Last time we ended on the depletion fraction. The amount of water the crop can pull from the soil before yield suffers. That number moves with evaporative demand. And evaporative demand is what we are going to pin down here: how many millimetres the crop actually lost yesterday, and how you turn that figure into a depth of irrigation water to put back.
The term is evapotranspiration. It is the sum of evaporation from the soil surface and transpiration through the crop. Agronomists call it ET. For scheduling, you need the crop‑specific version, ETc. The arithmetic is simple: ETc = ET0 × Kc. ET0 is the reference evapotranspiration, a measure of atmospheric demand for a standard grass surface. Kc is the crop coefficient, a multiplier that adjusts for your crop’s growth stage and canopy cover. Subtract any effective rainfall that fell, and the remainder is the depth you need to apply.
ET0 in Kenya: why a single national number is useless
Reference evapotranspiration varies sharply with location and time of year. A study comparing two Kenyan stations makes this concrete. In Voi, an arid lowland, the March peak of 175.3 mm amounts to roughly 5.7 mm per day (175.3 ÷ 31). Kitale, a humid highland, peaks at 157.1 mm in March, or about 5.1 mm per day. By June, Kitale drops to 121.5 mm, a daily average of 4.1 mm. Two comparisons fall out of that, and they are different questions. In March, with both stations at their peak, Voi asks for about 12 percent more than Kitale (175.3 / 157.1 = 1.12) on the same crop on the same day. And Kitale alone swings 23 percent between its March peak and its June low (121.5 / 157.1 = 0.77), which is the same block asking for a different depth three months apart.
If someone hands you a single ET0 number for all of Kenya, it could be off by over 50 mm in a single month. Compare Voi’s March high of 175.3 mm with Kitale’s June low of 121.5 mm. A blanket figure misses half the truth. Both sets of numbers were calculated with the standard FAO‑56 Penman‑Monteith equation on daily data from 2000‑2009, so the difference is purely climatic, not methodological.
Reference evapotranspiration for two stations, FAO‑56 method, 2000‑2009. Voi is arid lowland; Kitale is humid highland.
The crop coefficient: where the table ends and your block begins
Multiply your local ET0 by the crop coefficient, and you get ETc in mm per day. The FAO‑56 manual provides a table of Kc values by crop and growth stage (Table 12, Chapter 6). The values are split into initial, mid‑season, and late‑season stages. Look up your crop and its current growth stage there, and take that value as your starting point.
Then treat it as a rough starting point. The tabulated coefficients were derived for a standard climate: sub‑humid, wind speed of 2 m/s, minimum relative humidity 45%. Most Kenyan blocks do not see that combination. When you apply a default Kc without adjustment, the error can be large. A review of published studies found that the FAO‑56 initial‑stage Kc sometimes deviates by as much as 200% from the value you would get by calibrating to your own conditions (Allen & DeWalle, 2009, cited in this USDA analysis). That is not a rounding error. It is the difference between a schedule that tracks the crop and one that quietly runs on a number from somewhere else.
Another trap is mixing coefficients from older FAO‑24 studies with modern FAO‑56 ET0 values. FAO-56 Chapter 6 puts the overestimate of the older method at up to 25 percent under high wind and low humidity, so a Kc derived from FAO‑24 will systematically mis‑schedule if you plug it into a Penman‑Monteith ET0. If you are not certain which method a coefficient came from, you are safer looking up the current FAO‑56 table and adjusting from there.
Subtract effective rainfall to get the actual irrigation need
Not all rain counts. The portion that infiltrates the root zone and is not lost to runoff, deep percolation or evaporation from wet leaves is called effective rainfall. The FAO Irrigation Manual Module 4 provides formulas to estimate it. In practice, for a given day, you take ETc and subtract any effective rain that fell since your last irrigation. The result is the net irrigation depth you must replace. If the effective rain exceeds ETc, the surplus stays in the soil profile.
This is why tracking soil moisture between irrigations is not optional. Without it, you are guessing when the stored water runs out. Our probes report relative moisture as a percentage of sensor scale, not volumetric water content. As covered in an earlier post on reading a moisture number: once you have calibrated for field capacity and wilt point on your block, the depletion curve becomes a direct check on your ETc estimate.
What to do when the table Kc is wrong
The most practical defence is to watch the soil moisture curve. If it consistently shows the crop drying the soil faster than your schedule predicts, your Kc is probably too low. If you never see the curve dip before the next irrigation, your Kc is too high. The direction of error tells you whether to raise or lower the coefficient.
A more rigorous approach is to calibrate Kc locally. A 2003 study from Kenya demonstrated how to derive dual crop coefficients from satellite imagery for water allocation planning. It builds on the dual coefficient described in FAO‑56, which separates basal crop transpiration from soil evaporation; what the study adds is a route to deriving those terms from imagery rather than from a table. Similar remote sensing adjustments have been tested in other climates, such as on squash in Portugal (Lis Valley study), but none of these give you a ready‑to‑use number. They show you the route; you still have to walk it on your own block.
You can also tap the FAO's WaPOR portal. WaPOR version 2 publishes at three levels, 250 m for regional work, 100 m, and 30 m at level 3, with a latency of two days. Actual evapotranspiration is published dekadally. For many blocks, that is enough to correct a book value. Where that cadence is too slow for the way you irrigate, the answer is an on-farm weather station: a measured local ET0 removes the guesswork that a regional product leaves in.
The arithmetic, start to finish
Start with the FAO‑56 crop coefficient for your crop and growth stage. Look it up in Table 12 of the manual. Then adjust it. The direction of the adjustment is not something you can read off the table, and nobody has published it for your block. What the table does tell you is the climate it assumes: sub-humid, 2 m/s wind, 45 percent minimum humidity. Work out how your own block differs from that, then nudge the coefficient and let the soil moisture readings tell you whether you nudged the right way. The readings, not a rule of thumb, tell you whether the adjustment went the right way.
Multiply that local Kc by your daily ET0 from the best source you have: a nearby weather station, WaPOR, or your own pan. The result is the crop’s water loss in mm. Subtract any effective rainfall that fell, and that is the net depth to replace. The FAO Irrigation Manual Module 4 walks through the arithmetic step by step. It also covers how to account for soil moisture already in the profile, and how to convert a net depth into a run time using your system’s application rate. But the starting point is always an honest evapotranspiration number. If that number is wrong, everything that follows is waste, either of water or of yield.
For more on the technologies that can deliver this data, see NuaSense’s overview of smart irrigation in Kenya, which explains why drip and micro‑irrigation have been adopted on smallholder plots and where costs still block uptake. And if you are still working out when to plant, the post on Kenya’s planting cycles lays out the long and short rains by region and how the windows are shifting.