• The larger soil pores allow soil to drain
• The drainable porosity varies greatly between soil types as well as within soil types
• Water can drain across the soil surface as overland flow
• Soils can drain through the soil matrix or through large cracks and holes
• Sideways water flow in subsoils leads to springs and seeps

The amount of water that will move through the landscape, whether it is over the surface, or through the soil into the groundwater or streams is influenced by the position in the landscape, the soil type, soil structure, the amount of soil compaction and the climate. An understanding of how water is stored in and moves through different soils and landscapes can help in designing a drainage plan.

The soil bulk volume consists of both solids and pore space. The proportion of the soil volume that's pore space depends on soil texture (the amount of sand, silt and clay) and structure (how the soil particles are packed together), but typically varies between 35 and 55 percent. Water is held in the larger soil pores by capillary forces while stronger adsorptive forces hold water as a film that surrounds soil particles and the finer soil pores. After a rainfall event that fills up all the soil pores, the largest of pores drain by gravity until the capillary forces can hold onto the water in the pores against the pull of gravity. The soil is now at field capacity. The actual soil moisture content at field capacity varies with soil texture and soil structure, typically ranging from 15 to 48 percent by volume. Plants can easily extract water from a soil when its moisture is at or near field capacity. The soil dries out by plant roots extracting the water and transpiring it through their leaves or by evaporation at the soil surface drawing up water from deeper soil pores. The soil dries until plants can no longer extract any water from the soil. The soil has now reached the wilting point. Soil moisture content at wilting point is the plant-available water. Poorly drained soils may have water tables at or very near the soil surface for extended periods of time. Under these conditions, the proportion of air-filled pores in the soil profile is vert small, so the soil lacks proper aeration to support plant growth.

Figure 7 shows the different categories of soil pores and water in a typical Tasmanian Sodosol that is duplex by having a sandy loam topsoil overlying a heavy clay subsoil. The volume of pores in the soil that drain by gravity is known as the drainable porosity that is calculated as the difference between the total volume of pores and the volume at field capacity. These pores are the larger pores (macropores) between 3 mm and 0.003 mm (30 μm) in diameter. It is the drainable porosity that determines how much water can be drained from the soil by trench, mole or underground drains. After draining, the soil has more pore volume available for water infiltration during the next rain because of the larger volume of empty pores. Consequently, more infiltration and less runoff may occur with an artificially drained soil compared to a poorly drained soil, depending on the next rain's intensity and duration. A very intense rain may not produce much infiltration in either case. Drainage does not increase or decrease the amount of plant-available water that is able to be stored in the soil profile. Drainage only removes the drainable water from the soil as the remaining water is held strongly enough to resist drainage by the force of gravity.

The relationship between the different sizes of soil pores is not consistent across soil types and can vary dramatically which effects both the amount of water stored and the drainage capacity of the soil. The pore sizes and volumes of readily available water stored in the topsoils (0 – 30 cm depth) of several Tasmanian soils was determined for the purpose of understanding irrigation scheduling and drainage (Figure 8). The values for the duplex soils appear to fall into two distinct classes. However, the volumes of drainable porosity did not show the same groupings (Figure 3) and so the responses to drainage will be different between soils even on the same soil type.

For most plants to grow in soil, a proportion of the pore space needs to be air-filled to allow the influx oxygen into, and the efflux of carbon dioxide (CO2) from the soil. Plant roots, microorganisms and chemical reactions all consume oxygen and release CO2 with the major effect of poor aeration on plant growth being a lack of oxygen (Glinski and Stepniewski 1985). This same air-filled porosity is the space that allows for drainage of water by gravity after a saturating rainfall (drainable porosity).

Although critical values for the air-filled porosity to sustain plant growth are unlikely to exist (Cook and Knight 2003), as the total air-filled porosity decreases to 10% or less, the oxygen diffusion rate into the soil is inhibited, causing injury to roots and their inability to function (Engelaar & Yoneyama, 2000). Also, connectivity between larger pores will be reduced resulting in slower effective drainage. Compaction of soils that destroys soil structure, results in the loss of a greater proportion of the larger pores (drainable porosity) than the smaller pores thus reducing the ability of the soil to effectively drain. Five of the Tasmanian soils tested had drainable porosities of less than 4 % of soil volume (Figure 9) and it is critical to keep this porosity drained and air-filled for plant optimum growth but with such low volumes of these larger pores these soils will require specific artificial drainage as well as good stock and crop management practices.

Generalised recommendations of the drainage systems suitable for different soil types can be given (Chapter 5), however, differences in the volume of the different categories (sizes) of soil pores within each soil type mean that recommendations for effective drainage can only be given following detailed site investigation of individual paddocks (Chapter 5).

