!*==sbwus.spg  processed by SPAG 6.70Rc at 15:33 on 10 Dec 2012
!*------------------   SPAG Configuration Options   --------------------
!*--0323,76 000101,-1 000000102011332010100002000000210211210,136 10 --
!*--1100000011112111000000000000,10,10,10,10,10,10,900,100 200000000 --
!*--000000010000000000000,72,72 73,42,38,33 00011112110000100000000  --
!*----------------------------------------------------------------------
!++++ +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
!$Author: joelevin $
!$Date: 2011-03-24 11:33:26 -0500 (Thu, 24 Mar 2011) $
!$Revision: 11724 $
!$HeadURL: https://eweru-dev1.eweru.ksu.edu/svn/code/weps1/branches/WEPS_F90_update/weps.src/src/lib_erosion/sbwus.for $
!**********************************************************************
!     subroutine sbwus
!**********************************************************************
      subroutine sbwus(anemht,awzzo,awu,wzzov,brcd,wus)
      implicit none
!*--SBWUS17
!
!*** Start of declarations rewritten by SPAG
!
! Dummy arguments
!
      real :: anemht,awu,awzzo,brcd,wus,wzzov
      intent (in) anemht,awu,awzzo,brcd,wzzov
      intent (inout) wus
!
! Local variables
!
      real :: wusst,wusv
!
!*** End of declarations rewritten by SPAG
!
!
!     +++ PURPOSE +++
!     To calculate subregion, friction velocity, given station
!     anemometer height, surface roughness, wind speed; and subregion
!     aerodynamic roughness.
!
!     if standing biomass present, then calculate friction velocity
!     at surface below the canopy (wus).
!
!     +++ ARGUMENT DECLARATIONS +++
!
!     +++ ARGUMENT DEFINITIONS +++
!
!     anemht - parameter, anemometer height of input wind speed (m).
!     awzzo - parameter, surface aerodynamic roughness at input wind
!             speed location (mm).
!     awu - input wind speed driving EROSION submodel (m/s).
!     wzzov - subregion aerodynamic roughness (mm).
!     brcd - biomass drag coefficient
!     wus - subregion soil surface friction velocity (m/s)
!           i.e. below canopy, if one exists.
!
!     +++ LOCAL VARIABLES +++
!
!     +++ END SPECIFICATIONS +++
!     note:  in BLOCK.FOR wzoflg should be set to 1 and anemomht
!             set to correct height if anemometer is at field site
!             to obtain correct values from SBWUS or read as
!             input data in stand-alone EROSION.
!
!     Calc station (input wind speed location) friction velocity
      wusst = awu*0.4/alog(anemht*1000./awzzo)
!
!     calc subregion friction velocity
      wus = wusst*(wzzov/awzzo)**0.067
!
!     if standing biomass, calculate wus below canopy
      if (brcd>0.0001) then
         wusv = wus
!
!        calculate friction velocity below canopy
 
         if (brcd>2.56) then         !check to avoid underflow
            wus = wusv*0.25*exp(-brcd/0.356)
         else
            wus = wusv*(0.86*exp(-brcd/0.0298)+0.25*exp(-brcd/0.356))
         end if
         wus = amin1(wus,wusv)
      end if
!
      end subroutine sbwus