fixed ponded head mass balance accounting error for when conceptual r…

[peterlafollette]

…eservoir is enabled and ponded head is allowed to be greater than 0

When a nonlinear reservoir is enabled and ponded_depth_max is set to greater than 0, fixed mass balance accounting.

To reproduce old error:
Change configs/config_lasam_Bushland.txt such that:

mbal_tol=0.001
ponded_depth_max=2[cm]

you will get a mass balance error. In the new branch you will not.

There are no changes to the simulated wetting fronts or model outputs, this fix is purely a mass balance accounting error. (You can actually see this in the identical model summaries if you leave out the mbal_tol line in the config.) So both the old and new simulations should be the same except for the substep mass balance accounting.

In particular, what was going wrong:

When ponded_depth_max > 0, ponded water can be carried into the next substep as volon_timestep_cm. If preferential flow to the conceptual reservoir is active on that substep, the code partitions both precipitation and existing ponded water: a fraction is routed to the conceptual reservoir, and volon_timestep_cm is reduced to the fraction that remains on at the surface. So far so good.

The issue: the old local mass balance check used this already-reduced volon_timestep_cm as the initial ponded water storage term, meaning it missed the fraction that went to the reservoir. So it undercounted the water present at the start of the substep, while the conceptual reservoir storage/outflow terms still accounted for the fraction routed to the reservoir. As a result, the model could report a false substep mass balance error and abort.

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[ajkhattak]

looks good.