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HYDROLOGY COMPONENT RESULTS |
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What story do the hydrology results tell?Hydrology scores range from high in the northeast and some of the southeast blufflands, to very low in the southern and western part of the state. The primary drivers of these trends are low levels of perennial cover (year-round vegetation), and loss of water storage from wetland loss and drainage practices. Lower scores found in the Twin Cities metro region are due to high levels of impervious (hard cover) surface and high rates of water use. Why is the mean score important?Comparing the mean (average) score reveals statewide trends in hydrologic health, although it may mask any extreme values. The mean can also be used to compare similar watersheds, such as upstream or downstream within the same basin. Hydrology Results (Mean) Why is the minimum score important?The minimum is the lowest of the five hydrology index scores for each watershed. Like the lowest grade on a report card, it may indicate an area in need of focus and effort to improve overall watershed health. It may also help identify the most impacted or limiting aspect of the system. |
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What hydrology information was
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What story do the hydrology results tell across Minnesota?
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Perennial Cover Data Layers: National Land Cover Data (2001) 1890's Land Cover |
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Impervious Cover Data Layers: Impervious Land Cover (2000 Satellite Data) MN DNR Watersheds, All Catchments (2009) |
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Water Withdrawal Data Layers: State Water Use Database (MN DNR, 2009) County Well Inventory (2007) |
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Hydrologic Storage National Wetland Inventory Restorable Wetland Inventory (1992) Ssurgo Hydric (saturated) soils (NRCS, 2009) MN DNR Lakes and MN DNR Streams |
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Flow Variability Data Layers: USGS Stream Flow Records (1980-2010) Indicators of Hydrologic Alteration (IHA) |
The five hydrology index values were combined into one mean (average) hydrology score for each watershed. This mean value masks some of the variation found in each individual index, but serves to illustrate an overall gradient in results by basin and region.
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The lowest mean hydrology scores are near the Twin Cities. Very low scores for Water Withdrawal and Impervious Surface associated with dense human development drive this trend. Low mean scores are also found in the Blue Earth, middle Minnesota, and lower Red River watersheds. In this region, low Perennial Cover and Hydrologic Storage scores bring down the average score.
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Of the five hydrology index values, the index with the lowest value was identified and mapped across the state. The map on the left shows the lowest hydrology index value in each watershed. The map on the right identifies which index scored the lowest in each watershed.
Across Minnesota the minimum and mean hydrology scores generally decrease from north to south and from east to west. The lowest mean score is found near the Twin Cities, reflecting the very low Water Withdrawal and Impervious Surface scores associated with high population density and high rates of development. Low scores are also found in other watersheds in southern and western Minnesota, particularly in the Minnesota River and Red River basins where loss of perennial cover and hydrologic storage is predominant.
Mean hydrology scores are generally high in the northeastern third of the state, but the Flow Variability and Hydrologic Storage index values reveal some impacts that likely reflect mining activity and flow manipulation. Mean scores are also high in the extreme southeast bluffland area. The mean scores decrease quickly to the west as the steep valleys give way to urban and agricultural uses, which is established by the Perennial Cover and Hydrologic Storage indices.
Low mean and very low minimum hydrology scores are found in the southern and western portions of the state. These scores reflect low perennial cover and significant losses of hydrologic storage due to ditching, drainage and wetland removal.
Large amounts of impervious surface together with very high rates of water withdrawal resulted in low overall scores in the Twin Cities and St. Cloud watersheds. These index values also reflect the difficulty in comparing intense urban use of resources with the relatively low use in outstate Minnesota. Due to the large disparity in values being ranked on the same 0-100 scale, very high scores are assigned in outstate Minnesota. These scores mask important concerns about impervious surfaces and water use in these outstate watersheds. Additional data needs to be evaluated when reviewing these index values for non-metro watersheds.
The range of Flow Variability scores are relatively narrow across Minnesota. However, it should be noted that this Flow Variability index is a combination of several sub-index values that should be reviewed independently in order to determine which part(s) of the hydrologic cycle may be impacted in each watershed.
Relative scores that better reflect the variation in impervious surface and water withdrawal for outstate Minnesota could be developed. The flow variability index could utilize computations other than the mean of the sub-index values to create the score.
