The effect of flow above the water-table on tile drain design in Murrumbidgee Irrigation Area soils

Abstract

Most tile drainage systems in the Murrumbidgee Irrigation Areas are installed in soils that are relatively impermeable at a short distance below the drain level, and these systems are designed to discharge excess water at a rate much less than the average hydraulic conductivity of the soil profile. The contribution of unsaturated flow above the water-table under these conditions may be considerable, and the effect of ignoring this contribution upon drainage design is therefore investigated. Capillary conductivity-suction relationships of Murrumbidgee Irrigation Area sods are characterized by the absence of a critical suction value, above which capillary conductivity declines rapidly from the saturated hydraulic conductivity. The presence of a critical suction value, however, is a basic assumption in acceptable analyses of capillary flow and immediate application of these analyses in our case is not justified. An approximate analysis is therefore presented, dividing flow above the water-table into a saturated capillary fringe as conventionally defined and a region above this fringe, where flow is in the unsaturated state and mainly vertical. The latter region is evaluated from the capillary conductivity-depth profile during vertical infiltration from the soil surface to the water-table. It is found that flow above the water-table is accounted for by adding a layer of 10 cm or less to the saturated flow region. The maximum error in design spacing, resulting from neglect of flow above the water-table, is about 6%. Capillary flow may therefore be safely ignored for practical design purposes. If necessary, more accurate solutions may be obtained numerically by solving the Laplace equation for the stream function rather than for the potential function.