c
c
      subroutine crust
     i              (crstrmd,perctill,
     o               cro,cover)


c     + + + PURPOSE + + +
c
c     This subroutine destroys the surface crust after a tillage event.
c
c     + + + KEYWORDS + + +
c     crust, tillage (primary/secondary)

*$noreference
      include 'p1werm.inc'
*$reference

c
c     + + + ARGUMENT DECLARATIONS + + +
c
      real perctill, cro, crstrmd, cover
c
c     + + + ARGUMENT DEFINITIONS + + +
c
c     cover		- variable containing the loose cover material
c     cro		- Fraction of surface crusted before operation
c     crstrmd	- Fraction of crust removed (0<=crstrmd<=1)
c     perctill	- Fraction of the surface tilled
c
c     + + + ACCESSED COMMON BLOCK VARIABLE DEFINITIONS + + +
c
c
c     + + + PARAMETERS + + +
c
c     + + + LOCAL VARIABLES + + +
c
      real crf
c
c     + + + LOCAL VARIABLE DEFINITIONS + + +
c
c     crf		- Fraction of surface crusted after the operation
c
c     + + + END SPECIFICATIONS + + +
c
c	  Perform the calculation of the surface crust destroyed.
c
      crf = (1.0 - crstrmd)*perctill*cro
c     check to see if the loose material on the surface is still there
c     if enough of the crust is removed set cover to zero (loose material)
c     This was done according to L. Hagen
      if (crf .lt. 0.01) then
        cover = 0.0
      else
        cover = cover*crf/cro
      endif

      cro = crf

      return
      end