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## Lateral Sub-surface Runoff

Lateral sub-surface runoff occurs when the soil column is saturated. This is described by the equation:

q_drai = Θ_ice K_baseflow tan(β) Δz_sat^N_baseflow

Where:
- K_baseflow is a calibration parameter
- β is the topographic slope 
- N_baseflow = 1
- Δz_sat is the thickness of the saturated portion of the soil column

The saturated thickness is determined by:
Δz_sat = z_bedrock - z_∇
Where z_∇ is the water table depth, determined by the first soil layer above bedrock where the volumetric water content drops below a threshold (default 0.9).

The specific yield, S_y, is calculated based on the soil properties and water table location:
S_y = θ_sat (1 - (1 + z_∇/Ψ_sat)^(-1/B))
Where B is the Clapp-Hornberger exponent.

Two numerical adjustments are then made:
1. Any excess liquid water in each soil layer is added to the layer above, and remaining excess is added to drainage q_drai.
2. To prevent negative liquid water content, water is taken from lower layers to bring each layer up to the minimum liquid water content of 0.01 mm.

Finally, the surface soil layer liquid water and ice contents are updated for dew, frost, and sublimation.