      subroutine resbact
      
!!    ~ ~ ~ PURPOSE ~ ~ ~
!!    this subroutine models bacteria in reservoirs

!!    ~ ~ ~ INCOMING VARIABLES ~ ~ ~
!!    name             |units       |definition
!!    ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
!!    inum1            |none        |reservoir number
!!    inum2            |none        |inflow hydrograph storage location number
!!    res_bactlp(:)    |# cfu/100ml |less persistent bacteria stored in 
!!                                  |reservoir
!!    res_bactp(:)     |# cfu/100ml |persistent bacteria stored in reservoir
!!    reswtr           |m^3 H2O     |initial reservoir volume
!!    thbact           |none        |temperature adjustment factor for bacteria
!!                                  |die-off/growth
!!    tmpav(:)         |deg C       |average air temperature on current day
!!    varoute(2,:)     |m^3 H2O     |water flowing into reservoir on day
!!    varoute(18,:)    |# cfu/100ml |persistent bacteria
!!    varoute(19,:)    |# cfu/100ml |less persistent bacteria
!!    wdlpres          |1/day       |Die-off factor for less persistent bacteria
!!                                  |in reservoirs
!!    wdpres           |1/day       |Die-off factor for persistent bacteria in 
!!                                  |reservoirs
!!    ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

!!    ~ ~ ~ OUTGOING VARIABLES ~ ~ ~
!!    name        |units         |definition
!!    ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
!!    res_bactlp(:)|# cfu/100ml  |less persistent bacteria in reservoir/outflow
!!                               |at end of day
!!    res_bactp(:) |# cfu/100ml  |persistent bacteria in reservoir/outflow at
!!                               |end of day
!!    ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

!!    ~ ~ ~ LOCAL DEFINITIONS ~ ~ ~
!!    name        |units         |definition
!!    ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
!!    jres        |none          |reservoir number
!!    netwtr      |m^3 H2O       |net amount of water in reservoir during 
!!                               |time step
!!    totbactlp   |10^4 cfu      |mass less persistent bacteria
!!    totbactp    |10^4 cfu      |mass persistent bacteria
!!    wtmp        |deg C         |temperature of water in reach
!!    ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

!!    ~ ~ ~ SUBROUTINES/FUNCTIONS CALLED ~ ~ ~
!!    Intrinsic: Exp, Max
!!    SWAT: Theta

!!    ~ ~ ~ ~ ~ ~ END SPECIFICATIONS ~ ~ ~ ~ ~ ~

      use parm
      implicit none

      real, external :: Theta

      integer :: jres
      real :: totbactp, totbactlp, netwtr
      real :: wtmp

      jres = 0
      jres = inum1

      !! calculate temperature in stream
      !! Stefan and Preudhomme. 1993.  Stream temperature estimation
      !! from air temperature.  Water Res. Bull. p. 27-45
      !! SWAT manual equation 2.3.13
      wtmp = 0.
      wtmp = 5.0 + 0.75 * tmpav(jres)
      if (wtmp <= 0.) wtmp = 0.1


!! daily mass balance
      !! total bacteria mass in reservoir
      totbactp = 0.
      totbactlp = 0.
      totbactp = varoute(18,inum2) * varoute(2,inum2)                   
     &                                        + res_bactp(jres) * reswtr
      totbactlp = varoute(19,inum2) * varoute(2,inum2)                  
     &                                       + res_bactlp(jres) * reswtr

      !! compute bacteria die-off
      totbactp = totbactp * Exp(-Theta(wdpres,thbact,wtmp))
      totbactp = Max(0., totbactp)
      totbactlp = totbactlp * Exp(-Theta(wdlpres,thbact,wtmp))
      totbactlp = Max(0., totbactlp)

      !! new concentration
      netwtr = 0.
      netwtr = varoute(2,inum2) + reswtr
      if (netwtr >= 1.) then
        res_bactp(jres) = totbactp / netwtr
        res_bactlp(jres) = totbactlp / netwtr
      else
        res_bactp(jres) = 0.
        res_bactlp(jres) = 0.
      end if

      return
      end