Oaks ’n’ Folks – Volume 13, Issue 1- February. 1998
Establishing Livestock Carrying Capacity From GIS & Range Science Research
The carrying capacity ratings used today for rangeland enrolled in the Williamson Act in Tulare County were established in the 1950s. In 1995,the State Board of Equalization reported that these ratings are 40% below the actual carrying capacities. The County Assessor contacted the Integrated Hardwood Range Management Program to determine estimates of livestock carrying capacity through science-based methodology to ensure the sustainability of the Williamson Act program for grazing land within the County. Four landowners agreed to cooperate in this project by providing historic grazing information and access to their land for field verification of various estimates. The selected parcels are representative of the variety of canopy cover, slope,and elevation of grazing land under the Williamson Act within the County.
Maps the databases were developed using the ArcInfo geographic information system (GIS). Existing resources were used to create slope and canopy cover maps for the pilot sample of parcels. Slope maps were created by using United States Geological Survey (USGS) Digital Elevation Models. Maps of oak, pine,and brush canopy cover were generated by using existing aerial photography,and manually mapping areas into canopy cover classes onto a mylar overlay. These canopy cover classes correspond to canopy cover classes in a “grazing capacity scorecard” developed for Tulare County, and experience of the investigators (see Table 1 below). A zoom transfer scope was used to match the landscape features from the aerial photo mylar to those of a mylard eveloped from topographic maps.
Both the mylar aerial photo sheet and the mylar topographic maps sheet were digitized to record the information. ArcInfo was used to combine the values in the cover class coverage (from aerial photos) and the slope class coverage (from USGS Digital Elevation Models). ArcInfo was then used to analyze the data to produce tables with carrying capacity estimates and maps to graphically display the information. Carrying capacity estimates generated through this procedure were referenced with parcels of known carrying capacity based upon historic grazing and residual dry matter information.
Thirty-three parcels were analyzed during this project. These ranged in size from about 30 acres to over 775 acres, representing a total of over 11,000 acres, or about 2% of Tulare County’s Williamson Act grazing lands.Individual parcel analyses were generated detailing the animal unit months(AUMs) per acre, acres per class, and total AUMs by slope and canopy cover classes present.
Tulare County currently rates each parcel based on the number of acres required to provide the forage needed to support one animal unit for one year (AUY). Existing ratings for the 33 parcel pilot sample range from 15 to 50 acres. The estimates produced with the developed methodology ranged from 13 to 72 acres per AUY. These estimates fall within the range anticipated,based on field-0-derived forage production estimates, residual dry matter requirements, and historic grazing rates.
A direct comparison of acres/AUY estimates with current ratings showed that over 40% of the ratings were either similar to or lower than estimates,while approximately 60% were higher (lower ratings indicate more acres/AUY are required, higher ratings indicate that fewer acres are required). When estimates and ratings were within 10% of each other, they were considered similar.
The old grazing capacity ratings for tax purposes averaged 27 acres per AUY. Estimates form the scorecard method on the same parcels averaged 20 acres per AUY, about 25% higher than the old figures, but considerably less than the 40% difference claimed by the Board of Equalization.
There was no correlation between size of parcel and carrying capacity estimates, nor between the parcel size and whether estimates ere higher or lower than the current ratings. There also is no uniform adjustment tocurrent ratings that would bring them in line with our estimates. To accurately assess the carrying capacities of grazing capacities of grazing lands as enrolled in the Williamson Act, each parcel will have to be individually assessed.
Table 1. Grazing capacity scorecards. Figures are AUMs per acre for different canopy cover and slope classes in Tulare County.
- Less than 12″ average annual precipitation
|Slope Classes (%)||% Canopy Cover|
More than 12″ average annual precipitation
|Slope Classes (%)||% Canopy Cover|
prepared and edited by Richard B. Standiford and Pamela Tinnin
U.C. Integrated Hardwood Range Management Program