Measurement of surface-soil hydraulic properties: Disk permeameters, tension infiltrometers, and other techniques

In this chapter we describe the development of in situ techniques to measure the hydraulic properties of the soil surface and to assess quantitatively the contributions of preferential flow paths to, and the impact of soil management practices on water entry into field soils. Both confined, one-dimensional techniques (tension infiltrometers, rainfall, sprinkler, and drip infiltrometers) and unconfined, threedimensional methods (the disk permeameter and multi-sized surface sources) are discussed. The assumptions and theory underpinning their use are presented. Applications of the three-dimensional techniques to measure sorptivity, S₀; hydraulic conductivity, K₀; and the hydraulic conductivity relation, K(Ψ), are given. Measurements of the soil structural parameters; mean characteristic pore size, λ₀ and macroporosity area, A_M are described. The relevance of these parameters to water entry, erosion, and plant growth are demonstrated. The limitations of all techniques are also discussed.