Net Irrigation After Rain: How Effective Rain Changes What to Water

Net irrigation after rain is the water still needed after effective rain is taken off weekly crop use. For the tomato example on the calculator page, weekly use is 403 litres on 10 square metres, effective rain of 10 mm adds 100 litres, and net irrigation is 303 litres. Use that net figure as the amount to irrigate that week.

Net Irrigation After Rain in Plain Terms

Net irrigation after rain answers one weekly question. How much water does the crop still need once rain has done part of the job. It is not the total weekly use. It is the remainder, after the calculator takes effective rain off the weekly total.

Rain does not change how much water the crop uses. It changes how much of that use still has to come from irrigation. A week with no effective rain leaves the full weekly use to irrigate, and a week with useful rain leaves less.

What Effective Rain Is

Effective rain is the part of the rain that stays in the root zone and is available for the crop to use. Total rain is the depth that falls from the sky. Effective rain is the smaller share that counts toward crop water use. The calculator asks for effective rain, not total rain.

The calculator page states that effective rain is less than total rain on hot or heavy storms. Hot weather can dry rain from leaves and soil before roots can take it up. A heavy storm can send water away as runoff, or push it below the root zone.

The page gives no method to convert total rain to effective rain, so this blog does not invent one.

How the Calculation Works

The calculator uses four formulas. First, daily crop water use, written ETc, equals the crop coefficient, written Kc, times reference evapotranspiration, written ET0. ET0 is the water used by a well watered grass surface under local weather. Kc adjusts that figure for a crop and growth stage. The Kc values come from the FAO 56 guidelines and assume a healthy, well watered crop.

Second, weekly use equals ETc times 7, treating daily ETc as the same on each of the 7 days. Third, net irrigation equals weekly use minus effective rain. This is the rain credit, a subtraction in depth or in volume once both figures use the same unit. Fourth, volume equals depth times area. One mm over 1 square metre equals 1 litre.

The Worked Example Explained Step by Step

The calculator page gives one worked example: a tomato crop at full canopy, Kc 1.15, ET0 5 mm. That is the only crop and stage with a Kc value stated in the page text, so no Kc value for any other crop or stage is invented here.

Step 1 is daily depth: 5 mm times Kc 1.15 gives 5.75 mm a day, rounded on the page to about 5.8 mm a day. Step 2 is daily volume: 5.75 mm times 10 square metres gives 57.5 litres a day, which matches the page exactly.

Step 3 is weekly volume: 57.5 litres a day times 7 days gives 402.5 litres a week, rounded on the page to 403 litres. Step 4 is the rain volume step, not spelled out on the page and shown here explicitly: 10 mm over 10 square metres gives 100 litres, because 10 times 10 equals 100.

Step 5 is net irrigation: 403 litres minus 100 litres gives 303 litres, which matches the page. That 303 litre figure is the net amount for that tomato bed for that week.

Net Irrigation for Different Rain Amounts

The table below keeps the same tomato example, Kc 1.15, ET0 5 mm, and area 10 square metres, and changes only the effective rain. Weekly use stays at 403 litres, or 40.25 mm.

Effective rain mm	Rain volume litres	Net depth mm	Net irrigation litres
0	0	40.25	403
5	50	35.25	353
10	100	30.25	303
15	150	25.25	253
20	200	20.25	203
25	250	15.25	153
30	300	10.25	103

Every row follows the same arithmetic. Rain volume is the rain depth times 10 square metres, and net irrigation is 403 litres minus the rain volume. Each extra 5 mm of effective rain takes off 50 litres on this area.

How Weekly Use Changes With ET0

Weekly use itself changes with ET0, because ETc equals Kc times ET0. The table below keeps the same tomato crop, Kc 1.15, area 10 square metres, and effective rain 10 mm, and changes only ET0.

ET0 mm	Daily ETc mm	Weekly use litres	Net irrigation litres
3	3.45	242	142
4	4.60	322	222
5	5.75	403	303
6	6.90	483	383
7	8.05	564	464

Check one row. At ET0 4 mm, daily ETc is 4 times 1.15, which is 4.60 mm a day. Weekly volume is 4.60 times 10 square metres times 7 days, which is 322 litres. Take off 100 litres of rain and net irrigation is 222 litres. The rain credit stays at 100 litres in every row, but the net figure rises with ET0 because weekly use rises.

