c$Author: fredfox $
c$Date: 2001-09-27 15:45:20 $
c$Revision: 1.3.6.1 $
c$Source: /weru/cvs/weps/weps.src/hydro/psd.for,v $

      subroutine   psd (sandm, siltm, claym, pgmd, pgsd)

c     + + + PURPOSE + + +
c     This subroutine calculates the soil geometric mean diameter and
c     geometric standard deviation from percent sand, silt, and clay
c     using geometric mean diameters within each of the three soil
c     particle size fractions.
c     From: Shirazi, M.A., Boersma, L. 1984. A unifying Quantitative
c     Analysis of Soil Texture. SOil Sci. Soc. Am. J. 48:142-147
c     DATE:  09/29/93

c     + + + KEY WORDS + + +
c     Geometric mean diameter (GMD), Geometric standard deviation (GSD)

c     + + + ARGUMENT DECLARATIONS + + +
      real claym
      real pgmd
      real pgsd
      real sandm
      real siltm

c     + + + ARGUMENT DEFINITIONS + + +
c     sandm  - Mass fraction of sand
c     siltm   - Mass fraction of silt
c     claym  - Mass fraction of clay
c     pgmd   - Geometric mean diameter of psd (mm)
c     pgsd   - Geometric std deviation of psd (mm)

c     + + + PARAMETERS + + +

      real   sandg, siltg, clayg

      parameter   (sandg = 1.025, siltg = 0.026, clayg = 0.001)

c     sandg - percent sand
c     siltg - percent silt
c     clayg - percent clay

c     + + + LOCAL VARIABLES + + +
      real a, b

c     + + + LOCAL DEFINITIONS + + +
c     a, b   - Temporary variables

c     + + + END SPECIFICATIONS + + +

c     calculate geometric mean diameter
          a = sandm*log(sandg)+siltm*log(siltg)+claym*log(clayg)
          pgmd = exp(a)

c     calculate geometric standard deviation
          b = (sandm*log(sandg)**2.+siltm*log(siltg)**2.+
     &         claym*log(clayg)**2.)
          pgsd = exp(sqrt(b-a**2.))

      return
      end