Why rain-fed crops still run short

Supplemental irrigation means watering a crop that lives mostly on rain, only when the rain falls short. A system installed for the dry season can do this job too, if you know when to switch it on.

In Nigeria the rains begin around February in the south and June in the north. The first storms can be a false start: a dry spell can follow them, and a dry spell of two weeks or more can be critical for germination and young plants. Dry spells come later in the season too. NiMet, the Nigerian Meteorological Agency, publishes a Seasonal Climate Prediction early in the year that says where the rains are expected to start early or late; its 2026 edition also warned of a long August break in parts of the south-west.

A month's total can mislead. FAO's example is a month with 100 mm of rain: falling as two storms, at the start and the end of the month, it leaves the crop dry in between, and it may wilt; spread over the month in small showers, it may need no irrigation at all.

Sources: Ozoemene, Nwosu, Obikwelu and Duru (2022), Journal of Geography, Meteorology and Environment 5(1), Nnamdi Azikiwe University: dry spells in Sokoto State; Voice of Nigeria (2026), NiMet unveils 2026 Seasonal Climate Prediction; FAO (1985), Irrigation Water Management Training Manual No. 1: Introduction to Irrigation, chapter 4.

Measure the rain

Rainfall is measured as the depth of water it would leave on flat ground, in mm. Each mm puts 1 litre on every square metre. A gauge on your own land tells you what fell on your crop.

Choose level, open ground. Keep the gauge away from buildings, trees and walls: no closer than four times their height. Avoid exposed spots such as hilltops.
Fix the gauge firmly, with its rim level and about 30 cm above the ground.
Read it at the same time every morning. Read the water at the bottom of its curved surface, then empty the gauge and put it back.
Write the reading down as the previous day's rain. FAO records readings taken up to 9 in the morning against the day before.
Keep the collector clean of dust and leaves, and check it for leaks.

A proper gauge has a funnel and a narrow neck to cut evaporation. If you use a straight-sided tin instead, the depth of water in it is the rainfall, but read it early, before the sun dries it.

Sources: FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, Annex II, rainfall measurement; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 1, units.

How much of the rain counts

Only the rain that stays in the root zone helps the crop. FAO calls it effective rainfall: the rain that falls, minus what runs off, what evaporates from the surface and what drains below the roots.

Small showers: FAO 56 treats daily rain of less than about a fifth of that day's ETo (the weather's drying power, explained in our crop guide) as evaporated, so leave it out of the count.
Heavy storms: the root zone can hold only so much. For the count, FAO 56 treats water beyond full as draining below the roots the same day. Heavy rain also runs off slopes, and off fine soils whose surface the big drops have sealed.
Soil: in sandy soil rain soaks in quickly but much of it drains below the roots; clay takes it in slowly but keeps more of it in the root zone.
Rain soon after earlier rain: the soil is already wet, so less soaks in and more runs off.
For planning a season, not this week's watering, FAO gives two rough formulas for monthly rain on slopes up to 4 to 5 percent: effective rain = 0.8 × rain − 25 mm when a month has more than 75 mm, and 0.6 × rain − 10 mm when it has less, never below zero.

Sources: FAO (1985), Irrigation Water Management Training Manual No. 1: Introduction to Irrigation, chapter 4; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 8, soil water balance and Table 22; FAO (1986), Irrigation Water Management Training Manual No. 3: Irrigation Water Needs, chapter 4.

Keep a root-zone count

Think of the root zone as a tank. A soaking rain fills it. The crop draws it down every day. Rain tops it up, but never past full. The refill point is how far it may go down before the crop starts to suffer.

Work out the refill point once: the usable water per metre of soil, times the root depth, times the share the crop may use before stress (FAO's p). Our soil guide gives the figures.
Start the count at zero after a soaking rain or a full irrigation, when the root zone is full.
Each day, add the crop's water use: ETo times the crop factor.
Take off the rain from the gauge, leaving out the small showers and, on slopes or crusted soil, the part of a heavy storm that ran off. The count never goes below zero, because a full root zone cannot get fuller.
When the count reaches the refill point, irrigate. Give back no more than the count, or the extra drains below the roots. A drip line wets only a strip of the bed, so it gives back less; the example below shows how much.
Every few days, check the count with the feel test and correct it.

Soil water figures and the feel test

Sources: FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 8, soil water balance and Table 22; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 3; FAO (1988), Irrigation Water Management Training Manual No. 5: Irrigation Methods, chapter 6.

Full, refill point, too dry

Rain gauge: read it each morning and write down the rain
Full after rain: a soaking rain fills the root zone
Water beyond full drains below the roots and is lost to the crop
Refill point: when the count reaches it, switch the drip on
Too dry: wait past the refill point and the crop starts to suffer
The root zone: the soil the roots draw on, which the crop empties a little every day

The gauge tells you how much rain fell; the root zone goes from full after rain to the refill point, when you irrigate, and then to too dry if you wait.

A dry spell, day by day

This count follows a tomato crop on drip through twelve days with a soaking rain, a shower and a light rain, and then compares a sandy soil.

