• There is no substitute for walking over the ground when it is at its wettest
• Check where water is coming from; drain the landscape
• Check plant growth and soil surface conditions
• Dig a soil pit to determine soil properties that will influence drainage design
• Different soil types require different drainage solutions

Diagnosing your waterlogging problem is the key to achieving success with any drainage. There is a lot to be aware of and to look for when on the paddock trying to diagnose what is causing waterlogging and trying to devise the best drainage solution for the site (Figure 1). The author has taken many phone calls from landowners wanting drainage advice thinking that if they describe what they think the situation is, that advice will readily be given over the phone. Unfortunately this is not able to be done as each site has its own unique characteristics which must be considered when formulating a drainage plan. Also, rarely will a site be able to be drained in isolation from other nearby sites.

The best way to identify how bad a waterlogging problem is to get out onto the ground in your gumboots during the wettest time of year when soils are saturated, usually August and September in Tasmania. Drainage rule No. 1 – Design your drainage in the wet, install drains when dry. In winter it is easier to identify the limits of wet areas, particularly seepage areas, and to identify soil horizons on which a perched water table occurs. However, waterlogging can occur at any time of year due to a heavy rainfall event or problems from irrigation.

The on-the-paddock process of diagnosing a waterlogging problem, the causes and potential drainage solutions should be a continuous and iterative one as one walks across the landscape (Figure 25). The conditions to look out for include:

• Check the frequency, duration and extent of waterlogging and flooding.
• Visually scan the landscape in order to identify geomorphic units and topographic break points between units (e.g. slopes meet flats). This requires an understanding of the processes and factors involved in landscape, landform and geomorphic unit formation.

» Geology – rock type (igneus/sedimentary/metamorphic), tectonic activity (uplift,

faults, tilting)

» Erosion – hills & valleys, landslide/slump/wind/water

» Soil parent materials – weathered rock, sediments (river or geologic), swamp deposits

» Depositional landforms – floodplains, levees, back-swamps, meander channels, fans,

» slumps, dunes

» Slope angle and length

• Visually check to see if water is lying on the soil surface and ascertain if this might be due to:

» Seepage – drain the landscape

» Flooding – surface drainage

» Pugging – drain the soil (Figure 26) feels like it will break your ankles when walking over it

» Regionally high water table – drain the soil

• If seepage is a likely problem, determine:

» What position in the landscape does seepage start, i.e. position on slope / break in slope?

» Can the landscape be sectioned into geomorphic units that have different water content /drainage properties?

» Determine if lower lying areas can be isolated from up-slope water with interception or cut-off drains. Would an interception drain collect water from the upslope geomorphic unit, thus alleviating seepage into the lower geomorphic unit?

» Where and at what depth does an intercept drain need to be placed to optimise interception?

» Where will the intercept drain outfall be to provide sufficient gradient?

• Determine likely outfall points - river, property boundary, valley floor, gully endpoint, deep drain. Outfalls are critical from within paddocks, at the edge of paddocks and off the whole farm.

» Check the potential outfall for backflow caused by tides or stream/river floods. If present, recommend flood flap or angled drain entry for venturi effect.

» Check outfall for erosion protection – may need to embed pipe & compact back-fill, or use upstream log to divert flow around outfall.

» If there is not sufficient gradient to an outfall, consider installing a sump and pump.

• Determine direction of fall and likely gradient.

Is the gradient sufficient for the selected drain type?

Swamp <0.2% trenches + hump & hollow or raised beds

Flood plain 0.1 – 3 % trenches + underground pipes &/or moles

Terraces 0.2 – 5 % underground pipes

Low hills > 5 % grassed waterways.

A drain with a flat wide base will disperse the flow of water across the drainage line, reducing the erosive power of the water. These drainage lines need to be 3.0 metres wide and at least 100 mm deep.

• A drain with a flat wide base will disperse the flow of water across the drainage line, reducing the erosive power of the water. These drainage lines need to be 3.0 metres wide and at least 100 mm deep.

If there is standing water over most of the soil surface, then an open trench drain is probably needed in order to get rid of the large volume of water on the paddock (Figure 27).

• Check how wet the soil is beneath your boots. Is it firm or soggy?
• Where is the break point between firm & soggy as one walks over the landscape?
• If you are standing in surface water on a paddock, an open trench drain is probably required.
• Are there areas with poor crop establishment and growth? Are sedges & rushes growing, and patches of excessive weed growth? Is wintergrass (Poa annua) or moss growing on the soil surface?
• Are there deep machinery or wheel ruts that fill with water, e.g. from a centre pivot irrigator (Figure 28). On flat ground with a low gradient (e.g. 1:1500) water will back up 150 m from only a 1 cm deep irrigator rut.
• Are there grey or white coloured topsoils in areas of the paddock that are visible when the paddock is cultivated? (These areas stay wet for longer)
• Are there signs of salinity such as salt tolerant plants or surface scalds with no plants growing? (Figure 29, Chapter 9)

