By Jeffrey S. Briberg, LPG, REM
Over the next three years rural and exurban landowners will convert a small but meaningful percentage of marginal land from row crops to solar fields and community solar gardens. As this transition happens, choosing a diverse native landscape for these solar sites will provide clean, renewable energy, and also provide important storm water management. Noting the numerous co-benefits of vegetative ground cover on solar sites, the National Renewable Energy Laboratory recently began a three-year study to document best practices and help meet SunShot cost and deployment targets.
A native landscape is one planted with a diverse mix of multiple species of low-lying deep-rooted grasses, flowers, and sedges instead of using gravel or a monocrop of a shallow-rooted non-native plant species, like turf grass. Ecologists have proven that increasing plant richness by the addition of legumes and forbs to the traditional grasses enhances the functional diversity and stacks benefits provided by these systems.
Native Landscapes for Improved Storm Water Management
One of the services WSB provides to clients is to conduct site layout/design, vegetation establishment and management plans, and storm water modeling that meets storm water regulations. We ran Hydro CAD model runoff calculations for a range of flow conditions for a typical 2 MW project that is converting 8 acres of cropland to a solar site using a native landscape (low mow grasses plus clover in the service lanes, shade-tolerant grasses with seven flowering plants behind the panels).
The model uses the assumption that we build 1.2 acres of impervious service roads, 50 percent of the panel area is impervious surface (an assumption required by some state agencies even though all the space around and under the panels is a permanently vegetated, pervious surface), and 100 percent pervious surfaces with perennial plantings for all lanes, buffers etc. This condition is compared with a baseline of cropland that is barren of vegetation from mid-October to mid-May, compared to low-cut, shallow-rooted turf grass, and compared to a deep-rooted native plant mix.
|Native plant mix
|Base line condition
The storm water runoff is reduced 23 percent for the 2-year storm (2.9 inches of rain) and 8 percent for the 100-year storm (7.8 inches of rain). The runoff reduction is dependent on soil types, slopes and existing land cover, but, we find these results are very typical.
We believe the actual results will be better than the model suggests because the panels are not actually impervious, they just focus the drainage long a drip-line at the bottom of the panels where the deep rooted perennial vegetation allows infiltration whenever the ground is not frozen (see Hydrologic Response of Solar Farms, Journal of Hydrologic Engineering, May 2013 for hydrographs and additional research).
Further, we expect a mix of prairie plants to provide superior hydrologic performance compared to monocrop turf-grasses that are common on solar sites in some areas of the country. In 2008, the U.S. Geological Survey completed a five-year storm water study in cooperation with a consortium of 19 cities and towns in the area of Madison, Wisconsin that revealed “striking differences between turf and prairie vegetation.” The study found “prairie vegetation had greater median infiltration rates than those with turf grass,” and roots in the prairie vegetation plot were “found to a depth of 4.7 feet compared with 0.46 feet in the turf.”
In addition to superior storm water management, native plants improve the soil with organic matter over the 20 to 30-year life the project, allowing microorganisms and soil fauna to recover after years of intensive compaction, pesticide and fertilizer application. And, over time, native plants out-compete weeds allowing ground cover to be maintained with just a single annual mow, reducing operating costs.
The growth of solar installations nationwide can have other positive benefits besides clean power and reduction of greenhouse gasses. Flood attenuation from storm events and water quality improvements are something every community needs.
As an increasing number of solar projects are built in areas that receive significant annual rainfall — and considering that the trend is for individual storms to increase in intensity over the next 20 years — using native landscapes for storm water management will reduce uncertainty and operational costs, while providing additional secondary benefits.
Jeffrey Broberg is a licensed professional geologist, and registered environmental manager for WSB Associates of Rochester, Minnesota. He is a board member of Fresh Energy, an independent energy nonprofit.
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