1. In Stella you can clamp your state variables to make sure that they never become negative. For example in this model the Surface_Water is non-negative. Note that the "non-negative" option is checked in the variable definition box. Since by default the variables get clamped in Stella this may be sometimes somewhat confusing and may hide some of the errors, when the variable is actually negative, but you do not see it. It is good practice to make sure that your processes (flows) are described properly, and do not deplete state variables beyond levels that are intended. Uncheck the non-negativity in this model and see what realy happens to the Surface_Water. Redefine the flows in the model to make sure that Surface_Water does not go negative.
Obviously soil saturation is going to be a major determinant of the rate of surface water infiltration, but since we don't have a soil component in our model (yet, I hope), I just made infiltration a function of surface water: infiltration = .5 * surface_water
2. Let us supposed that the whole area got paved. How do we
describe this in the model? What happens to Surface_Water? Does the
result look plausible? Are there any other processes that we may be
Assuming that the pavement is totally impermeable, surface water simply accumulates as the integral of rainfall. Obviously this is unrealistic, surface water will leave the site by flowing to lower elevations.