In continuous simulations, VO applies a modified SCS (Curve Number) soil moisture accounting approach to represent the dynamic interaction between rainfall, infiltration, and soil water storage.
Unlike event-based simulations, where initial abstraction and Curve Number (CN) are static, continuous modelling in VO maintains a time-varying soil moisture balance, allowing the model to account for antecedent conditions from previous events.
The infiltration process in continuous mode is not based on Horton or Green–Ampt methods. Instead, infiltration and runoff are governed by the SCS rainfall–runoff relationship, dynamically updated at each time step according to the available soil storage.
Soil Storage and Runoff Computation
The basic form of the modified SCS equation used in continuous mode is:
where:
Q = direct runoff (mm)
P = total rainfall (mm)
Ia = initial abstraction (mm) (typically related to depression storage and interception)
S = available soil storage (mm)
In continuous mode, S is updated at every time step to reflect changes in soil moisture, evapotranspiration, and snowmelt infiltration.
The total potential soil storage Smax is calculated from the Curve Number for AMC I, CN I, using:
At each time step, the model computes the current available storage (St) as a function of the antecedent soil moisture deficit. The runoff fraction is then determined by substituting St into the modified SCS relationship.
This formulation allows VO to continuously track the wetting and drying cycle of the soil profile without manually resetting parameters between storms.
Seasonal Variability
VO supports the seasonal adjustment of climate and vegetation-related parameters to represent long-term hydrologic conditions better.
Users can define:
- Monthly Evaporation (mm/month) — to represent seasonal potential evapotranspiration; and
- Monthly Growth Index (dimensionless) — to capture vegetation growth and transpiration effects.
These parameters can be edited through the Simulation Engine → Soil tab by clicking the Engine Option under Simulation tab.
A monthly table and corresponding plot allow users to visualize and input variable values.
Soil hydraulic properties (e.g., texture, conductivity) remain constant through the simulation and are not seasonally adjusted.
Frozen Soils Representation
VO includes a simplified representation of frozen soil conditions, which limits infiltration and increases surface runoff when freezing occurs.
The model applies a Soil Storage Capacity Reduction Ratio due to Frozen Soil (Rf) to reduce the effective soil storage when frozen conditions are active:
Where:
Seff = effective soil storage under frozen conditions (mm)
Rf = soil storage reduction ratio (default = 0.12)
Frozen soil conditions are triggered automatically during periods of snowpack presence and sub-zero air temperatures, as determined by the continuous snowpack routine.
The reduction is applied dynamically to represent the limited infiltration capacity of frozen or partially frozen ground.
