How much of the land area in each of Minnesota's watersheds is covered in perennial (year-round) vegetation?
Open the Perennial Cover Health Score in the WHAF Explorer.
Why is this important for hydrology?
The amount of vegetation covering the land surface affects the way water moves across the entire landscape. Vegetation influences how water is stored and how fast it flows across the surface, the rate that water infiltrates into the soil and recharges groundwater, and the amount of water utilized and slowly released by growing plants.
The historic landscape had permanent vegetation covering most land area in Minnesota, providing balance to the water cycle and the timing of water movement. For example, permanent vegetation protects the soil surface during intense rainfall, slowing water movement and reducing the loss of soil particles. Perennial vegetation transpires water, utilizing vast quantities of water that are processed and released as water vapor over time. Roots and decaying vegetation also improve soil structure below the surface, further increasing the permeability of the soil surface and reducing the amount of runoff.
The loss of perennial vegetation is frequently accompanied by replacement with either annual crops or developed surfaces, both of which are much less efficient at absorbing and utilizing rainfall, resulting in larger peak and total flows. Bare-soil cultivation exposes the surface to rainfall impacts, generally resulting in soils that absorb less rainfall, which leads to greater and faster runoff. Developed surfaces such as roads and buildings block water from infiltrating the soil, causing it to travel faster and with greater force over the surface.
Over time, with less vegetation in a watershed to hold and process water, the hydrologic cycle shifts to accommodate changes in surface water runoff and infiltration. Water flows with greater force, causes more erosion and carries more sediment with higher contaminant loads from human activities and land uses.
- Creating the index
Input data
Source data for this score can be accessed here. The MRLC website provides further access to data, metadata, and additional resources.
Calculating the index
Current perennial cover estimates were derived from the Multi-Resolution Land Cover Consortium (MRLC) 2021 National Land Cover Dataset (Dewitz, 2023). The MRLC is a partnership among federal agencies to develop uniform, documented, high-quality land cover data for the U.S., based on satellite images. The data represent the primary land cover class represented within each 30 meter pixel.
The perennial cover index score is derived from the percent of the watershed land area that currently has perennial cover (after removing the open water land cover class). The land cover classes considered to be perennial vegetative cover are: forest (deciduous, evergreen, and mixed), shrub/scrub, grassland/herbaceous, pasture/hay, and wetlands (woody and emergent herbaceous). This index score is calculated at both the catchment and major watershed scales.
EQUATION:
Land area with perennial cover/(total watershed area - open water) = % Perennial Cover
The percent perennial cover in each watershed area is multiplied by 100 to generate a potential range of values from 0 to 100. These values were used directly as the index score.
Time series
This index is available as a time series of values based on the years available in the NLCD dataset. Index Values for Perennial Cover in 2021, 2019, and 2001 are delivered in the WHAF Explorer. Additional Perennial Cover index values (2001, 2004, 2006, 2008, 2011, 2013, 2016, 2019, and 2021) are available by downloading the Watershed Health Assessment Scores data. The change in perennial cover index is also calculated at the catchment scale and can be viewed in WHAF Explorer (change from 2001-2021) as well as via download for additional time frames.
- Index results
Interpretation of results
The index spanned nearly the complete range, with lower values in the southern and western portions of Minnesota and higher values in the northern and eastern portion. All of the southwest and western portions of Minnesota have less than 40% perennial cover remaining. The northeastern portion of the state presents a pattern of gradually increasing perennial cover with the majority of the forested portion of the state retaining 80% or more perennial cover.
Viewing catchment level results reveals a pattern of important corridors of remaining perennial cover in the riparian zones around major rivers, particularly the Minnesota River. This level of detail also reveals that many watersheds with scores in the middle range show a gradation of perennial cover within their boundaries. In the Red River Basin, it is the headwater areas that have more vegetative cover, with less vegetation moving toward the river. In the lower Mississippi Basin this trend is reversed. The headwater uplands are cultivated and the blufflands near the river hold more forested cover.
Perennial vegetation has been eliminated from much of western and southern Minnesota, primarily through conversion to agricultural land uses. This high proportion of converted land use is largely due to the combination of rich soils, low relief, and drainage, making land accessible for farming. Most other areas of the state lack at least one of these three characteristics. The watersheds with greater relief, such as those along the Mississippi River, have more remaining perennial cover due to their steep slopes that are less suited for farming and development. The north central and northeast part of Minnesota have retained much of their perennial cover, due to shallow, rocky, or sandy soils, or large bog and wetland complexes that are difficult to drain. These features reduced the rate of conversion to farming.
Urban and suburban development removed additional perennial vegetation. Although the impacts cover a small area of Minnesota they are dominant in watersheds near the Twin Cities Metropolitan area. However, urban landcover is generally less than 20% in the most affected large watersheds.
The highest percentages of perennial vegetation are in the largely forested northeastern portion of the state, through the central and eastern portions of the state, including substantial grassland in the Pine Moraine and Outwash Plains of eastern Becker County and the Mille Lacs Uplands in eastern Chisago County.
A high percentage of perennial vegetation does not imply that vegetative communities are those that were present historically. This index simply quantifies how much perennial vegetation is on the landscape now. Much of the forested regions of northeast Minnesota have been harvested, some farmed and abandoned to forests, and some affected by higher or lower fire frequencies, invasive species, or other non-natural disturbance regimes. These alterations undoubtedly will have changed perennial vegetation composition and structure.
