The Candover valley is known for the risk of flooding including sub surface flooding – reference is made to the government surface flooding map. Generation of overland flow (runoff) occurs through a variety of mechanisms that are primarily controlled by the infiltration capacity of the surface (soil, vegetation) and the rate of precipitation. The infiltration capacity of the surface is influenced by the porosity of the surface and by how much of the soil is already full of water (saturation). The generation of runoff is strongly determined by the amount of precipitation falling at a given time on that surface – the precipitation rate, and over longer periods of time by the degree of soil saturation – a function of rate of drainage of soil and total time integrated rainfall. Thus, in wet winters, as experienced this winter, progressive rainfall accumulations over weeks, result in saturated soils and progressively higher runoff rates. Similarly, convective storms with high precipitation rates, can generate very high runoff simply because the rate of precipitation is far greater than the rate of infiltration.