Soil moisture variability: a field application of time domain reflectometry

Soil moisture variability and available water content are influenced by grain size distribution, organic content, bulk density and soil structure. Each of these soil character!' sties is discussed in relationship to one another and soil moisture variability. A mathematical model is derived for estimating moisture content at field capacity, permanent wilting point and maximum available water content. Seven soil classes are included, ranging from clay to sandy loam.A brief description of four methods for determining moisture content are presented: Time Domain Reflectometry (TDR), gravimetrics, porous resistance blocks and neutron scattering. The advantages and disadvantages of each method are discussed.The design of a field study using TDR for determining in situ soil moisture content is presented. TDR lends itself to this application, since it is a non-destructive procedure allowing for repetitive moisture determinations at the same location. Field portability and ease of operation enhance its use.