Africa Estate Farm Tools
Water & irrigation: what can it irrigate, and what does pumping cost?
Two questions every irrigation farm raises. Crop water need: what the crop takes by FAO 56, what the rain gives back, what the system must apply and how many hectares the registered water covers. Pumping cost: the kilowatts a borehole or river pump draws, the hours it runs and what a season costs at your tariff.
1. The crop (FAO 56)
2. The climate of the farm, month by month
Reference evapotranspiration (ETo, mm per month) from FAO CLIMWAT 2.0, SAPWAT 4 or your weather station; rainfall (mm per month) from your rain gauge or the rainfall tool. The tool ships no regional values: a station average is not this farm.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ETo mm | ||||||||||||
| Rain mm |
3. The system and the water
Assumptions used
- ETc = Kc x ETo per day on the FAO 56 curve; crop coefficients and stage lengths: FAO Irrigation and Drainage Paper 56, Allen et al. (1998), Tables 11 and 12.
- Effective rainfall: USDA Soil Conservation Service method, as used in FAO CROPWAT 8.0: Peff = P x (125 - 0,2 P) / 125 up to 250 mm a month, 125 + 0,1 P above, pro rata over the season days in the month.
- Application efficiency: FAO Irrigation Water Management Training Manual No. 4 (1989), indicative field application efficiencies.
- Pump power = Q x H x 9,81 / (3 600 x efficiency); efficiency 0.66: Nebraska Pumping Plant Performance Criteria (pump 75%, electric motor 88%), as cited by K-State Research and Extension and Oklahoma State University; USDA NRCS Standard 533.
- Tariff: Eskom Tariffs and Charges Booklet 2026/27 (Landrate, Ruraflex), effective 1 April 2026; typed from your account, never assumed.
The answer
Choose the crop and type the monthly ETo (and rainfall) for the farm to see its water need.
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What this does not tell you
- A water need is not a water authorisation: the National Water Act 36 of 1998 decides what you may take; read the water rights guide.
- ETo and rainfall are what you typed; a station average is not this farm.
- Kc values are FAO 56's sub-humid standard; a hot, dry, windy site runs higher at mid-season (FAO 56 equation 62).
- A pump test gives your real efficiency; 0,66 is a well-running average system.
Sources
- FAO Irrigation and Drainage Paper 56, Allen et al. (1998), Tables 11 and 12 (1998-01-01)
- FAO Irrigation Manual Module 4, Crop water requirements and irrigation scheduling (2002) (2002-01-01)
- USDA Soil Conservation Service method, as used in FAO CROPWAT 8.0 (2009-01-01)
- FAO Irrigation Water Management Training Manual No. 4 (1989), indicative field application efficiencies (1989-01-01)
- The Engineering ToolBox, Pump Power Calculator (hydraulic and shaft power) (2026-10-19)
- Nebraska Pumping Plant Performance Criteria (pump 75%, electric motor 88%), as cited by K-State Research and Extension and Oklahoma State University; USDA NRCS Standard 533 (2017-02-21)
- Eskom Tariffs and Charges Booklet 2026/27 (Landrate, Ruraflex), effective 1 April 2026 (2026-04-01)
Estimate, not advice. Confirm every figure with your bank, attorney and tax practitioner before a decision.