Internal Loading Rates
Internal Loading Rates reflect nutrient recycling from bottom sediments. Rates are normally set to 0, since the pre-calibrated nutrient retention models already account for nutrient recycling that would normally occur (at least in the collection of reservoirs used for model calibration (Walker, 1999, Chapter 1; Walker, 1985 )).
Nonzero values should be specified with caution and only if independent estimates or measurements are available.
In some studies, internal loading rates have been estimated from measured phosphorus accumulation in the hypolimnion during the stratified period. This procedure should not be followed unless there is evidence the accumulated phosphorus is transported to the mixed layer during the growing season.
Specification of a fixed internal loading rate may be unrealistic for evaluating response to changes in external load. Because they reflect recycling of phosphorus that originally entered the reservoir from the watershed, internal loading rates would be expected to vary with external load.
This option is included at the request of model users but is not endorsed by the author. In situations where monitoring data indicate relatively high internal recycling rates to the mixed layer during the growing season, a preferred approach would generally be to calibrate the phosphorus sedimentation rate (specify calibration factors< 1). There is some risk that apparent internal loads actually reflect under-estimation of external loads.
Rates are entered on Segments input screen (third tab). Recycled nutrients are assumed to be in bioavailable form, so separate estimates of ortho P and inorganic N internal loads are not required.
Rates are entered in units of mg/m2-day. These are mean values for the entire averaging period, as defined on the Edit Global Variables screen. For example, if an internal loading rate of 10 mg/m2-day is assumed to occur over half of the year and an averaging period of 1 year is being used to formulate the mass balances, a rate of 5 mg/m2-day would be specified on the input screen.