The amount of water leaving the soil must be at least equal to the amount entering it. The water should not be allowed to accumulate, but perennial irrigation invariably raises the water table. In some areas the groundwater tables are rising at a rate of 3 to 5 metres a year. This is caused primarily by the water lost through seepage from irrigation channels or simply by overuse of irrigation water. As waterlogging sets in, so the inevitable process of salinization begins.
Between 1990 and 1997, the water table under the city of Milan rose 11 metres in some areas, leaving the water just four metres below the surface. In the city centre, where buildings are taller but also much deeper, the dry belt is only 10 metres. The reason for the rising water table is a huge decrease in the use of water, due to the closing of the city plants of some large industrial firms and a sharp fall in population. Some 11,000 underground parking spaces have become unusable because of flooding and damp caused by the rising water table. Cellars and basements and about 10,000 residential and office buildings could be at risk. The underground railway system is also badly affected. Other cities including London, Barcelona and Paris are afflicted by the same problems, though not as seriously.