# Evaporation model (Water Overlay)

Water can evaporate from the hydrological model over time. Multiple forms of evaporation are implemented, namely surface and underground evaporation.

Both forms of evaporation can be configured directly by setting the weather's evaporation rate. If the evaporation rate is set to 0, no evaporation will take place in any form.

The weather's evaporation rate is defined as a period during which a certain rate of evaporation will take place, similar to rainfall. Multiple periods of evaporation can be defined, and at any specific moment during the simulation an exact evaporation rate is defined by the weather.

Per form of evaporation, the weather's evaporation rate is used as a base for determining the exact amount of evaporation per timestep.

During a specific timestep, if the evaporation rate indicates that more water should evaporate than is currently present in the relevant location, the amount of evaporation which takes place is equal to the amount of water which is present and available to evaporate. In the next timestep, the evaporation rate is calculated anew. Excess evaporation potential from a previous timestep is discarded.

It is possible to set up a dynamic evaporation definition, which allows for the evaporation rate to vary over time. The default setup for evaporation consists of a single period of evaporation. More periods with varying amounts of evaporation (including an evaporation rate of zero) can be defined.

## Surface evaporation

Water situated on the surface is conceptually able to evaporate. The amount is based on the weather's evaporation rate and the overlay's SURFACE_WATER_EVAPORATION_FACTOR. The product of these two attributes results in the actual rate of evaporation. The water in each individual cell at the surface is then subject to the calculated rate of evaporation.

## Underground evaporation

Water can evaporate from the underground if the cell's surface meets either of the following criteria:

In other words: if a construction is present the construction's properties are used, otherwise the terrain's properties are used.

Conceptually, water can evaporate from the underground via crops and foliage. These can draw water from the unsaturated and saturated zones in the underground, if their roots reach deep enough and the terrain or construction have a positive evaporation factor. Water is drawn from the underground and evaporated into the air, effectively removing it from the hydrological model. Water for evaporation can only be drawn from sections of the underground which are within reach of the roots, as defined by the root depth.

Evaporation can only take place if the roots of the terrain or construction can reach underground water. The depth the roots can reach is defined by either the construction's ROOT_DEPTH_M or the surface terrain's ROOT_DEPTH_M.

The underground evaporation can be configured directly by changing the terrain type's WATER_EVAPORATION_FACTOR and construction 's WATER_EVAPORATION_FACTOR. If these attributes are set to 0, no underground evaporation will take place.

Similar to the surface evaporation, the rate of underground evaporation is also determined by the weather definition's evaporation rate. Multiplying the weather's evaporation rate and the evaporation factor of the construction or terrain (as applicable) gives the actual evaporation rate for the underground.

Underground evaporation first draws water from the saturated region and then from the unsaturated region. If, according to the calculated evaporation rate, a sufficient amount of water can evaporate from the saturated zone, no water will evaporate from the unsaturated zone. Otherwise, all water in the saturated zone which is in reach of the roots of the construction or terrain (as applicable) is evaporated, and the remainder of the amount of water which should evaporate according to the evaporation rate is evaporated from the unsaturated zone. The sum of evaporation in the saturated zone and the unsaturated zone will never exceed the calculated evaporation rate.

## Example for dynamic evaporation

See the table below for an example of dynamic evaporation:

Time key (minutes) Evaporation rate (mm/d)
35 1,5
42 1,9
120 0,5

For the sake of simplicity, we will assume a surface water evaporation factor of 1. We will also assume that there is sufficient water to evaporate.

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, water will evaporate at a rate of 1,5 mm per day. Uniformly this gives 1,5 / (24 * 60 * 60) = 0.000017 mm per second.
• The second period starts at t=35 minutes and ends at t=42 minutes. In this period, water will evaporate at a rate of 19 mm per day. Uniformly this is 1,9 / (24 * 60 * 60) = 0.000022 mm per second.
• The last period starts at t=42 minutes and ends at t=120 minutes. In this period, water will evaporate at a rate of 5 mm per day. Uniformly this is 0,5 / (24 * 60 * 60) = 0,000006 mm per second.

In total it can be expected that 1,5 * ( (35-0) / (24*60) ) + 1,9 * ( (42-35) / (24*60) ) + 0,5 * ( (120-42) / (24*60) ) = 0.036 + 0.009 + 0.027 = 0,0728 mm of water will have evaporated in a total simulation time of 120 minutes.

If, during any timestep, the amount of water present is less than what is calculated to evaporate, the water present will evaporate, but because there was not enough water to evaporate, the actual total amount of evaporation will be less than the amount that was calculated to potentially evaporate.