4.2 Soil Water Potential for Systems at Equilibrium

Perhaps the most fundamental concept for understanding soil water flow is the fact that differences in soil water potential drive soil water flow. Intuitively, we might assume that water always flows downward through the soil, or perhaps we may assume that water always flows from wetter soil to drier soil. But reality can be surprising, and these intuitions can mislead us. In fact, water can and does flow upwards and horizontally in soil under certain conditions, and water can flow from drier soil to wetter soil under certain conditions. Therefore, to understand water movement in soil, we must supplement our intuitions with a solid understanding of the fact that differences in soil water potential ultimately drive soil water flow.

When soil water potential is the same throughout the soil, the soil is said to be in hydraulic equilibrium, and no water flow occurs. Although the total soil water potential is uniform throughout a soil that is at hydraulic equilibrium, the component potentials such as gravitational potential and pressure potential often vary throughout the soil. In fact, if we neglect any differences in osmotic potential, then for soil at hydraulic equilibrium, the variations in gravitational potential and pressure potential will perfectly offset one another. In this way the total potential is the same throughout the soil. Before you can solve soil water flow problems, you need to be able to correctly determine soil water potentials for systems in equilibrium. To do that you can use diagrams and tables like the ones shown in Fig. 4‑3, as explained in the accompanying video. Please get a pencil and paper and watch the video now, taking time to create your own tables and fill in the blanks as you go.