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.

Dynamic rainfall

It is possible to create a rain scenario in which the amount of rainfall per second varies over time. See the table below for an example:

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.

Notes

The rainfall table is stored as an attribute array with the name RAIN_M. This attribute is located in the Weather associated with the Water Overlay.
The Water Overlay wizard is the easiest way to adjust rainfall.
The time keys are defined in seconds.
The last time key also determines the total simulation time.
Zero rain within a period is allowed, but negative is not. Negative values are ceiled up to zero.
The next time key should always be larger than the previous time key. Otherwise the rain calculation model will stop applying rain from the moment it reaches that time key.