Overseer uses two categories of climate data; annual input data and climate distribution patterns. The methodology is described in detail in the Technical Manual Chapter on Climate (Wheeler, 2017). As the model has evolved, the use and importance of climate data within the model has changed. This is because Overseer now uses a monthly time step in the N model, and a daily time step in the hydrology model.
Climate data inputs include:
- rainfall (mm/year)
- potential evapotranspiration (PET, mm/year)
- averageannual temperature (°C).
These datapoints are all provided by a dataset from NIWA. This is a rolling 30 year dataset, which is updated every ten years. The current dataset covers the period 1991-2020. The appropriateness of continuing to use this 30 year dataset to establish a single value for each datapoint was examined in "Assessment of the impact of using different temporal climate data on the N-loss estimated by the Overseer science model" Jean-Paul Tavernet and Jacquie Harper (2022)
Rainfall is an important driver of drainage and N cycling. OverseerFM links to data derived from the virtual climate network (VCN) which provides annual climate data inputs for a given reference point on a 500m grid. Annual PET (mm/year) and annual average temperature now have values based on the monthly temperature over 30-years (1991-2020) data for the datapoint. In older climate datasets these values were set at the region or nearest town.
Rainfall distribution is based on the pattern of average rainfall over 30-years (1991-2020) data for the individual datapoint in the Vitual Climate Network. For rainfall, a daily pattern is also determined based on a ‘typical’ year for fifteen annual intensities and seasonal pattern categories, with the daily pattern within a category based on rainfall for a typical mid-range site in the VCN. Monthly and daily PET distribution is based on a Fourier series for wet or dry days. Monthly temperature is based on the pattern of average monthly temperature over 30-years (1981-2010) data for the region or nearest town. Daily temperatures are not used in Overseer.
, annual average temperature refers to the daily 9 am air temperature averaged over 365 days of the year, and the average annual temperature’ is the annual average temperature averaged over several years.