The model calculates nitrogen captured by each wetland using the nitrogen load and multiplying this by the removal efficiency of the wetland. The nitrogen load is made up of nitrogen from rain, surface runoff and sub surface runoff (drainage) from the catchment, with surface runoff adjusted depending on the catchment’s convergence.
For natural wetlands N removal efficiency is a fixed value based on the condition type. For artificial wetlands, N removal efficiency is calculated using the artificial condition type (1,2 or 3), the temperature (average temperature across the farm), the area of the wetland and a function of runoff and rain inflows.
Rutherford et al. (2008) reported that P removal in natural wetlands is considered to be small compared with filters. A value of 10% P removal by natural wetlands may be appropriate, although the uncertainty in this estimate is high. Furthermore, they also went onto conclude that artificial wetlands may remove P as well as N. However, the percentage removals for P are lower than for N and there is a very high variability. Consequently OVERSEER currently does not estimate P removal from wetlands. Phosphorus removed by sediment traps is also not included.