The role of shallow drains in removing excess water from texture-contrast soils

Abstract

Variability of flow in shallow drains installed in a grazing catchment in South Australia was partly caused by the enormous variability each winter in the saturated thickness of the texture-contrast soils and their hydraulic conductivity. High variability has been found in other trials of shallow drains but the causes were not well understood. This study found that perched watertables are not always found on top of the B horizon. The depth and thickness of saturation varies with landscape position (between texture-contrast soil types) and also varies seasonally at one landscape position. Because drains are on a gradient, they pass through a variety of texture-contrast soils. These soils may have different soil chemistry, physical properties and depth of saturation. The presence of sodic subsoils with low hydraulic conductivity has a major impact on drainage volumes, particularly throughflow. In some years the drains intercepted deep, perched water or groundwater, which rose into the collection zone of the shallow drain. Flow from the drains in the study catchment was compared to that from other catchments with texture-contrast soils. Drain flows are highly variable and dependent on localised soil conditions, making flow prediction difficult. Despite this, a clear relationship was found between average annual catchment drain flow and annual rainfall. Drains installed across waterlogged paddocks with texture-contrast soils in southern Australia will average 10 mm flow (2.5% of annual rainfall) when annual rainfall is 400 mm, increasing to 100 mm (14%) when rainfall is 700 mm.