Small Farm: Water Infiltration and Drainage of Soils

Waterlogging is a serious problem affecting many areas of agricultural land. Before we can correct the problem, we need to understand what is happening. As waterlogging is due to restricted water movement through the soil, we need to understand the behaviour of water in the soil profile before we can identify the most appropriate course of action.

Infiltration and hydraulic conductivity

Both of these terms have been used to varying degrees to describe water movement in the soil. However, they are subtly different.

Infiltration rate:

The rate at which water can enter the soil when water supply is not limiting.

Hydraulic Conductivity:

The infiltration rate plus other constraints associated with water movement through soil.

Infiltration

As infiltration refers to water entry into the soil, it is largely determined by the condition of the soil surface. The main influencing factors are:

Soil texture

As soil particle size decreases, there is less distance between soil particles. Hence, as the inter-particle distance gets less, water infiltrates slower.

Soil structure

Well aggregated soil promotes high infiltration rates as there are more ‘gaps’ for water to flow through. Excess cultivation can be detrimental to soil structure. The surface crusts that can sometimes result from high cultivation levels may cause sealing of the soil surface, restricting water entry. Soil sodicity also results in poor soil structure, as the aggregates break down and disperse.

Soil management

How the soil is managed has a large bearing on infiltration rates. As mentioned previously, cultivation can have a large impact on infiltration, as can the organic matter levels in the soil. Not only does organic matter help improve soil structure through promoting soil aggregation, but it also aids in ‘trapping’ water, and allowing it to infiltrate slowly. This is termed ‘soil roughness’ – basically stopping water from running off and pooling.

Hydraulic Conductivity

The hydraulic conductivity of a soil is determined by soil profile characteristics. These include:

Soil moisture

If the soil already has a high soil moisture content, infiltration will be low, as most of the soil pores will already be filled.

Soil texture

As mentioned previously, water will infiltrate into sand quicker than into clay. Water will also move through sands quicker than clays. However, the flip side to this is that sandy soils will not store as much water as clay soils – requiring greater inputs of water to maintain moisture contents.

Porosity

Soils that are well structured are likely to have greater numbers of pores than poorly structured soils. Hence, soils with high porosity are likely to have a greater hydraulic conductivity.

Soil type (duplex vs gradational)

There are three broad classes of soil types:

• duplex
• gradational
• uniform

The flow characteristics of each of these soil classes are different, for example, duplex soils have greater amounts of lateral flow than gradational or uniform soils.

Water flow through soil

Lateral Flow

Common in duplex soils

Vertical Flow

Common in gradational and uniform soils

Duplex soil profile

There is a sharp change in texture between the A and B horizons (eg. Loam overlying clay). This contrast inhibits water flow down the profile – may result in waterlogging in the A horizon and lateral flow. This profile is common in sodic soils.

Gradational soil profile

Gradual change in texture down the profile. As the texture gets heavier water movement slows down. However, this profile still drains better than a duplex soil.

Uniform soil profile

Little change in soil texture down the profile. This profile drains well, with good water movement downwards. However, accessions to the watertable may be quite high, hence depth to the watertable should be monitored regularly.

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