Agricultural weather data integration spans three distinct timescales: past (climatology, the last 24–72 hours of record), now (nowcasting, 0–6 hours) and forecast (6 hours to 14 days). Each scale has its own sources, accuracy profile and field of application.

Numerical weather prediction models

Numerical Weather Prediction (NWP) models solve the fluid-dynamics equations of the atmosphere on a grid. Common global and regional models include:

  • GFS (Global Forecast System, NOAA): 13 km global grid, refreshed every 6 hours, output to +384 hours. Free open data.
  • ECMWF IFS / HRES (European Centre for Medium-Range Weather Forecasts): 9 km global resolution, widely regarded as the world's most accurate model. The ERA5 reanalysis is the reference for long-term climate work.
  • ICON (DWD, Germany): 13 km global, with a 6.5 km European nest.
  • HRRR (NOAA): a 3 km North American regional model, refreshed hourly.
  • WRF (NCAR): an open-source model suitable for 1–10 km regional runs.
  • MGM MKAP and its operational models are the official forecast sources of Türkiye's national meteorological service.

Ensemble forecasting runs the same model dozens of times with slightly perturbed initial conditions and interprets the spread as a probability distribution. ECMWF ENS uses 51 members, GFS GEFS 31 members. Probability of precipitation (PoP) and similar quantities are derived from this approach.

Nowcasting

For the 0–6 hour window, the most accurate source is weather radar. Doppler radar systems (S-band 2–4 GHz, C-band 4–8 GHz, X-band 8–12 GHz) record the motion of precipitation particles; extrapolation algorithms (e.g. SCIT, optical flow) project the rainfall pattern forward in time. Türkiye's MGM operates a Doppler radar network at 17 sites.

Reference evapotranspiration (ET₀)

The gold-standard calculation for irrigation water demand is the FAO-56 Penman-Monteith equation (Allen et al., 1998). It takes four meteorological inputs:

  • Temperature (Tmin, Tmax, mean)
  • Relative humidity or dew point
  • Wind speed at 2 m
  • Net radiation (from sunshine duration or global radiation)

ET₀ expresses the daily water evaporation, in mm, of a well-watered reference grass 0.12 m tall. Actual crop demand (ETc) is obtained from ET₀ × Kc (crop coefficient). Kc is around 0.8–1.0 for turf and around 1.15–1.20 for maize in its mature stage.

Relevant standards

  • WMO No. 8 (Guide to Meteorological Instruments and Methods of Observation): the reference for meteorological measurement.
  • ISO 17714:2007 — meteorological-station standard for air-temperature measurement.
  • ASCE-EWRI Standardised ET equation (2005): the hourly form of FAO-56.
  • FAO Irrigation and Drainage Paper No. 56 (1998): the international reference for ET₀ computation.
Rain detection: the two most common devices for real-time field-level rainfall verification are the tipping bucket (0.1–0.2 mm resolution) and impedance-based rain sensors (binary alarm threshold). When NWP forecasts fail to capture the local situation (isolated showers, slope effects), these sensors act as ground-truth references for the prediction.