1 The role of soil

The ability of the ground surface to absorb water is called its infiltration rate. Where the ground is paved or compacted, that value is very small (often less than 1 mm of water moving through the soil per hour). Where there is vegetation growing on undisturbed soil, it can be very large (over 30 mm per hour, allowing it to absorb all but the very heaviest rainstorms). The rate of infiltration can be assessed using a ring infiltrometer pushed into the soil (see Figure 1). The rate at which the water level in the inner ring falls is measured.

Figure 1 A ring infiltrometer. Both rings are filled with water to a depth of 5 cm and then the rate of fall in the inner ring is recorded.

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A diagram showing an inner ring which is 30 cm diameter and an outer ring which is 60 cm in diameter. They are pushed 15 cm into the soil, leaving 12 cm of both rings above the soil.

Figure 1 A ring infiltrometer. Both rings are filled with water to a depth of 5 cm and then the rate of fall in the inner ring is recorded.

A well-structured soil, such as that within an established floodplain meadow, can consist of more than 70% void (empty space) by volume, which can be filled either with air (when dry) or with water (when saturated). This space provides a huge store for water; perhaps three million litres per hectare. Soils are therefore able to mitigate flooding by absorbing water rather than it running-off into the river. Figure 2 illustrates how open the structure of an undisturbed soil can be.

Figure 2 A cross section through a piece of well-structured soil showing the extent of void, compared to a piece of poorly structured soil.

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Diagrams contrasting soil structure. The well structured profile is composed of rounded crumbs in its top layer with larger blocks beneath and then a gravel aquifer at the base. The poorly structured profile has a blocky structure in the upper layer over quite tightly packed prisms in the lower layer, constraining water movement to and from the gravel aquifer.

Figure 2 A cross section through a piece of well-structured soil showing the extent of void, compared to a piece of poorly structured soil.

A permeable soil can absorb yet more water by transmitting it to underlying porous rocks (aquifers) from where it can move slowly at depth out to sea without ever entering a river. The amount of water stored and transmitted beneath our feet as groundwater is under appreciated.

Figure 3 A typical hydrological cycle in a coastal catchment. Boxes represent stores of water, while arrows represent flows of water. Quantities of stored water are in units of trillion tonnes and flows are in units of trillion tonnes per year.

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A diagram showing a typical temperate catchment. The water cycle is shown with precipitation falling onto the mountains, of which 64% is evaporated back to the atmosphere, 17% percolates to groundwater and just 19% flows overland via a river to the sea. The diagram shows the various stores of water too, Snowpack and groundwater are the main terrestrial stores; biota, rivers and the atmosphere are relatively inconsequential.

Figure 3 A typical hydrological cycle in a coastal catchment. Boxes represent stores of water, while arrows represent flows of water. ...