THE MEDITERRANEAN REGION: an Hydrological Overview
by Agim Selenica,
1. A Geographic and Climate Perspective
The Mediterranean Sea covers about 2.5 million km², with an average water depth of about 1.5-km. The length of the Mediterranean coastline totals about 46,000 km, of which 19,000 km represent islands coastlines. The entire coastal region covers an area of nearly 1.5 million km², 17% of the total area of the bordering countries: Spain, France, Monaco, Italy, Slovenia, Croatia, Bosnia, Yugoslavia, Albania, Greece, Turkey, Cyprus, Syria, Lebanon, Palestinian Territories, Israel, Egypt, Libya, Malta, Tunisia, Algeria, and Morocco.
Climatically, the Mediterranean is characterised by generally warm temperatures, winter-dominated rainfall, dry summers and a profusion of microclimates due to local environmental conditions. The north tends to be relatively temperate and damp, whereas the south is hot and arid. The strong summer-winter rainfall contrast that characterises the Mediterranean climate is associated with pronounced seasonal cycles in most climatic variables. July, August and September are warm and dry, whereas the winter is characterised by cyclonic disturbances, is also influenced by local orographic effects.
One result of the seasonal rainfall and high evaporation is that water shortages are endemic. The problem is particularly striking in the southern parts of the Mediterranean in contrast to seasonal shortages in the north (corresponding to the dry months). The dry season in some southern countries exceeds six months, meaning that water shortage is a permanent handicap for sociological and economic development.
2. The Marine and Coastal Environments
Because of the seasonal rains, high rates of evaporation and the correspondingly low runoff of the relatively short rivers, the Mediterranean has a deficient hydrological balance, one that has increased with the damming of the Nile and other rivers. As a result, Atlantic water that enters the Mediterranean through the Strait of Gibraltar exits as more saline (by nearly 10 %) subsurface water. This basin-wide circulation, combined with the Mediterranean climate and low land runoff, explains the low biologic productivity throughout the sea.
Although alluvial and coastal plains are few and not extensive (the Nile Delta being far the largest) most coastal plains have demographic and economic importance ranging from agriculture to industry/ commerce to recreation to historical/archeological significance. Most areas still contain partly to little-modified natural ecosystems of irreplaceable value. Because of their ecological fragility, related to the land-use transition, and their economic importance, these coastal lowlands are particularly vulnerable to climatic changes that can affect hydrology, sea-level rise and ecosystems. Anthropic activities can also affect these areas because of pollution and sediments flows from upstream catchments.
3. Population and Development
In 1985 the countries bordering the Mediterranean Sea had a combined population of 352 million people, of which 37% lived directly in the coastal zone, meaning that population densities were generally three times greater in coastal than in non-coastal areas. Coastal population densities range from greater than 1000 km² in the Nile Delta to less than 20/km² along coastal Libya. According to some projections, the population in the Mediterranean is expected to reach 430 million in the year 2000, and 545 million in 2025 (UNEP,1987a). Increasingly the population will urbanise and it is expected that by the year 2025 about 75% of the population will be urban. The economic and environmental burden on cities, therefore, will increase substantially.
Population growth, however, shows major differences between North and South. The European countries have nearly stable population, annual growth rate often less than 1%. In contrast, population growth in southern countries ranges from 2 to 3% per year. As a result the population in the coming years will increase and became younger in the South. With this shift will come increasing problems concerning education and job-creation in southern countries.
The wide variation in political and economic systems as well as historic differences have led to great discrepancies in the level of development between Mediterranean countries. The highly developed industrial countries in the north (France, Italy and Spain) and countries on the way to becoming industrialised (Greece, Yugoslavia and Turkey) stand in stark contrast to the countries in the South.
Mediterranean countries mostly lack natural resources. Libya, Algeria and Egypt are considered moderate-sized petroleum producers, Morocco is the world' third-largest producer of phosphates, Albania the third largest producer of chrome, and Spain the second largest producer of mercury. Water resources are relatively plentiful in the North but scarce in the South. Forests have limited economic significance, but are important for the preservation of soil as for recreation and landscape.
Mediterranean agriculture is characterised by multi-faceted crops, particularly olives, citrus fruits, grapes and hard grain; the main livestock is sheep. Increasingly, irrigation is needed in the South to maintain or increase crop production. While coastal regions tend to have little agricultural land, it often is of high quality, particularly around delta areas. However, Mediterranean agriculture is also characterised by long-term misuse and overexploitation. In part this is due to generally poor soils, lack of rain and the increasing population pressures (particularly in the South).
In recent years, efficient farming and growing urbanisation in the north has lead to increased abandonment of farmland and rangeland and the corresponding advance of forests. This contrast strongly in the South and east where marginal areas, such as arid steppes and rangelands, are being cleared for grain production ; unfortunately, the lack of water resources and especially continually flowing rivers has restricted the use of irrigation. One result has been the increased desertification in North Africa and the Near East. Given the trends of the past 40 years, in the near future virtually all tillable land in the southern and eastern part of the Mediterranean basin will be cultivated for cereal production, even through the risk will be high and the yields low.
