c$Author: fredfox $
c$Date: 2001-09-27 20:37:46 $
c$Revision: 1.2 $
c$Source: /weru/cvs/weps/weps.src/hydro/volwatadsorb.for,v $
      real*4 function volwatadsorb(bulkden, clayfrac, orgfrac,
     &                             claygrav80rh, orggrav80rh )
c     computes the volumetric water content of the soil at 80 percent
c     relative humidity based on basic soil properties and the clay
c     adsorption isotherms on clay minerals by Berge, H.F.M. ten, 1990
c     with the addition of the organic matter isotherm from Rutherford
c     and Chlou, 1992

c*** Argument declarations ***
      real*4 bulkden, clayfrac, orgfrac, claygrav80rh, orggrav80rh
c     bulkden        - bulk density of the soil (kg/m^3)
c     clayfrac       - fraction of the mineral soil which is clay (kg/kg)
c     orgfrac        - fraction of the total soil which is organic (kg/kg)
c     claygrav80rh   - Gravimetric water content of clay at 0.8 relative
c                      humidity (parameter A in reference)
c     orggrav80rh    - Gravimetric water content of organics at 0.8 relative
c                      humidity (parameter A in reference)
c*** Include files ***
      include 'hydro/vapprop.inc'

      volwatadsorb = (bulkden  / denwat)
     &             * ( clayfrac * (1-orgfrac)*claygrav80rh
     &             + orgfrac * orggrav80rh )
      return
      end

c     not used but retained if needed

c      real*4 function volwat_rh( relhum, theta80rh, thetaw, soiltemp )
c     returns the volumetric water content of the soil based on
c     the relative humidity using the approximation of water adsorption
c     isotherms on clay minerals by Berge, H.F.M. ten, 1990

c*** Argument declarations ***
c      real*4 relhum, theta80rh, thetaw, soiltemp
c     theta      - present volumetric water content
c     thetaw     - volumetric water content at wilt (15 bar or 1.5 MPa)
c     theta80rh  - volumetric water content of soil at 0.8 relative humidity
c     soiltemp   - soil temperature (C)

c*** Include files ***
c      include 'vapprop.inc'

c*** local declarations ***
c      real*4 relhumwilt

c      if( relhum .le. 0.8 ) then
c          volwat_rh = theta80rh*relhum/0.8
c      else if( relhum .lt. 1.0 ) then
c          relhumwilt = exp( (potwilt * molewater * gravconst)
c     &               / (rgas * (soiltemp + zerokelvin)) )
c          if( relhum.lt.relhumwilt ) then
c              volwat_rh = theta80rh+(thetaw-theta80rh)
c     &                  * (relhum-0.8)/(relhumwilt-0.8)
c          else
c              volwat_rh = thetaw
c          endif
c      endif
c      return
c     end