3.2.2  Water flow through soil to roots

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Soil is porous and holds water in its pores by capillary forces. As a soil dries, the larger pores drain and the remaining pores hold water ever more tenaciously. Water in these pores is under suction (negative hydrostatic pressure, P — Section 15.1) and this suction typically ranges from about 10 kPa to about 2 MPa in soils supplying water to plants. At suctions of less than 10 kPa, water is held in such large pores that it is likely to drain quickly away; at suctions greater than 2 MPa, most plants are at their limit for exerting sufficient suction to extract the water.

Flow of water through soil is induced by gradients in hydrostatic pressure, P. The rate of flow, F (m s–1), depends on both the gradient in P and on hydraulic conductivity, K (m2 MPa–1 s–1), of the soil, thus:

where x (m) is distance. This equation is Darcy’s Law. Conductivity, K, varies enormously, by about a million-fold, over the range of available water content. This large range comes about because water flows much more easily in a large pore than in a small one (Poiseuille’s Law — Section 5.2).