function[Qx_b,Qy_b]=fo_diff_backward(Q,JM,KM,dx,dy)

Qx_b(JM,KM,:)=(Q(JM,KM,:)-Q(JM-1,KM,:))/dx;
Qy_b(JM,KM,:)=(Q(JM,KM,:)-Q(JM,KM-1,:))/dy;

