  We study the local discretization error of Patankar-type Runge-Kutta methods
applied to semi-discrete PDEs. For a known two-stage Patankar-type scheme the
local error in PDE sense for linear advection or diffusion is shown to be of
the maximal order ${\cal O}(\Delta t^3)$ for sufficiently smooth and positive
exact solutions. However, in a test case mimicking a wetting-drying situation
as in the context of shallow-water flows, this scheme yields large errors in
the drying region. A more realistic approximation is obtained by a modification
of the Patankar approach incorporating an explicit testing stage into the
implicit trapezoidal rule.
