POPflux_u = 0. _d 0
POPflux_l = (POPflux_u+P_spm(i,j,k)*drF(k)
P_reminp(i,j,k) = (POPflux_u + P_spm(i,j,k)*drF(k)
P_reminp(i,j,k) = POPflux_u*recip_drF(k)
POPflux_u = POPflux_l
_RL POPflux_u