4 water resources and climate in the future
It is expected that water resource problems associated with desertification, flooding, erosion and sediment transport, water contamination, over-exploitation of aquifers, etc., will become more acute in the future, and this will make the sustainable development of water resources even more difficult. Modifications to the climatic system will merely serve to complicate matters.
The water cycle plays an extremely important and reciprocal role in the climatic system, both conditioning the climate and being affected by it. Changes in precipitation can bring about changes not only to the runoff magnitude and temporality, but also to the frequency and intensity of storms and droughts. Temperature changes undoubtedly cause alterations to the evapotranspiration, soil moisture and seepage to the deepest layers. Such changes in the surface water-content modify the vegetation cover, which brings about a chain reaction, affecting cloud formation, the Earth's albedo and precipitation.
Studies carried out into the impact of possible climatic change on water resources in semi-arid zones should be pointed out by way of example. According to the first report on scientific evaluation of the IPCC (1992), a temperature increase of 1º C to 2º C, together with a 10% precipitation reduction, could lead to a 40% to 70% reduction in renewable resource. Furthermore, a temperature increase could cause snow to melt earlier, increasing the winter runoff and reducing the thawing processes in spring and summer. Even in the areas where precipitation increases, the greater evaporation could lead to lower runoff rates.
A variation in the risk and intensity of droughts is the most serious negative impact of climatic change on water resources. A reduction in water availability could lead to desertification in zones where the balance is particularly fragile.
Climatic change can have considerable repercussions on the flood regime. The predicted variation in storm magnitude and frequency would give rise to a spectacular increase in runoff in short periods of time, which would aggravate the already catastrophic effects of flooding, thus making it necessary to review present techniques for water resources estimation, prevention, prediction and management.
It is only to be expected that a worsening in the summer would adversely affect the water quality and it would increase salinity in the water and the soil. Water scarcity would make effluent dilution more difficult, causing, amongst other phenomena, a further increase in reservoir eutrophication. The scarcity of the resource ought to lead to the deployment of wastewater for other purposes.
The impact on aquifers of potential climatic change would take the form of a reduction in natural recharge (renewable resource), giving rise to a reduction and eventual disappearance of natural outlets (rivers, springs, wetlands etc.). Furthermore, the possibilities for management would become reduced and more complicated, being excessively concentrated on specific aquifers, with the evident risk of over-exploitation. A reduction in the capacity of the resources to be renewed would cause a lowering of water quality and a drop in the groundwater table which, in coastal areas would lead to an increase in seawater intrusion; the latter problem could be further aggravated by a rise in the sea level.
The degradation and even disappearance of vegetation cover, especially that of the woodland type, together with an increase in torrential rain and a reduction in soil moisture, would increase erosion and soil loss, thereby reducing the useful life of the reservoirs by causing them to silt up, all of which would have a negative effect on harnessing water resources.
As a result of the aforementioned, an attempt must be made in the near future in order to obtain a greater insight into the possible effects of climate change upon the quantity and quality of water resources, on the water requirements of the various human activities and the availability of this particular resource, especially in the semi-arid zones which are the subject of this report.
In Portugal, according to the First European Climate Assessment and with the help of ESCAPE model, it was possible to predict the following changes between 1990 and 2050:
Mean Precipitation (summer): -10 to -15%
Annual Mean Temperature (south of Tejo): +1,5 to 2ºC
Annual Mean Flow: Minho (0 to -5%), Centro (-5 to -10%), Sul Tejo (-10 to -20%) and Guadiana e Algarve (-20 to -100%)
In Sardinia, water resources have diminished due to lower precipitation. For example the water collected in the Flumendosa reservoir has dropped to 30%. Average discharge over the last twenty years is roughly 60-65% of the average for the previous 50 year period and over the last nine years, it is only 45%.
In Spain, studies have not thus far been made to assess the possible effect of changes in precipitation and evapotranspiration on water resources. However, guidelines for these studies are now being prepared by CEDEX for the General Directorate of Hydraulic Works and Water Quality.