Waterbody evaporation and precipitation: use area as variable instead of average

[shartgring]

Currently, the precipitation and evapotranspiration are calculated as averages for the river cells in which the waterbody is located. It may be a nice addition to include the effect of a varying surface water area when it can be calculated if the hypsometry is provided via an SH-table.

This may be relevant for large reservoirs in regions with strong seasonality. I overheard the example of regulated waterbodies in Spain where minimizing the area of the surface water during parts of the dry season was considered in deriving the operation rules. It may also help by comparing strategies for reservoirs: currently an operation plan optimized such that Qin = Qout will perform similarly for a full and empty reservoir, while in practice it can vary (depending on climate conditions). Or it could be relevant if wflow is used to determine when a reservoir dries out? Note: I have not tried any calculations myself, so I am not sure if it is significant ;)

I think that in practice implementation may be a bit challenging as evapotranspiration and precipitation are given as an input to the reservoir/lake function and not calculated inside these functions. And the relationship of the area is not directly given when the SH-relation is used. Maybe an additional column for area (A) can be derived when the waterbody is initialized based on the SH-relation?