3.9. desertification and erosion in basins
Desertification is a recently coined term, whose meaning has not yet fully crystallised either from the technicians or from societys viewpoint. According to Puigdefabrégas (1995), desertification refers to a situation in which the lands productive capacity deteriorates and the socio-economic systems that exploit that system start to crumble. These zones are affected by a water shortage during at least one season of the year, are fraught with problems resulting from negative human activity owing to over-use of the resources and finally, are suffering from or have suffered from acutely dry periods or dry periods whose duration is greater than normal for that region.
Desertification can cause a reduction of infiltration into the soil and thus, a greater surface flow, with a corresponding increase in the maximum flood discharges. Desertification also causes modifications to the vegetation cover, which is currently undergoing rapid changes as a result of deforestation, either for providing fuel or for obtaining more arable land. The new vegetation cover, when it does exist, consists of either crops or poor vegetation. The soil is unprotected and the erosion caused by an increasing surface flow on the ground becomes even more serious, thus starting a spiralling process. Another consequence is a reduction in the storage capacity of reservoirs which receive and retain a larger amount of sediments. These reservoirs can produce environmental impacts downstream, given that siltation and flow regulation do not allow the periodic flooding of plains with nutrients, contributing to soil degradation.
Of all the countries on the Northern shores of the Mediterranean Sea, Spain is the one whose conditions make it most susceptible to suffer the effects of desertification, especially in the semi-arid areas, because of the following reasons: a mountainous morphology with steep slopes, heavy rainfall with considerable erosion ability, climatic conditions that range from sub-humid to semi-arid, temperatures and rainfall featuring highly variations from one year to the next, over-exploited systems because of the delicate balance between availability and water resource consumption, etc. (Puigdefábregas, 1995).
In Portugal, the most frequent type is water erosion because of the climatic conditions, the irregularity and concentration of the rainy period with intensive events immediately after the dry season, when the soil is dry and less protected. European studies, show that the most affected areas of Portugal are the mountainous regions in the north, although the erosion phenomenon in plains such as Alentejo or in Algarve has increased.
Considering the semi-arid areas of Portugal, 69% of them have a high risk of erosion and only 5% a low risk.
Since the Stockholm Conference of 1972, several projects have been developed concerning erosion processes but they do not form a global monitoring program. There are, however, some experimental centres in Alentejo and Algarve regions (southern Portugal) where long runs of erosion and siltation data have been measured at seven hydroelectric dams.
In Sardinia, different desertification phenomena are caused by soil erosion. Desertification occurs after fires, on slopes subjected to ploughing, and because of overgrazing and wood clearance. Good farmland is also getting decreasing because of urbanisation and contamination.