per day or 1 million gallons per year (Water Appropriations Permit Program). Permit holders are required to submit an annual report of their water use.
Groundwater and surface waters interact in all types of landscapes. Surface waters may either receive water or send water to a groundwater aquifer, or they may do both under different conditions (Winter et al. 1998). Where groundwater and surface water are connected, groundwater pumping will alter this movement by capturing groundwater flow before it discharges to the surface or by increasing the rate at which surface water moves into the aquifer (Bartolino and Cunningham, 2003). In connected systems, groundwater withdrawal will eventually result in a decrease in streamflow or lake water levels (Bredhoeft and Durbin, 2009).
Groundwater discharge often sustains the water level in streams, lakes and wetlands, especially during dry weather. The contribution of groundwater to all stream flow in the U.S. is estimated to be as large as 40% (Alley et al., 1999), loss of which is critical to aquatic ecosystems. The strength and speed of the connection between groundwater and surface water depends on many factors including climatic conditions, extent of human water use and land use practices. Thus, groundwater and surface water are more effectively managed as a single unit (Winter et al., 1998).
Water Withdrawal Health Scores
- Creating the Index
Calculating the Index
The Water Withdrawal Index provides a comparison between stream discharge and reported water use. The catchment index value considers these inputs for each catchment and its contributing upstream catchments.
Stream discharge or Mean Annual Discharge (MAD), is estimated using a predictive model that factors in rainfall, temperature, geology, and the size and location of the watershed.
Mean Annual Discharge (MAD) calculation:
- Daily streamflow data was downloaded for 82 USGS stream gages located throughout Minnesota and in neighboring states. The majority of gages (49) having a complete record of 31 years from 1988-2019.
- Annual total precipitation and mean annual temperature data were downloaded from the PRISM Climate Group web site (www.prism.oregonstate.edu) as gridded raster data at a resolution of 4 km. The mean raster value was extracted for each gage’s watershed.
- Drainage area for the stream gages was calculated using USGS StreamStats web application and ranged from 7.7 mi2 to 36,800 mi2 with a median of 905 mi2.
- Model detail available upon request.
Reported Water Use calculation:
- Water use is calculated from data reported to the DNR by water appropriation permit holders.
- Water use is reported by permit location on an annual basis as millions of gallons per year (MG/year). Water use is summed for each catchment’s total contributing area (that catchment and upstream catchments) on an annual basis.
- Water use values are adjusted by the consumption rate for each water use category (the percent of water estimated to be consumed by each type of use). Once-through water for power generation was not included in the index.
Water Use Vulnerability calculation:
- Percent water use is calculated as ((volume of water use / MAD) * 100).
- In a catchment where water use is greater than zero but the MAD prediction is zero the percent use is set to NA (i.e., no value).
- The maximum output Water Use Vulnerability score is 100. In cases where water use is greater than MAD, the output values are set to 100.
Water Withdrawal Index calculation:
- To calculate the Index, the mean of the most recent 5 years of the Water Use Vulnerability score is calculated and that value is subtracted from 100.
- This provides a range of values that are consistent with the WHAF index scoring approach, where a value of 0 is a poor health/high risk condition, and a high value of 100 is a good health/low risk condition.
- Index Results
Interpretation of results
This index helps locate where water use is high relative to discharge, a scenario that causes stress to the available water supply and the health and resilience of the watershed.
When water withdrawals from a watershed are high, a reduction in discharge from a watershed is expected. This index includes withdrawals from surface waters and groundwater, including groundwater withdrawals from both surficial and confined aquifers. Surface water levels will respond at different time scales to these different withdrawal points. Nevertheless, the index serves as an important early warning system for identifying regions of the state where there is an imbalance between water use and water availability.
If water use exceeds available water, surface water levels will lower in response, leading to a loss of stream and lake habitat. Studies show that a reduction in streamflow below 10% often has a minimal impact on biotic habitat, while a reduction of 20% is considered a significant impact (Acreman and Ferguson 2010; Richter et al. 2012). A Water Withdrawal Index score of 80 represents an estimated condition where 20% of discharge volume is being consumed, a relatively high level of reported water use that may be unsustainable if it persists. While this index shows a high health condition for much of the state, it is important to remember that a value of 80 represent a potential resource concern that warrants closer investigation.
In addition to the Water Withdrawal health score, the WHAF provides two additional data sources to help interpret patterns of water use. The Change in Water Withdrawal Index is available through the Add Data tool in the map application; this dataset displays the difference between the overall average water use vulnerability (1990-2019), and the recent 5 year average (2014-2019). This map layer helps to highlight locations where recent water use has greatly increased or decreased relative to the entire period of record. The second resource, Water Withdrawal Charts, is available through the WHAF Land Cover application; this resource provides charts and a table of values providing the entire record of data for a single catchment location.
The results of this index indicate that some catchments may be vulnerable to water overuse. When high water use is coupled with limited water availability, some areas in Minnesota have the potential for water stress. There are many factors that can lead to water availability shortfalls in any given year, such as precipitation patterns; but the vulnerable catchments are at greater risk given current water use patterns.
Relationship to other health components
Water use and the resultant streamflow depletion can decrease water quality by concentrating contaminants in depleted surface waters and by increasing the flow of surface water into groundwater drawing with it contaminants.
Reduced streamflow, particularly during the low flow months, can result in a longitudinal disconnection of stream habitats and strand aquatic organisms. Decreased streamflow will also increase sedimentation which can vertically disconnect the hyporheic zone with the streambed. Decreased flows and water surface elevation can adversely impact riparian vegetation and result in destabilized streambanks, altering the connectivity between the stream channel and the floodplain.
A change to the stream flow regime, such as decreased flow, will cause stream instability, function loss, and impairment. The consequence of river instability is loss of aquatic habitat, land loss from bank erosion, increase in vegetation encroachment and channel capacity, decrease in sediment supply and transport, and loss of ecological function.
Aquatic habitat is a direct function of streamflow with the habitat of many species inhabiting riffles and raceways being positively related to streamflow. Summer habitat conditions are often considered a bottleneck for aquatic population being a time of maximum feeding and growth but also a time of low streamflow. As such, any additional decrease in streamflow will likely result in decreased habitat and additional stress on aquatic populations. Evidence that a 10% flow alteration is likely to have a negligible effect on most taxa, stream types, and hydrologic conditions is generally agreed on by experts (Acreman and Ferguson 2010). A high degree of ecological protection will be provided when daily flow alterations are no greater than 10%; a high level of protection means that the structure and function of the riverine ecosystem will be maintained with minimal changes (Richter et al. 2012). Alternately, water appropriations of 20% or greater will likely result in moderate to major changes in natural structure and function of ecosystems.
- Supporting Science
Scientific literature support
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. 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 index
Confidence in this metric is moderate. Although the best available data were used, the depth of analysis possible is still limited. Although water use permit data are available statewide, the index uses the 'maximum permitted use', not a measure of actual water used. In addition, the comparison of water use to total water available relies on estimates of runoff. Additional groundwater level data and stream flow data could improve this index and more accurately reflect vulnerability associated with water use and available water quantity.
To more accurately connect water use and supply, water origin and available volumes should be mapped, such as watersheds above surface withdrawal sites, and aquifer volumes, recharge areas, and flow rates. This index could be refined by differentiating between deep and surficial aquifer sources for groundwater permits to improve the estimate of impact to available water from groundwater withdrawals.
The current index incorporates self-reported annual consumption by permit holders. Measuring actual water consumption would improve the accuracy of the index. Also, refinements to the accuracy of consumption coefficients for different permit types would further improve estimates of water consumption.