2-Seasonal rainfall patterns

Characterise the seasonal pattern and variability of rainfall and establish water use efficiency

This procedure covers:

• characterising rainfall and water use efficiency
• understanding water balance
• applying water use efficiency information

Guidelines for characterising rainfall and water use efficiency

Build a record of the farm’s annual total and normal monthly rainfall distribution

Rainfall is a key determinant of pasture growth on your farm. To know the potential of your grazing land to produce pasture, you must understand the pattern of rainfall on your land. Assess historical rainfall records (for the farm, district or regional centre) and note down the normal monthly rainfall distribution and the extent of variability from year to year (see Tool 2.5).

Knowing the rainfall characteristics on your farm can help you to:

• improve pasture growth and financial and environmental management through efficient use of rainfall in all seasons, particularly during periods of lowest pasture growth
• reduce costs and maximise returns from pasture re-sowing, by selecting the most appropriate pasture species for the normal pattern and variability of plant-available water and sowing at the right time.

Use long-term rainfall records to determine the annual pasture production from grazing land

Tool 2.5 provides guidance on how to source, collate and analyse rainfall data for application in this procedure.

Understanding water balance

To maximise the potential of rainfall in a grazing system, it is necessary to understand the water cycle and what happens to rain after it falls (see Figure 1)

When rain falls on a pasture paddock it either:

• moves across the soil surface (run off)
• is caught on bare soil, litter or plants and evaporates (evaporation)
• infiltrates the soil and is used by plants (transpiration)
• infiltrates the soil, drains below the roots and joins the water table (deep drainage)

When soils have reached saturation point or the soil surface is hard or crusted and infiltration is slowed down, water is forced to move across the surface and becomes runoff. Some runoff is encouraged as it fills dams and enters streams and rivers. Excessive runoff means less water might be available for plant growth. Soil and nutrients might also be removed, impacting on pasture growth.

Low plant density, compacted soil surfaces, low litter levels and saturated soils all increase the amount of run-off.

It is important to maximise plant water use because it is one of the drivers of pasture production. Plant water will be reduced if groundcover is low, annual species dominate, pastures are overgrazed, set stocked or nutrients aren’t applied to maintain fertility.

Water that drains past the root zone into the water table falls into the category of deep drainage. This water is lost for pasture growth and could cause increased salinity levels and possibly contaminate the groundwater with nutrients.

The amount of water vegetation uses determines how much water goes into deep drainage. Annual pasture species and crops use the least amount of water. Perennial plants use more water due to their longer periods of growth.

Temperature also influences pasture growth potential. Unlike rainfall, it varies only slightly from its normal seasonal pattern throughout the year. Its major impact is on pasture production as all species have different upper and lower limits for growth. It also needs to be considered when selecting new temperate pasture species and Procedure 4 refers you to agronomy advice to assist with selection and appropriate management of pasture species.

While sunlight is a critical element in plant growth, it is not considered to be a climatic variable that limits plant growth in southern Australia. Physical disruption of sunlight through slope and aspect (south facing slopes) is a factor and is covered in Procedure 1 of this module. It increases in importance with increasing latitude. Growth and quality can also be limited through shading from other plants, and should be considered in Procedure 1 of this module (eg heavily timbered land) and in pasture utilisation practices (see Module 3: Pasture utilisation).

Plant-available water capacity (PAWC)

The difference in yearly pasture production is better explained by the variation in plant available water capacity (PAWC) than by total rainfall. PAWC depends mainly on texture (clay content), the ability to hold water and rooting depth.

Source: Better soils: A project of the Agricultural Bureau of South Australia

PAWC is a function of both plant rooting depth and the soil moisture storage capacity. This combination determines how much water can be held in the root zone available for plant use. Rooting depths are:

• 0.6–1.0m for sub clover, annual and perennial ryegrass
• 1.5–1.8m for perennial pasture grasses
• over 3.0m for lucerne, chicory and trees.

Deep, heavy clay soils can take in and hold a lot of water (ie a large PAWC) allowing a perennial pasture to use up to 100mm/year more than on a sandy soil. Lighter, sandy soils not only have a small PAWC, but also tend to have very high infiltration rates. This results in little or no run-off, greater deep drainage and pasture plants wilting more quickly between rainfall events.

In southern Australia, the more common duplex soils tend to have a balance between drainage and run off. In the northern, summer rainfall zone these same soils produce a lot of run-off and very little deep drainage.

Water use efficiency

Water use efficiency (WUE) is the amount of rainfall (water) that is converted into pasture growth

Water use efficiency is a measurement used to assess the opportunities for improved pasture growth through better soil nutrition and pasture composition (see Tool 2.6, Procedure 3 and Procedure 4). Knowing rainfall variability and water use efficiency makes it possible to manage soil fertility, pasture composition and grazing management to maximise the use of rainfall for pasture growth. Water use efficiency is an indicator of the amount of rainfall (water) that is converted into plant growth. Calculate water use efficiency for the pasture zones defined in Procedure 1 by following the guidelines provided in Tool 2.6.

