Add the land use type "SEALED" to HydPy-H-Land

[tyralla]

While experimenting with the "contributing area" approach of application model hland_v1 (following HBV96), we concluded that we should allow defining zones as "sealed areas". Then one could hopefully improve the model's skill in predicting small summer flood events without introducing an inverse relationship between flood generation and soil moisture.

My first idea was to let sealed areas work like FIELD or FOREST with zero field capacity. This simple approach would increase the amount of water available for runoff generation. However, it does not need to become direct runoff by necessity. For dry summer conditions and with a moderate fraction of sealed areas within a subbasin, runoff generated on such sealed zone would fill the subbasin-wide upper zone storage only a little. Hence, much of this runoff could percolate to the lower zone layer and become base flow instead of direct runoff.

I see four alternatives for handling the runoff generated on sealed areas:

1. Route it directly to the subbasin outlet
2. Pass it into the unit hydrograph
3. Pass it to a separate (linear) runoff concentration storage
4. Combination of alternatives 3 and 4

Alternative one would not work well for large subbasins. I would go for alternative two, which does not introduce any new parameters. If we realise that we need more flexibility, we could introduce another way to model the runoff concentration of sealed surface flow later.

[tyralla]

When implementing one of the alternatives suggested above (we decided on alternative two for now), we have to carefully deal with some fluxes and states' "spatial extent". Due to the incomplete spatial extent of the upper zone layer, the difficulty is already present in hland_v1, so I start from here.

All water fluxes entering or leaving a zone are given in millimetres. For zones, the relation between volume and height is H_z = V_z/A_z; for the upper zone layer, it is H_u = V_u/A_u; and for the lower zone layer, it is H_l = V_l/A_l. When passing generated runoff from a zone to the upper zone layer, we need cannot use the relative size of the zone regarding the total size of the subbasin (A_t) for weighting. Instead, we require the total size of all zones above the upper zone layer (all zones except internal lakes). See method Calc_InUZ_V1.

When we introduce sealed areas, we first need to consider that the spatial extent of the upper zone layer further decreases. Strictly speaking, we require the total size of all zones that are neither internal lakes nor sealed areas for calculating InUZ. Second, the extent of the lower zone layer also decreases potentially.

[tyralla]

Our decision to pass runoff generated on sealed areas does not work for the current configuration H-Land Version 3 (HBV96/PREVAH). As far as I am aware, PREVAH does not implement a unit hydrograph, so I also did not implement it in version 3. To apply our sealed area concept consistently, we should add it to version 3. I suggest passing all runoff components to the unit hydrograph as in Version 1 (in agreement with HBV96).

[tyralla]

Implemented by commit aba8abf.