OT_219: FAO Irrigation Water Management Training Manual 3 (1986), CWR + effective-rain method

OT_219: FAO Irrigation Water Management Training Manual 3 (1986), CWR + effective-rain method

Source

https://www.fao.org/4/s2022e/s2022e00.htm — original source (opens in a new tab; the file is not redistributed)

Context: ni · Feeds D02 + D03 (effective-rain method + indicative CWR)

Summary

The FAO training manual on irrigation water needs, retrieved for its effective-rainfall method: how to convert gross rainfall into the share a crop actually uses. This is the loss factor the model applies to its GIS regional rainfall, and it is the leg GIS does not supply. It also tabulates indicative seasonal crop water needs that corroborate the FAO-56 Kc approach (OT_218).

Key claims

- claim: "EFFECTIVE-RAINFALL FORMULA (for slopes up to 4-5%). Effective rainfall Pe = 0.8 x P - 25 when monthly rainfall P is over 75 mm; Pe = 0.6 x P - 10 when P is under 75 mm (Pe not below 0). At typical NZ growing-season rainfall (~80-120 mm/month) this gives an effective fraction of about 0.55-0.66, i.e. lower than the model's assumed 0.75 (the higher USDA-SCS/CROPWAT method gives 0.76-0.84). [verified against the raw]"
  source_location: "FAO Training Manual 3, Chapter 4 (Effective Rainfall), section 4.2."
- claim: "INDICATIVE SEASONAL CROP WATER NEEDS (mm/season, Table 14): potato 500-700; bean 300-500 and pea 350-500 (faba proxy); cabbage 350-500 and onion 350-550 (field vegetables); sunflower 600-1000 (rape lower). These corroborate the FAO-56 Kc x ETo estimates. [verified against the raw]"
  source_location: "FAO Training Manual 3, Chapter 3, Table 14 (indicative crop water needs)."

Model use

• The effective-rain leg of the applied-irrigation calc in D03: applied_irrigation = CROP_CWR_MM - (effective_fraction x GIS_rainfall). The GIS rainfall layer is the input; this source supplies the fraction (or, more rigorously, a daily soil-water balance as in OT_220). The FAO fixed formula favours ~0.6-0.65 for NZ; the assumed 0.75 is only defensible on the USDA-SCS method. OT_219

Notes

• Multi-file. Chapters 3 (crop water needs) and 4 (effective rainfall).
• Provenance. FAO first-party training manual, verified against the raw.

Connections

Links to

Sources (2): OT_218 · OT_220

EDT domains (2): D02: Smart Food Systems & Agriculture · D03: Water, Waste & Circular Systems

Referenced by

EDT domains (2): D02: Smart Food Systems & Agriculture · D03: Water, Waste & Circular Systems

Sources (1): OT_218