How to estimate future forest cover in a watershed

Center for Watershed Protection USDA Forest Service, Northeastern Area, State and Private Forestry Center for Watershed Protection USDA Forest Service, Northeastern Area, State and Private Forestry

How to estimate future forest cover in a watershed


future land cover projections

• GIS-based method to estimate future land cover in a watershed

• “Leafout analysis” focuses on forest cover• Local governments and watershed groups can

use method to:• evaluate the effect of different watershed protection

scenarios on future land cover• predict future stream health • set numeric land cover goals• develop watershed protection recommendations


why land cover is important to watersheds

• Impervious cover – excellent indicator of stream health

• More than 200 studies on relationship between IC and various measures of stream quality

• Forest and wetlands are a beneficial cover type from hydrologic, water quality, and habitat standpoint

• Other pervious areas – majority is often maintained as turf, may include cropland, pasture or grasslands


link between forest cover and stream health

• Stream health rating of Excellent requires no more than 6% IC and at least 65% riparian forest cover (Goetz et al, 2003)

• Stream health rating of Good requires no more than 10% IC and at least 60% riparian forest cover (Goetz et al, 2003)

• Watersheds with at least 65% forest cover usually had a healthy aquatic insect community (Booth, 2000)


steps for estimating future land cover

1. Identify developed and undeveloped parcels2. Calculate the area of each zoning category for

undeveloped land 3. Identify and subtract protected land from undeveloped

land for each zoning category 4. Multiply the remaining buildable land in each zoning

category by the corresponding land cover coefficients5. Calculate the area of each land cover type for

developed land and protected land6. Sum future land cover on buildable, developed and

protected land


GIS data needs for future land cover analysis

• Property boundaries and parcel data• Current land cover – impervious, forest, turf,

etc• Zoning• Protected land – conservation easements,

conservation zones, parkland, stream buffers, steep slopes, wetlands, erodible soils, floodplains (will vary with local regs)

• Land cover coefficients (more on this later…)• Recent aerial photos may also be helpful


assumptions

• Full buildout of the watershed will occur based on allowable zoning (e.g., no rezoning)

• Current land cover on developed land will remain the same with the future buildout

• Protected land will remain the same in future buildout scenario

• Buildable land will be converted to impervious cover, forest, or other pervious cover, as dictated by land cover coefficients


No buildings. Parcels not

selected

Parcels intersect with buildings so they are selected

as “developed” parcels

Step 1: identify developed and undeveloped parcels


Step 1: results


Step 2: calculate the area of each zoning category for

undeveloped land


Step 3: identify and subtract protected land from undeveloped land for each

zoning category

• Protected land includes specific parcels that are under permanent conservation (e.g., easements, conservation zones, parkland)

• Also includes types of land that must be preserved during development according to local regulations: stream buffers, wetlands, steep slopes, erodible soils, floodplains…

• This will vary locally and requires a little research or local knowledge


protected lands

undeveloped land…


…minus protected land…


…equals buildable land


Step 3: results

buildable land by zoning category


Step 4: multiply the remaining buildable land in each zoning category by the

corresponding land cover coefficients

• Land cover coefficients represent the fraction of a particular land use that is a certain land cover type

• Land cover coefficients vary by land use type and intensity

• Impervious cover coefficients are available from Cappiella and Brown (2001)

• Forest cover coefficients are an important data gap

impervious cover

coefficients vary by land

use

Land use ICC

Agriculture .019

Open urban land .086

2 acre lot residential .106

1 acre lot residential .143

½ acre lot residential .212

¼ acre lot residential .278

1/8 acre lot residential .326

Townhomes .409

Multifamily residential .444

Institutional .344

Light industrial .534

Commercial .722

Source: Cappiella and Brown (2001)


factors affecting forest cover coefficients

• Land use type and intensity• Age of development• Local forest conservation regulations• Pre-development land cover (agriculture

versus forest)• Local clearing and grading practices• Reforestation and landscaping

requirements

forest conservation regulations influence forest cover coefficients

Photo courtesy of Harford County DPW

Photo courtesy of Randall Arendt

Forest cover coefficient = .057

Forest cover coefficient = .40


options for deriving forest cover coefficients

• Assume the worst: 95% of the site (except for protected lands) will be cleared

• If local forest conservation regulations exist, use the required conservation thresholds

• Use data from Urban Ecosystems Analyses www.americanforests.org or Urban Forests Effects Model www.ufore.org for your area

• Do your own detailed calculation using methods described in Cappiella and Brown (2001)

http://www.americanforests.org/

http://www.ufore.org/

Zoning Category

Buildable Area (acres)

Forest Cover

Coefficient

Future Forest

Cover on Buildable

land (acres)

Agriculture 765 .168 128.5

Open Urban Land

50 .268 13.4

LDR 732 .321 235.0

MDR 645 .248 160.0

HDR 220 .203 44.7

Institutional 88 .134 11.8

Commercial 130 .067 8.7

Total 2,630 602.1


Step 5: calculate the area of each land cover type for developed land

and protected land

• Impervious cover – good indicator of stream health

• Forest cover – most beneficial type of cover from hydrologic, water quality, and habitat standpoint

• Other pervious areas – majority is often maintained as turf, may include cropland, pasture or grasslands


sources of land cover data

• Maryland DNR Urban Tree Canopy assessment (includes impervious surfaces from IKONOS data)

• Chesapeake Bay Program land cover data (includes 2000 impervious surface data and RESAC land cover data http://www.chesapeakebay.net/data/index.htm

• Local government impervious surfaces (roads, parking lots, etc) and forest cover data

http://www.chesapeakebay.net/data/index.htm

common scale issue: Map B contains alot more detail on forest cover while Map A is fairly generalized

BA

Impervious cover layers


Step 5: results

Step 6: sum future land cover on buildable, developed and protected land

Current land cover on developed and protected land:•Impervious: 1,917 acres•Forest: 639 acres•Turf: 1,278 acres

Future land cover on buildable land (estimated using land cover coefficients):•Impervious: 680 acres•Forest: 600 acres•Turf: 1,350 acres

+


Step 6: results

Future land cover in the watershed=

future land cover spreadsheet


you’ve got results: now what?

• Identify ways to increase future forest cover• Repeat Steps 1-6, making adjustments for

these measures• May need to run several scenarios to find

acceptable and realistic future forest cover• Set/adopt forest cover goal• Implement techniques to maintain or increase

forest cover


techniques to maintain or increase forest cover

• Protect priority forests• Prevent forest loss during development

and redevelopment• Maintain existing forest canopy• Plant trees during development and

redevelopment• Reforest public land• Reforest private land


additional resources• Urban Watershed Forestry Manual Part 1:

Methods for Increasing Forest Cover in a Watershed. Available for free download from www.cwp.org

• Impervious Cover and Land Use in the Chesapeake Bay. Available for free download from www.cwp.org

• USDA Forest Service’s Urban Forests Effects Model (UFORE) www.ufore.org

• American Forests’ Urban Ecosystem Analysis www.americanforests.org


group exercise: estimating future forest cover in the Green River

Watershed

• Identify group reporter• Read the Green River Watershed scenario• Select a combination of Urban Watershed

Forestry techniques to implement in the Green River Watershed

• Report your results to the entire group

Be ready to report out in 30 minutes!

Technology

How to estimate future forest cover in a watershed