# Inlet formula (Water Overlay)

Inlets are hydraulic structures can pump water in and out of the project area and are connected with a water body outside the project area.

The amount flowing in or out of inlets is calculated for the cell the inlet resides on.

Case 1: Inlet:
When calculating inlets, first the capacities are calculated.

If a lower threshold ${\displaystyle T_{l,t}}$ is defined:

${\displaystyle Q_{lth}=max(0,T_{l,t}-w_{t})}$

If a positive inlet q Qt is defined:

${\displaystyle Q_{t}=\Delta t\cdot Q_{t}}$

If a capacity Qtotal is defined:

${\displaystyle Q_{c}=Q_{total}-\sum _{0..t-1}Q_{i}}$

After calculating the capacities, the actual water inflow is calculated. If any of the terms are undefined, they are not included.

Δw = max( 0 , min( Qlth , Qt , Qc ) ) / A

Case 2 Outlets:
When calculating outlets, first the capacities are calculated.

If an upper threshold Tu,t is defined:

Quth = min( 0 , Tu,t - wt )

If a negative inlet q Qt is defined:

Qt = Δt * Qt

If a capacity Qtotal is defined:

Qc = -Qtotal - ΣQ0..t-1

After calculating the capacities, the actual water ouflow is calculated. If any of the terms are undefined, they are not included.

Δw = min( 0 , max( Quth , Qt , Qc) ) / A

Where:

• Q0..n = The calculated previous amounts of flow.
• Qlth = The desired lower water level at the inlet's location, based on the LOWER_THRESHOLD of the inlet.
• Quth = The desired upper water level at the outlet's location, based on the outlet's UPPER_THRESHOLD of the inlet.
• Qt = The amount of water inflow (or outflow when negative) possible based on the INLET_Q of the inlet and timestep Δt.
• Qc = The total amount of water inflow (or outflow when negative) allowed based on the total capacity of the inlet.
• A = Size of the inlet/outlet area in m2, based on the amount of cells and cell size. Current implementation only has one cell.
• Δw = The amount of water flow which takes place.

## Related

The following topics are related to this formula.

Structures
Inlet
Models
Surface model