Grazing Impacts on Wetland Surface Microtopography

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Wyoming’s high desert riparian areas cover less than 1% of the landscape but account for up to 20% of forage. Livestock spend disproportionate time in these areas, inflicting disproportionate ecological impact by grazing and trampling, cited causes of increased riparian erosion and hummocking [citations 1-5]. Wetland hummocks are elevated soil pedestals separated by inter-hummock channels draining the wetland and eroding the organic soils that are the “sponge” for holding water in the system and prolonging streamflow that wildlife (and humans) depend on. Bureau of Land Management (BLM) and Agricultural Research Service researchers used one handheld camera to capture nadir stereo images 1.5 -m above transects located both inside and outside long-term grazing exclosures in four wetlands. They processed images into 5-mm digital elevation models (DEM), detrended slope from the models by subtracting a modeled slope surface from the actual surface [6], and used the standard deviation of all resulting grid cell elevations as a surface roughness indicator. Data were verified by an independent erosion bridge field method. Surface roughness outside grazing exclosures was 50% higher than inside exclosures (p 0.0001, n = 4), supporting a conclusion that livestock grazing significantly contributes to hummock formation in these high desert wetlands and that changes in grazing management are needed to ameliorate the situation. The utility of the monitoring method used could be enhanced by adaptation to aerial platforms.

[1] Corning RV (2002) Diminished Sweetwater River flows from the high cold desert region of Wyoming. A whitepaper included in Part III of the 2002 Green Mountain Common Allotment Evaluation, Lander Field Office, Bureau of Land Management.
[2] http://babel.hathitrust.org/cgi/pt?id=uva.x005020773;view=1up;seq=1
[3] https://fishandgame.idaho.gov/ifwis/idnhp/cdc_pdf/ecentful.pdf
[4] http://rydberg.biology.colostate.edu/facwet/winter_2004.pdf
[5] http://www.rala.is/umhvd/hhagar/edefault.htm
[6] Smart G, Duncan MJ, Walsh JM (2002) Relatively rough flow resistance equations. J Hydraulic Eng 128:568-578

Textured surface model shown above a slope-detrended DEM of a 1.5-m strip of degraded hummocked wetland. Arrows connect features common in the surface model and DEM. Blue squares above the surface model show the camera positions (seen in oblique in the upper left quadrant of the figure).
Textured surface model shown above a slope-detrended DEM of a 1.5-m strip of degraded hummocked wetland. Arrows connect features common in the surface model and DEM. Blue squares above the surface model show the camera positions (seen in oblique in the upper left quadrant of the figure).

Sensor
Platform
Author Name
Samuel Cox
Author Email
secox@blm.gov