Soil Water Measurement - J David Cooper

Soil is vital for production of most of our food and it also plays a crucial role in shaping our environment. Rising global population and the attendant demand for more food is putting unprecedented pressure on soil resources, whilst this same pressure, often coupled with poor management, leads to loss of soil through wind and water erosion, and areas of land becoming less fertile and even sterile through salinisation, loss of organic matter and pollution. Soil water plays an important role in all of these. Measurement of soil water and its interaction with vegetation and the weather is, therefore, important for improving agricultural productivity and the environment through work in hydrology, pollution, water resources, geotechnics, civil engineering, hazardous and other waste disposal.

KEY CONCEPTS OF SOIL WATER
Three key concepts underlie the behaviour of soil water:

Water Content
The most fundamental property of water in soil is the amount stored. Most often, we need to know the volume of water held in a defined volume of space, normally termed the volumetric water content.

Soil Water Potential
Ultimately, water is held in soil by the attractive forces between soil particles and the water molecules. In simple granular soils, capillarity is the dominant phenomenon. However, in many claydominated soils and at low water contents in other soils, more explicit descriptions of surface interactions are required. In swelling and shrinking soils, water can intercalate clay particles, temporarily forming part of the clay structure. In all cases, however, the lower the water content, the tighter the water is bound into the soil. This has two main consequences. Firstly, as soil dries, it becomes increasingly difficult for water to be removed from the soil. Secondly, water will tend to move from places which are wet to those where it is drier. This is usually quantified by defining a soil water potential as the amount of work required to place an infinitesimal amount of free water in the soil at a given location. Since work is usually needed to remove water from the soil, this quantity is normally negative. The concept of water potential also allows the effects of gravity and osmotic forces to be incorporated into the same formulation. As in conventional mechanics and electrical problems, the force moving water can be defined by the gradient of the potential.

There is a characteristic relationship between water content and water potential for each soil and, often, for each location in the soil. This is rarely unique, since the relationship is hysteretic.

Hydraulic Conductivity
To quantify the movement of water through soil, a relationship is needed between the potential gradient and the water flux induced. A proportionality constant relating the two has been found to describe the process well: