# Rain model (Water Overlay)

Rain is modeled as an amount of precipitation that falls within a defined period of time, defined as a weather definition It is configured as a specific amount of water and a specific duration. These values are converted to an amount (mm) of precipitation per second. e.g. 15 millimeter of rain in 45 minutes matches would be converted to 0,0056 millimeter per second. The amount of rainfall is further converted for the appropriate amount of rain per calculated timestep. At the end of the defined period, exactly the configured amount of rain will have fallen on each grid cell.

For example, 15 millimeter of rain that falls in 45 minutes. In the Water Module this amount can be converted to an amount (mm) of precipitation per second. Moreover, the amount of rain per second that falls remains the same within the period it was defined for. At the end of the defined period of rain, it can be expected that the defined amount of rain should have fallen on each grid cell.

Rain is always added to the surface layer of a grid cell and each grid cell receives the same amount of rain per time step. For any specified period of rainfall, the amount of rain added to the grid cell is consistent. However, multiple periods of rain can be configured with varying durations and rain amounts.

It is possible to set up a dynamic rainfall definition, which allows for the amount of rainfall per period of time to vary over time. The default setup for rain consists of a single period of rain, and a single period without rain. More periods with varying amounts of rain (including zero rain for dry periods) can be defined.

## Example for dynamic rainfall

See the table below for an example of dynamic rainfall:

Time key (minutes) Amount of rainfall (mm/m2)
35 15
42 19
120 5

This format can be interpreted as follows:

• The first period is starts at t=0 minutes and ends at t=35 minutes. In this first period, 15 mm/m2 of rain will fall. Uniformly this gives 15 / ((35 - 0) * 60) = 0.00714 mm per second.
• The second period starts at t=35 minutes and ends at t=42 minutes. In this period, 19 mm/m2 of rain will fall. Uniformly this is 19 / ((42 - 35) * 60) = 0.04524 mm rain per second.
• The last period starts at t=42 minutes and ends at t=120 minutes. In this period, 5 mm/m2 of rain will fall. Uniformly this is 5 / ((120 - 42) * 60) = 0,00107 mm rain per second.

In total it can be expected that per m2 15 + 19 + 5 = 39 mm rain has fallen in a total simulation time of 120 minutes.