Prescott Index derived from 1" SRTM DEM-S

Brief description

The Prescott Index is a measure of water balance that has proven to be a useful in soil mapping both to stratify study areas for sampling and as a quantitative predictor of soil properties (Prescott, 1949; McKenzie et al, 2000). The index was designed to give an indication of the intensity of leaching by excess water and is calculated using long-term average precipitation P and potential evaporation E, both expressed as mean monthly values in mm (mean annual values divided by 12):

PI = 0.445P / E^0.75

The evaporation was estimated from temperature and net radiation; the net radiation was computed by the SRAD solar radiation model using the smoothed 1 arc-second resolution DEM-S (ANZCW0703014016) and includes both regional climatic influences and local topographic effects.

Precipitation and temperature were obtained from national climate surfaces averaged over the same time period as the climatic information used in the radiation calculations (1981-2006).

The Prescott Index has no units. Larger values indicate wetter conditions.

The 3 arc-second resolution version of the Prescott Index has been produced from the 1 arc-second resolution surface, by aggregating the cells in a 3x3 window and taking the mean value.
Lineage: Source data
1. Mean monthly net radiation calculated by SRAD using the 1 second DEM-S
2. Precipitation at 0.05 degree resolution for the period 1981-2006 (Bureau of Meteorology http://www.bom.gov.au/jsp/awap)
3. Temperature at 0.05 degree resolution, calculated from monthly minimum and maximum air temperature for the period 1981-2006 (Bureau of Meteorology http://www.bom.gov.au/jsp/awap)

Prescott Index calculation
Mean annual precipitation for the period 1981-2006 was calculated then divided by 12 to give a single monthly average. A single average monthly temperature was calculated from the mean monthly minimum and maximum temperatures for 1981-2006. Both the precipitation and temperature surfaces were then resampled to 1 arc-second resolution.

A single mean monthly net radiation was calculated from the 12 net radiation surfaces produced by SRAD.

Calculation of Prescott Index requires monthly potential evapotranspiration (mm/month) as an input. The equation used to calculate PET from net radiation is the Priestley-Taylor equation (Priestley and Taylor, 1972) expressed as mm/month:
PET = (6.226 + 0.2670T - 0.002130T^2) * RN

Finally, Prescott Index was calculated from mean monthly precipitation and PET:
Prescott = (0.445 * precipitation) / (PET ^ 0.75)

The Prescott Index calculation was performed on 1° x 1° tiles at 1 arc-second resolution and the 3 arc-second resolution version was produced by aggregating the 1” cells in a 3x3 window and taking the mean value.