In order to correctly diagnose what is causing a drainage problem and design suitable drainage, a pit needs to be dug to identify any soil conditions that are contributing to the problem. The soil properties and conditions to look for include:

• Determine if topsoils are pugged or compacted with clods compacted in sheets parallel to the soil surface when compared to soil beneath nearby fence lines. If compacted, consider ripping or tillage.
• Check to see if there is a surface organic mat of dead grass/organic matter. If present, consider tillage or “hoof and tooth” to break up the organic mat.
• The depth of plant roots (shallow in poorly drained or compacted soils).
• Depth and thickness of any layers of contrasting texture or hardness, e.g. sandy loam over clay that indicate the overall soil type – uniform / gradational / duplex. Duplex soils often have a ‘spewy’ later at the interface between the sandy loam topsoil and clay subsoil.
• Determine soil type in each geomorphic unit by digging a hole or by inferring across landscape units.

Uniform sandy - trench, hump & hollow, underground pipe (with sleeve)

loams & clays – all drain types

Gradational all drain types

Duplex trench, underground pipes on slopes, raised beds

• Determine if and at what depth the water table is beneath the soil surface (Figure 30).

• Does the soil smell sulfurous at any depth (indicates anaerobic conditions)?
• Where does the water flow into the hole from?

from the bottom, indicates a ground water problem

from a particular layer, indicates perched water

from the surface, indicates surface sealing or perching.

• Determine clay content at 30 – 80 cm below soil surface

< 35 % clay NOT suitable for mole drains

>35 % clay suitable for mole drains

• Determine soil colours and mottles (see below) to indicate drainage status.
• Determine permeability of subsoils for drain type and spacing. A hand auger hole can also be a useful way to diagnose how permeable a subsoil is which may provide indicators of the options for drainage types and spacing (Table 1).

• Determine if subsoils crack (Vertosols) to moling depth, i.e. 40 – 60 cm. If so, do NOT recommend mole drains.
• Determine if subsoil clays are dispersive, i.e. sodic. If dispersive, all drains > 30 cm depth to be avoided and do not recommend mole drains. Preferred drain types on sodic soils are shallow surface drains & raised beds.
• Determine if subsoils have hard resistant layers that may need deep ripping.
• Are there hard concretions of various sizes and shapes, or soft black segregations that often indicate poor drainage? Pedogenic ironstone gravels indicate fluctuating anaerobic/aerobic conditions.
• Test to check if the soil is sodic, particularly subsoils. A small air-dry sample may need to be tested for dispersion in distilled water or tested for sodicity with a laboratory test (Chapter 9).
• Determine if there are saline tolerant plants growing or bare saline scalds.
• Determine if water is concentrated by draining from infrastructure - buildings, hard traffic areas, tracks, feed pads. Intercept this water with a cut-off trench drain or shallow waterway.
• Determine which areas should be drained first – consider position in the landscape, potential associated with different soil types, position of arterial drains and potential for adding on underground pipes and mole drains.

Colours of the topsoil that are visible when the paddock is cultivated can indicate different drainage status. Areas with grey or white coloured topsoils compared to other browner or redder coloured topsoils are likely to indicate poorer drainage as the paler coloured areas tend to stay wetter for longer. The colour of the soil seen in a pit can indicate the drainage status of the soil (Figure 31). There are two types of soil colour to look for. The first is what is known as the matrix colour which is the overall background colour of the soil. Bright red or yellow colours down the whole soil profile indicate that the soil is well drained. Blueish-grey colours indicate permanently waterlogged soils that are described as gleyed. Combinations of bright colours and pale yellow and/or grey colours indicate intermediate states of drainage. The other type of soil colour is mottle colour. Mottles are spots or streaks of contrasting colour to the matrix or background colour. Mottles can range in colour from rusty red/orange through yellow to grey or white. The occurrence of mottles and the depth at which they occur are also indicators of the soil drainage status. (Figure 32).

Soil compaction, particularly on clay textured soils, that has been caused by heavy machinery traffic, a wet harvest of a root vegetable crop (potatoes or carrots), or heavy stocking in the wet,  can result in ponding of water at the surface (Figure 33). This situation can be readily diagnosed by digging a hole to inspect the soil profile to see if the water is perching on a compacted soil surface or if the problem is a shallow water table (Figure 30). This situation can occur even on some of Tasmania’s best soils – Ferrosols. Where the problem is identified as soil compaction, the solution does not require drains. Soil loosening with an appropriately designed deep ripper and improving soil structure with fibrous rooting perennial grass is the solution. Deep ripping should be undertaken when the soil has dried to be moist and not wet and plastic, but not too dry to be brittle causing the soil to break into large blocks (see Cotching 2009 for details on appropriate machinery design for deep ripping).

Different soil types require different solutions to drainage problems (Figure 34). Some generalised recommendations are given below.