The Mediterranean is not capable for supplying enough fish for its inhabitants. More than 50 % must come from outside areas. Most demersal stocks along the northern coasts of the Mediterranean are heavily fished, and the introduction of regulatory measures is necessary to maintain high levels of yields and catch. Coastal pelagic stocks, together with aquaculture, appear to be the best prospects for future development. Competition with coastal zones for other types of development, together with increased coastal pollution, however, will limit the extent to which aquaculture can develop.
Finally, it should be pointed out that because of both climate and historical/archeological significance, the Mediterranean continues to be the greatest tourist destination in the entire world. Conversely, tourism is the greatest consumer/user of the Mediterranean coast. In 1984, for example, there were about 100 million tourists in coastal areas. Total tourist facilities take up more than 2 million m² of space on the coast, and water consumption in 1984 amounted to 569 million m3 (UNEP, 1989). The number of tourists by the year 2000 could rise to 120-180 million, and 170-340 by 2025 (UNEP, 1987b). Such a growth will mean an increasing demand for coastal space as well as such necessities as electric power and water. Furthermore, the impact on certain habitats (particularly sandy beaches and dunes) will increase.
5. The Greenhouse Effect and Climatic Changes
The greenhouse problem can be viewed as one involving large uncertainties but with risks that are potentially high. Even the most well-defined projections, those for global-mean temperature, are the subject to considerably uncertainty. Global-mean warming projections for the interval 1990-2030 lie in the range 0.5-1.4 °C, a range which widens further if one takes account of uncertain future greenhouse-gas concentrations. At the low end of the range, the change is comparable to the global-mean warming that has occurred already this century (although the future warming would be more rapid). However, the likely observed change by 2030 is not the only important parameter, the warming commitment also needs consideration. In 2030, there is likely to be a substantial commitment to future warming, even if greenhouse-gas concentration could be held constant at their 2030 levels. Because of this warming commitment and because large increase in greenhouse-gas concentration are inevitable, we are already committed to significant changes in regional climate. When couple with changes in extreme-event frequencies that also must occur, the impacts could be considerable. At the high end of the commitment range (i.e. the low-probability/high-risk area), the projected global-mean temperature would far exceed anything previously experienced by human beings. There is clearly considerable cause for concern and a pressing need to reduce the uncertainties.
For the Mediterranean basin, General Circulation Model (CGS) result point to a warming similar in magnitude to the global-mean value, with no difference for any marked seasonal differences in the warming. As time progresses, the probability of periods of extreme warmth will increase, with attendant effects on human wellbeing and on "natural" events like forest fire. Furthermore, increased warmth will probably lead to greater evapotranspiration and (in absence of increased winter/spring precipitation reduced growing-season soil moisture levels, adding to direct thermal stresses on vegetation.
Projected precipitation changes vary so much from model to model that one cannot say, on the basis of model result alone, whether precipitation will increase or decrease. Depending on location, model used and season considered, projected changes between now and around 2050 range between +/- 1 mm/day. The mean precipitation rate for the Mediterranean basin as a whole is roughly 1 mm/day. Such large changes are undoubtedly unrealistic, and must reflect model deficiencies. However, the possibility of substantial changes (say, up to 30% over the next 50-100 years) must be considered as realistic.
6. Hydrological cycle, present and future
There is a rather clear topographic dependence on precipitation. A minimum is found along the Spanish east coast, whereas rather high values of precipitation occur along the Moroccan, Algerian, Yugoslavian, and Greek coasts. One could summarise the precipitation situation by saying that along the Spanish east coast the precipitation is 200-400 mm/year, along the coast of the Ligurian Sea 800-1200 mm/year, and along the Yugoslavian coast more than 1500 mm/year. In the southern part of the Mediterranean region the precipitation generally averages 100-400 mm/year. Precipitation clearly shows a minimum during summer, with exception of areas north of the 45° latitude, where seasonal precipitation is more evenly distributed.
Annual evapotranspiration along the northern coast is around 400-600 mm/year with a maximum during the first part of the year.
Mean annual runoff varies considerably throughout the Mediterranean. Along the Spanish east coast it amounts to about 100 mm/year, whereas along the Ligurian Sea values increases to 400-500 mm/year. In the Adriatic Sea it is around 300 mm annually but with somewhat higher values along the western part of the sea. Along the southern coast of the Mediterranean the runoff is highly variable. Runoff coefficient is about 0.5 to 0.6, except in southern Spain and in Tunisia, where it amounts to only 0.1 on average.
From the information about the changes in precipitation, based on GISS model, evapotranspiration and runoff are calculated using the water-balance method. Thus, in the northern part of the Mediterranean there will be small increase in precipitation and a rather high increase in evaporation as compared to other parts of Europe. Although the increase in runoff will be only 46 mm/year, water deficit in this part of Europe has strong implications for future agriculture and general land use. However, we must bear in mind that these conclusions refer to changes on a yearly basis. One must also consider small changes in timing or magnitude of seasonality, which can have important consequences. Severe stresses, for instance, may be placed upon agriculture if reservoir systems cannot store seasonal precipitation and runoff for later distribution