dimanche 4 janvier 2015

28- Water and irrigation -1- Water in agriculture

I have not mentioned it yet, but it is a fundamental issue when we speak about agriculture.
Indeed, there is no life without water. Agriculture exclusively works on the living beings, plants and animals. So there is no agriculture without water, and therefore, no water, no food.
In many parts of the world, rainfall is sufficient in quantity and distribution over the year, to meet the water needs of agriculture. For example, in France,
a country with highly developed and modern agriculture, only 7% of agricultural fields are irrigated. In Spain, the climate is much drier, so 20% of agricultural fields are irrigated.
This is one of the contradictions of agriculture. More dry is the climate, so there are less rain to restore water supplies, higher are the irrigation needs. The less available is the water, the more it is needed.

In fact, it's not really the way it happens. Most regions have an adequate average annual rainfall, in theory, to ensure the needs of agriculture. The problem lies in the distribution of rainfall and evaporation, normal physiological process of the plant. More the evaporation increases, more the plant needs water, to "provide the same work."

Let's consider two European cities, Rennes, France (Brittany), with an oceanic climate and Seville, Spain (Andalusia), with a warm Mediterranean climate.
Climate data, of total annual rainfall are:
Rennes between 400 and 1050 mm, 695 mm average.
Seville between 290 and 1090 mm​​, average 530mm.

The differences are not apparently important.
Yet the observation compared to the average rainfall curve, shows a very different distribution.

Rainfall in Rennes marks a relatively flat curve, while that of Seville shows a normal drought that lasts 4 to 5 months during the summer.
To this phenomenon, we must add the criterion of evaporation, or more accurately the hydric deficit, ie the difference between rainfall and evaporation.
In Rennes, it is around 200 mm per year, while in Seville it is around 600 mm per year. This means that in Rennes, the annual evaporation is 200 mm higher than annual rainfall, while in Seville, it is 600 mm higher.
A cornfield located around Rennes will not have the same requirements as the same cornfield near Seville.

I remind you that 1 mm of rainfall or evaporation is una quantity 1 liter of water per m2, which is equivalent to 10,000 liters per hectare or 10 m3 / ha.
So when I tell you that a crop requires 5,000 m3 of irrigation water per hectare per year, it means that it needs 500 mm of additional water that is not naturally available. It is through the irrigation water that it is possible to offset the deficit.

Okay. For the local agriculture, the consequences are obvious. Around Rennes, only 0.7% of the area is irrigated, while around Seville, the percentage goes to 37.7%.
In the world, only 20% of the cultivated area is irrigated, but it provides more than a third of global food production.

So far, everything is logical. It rains less in Seville at Rennes, the weather is also much warmer and evaporation is much higher.
The problem is that in regions where rainfall is seasonal, and agriculture has a great need for water in the dry season, it is imperative to have or to create water reservoirs. It is necessary to be able to accumulate in dams, the water in excess of the rainy periods, in order to dispose of it during dry periods.
This is the case in Spain, which has developed and carried out, many years ago, a water self-sufficiency policy.
And even if the political and social circumstances of the construction of these dams are highly questionable (Spain was under a dictatorship), the result is still there, nearly 30 years after the restoration of democracy, allowing the country to have a vibrant agriculture, and one of the most modern in the world in terms of irrigation and plant nutrition.

It is clear that an adaptation of structures is imperative in all regions where water scarcity is usual. But the possible adaptations, and irrigation methods are varied. And every situation has its own imperatives.
The first known adaptations correspond to what is now recognized as the invention of irrigation, and date back more than 5000 years BC in Mesopotamia.
             This picture is taken from the beautiful website http://www.sienteamerica.com/posts/2472-8-paisajes-de-montanas-en-terrazas

 There are many controversies regarding the use of water in agriculture and its consequences.
The first is that agriculture consumes a huge amount of fresh water. That's right, according to the FAO, over 70% of freshwater global consumption comes from agriculture.
It seems outrageous to some? No problem. Let's stop feeding us, the problem will be solved.
In fact all is not so simple. Can we reduce water consumption in agriculture without reducing yield? The answer is yes, doubtlessly.
But reducing agricultural water has costs, especially energy cost. You have to choose between saving water by expending more energy, or save energy by consuming more fresh water.
The problem is difficult and complex. I want to address these issues by trying to clarify some things about this vast subject.
It will be a new series simply called "water and irrigation."
I will talk about irrigation methods, monitoring of needs of plants, water quality, reserve management, pollution, plant physiology, etc.

The data used in this article come from Météo France, from AEMET, from the FAO, and from the inevitable Wikipedia, along with some personal information.

1 commentaire:

  1. Un asunto muy interesante. Espero, con interés, tus próximas publicaciones al respecto. Gracias.

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