dimanche 28 juin 2015

46- Water and irrigation -4- Why do we irrigate?


Before talking more about irrigation, it is important to understand what we are talking. So I will try to answer the question: what is it and what is it for?

Plants, like all living beings, need water. This is vital. Without water, no life, neither animal nor vegetal. But in the case of plants, water plays a multiple role, eventually more complex than in the case of animals.

The first role, as in all living beings, is to be the fluid that fills and bathes the cells and maintaining cell elements in activity.
Water is also the "sweat" of plants, that is to say that enables the plant to produce its own heat control through sweating, called also, in this case, evapotranspiration .
Water is also the vehicle of different minerals and hormonal elements through the plant in the form of sap, such as blood in animals.
But what distinguishes the plants of animals, as regards water, is that the plant has no independent way to eat. Eating is, in animals, the way to absorb minerals that their body requires, through more or less solid foods. The plant performs this vital action primarily by the roots, and partly by the leaves and young bark. But the roots only can absorb water, charged in dissolved minerals.
So the water represents, for the plant, the drink and the food.

Irrigation represented in  the tomb of Sennedjem, in Egypt, in the time of Pharaoh Ramses II, to the years 1275 BC, more than 3,000 years ago.

Irrigation is used in agriculture for more than 5000 years to allow plants, organized in most cases in monoculture, to continue their activity even during the months when rainfall is insufficient or absent.
The farmer must be able to provide water to his crops when nature can't do it. In this case it's to allow the plant to complete its cycle, to ensure the production of food (the lack of water could compromise it) and, in the case of perennial crops, to ensure their survival till next year.

Once this concept is understood, we enter into technical considerations that will concern the irrigation system, water availability, quality of available water, water doses, frequency, etc.
Good water management will help optimize resources while getting good agronomic and productive results.
Poor water management will instead cause all kinds of water stress, for lack or excess, along with agronomic problems, risk of groundwater pollution and bad productive results.
To cite one example that I know well, in my plain conditions in the region of Seville, I can drive the orchard until harvest, and prepare the next harvest with less than 3,000 m3 of water per hectare a year when I know orchards, under comparable conditions, or sometimes less difficult, consuming between 2 and 5 times more water.

Nutrients absorbed by the roots are always dissolved in water. Their origin, from soil or from natural or chemical fertilizers, does not change the way the plant can assimilate them.
Because they are soluble in water, not absorbed by the roots elements, may be driven to groundwater in case of excess of water or heavy rain. Thus, a poor water management also entails a significant risk of pollution of water resources.

In a climate where rainfall is fairly evenly distributed throughout the year, irrigation is not necessary because the soil water provision is naturally replenished by each rain event. However, in a Mediterranean, continental dry, arid or semi-arid climate, most plants are not able to sustain themselves without the help of the farmer. Irrigation is therefore necessary to support the plants during periods when the water reserve of the soil becomes insufficient.
A few years ago, here in Seville, a region where, without irrigation, there would be virtually no agriculture, I received a group of young students from a Danish agricultural school. They did not understand why the farm was equipped with pumping and piping. So I had to explain the climatic characteristics of the region, to help them to understand the value of a system that, obviously, is not necessary in their country.

Another difference between plants and animals is that these ones can move to go in search of water. Specifically, the farmer can locate the supply of water to the animals so that every liter of water is consumed, without any waste.
In the case of plants, it is different since the plant has to find water in the soil volume that it prospects with its roots. It is harder for the farmer to not waste water, because it is very difficult to monitor accurately, both the consumption of plants and the adjustment of irrigation volumes. This is where the technification of irrigation can provide solutions.

Be aware that irrigation is not a farmer's whim, but a real need in many, many areas of the planet. Without irrigation, food availability problems would be even more serious.
Also the rice, a major staple food in the world, need a permanent flood phase to occur, causing a high consumption of fresh water.
It is clear that without irrigation, food problems would be much more serious. Hunger often leads to rebellion. We can therefore consider that irrigation is an important factor of social peace.

 However, we must know the limits. Fresh water resources are not inexhaustible and it is essential to manage them consistently.

A major factor in wastage of fresh water resources is the mismanagement of irrigation. The farmer needs to know his crops, its needs at every moment and know (and be able) to increase or reduce water supply to suit the needs. So there is obviously on the one hand an important training/information issue for farmers, and secondly the awareness.
Good crop management can increase the productivity of each field. A well-irrigated field will produce more, so it will take less irrigated area to produce the same amount of food. To improve productivity through training and technification is therefore an important factor in saving water, and of sustainability for irrigated agriculture.

What are the problems that will push a farmer to irrigate too much?
They are many. We can talk
- Of irrigation system (flood, sprinkler or drip irrigation)
- Of water availability (wells or ponds that will yield water at will, or community networks with water turns that leave only limited time available)
- Of quality of plants water needs controls by the farmer (humidity control of soil, plants operating, climatic data, etc.)
- Of farmer's fear. Farmers are often afraid of the lack of water, more than the excess water. But excess water is usually more damaging than lack. This irrational fear also comes from the first symptom of an asphyxiated plant is very similar to the symptom of a plant suffering from drought.
- The type of soil. Some soils are difficult to manage because the water circulation is complicated. In general, farmers often prefer to take the risk of excess water, which is almost always a mistake.

I will return to these issues in several articles to come.

1 commentaire: