If a ground surface is made of impermeable material such as concrete or asphalt, the amount of rainwater deposited on it that is diverted to drains can be around 95% (Schuler, 1994) with the remainder presumably evaporating from the wet surface. In contrast, with a vegetated surface, runo can be very low, around 10%, with a high proportion being evapotranspired by plants, and the remaining water taken up by the soil (Arnold & Gibbons, 1996). Schuler (1994) gave a specific example of a meadow considered to be 1% impervious with a runo of 6%. Dierent proportions of vegetation cover and impermeable surface result in dierent levels of infiltration (Arnold & Gibbons, 1996) (Table 4.1).