Field values of the shape factor for estimating surface storage in furrows on a clay soil

Surface storage of water in furrow irrigation is an important parameter in volume balance analysis for computing different parameters such as infiltration characteristics. Surface storage can be estimated in several ways. A commonly used method is to multiply the cross-sectional area of flow at the inlet of a furrow by a shape factor, whose value is often assumed to vary between 0.7 and 0.8. In this study, field data for 13 furrow irrigation events were collected from a farm in northern New South Wales, Australia. These data were used to investigate the validity of the assumption regarding the value of the shape factor. The soil type at the experimental site has a high clay content (up to 67%) and develops cracks when dry. The values of the shape factor calculated for the irrigation events in the study varied from 0.96 to 1.8, and varied between furrows and from one irrigation to the next in the same furrow. They also varied considerably during a given irrigation event. It is therefore incorrect to assume a constant value of the shape factor between 0.7 and 0.8 in mathematical models of furrow irrigation.