Worked example, for illustration only

Tomato at full growth on loam, in a rainy-season dry spell

Assumptions

ETo of 4 mm a day on cloudy, humid days. FAO's indicative values for humid areas are 3 to 4 mm a day at a mean temperature of 15 to 25°C and 5 to 6 mm above 25°C; use your own figure if you have one.
Tomato at full growth, FAO crop factor 1.15, on beds 1.5 m apart with a drip line giving 3.33 litres per hour per metre (1.0 L/h drippers every 0.30 m).
Loam holding 150 mm of usable water per metre of soil, and roots drawing on the top 0.7 m, as in our soil guide.
FAO's share for tomato, 0.40, adjusted for a crop using 4.6 mm a day: 0.40 + 0.04 × (5 − 4.6) = 0.42.
Rain wets the whole bed; the drip line wets about 40 percent of it, as in our soil guide.
Drip gets about 90 percent of the water to the roots, FAO's indicative value. The rain record is made up for the example.

Working

Crop use: 1.15 × 4 = 4.6 mm a day. Refill point: 150 × 0.7 = 105 mm of usable water, × 0.42 = 44 mm that may be used.
Day 1: 35 mm of rain after a wet week fills the root zone; the rest drains away. Count: 0 mm.
Days 2 and 3: no rain. Count: 4.6, then 9.2 mm.
Day 4: a 0.6 mm shower, less than a fifth of the 4 mm ETo (0.8 mm), so it is left out. Count: 13.8 mm.
Day 5: 18.4 mm. Day 6: 6 mm of rain counts: 18.4 + 4.6 − 6 = 17.0 mm.
Days 7 to 11: no rain, 4.6 mm a day. Count: 21.6, 26.2, 30.8, 35.4 and 40.0 mm.
Day 12: 44.6 mm, past the 44 mm refill point, so the drip goes on.
First run, to wet the strip under the drip line again: 44.6 × 0.4 = 17.8 mm, × 1.5 m = 26.7 litres per metre of bed, ÷ 0.9 = 29.7 litres, which takes 29.7 ÷ 3.33 = 8.9 hours.
From day 13, the day's use each day: 4.6 mm × 1.5 m = 6.9 litres per metre of bed, ÷ 0.9 = 7.7 litres, which takes 7.7 ÷ 3.33 = 2.3 hours, about 2 hours 20 minutes. Stop when a soaking rain fills the root zone again, and start the count from zero.
The same days on sand holding 80 mm per metre: 80 × 0.7 = 56 mm usable, × 0.42 = 23.5 mm. The count passes that on day 8 (26.2 mm).

Result: In this example the loam carried the crop to day 11 after the soaking rain, helped by one 6 mm rain. On day 12 the drip went on, with a long first run to wet the soil under the drip line again and then the day's use each day. On the sand the same weather needed water from day 8. A gauge and a notebook showed when to switch on, without waiting for the plants to show stress.

Sources: FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 8, soil water balance and Table 22; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 6, Table 12; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 7, Tables 19 and 20; FAO (1986), Irrigation Water Management Training Manual No. 3: Irrigation Water Needs, chapter 3, Table 5; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, Annex I, Table 8; FAO (1988), Irrigation Water Management Training Manual No. 5: Irrigation Methods, chapter 6.

Check by feel, not by the leaves

The count is an estimate, so check it. Dig to the middle of the root zone in three or more places and use the USDA feel test. At the example's refill point, a little over 40 percent of the usable water is gone; the soil should still feel moist and form a ball, which is the feel test's 50 to 75 percent class. If it forms only a weak ball, or crumbles, the count is behind: water now and correct it.

Do not wait for wilting. By the time leaves change colour or curl, the crop has been short of water too long; with young plants above all, water before any sign shows.

The full feel test, by soil

Sources: USDA-NRCS (2001), Estimating Soil Moisture by Feel and Appearance: Program Aid 1619, copy hosted by Texas A&M AgriLife; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 3.

Watering around the rain

Never water a full root zone. Water beyond full drains below the roots and is lost to the crop.
Watering on fixed dates in the rainy season gives too much water in wet years and too little in dry ones. Go by the count.
Keep tomato watering steady through a dry spell: fruit cracks when rain follows a long dry period.
Gardens and estates on a controller: a rain sensor stops the controller watering while it rains. It does not measure the soil, so check the soil yourself during a dry spell.

See a rain sensor in the store Water needs by crop and stage

Sources: FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 8, soil water balance and Table 22; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 3; UC IPM, University of California (checked 2026), Fruit cracks on tomatoes.

Trademarks belong to their owners. ASFA IRRIGATION NG LIMITED is an independent company; no partnership is implied.

See drip irrigation See valves and automation See farm irrigation

Next step

Want help deciding when to water?

Send your crop or site, your soil, your water source and the system you have through the form or on WhatsApp, and we will suggest when to switch the irrigation on and for how long.

Request a quote Start on WhatsApp

Supplemental irrigation in the rainy season: when to top up.