Relationship to other health components
Connectivity
Vegetation provides documented corridors for dispersal for a broad range of terrestrial organisms, and so conversion affects connectivity, particularly for less mobile organisms. Impacts are greatest for terrestrial organisms, but because near-stream conversion also may affect temperature, sediment, and other important aquatic variables, stream segments may have reduced connectivity.
Water quality
When perennial vegetation is removed, it is most often replaced by annual plants and human development. As a result, higher runoff rates carry more nitrogen, phosphorus, sediment and other contaminants to waterways. For example, 53% of runoff from croplands in the Root River watershed occurs from March, April, and May when the land is bare (MN Department of Agriculture, 2019).
Geomorphology
Soils with perennial vegetation have better water holding capacity. Stream entrenchment and accelerated erosion occurs when perennial cover is replaced by annual cropping systems and impervious surfaces.
Biology
A landscape with perennial cover provides a continuous mosaic of habitat and is more likely to provide for the life cycle needs of diverse plant and animal species. Small patches of perennial habitat within a landscape dominated by annual crops or human development limits species richness and abundance.
- Supporting science
- There is strong support for this index, with causes, impacts and mechanisms identified in studies in Minnesota, the region, and the world.
Numerous studies have documented the impacts of forest cutting on stream and lake hydrology. For example, Detenbeck et al. (2000) found an exponential increase in peak snowmelt runoff as landscape clear-cut approached 50%, and Verry (1983), working in northern Minnesota, documented rainfall increased peak stream discharge by 250% for two years after clearcutting, then decreased toward precutting levels as forests regenerated. Storm flow peak water heights and flow volume doubled. In another study, Detenbeck et al. (2005) showed 10% reductions in forest cover lead to similar increases in water yield. Clearcutting may cause water tables to fluctuate, ranging from 9 cm higher to 19 cm lower than in peatlands with mature forests, and increases streamflow from upland forests in Minnesota by 30 to 80% (Verry, 1986).
There is ample literature documenting that conversion to agricultural land uses substantially affects stream hydrology. For example, Fitzpatrick et al. (1999) combined geomorphic field evidence with hydrologic and sediment-transport modeling to show that historical clear-cut logging, followed by agricultural activity, significantly altered the hydrologic and geomorphic conditions of North Fish Creek in Wisconsin. The upper main stem channel bed degraded (eroded downward) at least 3 meters and the channel capacity at least doubled after European settlement. The post settlement sedimentation rate on the flood plain and in the channel was 4-6 times higher than pre-settlement rates. Knox (1977) found that the conversion of natural land cover to agricultural land uses in the Platte watershed of southwestern Wisconsin caused a three to fivefold increase in the magnitudes of 1-5 year floods. Cultivation leads to loss of natural cover that intercepts rainwater and protects soil from erosion and leads to a reduction in soil infiltration capacity. The introduction of corn into the Platte watershed initiated a period of severe runoff and soil erosion beginning in 1870 and extending through the 1940s. Discharge estimates are 480 cfs for the present day channel, which is more than double estimates of 1830s discharge of 225 cfs. Udawatta et al. (2008) concluded that prairie restoration decreases soil bulk density parameters, indicating increased soil water infiltration. The observed bulk density of the native prairie was 84% of the cropped sites. In another study, Schilling et al. (2008) has documented how historical land use change has impacted the annual water budget in many Midwestern basins by decreasing annual evapotranspiration and increasing streamflow and baseflow.
Urban development also strongly affects stream hydrology. Runoff from developed areas has increased and groundwater discharge has been shown to decrease as replacement of vegetation with development reduces infiltration (Lin et al. 2008). In that study, land use change increased estimated peak differences in streamflow, surface runoff, groundwater discharge, and streamflow variability. Modeling studies support observations of conversion to urban and agricultural land uses. For example, Allan et al., (1997) used a model on the Saline River to demonstrate that increasing urban or agricultural land cover, at the expense of perennial vegetation, led to substantial increases in peak and total streamflow.
Confidence in index
This metric is strongly supported, resting on a well-developed theoretical foundation, with strong theoretical support in the technical processes of generating the base data and index and empirical tests of its application. There is a broad set of studies connecting changes in ecological, chemical, and physical characteristics of water bodies as a function of perennial cover in the contributing watershed. These studies have taken place across the world, in North America, the upper Midwest, and within Minnesota.
Current statewide data are generally of high quality and spatial resolution, and pre-settlement data, although coarse-scale, are adequate because nearly all native vegetation communities were perennial, and frequent, large-area disturbances did not change soil infiltration capacity, had ephemeral impacts, and/or left substantial canopy or intact surface cover.
Suggested enhancements
Data quality in forming the metric is very high, with relatively recent, high resolution data used to develop a map land cover classes. Classification accuracies for current landcover are above 90% for aggregated categories (Dewitz, 2023). Landcover data used is mapped for 30 meter (100 foot) cells. Newer methods that use multi-date, higher spatial resolution imagery may provide more accurate landcover classification, particularly for distinguishing grasslands from cereals and other row crops.
Additional information about specific land cover classifications summarized at various spatial scales are available through the WHAF: Land Cover application.