With the addition of roofed, asphalted and concreted ground surfaces, stormwater runoff to streams is increasing. This not only contributes to increasing flood risks, but also to drying of the land surface. Under cities and towns, groundwater levels are falling, evaporation is decreasing and the extent of the heat island is increasing, where the temperature is often more than 3 °C higher than the outside temperature of cities.

More than 5 million m3 of rainwater is drained annually from an area with 100,000 inhabitants. The cost of sewerage of this rainwater amounts to 5 million €, which was soaked into the soil, replenished the groundwater reserves and evaporated through the vegetation before sealing the earth’s surface, thus cooling the air, humidifying it, cleaning it and sanitising it.