Need
Reference evapotranspiration follows FAO Irrigation and Drainage Paper 56 (Allen et al., 1998), the Penman–Monteith equation. Crop need is the crop coefficient times that reference value.
The method
How the verdict is made.
Peak Root is a planning aid for irrigation in Greece. It tells you whether a plan holds for the dry season, and it shows the arithmetic.
Rain
Effective rain uses the USDA-SCS formula. Leaching is a planning constant of 10% of the deficit that remains after rain.
Root zone
Built from the soil's field capacity and wilting point, the root depth, and the crop's depletion fraction. Those soil constants are class midpoints, not a laboratory test of your field.
Where the climate comes from
Temperature, rain and sunshine come from climate normals of 11 Greek stations published by Meteostat. Humidity and wind are not in those files, so the app uses the FAO-56 missing-data defaults and labels them on every station page. Where sunshine is missing, radiation falls back to the Hargreaves estimate with a coastal coefficient of 0.19.
Cities without a normals file are not estimated. They are named in the app instead: Patras, Rhodes, Kos, Mytilene, Chios, Syros, Naxos, Santorini, Sparta, Nafplio, Volos, Ioannina, Kozani, Alexandroupoli and Tripoli.