Legend: 1. Evaporation; 2. Runoff; 3 Useful water; 4 Water infiltrated towards the water tables
In 1947, American climatologist and meteorologist Helmut Landsberg asserted that modern cities produce "a special microclimate which closely resembles that of a mineral desert."
According to him, the runoff generated by the waterproofing of urban surfaces transforms the local climate of cities and produces hydrological conditions similar to those observed in a desert or a wadi.
In the summer, Landsberg explained that runoff prevented cities from benefiting from the evaporation of water for their cooling. He then described, without really knowing it, the main cause of the urban heat island phenomenon.
Landsberg estimated that the equivalent of a cooling power of 5,000 Watts per m2 per year was thus lost due to runoff.
While floods during periods of drought are increasing and we now know that more than 50% of the precipitation captured by a city runs off (see image), the importance of integrated water management in cities remains too underestimated.
Regardless of whether we are talking about a “porous” or “spongy” city, de-waterproofing has become a climate imperative to fight against UHI and more generally so that our cities are no longer deserts.
Sources: H. Landsberg, C. Piel
Image: SYMASOL, “Rainwater management. Guide to implementing alternative techniques »