Solar Radiation

The unit models the variation in solar radiation during the simulation.

Description

When using the PHYTOPLANKTON module, the user has to specify the amount of solar radiation at the water surface. This affects the light induced growth limitation factor for the algae, and also for macrophytes and benthic algae, if these are simulated as well. The solar radiation is specified in the SOLAR RADIATION module. The light limitation factor is a function of the solar radiation, the suspended solids and algal concentrations (see below).

The solar radiation is specified in terms of the radiant energy incident on a unit area over the period of one hour (J/cm²). Solar radiation does not occur at night. The user has a choice of setting solar radiation in three different ways. It may be set as a constant (use keyword 'CONSTANT') value for the whole run using a fixed daytime period. The daytime period is specified as a number of hours which are centred on midday. Solar radiation may be allowed to vary on a daily basis with different daytime periods (use keyword 'DAILY'). This allows the user to account for the effect of weaker sunlight and shorter periods of daylight during the winter. Alternatively the user may specify hourly values for each day between 5:00 and 20:00 hours (use keyword HOURLY). This allows the user to account for the effect of stronger sunlight during the middle part of the day.

Equations

The light induced growth limitation factor for algae is calculated from the following equation (see PHYTOPLANKTON):
                                                                                                         (1)

Where:

μ _light= light limitation factor at some depth, z, below the surface

I = light intensity at some depth, z, below the surface (J/cm²)

I _max= light intensity which will produce the maximum productivity (J/cm²)

The light intensity at depth, z, is given by the Bear-Lambert law:
                                                                                                                 (2)

Where:

I_o= light intensity at the surface (J/cm)

k = light attenuation factor which is a function of the suspended solids and algae concentrations
                                                                                                  (3)

Where:

SS = suspended solids concentration (mg/l)

A = algal concentration (mg/l) (given as data in the PHYTOPLANKTON unit)

k₂= extinction coefficient relating to algal concentrations

Because of the effect of salinity on the flocculation of mud particles the light attenuation factor is calculated by the following equation when the salinity exceeds 2ppt:
                                                                                                  (4)

The equation for light limitation is integrated over the depth to give an average value for the whole water column.

General

The SOLAR RADIATION module must be used in conjunction with the PHYTOPLANKTON module. For HOURLY data the model assumes that there is no daylight between 8pm and 5am. DAILY and CONSTANT data allow the day to be extended or shortened. HOURLY data can be shortened by giving zero values for the extra hours of darkness, but cannot be extended.

The value of solar radiation given for DAILY and CONSTANT data represent the amount of energy (in J/cm²) over a typical hour during the day. It is not the total daily amount.

To convert data in W/m² J/cm² over an hour (as required by the 1D Solver) multiply by 0.36 (= 3600/10000).