c     file name: psd.for

      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     DATE:  09/29/93
c     DATE MODIFIED:  12/10/93

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

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

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

c     + + + PARAMETERS + + +

      real   sandg, siltg, clayg

      parameter   (sandg = 0.316, siltg = 0.01, clayg = 0.0002)

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.+
C *** a**2 removed per LH 7/9/99
C ***     &         claym*log(clayg)**2-a**2)
     &         claym*log(clayg)**2.)
          pgsd = exp(sqrt(b-a**2.))

      return
      end