Calculation of irrigation with precision

Little by little agriculture is becoming a science. It is not as exact as other branches in which exact quantities predominate (here you can play with many variables) but little by little many concepts are being unified. One of the most interesting formulas when it comes to achieving maximum crop performance with maximum economic savings is to know the net irrigation needs . Do you want to know how?


When the earth looks more or less dry, water is added until we see that the earth reaches a point where it does not swallow any more, just before its flooding. Or we add water until we think it is enough. This technique is not perfect but it works , and any of our readers can tell you that by this time of summer they will surely have large plants of pepper, tomato and what is worth their salt at this time. Is it a lie?

But hey, it doesn’t end here. There are more technical ways to know the exact amount of irrigation a crop needs. You may think that it does not make much sense when establishing a small or family garden, and we are not going to lie to you, it is true. But when you have large plots of crops , large orchards , large areas of fruit trees, or you do not have access to large amounts of water to irrigate, every drop counts (as the water saving advertisement would say).

As we did with the calculation of the irrigation of the pepper, tomato, eggplant and zucchini ,  what we intend is to establish a basic guide to guide us with the daily amounts of water that the crop demands, depending on which it is, and bearing in mind variables such as the place where we are and the date we are, since, as seems logical, the same water needs are not demanded in winter as in summer.


Here it is important to have a clear concept, evapotranspiration. Commonly explained, evapotranspiration is the amount of water that a plant or crop loses either by direct evaporation or by transpiration. That is, they are the amounts of water that our plants need to recover to continue growing without problems.

As many variables are involved, what is done is to compare the evapotranspiration of a reference crop of which a lot of data is known with our crop, so that we apply a correction coefficient depending on the species. An olive tree is not the same as a pomegranate, right?

All this paraphernalia is summed up in a simple and straightforward formula:


Here it gets quite complicated when we see that to calculate evapotranspiration we need to solve a formula that is difficult to digest. Although here the main problem lies above all in obtaining all these data:

At this moment we are rethinking closing Gardenprue and continue watering as usual, but thanks to computer systems we can forget about this previous formula… for now!

The Ministry of Agriculture of Spain offers a great tool (SIAR) to know the evapotranspiration of a certain crop, in a certain area and, of course, in a certain time. For the clueless say that you have the link back inserted in the sentence;). It is not necessary to explain how it is used, it is very interactive.


Anyone could say that millimeters (mm) measure distances and liters (L) quantities and cannot be related. But it is not like that, since the mm are a great instrument to measure precipitation and it will surely sound to you when you hear on the radio or television that 100 mm of water has fallen. This means that in an area of ​​1 square meter, if there were a container with these measurements, the rainwater would reach up to 100 mm in height, that is, 10 cm. And turning it to volume, 10 centimeters in height of water is 100 liters. Imagine a tank 1 meter high and 1 meter on each side, if you fill it with water it would have 1,000 mm of water height, or what is the same 1,000 liters.

In this case it gives us Kc or the crop coefficient, Eto as the reference evapotranspiration as mentioned in the attempt to explain the first formula, Etc is the crop evapotranspiration, and Pe is the precipitation, so that they are subtracted from each other, since precipitation is considered as water gain. What you have to check is the last column where it says Etc-Pe, which is the result of what we have to water. 3.00 mm would equal 3 liters per square meter. Here you have to see the planting framework of the crop, since if 3 plants enter that square meter you would have to water that day with 1 liter each. Interesting right?

Now you just have to look for your area and the crop you want to find out about and draw conclusions! You may not find it within your area because it may not be typical, but you have many more tools on the web, such as SIAM.

I hope I haven’t gotten you too bored with all these concepts. Greetings and see you next day! 😉

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