Water flowing over the surface of the landscape (overland flow) occurs when rainfall or irrigation intensity exceeds the soils surface infiltration rate. When this occurs on flat land the water ponds and there is a lag time after rainfall has ceased until all the ponded surface water can infiltrate into the soil. On sloping land ponded water will move downslope, creating surface runoff or overland flow. Natural soil porosity can influence the rate of infiltration but the effects of compaction by animals or machinery can have dramatic effects by slowing or even preventing any water infiltration.

The other main scenario that drives overland flow is when a soil is fully saturated, often as a result of a high water table or a slowly permeable subsoil layer that restricts drainage and all the drainage porosity pores are filled with water. The soil has no capacity for more infiltration and so water ponds or flows downslope (Srinivasen et al. 2002). This surface water flow will only stop once the water source is removed. However, saturated soil profiles can only be drained by evapotranspiration or artificial drainage, either open trenches or underground pipes (Hillel, 1998). This is why one of my rules of drainage is: ‘Drain the landscape, then drain the soil’. Installation of grassed waterways or shallow spoon drains is designed to capture overland flow and transport it safely off the paddock. Grassed waterways and surface spoon drains do not drain soil water.

In general, subsurface drainage tends to decrease surface runoff and decrease peak surface runoff rates compared to surface-drained or undrained land because water flows more slowly through the soil to reach the drainage system than it would as surface runoff. This is not the case for hump and hollow drainage which will result in larger and faster surface water flows off the paddock.

Subsurface land drainage provides a pathway for excess or drainable water to leave the soil. How well and how fast a soil drains is determined by both the volume of the larger pores (drainage porosity) as well as how connected the pores are to each other. In general, the finer a soil texture the less continuity of pores. Hence a sandy soil will have a greater drainage capacity than a fine-grained clay soil. When a soil is full of water or is saturated, water flow is largely governed by how many large pores there are (drainage porosity), how dense the soil is (bulk density) and soil structure. If the soil structure is poor or is destroyed by compaction, the connectivity between pores is reduced making the soil harder to drain.

In saturated soils the force of gravity creates a hydraulic gradient that drives water downward. Water can drain through the soil in one of two ways. Water can drain through the soil body in a relatively even manner, wetting the whole soil profile and this is called matrix flow. Matrix flow moves water through the finer soil pores within and around soil aggregates, rather than rapidly around soil aggregates. Matrix flow is often called a piston flow effect where water flowing from the soil surface displaces water that is deeper in the soil profile. This effect operates just like the hydraulics on farm machinery.

Water that moves down preferred pathways when soils are draining is called preferential flow (Hillel 1998) or bypass flow, as it results in a large proportion of the soil matrix being bypassed during the drainage process. Preferential flow typically takes place down large continuous cracks or a series of intermittent and somewhat connected soil cracks or channels with large pore space. These cracks and channels can be caused by earthworms or plant roots or as a result of wetting and drying cycles where the soil shrinks and swells opening large spaces around coarse prismatic or large blocky structures in the soil. In Tasmania’s duplex soils, these subsoil cracks are often filled in with sand that has migrated from the overlying surface horizons and thus provide a preferred drainage pathway compared to the dense heavy clays in the surrounding subsoil (Hardie 2011) (Figure 10). Installation of underground drains aims to create preferential flow paths that will drain the soil quickly. Water flows down preferred pathways in gravel backfill that is placed on top of underground pipe drains or to mole drains through cracks and fissures that are created during the installation of mole drains.

Water can move sideways in landscapes either over the surface or through the soil. Where the slope gradients are steep, overland flow can result in surface erosion and so interception trench drains and grassed waterways are appropriate drainage interventions. Lateral movement of water below the soil surface may be the result of an excess of either rainfall or irrigation that results in water perching in layered soils with subsoils having slower permeability than overlying horizons or on bedrock that has an inherent sloping gradient (Hardie et al. 2012). Subsoil water travels sideways above the horizons with slow permeability, often breaking out as springs or at changes in slope in the landscape (Figure 11). In Tasmania’s Midlands and some southern areas this can lead to waterlogging and salinity in lower parts of the landscape (see Chapter 9) where percolating water travels through sandy dune soils, basalt or dolerite rocks, or deeply weathered clays (Hocking et al. 2005). Lateral water movement can occur in a range of soil types and may occur simultaneously at multiple depths. The amount of lateral water movement and seepage outbreaks can vary from year to year due to variation in rainfall and irrigation between years and soil water storage remaining from previous seasons (Hardie 2011).