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 paramete r in volume balance analysis for computing different parameters such a s infiltration characteristics. Surface storage can be estimated in se veral 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 i s 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 norther n New South Wales, Australia. These data were used to investigate the validity of the assumption regarding the value of the shape factor. Th e 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 calcul ated for the irrigation events in the study varied from 0.96 to 1.8, a nd varied between furrows and from one irrigation to the next in the s ame furrow. They also varied considerably during a given irrigation ev ent. It is therefore incorrect to assume a constant value of the shape factor between 0.7 and 0.8 in mathematical models of furrow irrigatio n.