c$Author: fredfox $
c$Date: 2001-09-27 20:37:43 $
c$Revision: 1.2 $
c$Source: /weru/cvs/weps/weps.src/hydro/diffusive.for,v $
      real*4 function diffusive( theta, porosity, airtemp, atmpres )

c     calculation of the soil water vapor diffusivity in air (m^2/sec)
c     using teh mthods from Campbell (1985) to account for temperature,
c     pressure and air filled porosity

c*** Argument declarations ***
      real*4 theta, porosity, airtemp, atmpres
c     theta    - volumetric soil water content
c     porosity - total soil porosity (air + water volume fraction)
c     airtemp  - air temperature (c)
c     atmpres  - atmospheric pressure (kPa)

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

c*** Local declarations ***
      real*4 diffutp, airpore, poreb, porem
c     diffutp     - diffusivity adjusted for temperature and pressure (m^2/s)
c     soilairpore - soil air filed porosity (m^3/m^3)
c     poreb       - b coefficient for diffusivity air filled pore function
c     porem       - m coefficient for diffusivity air filled pore function
      parameter (poreb = 0.66)
      parameter (porem = 1.0)

      diffutp = diffuntp * atmstand / atmpres
     &        * ((airtemp+zerokelvin)/zerokelvin)**2
      airpore = max(0.0,porosity - theta)
      diffusive = diffutp * poreb * airpore ** porem

      return
      end