Applying information about water use efficiency

Knowing rainfall variability and water use efficiency enables precision management of the grazing system. This knowledge allows you to manipulate pasture species and soil fertility to achieve the best spread of pasture growth across grazed land in all seasons. It also enables you to anticipate and act to prevent under-and overgrazing when there is a high probability of rapid growth (eg in spring) or low growth (eg in winter).

• Undergrazing results in shaded pastures, lower growth rates and lower water use by pastures, which can lead to increased water loss through deep drainage.
• Overgrazing results in high rainfall run-off, poor infiltration rates and high evaporation caused by inadequate ground cover, low litter levels, surface crusting and poor soil permeability.

Understanding water use efficiency enables you to implement methods to improve soil water-holding capacity, such as building soil organic matter. This leads to reduced rates of deep drainage and pasture growth when soil moisture moves below the root zone.

Pasture composition and soil fertility have a major influence on water use efficiency. Good grazing management in the following areas can lead to more efficient water use and therefore higher rates of pasture growth:

• Groundcover reduces run-off and protects soil from erosion. Depending on rainfall (pattern and intensity) and soil type, maintain at least 70% ground cover (including leaf, dead and litter material plus dung) on grazed lower to middle slopes (landclass 3); and, a minimum of 100% on non-arable upper to steep slopes (landclasses 4 and 5). See Tool 2.1 for a description of landclass capability.
• Surface ponding and evidence of run-off indicate crusted or impermeable soil surfaces that lower rainfall infiltration rates. Tool 2.7 describes how to measure ponding and run-off.

Water use efficiency is influenced by pasture composition and soil characteristics

• Erosion or sediment deposits are further indicators of the need to apply management to slow or reduce run-off.
• Organic carbon (as measured in routine soil analysis) should where possible be maintained above 3% and up to a maximum of 12% of soil content to ensure improved soil water-holding capacity (see Tool 2.8).
• Soil biota (presence of living organisms) generally improves soil health and structure. This benefits water-holding capacity and infiltration rates, as well as helping to mobilise soil nutrients for uptake by plants.

1. Strategies to improve and maintain water use efficiencyAdopt tactical grazing (a range of grazing management methods) to meet animal production targets and pasture objectives at different times throughout one year, or over a series of years.

2. Assess soil nutrient status, as described in Procedure 3 and take corrective action where appropriate.

3. If soil nutrition status is within critical limits, assess pasture utilisation (refer to Module 3: Pasture utilisation for guidelines) and soil surface management and take the appropriate action.

4. Anticipate periods where analysis of annual rainfall patterns indicates an increased risk of overgrazing (low pasture growth rates) together with high-intensity rainfall events.

5. When pasture utilisation, soil nutrition and soil surface management are within the guidelines for maximising water use efficiency, or your analysis of rainfall against the pattern of pasture growth indicates that current pastures are unlikely to achieve the best possible seasonal growth throughout the year, evaluate the pasture composition using Procedure 4 of this module.

Tactics for managing the soil surface

• Maintain growing pastures near the start of growth phase II (about 1,200kg green DM/ha) for as long as possible to aid regrowth. This has the added benefit of ensuring the highest possible pasture quality for grazing stock.

• Maintain (or increase) ground cover to manage run-off by removing stock before minimum pasture mass limits are reached (1,200kg DM/ha).

• Aim for medium to high levels of litter (at least two or three handfuls in a 30 × 30cm area) to increase soil organic matter, protect the soil surface, decrease evaporation and increase water-holding capacity. Litter is preferably actively decaying plant matter, not old and inert material.

• Manage grazing practice to increase litter quality and breakdown rate.

• Avoing excessive cultivation and the application of soil biota-reducing chemicals to encourage build-up of soil biota, to improve soil structure (increased porosity or aeration), litter breakdown rates and incorporation of surface organic matter.

• Avoid grazing when soil is waterlogged to pervent pugging (where animals hooves work clay or loam soil into a soft, plastic condition with no porosity) 

• Create stock containment areas to remove stock from at-risk grazing areas.

• Change the pasture composition to deep-rooted perennials to ameliorate soils with declining structure.

What to measure and when

Use historical rainfall records to assess the normal monthly pattern and variability of rainfall throughout the year before beginning a program to improve pasture growth and quality.

• Collect and analyse records for each farm or farm sector where rainfall might be different across the farm. Where these records are not available obtain district or regional centre information from local sources as the starting point.

• Tool 2.5 provides a range of sources and methods to complete this analysis.

Assess water use efficiency annually. Tools 2.3 and Tool 2.7 describe the methods and measures for field observations. Collect the following measures from all pasture zones where water use efficiency is monitored:

• an estimate of daily pasture growth rates (kg DM/ha/day) for native and improved perennial pastures – as a guide, Tool 3.3 of Module 3: Pasture utilisation provides estimates of daily pasture growth rates (kg DM/ha/day) in a range of localities and regions across southern Australia

• percentage of groundcover and bare ground at the start and end of the growing season

• mass and quality of litter – aim for at least two or three handfuls in a 30 × 30cm area

• evidence of soil erosion or sediment deposition – observe routinely after rain;

• rate of surface ponding of water and run-off to dams – at least once every three years (see Tool 2.7for technique);

• organic carbon content (%) at increasing depth – initially and then every three years (refer to Procedure 3); and

• soil density or penetration – tested with a penetrometer.