The Same Example in Imperial Units, Rounded

The imperial figures below are rounded equivalents, not exact conversions, for the same tomato example in inches, square feet, and gallons. ET0 of 5 mm is about 0.20 inches. Daily ETc of 5.75 mm is about 0.23 inches. Weekly depth of 40.25 mm is about 1.58 inches. An area of 10 square metres is about 108 square feet. Daily volume of 57.5 litres is about 15.2 gallons. Weekly volume of 403 litres is about 106 gallons. Effective rain of 10 mm is about 0.39 inches, and its volume of 100 litres is about 26.4 gallons. Net irrigation of 303 litres is about 80 gallons. Work in one unit system at a time.

Imperial item	Rounded figure
ET0	0.20 inches
Daily ETc	0.23 inches
Weekly depth	1.58 inches
Area	108 square feet
Daily volume	15.2 gallons
Weekly volume	106 gallons
Effective rain	0.39 inches
Rain volume	26.4 gallons
Net irrigation	80 gallons

How to Use the Calculator Each Week

Use the calculator once a week. Choose the crop, choose the growth stage, enter ET0, enter effective rain, then irrigate the net amount shown.

Start with the growth stage, because Kc changes as the canopy grows. Next, get ET0 right for the week. ET0 is a weather figure, and it moves with sun, wind, temperature, and humidity.

Finally, read the net amount and use it as the irrigation target for the week. The page also says to add a little extra for irrigation inefficiency, with no amount given, so no amount is quoted here.

For planning the days inside the week, the Garden Watering Schedule Calculator can help turn a weekly amount into a watering pattern, and the Irrigation Scheduling Calculator covers timing in more detail. The Crop Water Requirement Calculator itself gives the weekly and net amounts used in this blog. If you collect rain for later use, the Rainwater Harvesting Calculator and the Rain Barrel Calculator cover how much rain a roof or a barrel can supply.

What This Blog Is Not

This blog is about the rain credit only. A fixed weekly need asks how much water a crop uses in a week when rain is not taken off at all, which is the weekly use figure, ETc times 7, on its own.

Net versus gross with efficiency asks how much water to apply when part of the applied water is lost before it reaches the roots. That question adds an allowance for inefficiency on top of the net need, and this blog does not do that calculation. Do not read the 303 litre figure as a gross figure. It is a net figure, after rain and before any efficiency allowance.

When Effective Rain Exceeds Weekly Use

The calculator page states no floor of zero if effective rain exceeds weekly use. The page does not say what the calculator shows in that case. The arithmetic gives zero or less, which in plain terms means no irrigation is needed that week, but that is a plain reading of the arithmetic, not a page rule.

Take the tomato example. Weekly use is 40.25 mm. If effective rain were 45 mm, the subtraction gives minus 4.75 mm. A negative irrigation amount has no practical meaning, so do not irrigate that week. As a suggestion only, this blog does not carry any surplus forward, because the calculator gives no rule for that.

Common Mistakes

Entering total rain as effective rain is the most common mistake. Entering the full total will understate net irrigation.

Forgetting the area is the next mistake. Ten mm of rain is 100 litres on 10 square metres, but 50 litres on 5 square metres. Always multiply by your own area.

Using last week rain for this week is another mistake. Start each week with fresh figures.

Treating the rounded figures as exact is a smaller mistake. Pick one route, show your working, and round only at the end.

Adding a guessed efficiency percentage is the last mistake to avoid. The page says to add a little extra for irrigation inefficiency, with no amount given, so label any percentage you add as your own allowance.

FAQs

Q: What is net irrigation after rain?
A: Net irrigation after rain is the water still needed after effective rain is taken off weekly crop use. For the tomato example, weekly use is 403 litres on 10 square metres, effective rain of 10 mm adds 100 litres, and net irrigation is 303 litres.

Q: What is effective rain?
A: Effective rain is the part of total rain that stays in the root zone and is available for the crop to use. It is less than total rain on hot or heavy storms.

Q: How do you work out net irrigation after rain in litres?
A: Work out weekly use in litres, then work out rain volume by multiplying effective rain in mm by area in square metres, and take the rain volume off the weekly use. In the tomato example, 403 litres minus 100 litres gives 303 litres.

Q: Why is the rain volume step shown separately in this blog?
A: The calculator page states the net figure of 303 litres but does not spell out the rain volume step. This blog shows it: 10 mm over 10 square metres equals 100 litres.

Q: What happens if effective rain is more than weekly use?
A: The calculator page states no floor of zero for that case. The arithmetic gives zero or less, which in plain terms means no irrigation is needed that week. That is a plain reading of the arithmetic, not a page rule.

Q: Can I enter total rain instead of effective rain?
A: No. The calculator asks for effective rain, which is less than total rain on hot or heavy storms. No method is given to convert total rain to effective rain.