On-farm tests for determining the most appropriate subsurface drainage system
Following are some practical on-farm tests which farmers and contractors can carry out to ascertain which drainage system is best suited to the particular soil type to be drained. These tests are best and most easily carried out in winter when the soil profile is saturated. However, the same tests can be carried out in dry conditions with greater difficulty but seek expert help if doing so.
If the on-farm tests are indecisive, proper soil tests carried out by a reputable laboratory should be done to indicate more accurately the soil’s permeability (bulk density), texture (sand, silt and clay) and suitability for mole drainage (slaking and dispersion tests). An experienced subsurface drainage expert should also be consulted in the designing stage to assist with selecting a system and developing a drainage plan.
1. Factors to consider while digging the holes
Testing a soil’s permeability and its suitability for either pipe or mole drainage involves digging holes, collecting soil for slaking/dispersion testing and measuring the rate of water inflow into the hole. These can be done by the landowner or contractor to aid in selecting the correct drainage system.
When the soil profile is saturated, usually in winter or early spring, dig several holes in the problem area to a depth of 1.0 to 1.5 m. Preferably use a hand auger of post hole size diameter (75 to 150 mm diameter). A mechanical auger, tractor driven post-hole digger or a shovel are less useful in picking up some critical factors mentioned below. Before digging, ensure that surface water is directed away from the hole to avoid confusing the observations.
As the hole is dug, take notice of:
- The depth of topsoil (usually a very permeable layer)
- The depth of plant roots (short in poorly drained and/or compacted soils, and in soils with little topsoil)
- The depth at which a compacted layer (harder to auger) may be located throughout the profile and its thickness
- The cause of the compacted layer (compacted soil due to animal traffic or equipment, coffee rock layer, etc.)
- Where the clay content increases substantially (not always easy to pick)
- Where the top permeable layer meets an obviously much less permeable layer (duplex soil type) and often characterised by a “spewy” later at the interface
- Where water begins to flow into the hole (near the top, middle or bottom of the hole)
- How fast the water enters the hole at various depths
- The clay content just above and at moling depths (400 to 600 mm). Collect soil samples at these depths for later testing for mole drainage suitability.
2. Determining the soil’s permeability (hydraulic conductivity)
This test will give a rough but useful indication of whether the soil is slowly or rapidly permeable. This test is best carried out a day or so after the hole has been dug to allow the water table to return to a stable level.
You will need a stopwatch (or second hand on your watch) and a flexible cloth-type tape measure with a small light float (eg. a small plastic vile) stuck to its end with the float base opposite the tape’s zero reading. Note the height of water in the hole. This is likely to be the water table height in the surrounding soil.
Have a set and easily seen mark on the side of the hole near its surface. Bale out most of the water from the hole, ensuring it and other surface water does not flow into the hole. Lower the float end of the tape into the hole until you can see or feel the float’s base sitting on top of the water in the hole.
Gently pull the tape taut, but maintain the float base on the water surface in the hole. Record the tape reading against the set mark immediately. As the water flows into the hole from the soil, the tape will need to be kept taut as the float rises.
Record the tape reading every 5 to 10 secs. Whether the water enters either very slowly or rapidly, alters the rate of recording. Record tape readings until about a quarter to one third of the removed water volumes flows into the auger hole from the soil profile. This will ensure that the water inflow will be relatively constant and a true indication of soil water conductivity. Repeat this in the same hole several times and do so in several holes in the area to be drained.
To measure the soil water movement (hydraulic conductivity), calculate the following:
Water flow rate = Rise in water level (mm)… x 360 x time taken (sec)… =………mm per hour water flow rate
Table 1 provides a guide of the rate of water movement through the soil through a saturated profile for a range of soil classes. The water flow rate (mm per hr) can be used to decide which drainage type may be most suitable.
As a rough “rule of thumb”, class 1 – 4 soils, being less permeable, may be suited to mole drainage (depending on clay mineral content, sand/stone presence, etc.) or moles over a collector pipe system. Classes 5 – 7 soils, being more permeable, may be most suited to subsurface pipe drainage. Unfortunately few dairying soils in southern Australian are of this nature.
Assuming the water inflow is entering the hole reasonably quickly and throughout most of the hole’s depth, or at the least, is entering in the lower section of the hole (below 500 mm), the soil will most likely be suitable for subsurface pipe drainage. If the topsoil is relatively deep (more than 200 mm) and/or is relatively permeable but less so at depth, say 600 to 800 mm, then moles over a pipe system might suit.
If water enters slowly throughout most of the hole, the soil’s permeability will be low. The earlier collected soil sample from moling depth will then need to be used to determine the soil’s suitability for mole drainage.
