explanation about the runoff part

[sunt05]

Q: How is (surface) runoff generated in SUEWS and at what timestep are these calculations performed? It would be nice if this explanation could include whether runoff is generated when the infiltration capacity is exceeded, when the soil is saturated, or both.

this was asked by Harro in UP-water project, the answer to which could be better added to the documentation to clarify related concerns.

[LllC-mmd]

SUEWS calculates the runoff over 7 surface types and mainly involves three steps:

1. Drainage calculation.

In this step, each surface type (excluding the water surface) within a grid calculates its total drainage to other surfaces based on pre-defined drainage equations. Then each surface type receives drainage distributed from other surfaces based on a pre-defined water distribution matrix denoted by WaterDist. https://github.com/UMEP-dev/SUEWS/blob/eade2e12a5d6867170c56cba5f6b915f6f502732/src/suews/src/suews_phys_waterdist.f95#L801-L820

The WaterDist is an input parameter of SUEWS and its meaning can be explained as follows:

• WaterDist[i, j] (i≤7) gives the fraction of water the i-th surface received from the j-th face’s drainage.
• WaterDist[8, i] gives the fraction of water from the i-th surface directly contributing to the potential runoff of the i-th surface denoted by water2runoff[i]. Here, we should note that in the SUEWS simulation, if the i-th surface is absent from the grid but WaterDist[i, j] still has a positive value, the corresponding drainage will directly contribute to the potential runoff of the j-th surface denoted by water2runoff[j].

2. Evaporation calculation based on latent heat calculation and available water volume.

3. Storage calculation.

This step is executed after the calculation of evaporation and determines the final runoff from each surface type:

• For impervious surface types (paved and building surfaces):

  • when the rainfall rate plus net drainage rate (which is calculated as the summation of drainages distributed to and received from other surface types) exceeds a pre-defined threshold (> 10 mm/h by default), runoff is generated for the excessive part.
  • The potential runoff of these surfaces will be directly added to the final runoff. https://github.com/UMEP-dev/SUEWS/blob/eade2e12a5d6867170c56cba5f6b915f6f502732/src/suews/src/suews_phys_waterdist.f95#L212-L259

• For pervious surface types (evergreen trees, deciduous trees, grass and bare soil):

  • When the rainfall rate plus net drainage rate exceeds a pre-defined threshold (> 10 mm/h by default), runoff is generated for the excessive part.
  • When current soil moisture plus the potential runoff exceeds its capability, runoff is also generated for the excessive part. https://github.com/UMEP-dev/SUEWS/blob/eade2e12a5d6867170c56cba5f6b915f6f502732/src/suews/src/suews_phys_waterdist.f95#L261-L331

• For the water surface, when water depth exceeds a threshold, runoff is generated for the excessive part. https://github.com/UMEP-dev/SUEWS/blob/eade2e12a5d6867170c56cba5f6b915f6f502732/src/suews/src/suews_phys_waterdist.f95#L332-L376