This article (click here) is very-one sided and incomplete; the problem is much worse than exposed. “Lining irrigation canals” is only one of many issues.
1. Large sections of the irrigation system are ill designed, and can never be efficient.
2. Many farmers over-irrigate because they don’t know much about the irrigation requirements of crops. The requirement depends on the type of soil, particularly the “water holding capacity ” of the soil. In the Algarve, there is little data on this. The daily water requirement should be calculated using measurements of evapotranspiration, usually obtained with a “Class A Evaporation Pan”.
In most countries, such evaporation pans are not only found in the met stations, but also on individual farms; I have never seen one in the Algarve.
3. The “crop factor” which varies from crop to crop, the growth stage and canopy cover of the crop. All of these variables are used in empirical formulas drawn up by the Ministry of Agriculture. I do not know anybody who uses these techniques here. The result is that many farmers over-irrigate, “just to be sure”.
In a well-designed and properly operated system, the irrigation efficiency should be between 90 and 95%; that is, 90% of the water leaving a source should end up at the plant, and be used by it. I estimate the irrigation efficiency in the Algarve to be about 40%. More than half the water is thus being wasted.
Prof. Dr. Eng. Willem C. Beets