This web site provides access to Relative Effective Annual Precipitation data for
Montana. The data was created by the Montana Natural Resources Conservation Service.
REAP is an indicator of the amount of moisture available at a location, taking into
account precipitation, slope and aspect, and soil properties. Two sites that receive
the same amount of precipitation may have very different effective precipitation
due to other site factors.
REAP values were developed for each Montana county by the Natural Resources Conservation
Service (NRCS) using a process that included:
- Adjusting the DAYMET precipitation data using available TAPS and other precipitation
station data to accommodate the 30 year data period used by the NRCS. DAYMET is
based on a more recent 18 year timeframe.
- Re-sampling the adjusted and verified 1 km DAYMET to the 10 or 30 meter DEMs.
- Sensitizing the adjusted data to the landscape to represent relative effective precipitation.
Depending upon the geographic location within Montana and varying with degree of
slope, actual precipitation for southerly aspects may be adjusted downward and northerly
aspects may be adjusted upward, also varying on degree of slope. Neutral aspects
(all slopes less than 15 percent as well as neutral compass points regardless of
slope) were not adjusted up or down.
- Calibration and refinement of the REAP data using the expert knowledge of field
soil scientists and results from mining the SSURGO soils dataset for elevations
associated with the cryic/frigid soil temperature regimes within a county.
DAYMET is a model that generates daily surfaces of temperature, precipitation, humidity,
and radiation over large regions of complex terrain. DAYMET was developed at the
University of Montana, Numerical Terradynamic Simulation Group (NTSG).
TAPS are Temperature and Precipitation Summary tables developed by the National
Water and Climate Center (NWCC).
The value in inches was calculated using a conversion factor of 0.3937007874. Refer
provided to associate the two values.