OverseerFM uses detailed farm management data and your farm's physical characteristics to estimate methane (CH4), nitrous oxide (NO2), and cardon dioxide (CO2) emissions for the farm by source. This allows you to understand where emissions are coming from and how much is being generated by different sources. The estimated value for each gas is converted to kg of carbon dioxide equivalent per hectare per year so they can be compared with each other and other GHG reporting.
Methane emission model
On pastoral farms methane emissions are a large contributor to greenhouse gas emissions. The model uses the dry matter intake from the animal model to calculate enteric methane. It uses dung deposited and effluent applications from the effluent system to calculate methane emissions from excreted dung.
For enteric methane the total dry matter intake of each enterprise is multiplied by an emission factor to give the total emissions. The NZI (New Zealand greenhouse gas emissions national inventory) emissions factors are used for dairy, sheep, beef and deer. Sheep applies a different factor for lambs (< 1 year).
Dung deposited on paddocks also uses NZI emission factors based on the enterprise type.
Emissions from effluent are calculated depending on the effluent system used. The emissions factors used are based on whether a 2-pond system was used and whether slurry was applied.
Any emissions from imported effluent and burning of crop residues is also calculated and added to total methane emissions.
Nitrous Oxide emission model
Nitrous oxide emissions are calculated by following NZ greenhouse gas emissions inventory (NZI) principles. Generically nitrous oxide emissions for a range of sources are estimated as a pool size multiplied by an emission factor.
Some work is underway to calculate farm specific emissions factors for excreta (based on soil hydrology); however currently the model uses the annual NZI emissions factors.
The following is a list of inputs to the nitrous oxide model.
Excreta - N deposited as dung and urine is calculated by the model based on the N intake of the animals, N removed in product and proportioned into dung and urine. Any excreta deposited on structures is removed with the remaining deposited on paddocks. This is multiplied by an annual NZI emission factor to give nitrous oxide emissions. A reduction in emissions is made if DCD is applied.
Fertiliser - Calculated from N applied, after volatilisation is removed, multiplied by an emission factor for N added to soil. A slightly higher emission factor is applied for nitrate fertilisers compare to urea. A reduction in emissions is made if DCD is applied.
Crop residues - Total of emissions from roots added (cultivation, end of crop), retained residues and from burning residues. The annual emission factor for N added to soil is used.
N fixation - Emissions from N fixation for legume crops is calculated based on the yield and the N added emission factor.
Effluent - N in Effluent and organic fertiliser applied also uses emission factor for N added to soil
Indirect - Emissions from urine volatilisation, urine leached, fertiliser volatilisation, background model leached, and background model volatilisation is calculated using specific emission factors. Emissions from direct loss to streams via drains, 2-pond system discharge and leaching from structures is also included.
Carbon Dioxide emission model
Direct and embodied CO2 emissions are estimated from existing model inputs that are required for other sub-models and relate to the emission required to produce and use a product on farm e.g. CO2 for electricity, direct CO2 from lime, CO2 from the manufacture of urea.
OverseerFM automatically generates GHG emission reports for the farm analysis based on the farm management information entered and using a series of default transport and fuel use settings. These are based on national averages and estimated for different farm types e.g. dairy, cropping etc. This means that the GHG report can be estimated with no additional inputs.
Direct and embodied CO2 emissions associated with ‘private’ residences or people living on the farm are not included. Respired CO2 and CO2 that accumulates or is released from soil are not included.