Irrigation scheduling is the decision of when and how much water to apply to an irrigated crop to maximise net returns. The maximisation of net returns requires a high level of irrigation efficiency. This requires the accurate measurement of the volume of water applied or the depth of application.

It is also important to achieve a uniform water distribution across the paddock to maximise the benefits of irrigation scheduling. Accurate water application prevents over- or under-irrigation. Over-irrigation wastes water, energy and labour, leaches nutrients below the root zone and leads to waterlogging which reduces crop yields. Under-irrigation stresses the plant, resulting in yield reductions and decreased returns. To benefit from irrigation scheduling you must have an efficient irrigation system.

Advantages of irrigation scheduling

The advantages of irrigation scheduling include:

• The rotation of water amongst paddocks to minimise crop water stress and maximise yields.
• A reduction in energy, water and labour costs through fewer irrigations.
• A lowering of fertiliser costs through reduced surface runoff and deep drainage.
• Increased net returns through increased yields and improved crop quality.
• A minimisation of water-logging problems.
• Assisting control of root zone salinity problems through controlled leaching.
• Additional crops through savings in irrigation water.

Water-balance irrigation scheduling

Water-balance irrigation scheduling is the day-to-day accounting of the amounts of water coming into and going out of the effective root zone of a crop. It is based on estimating the soil water content in the crop root zone viewed as a system (see Fig. 1 above).

Irrigation and rainfall add water to the root zone. Some water may be lost before entering the root zone as runoff. Some may drain below the root zone (and in some situations water can also enter the root zone from a high water table or water moving laterally through the ground). Water is also lost from the root zone through direct evaporation of water from the soil surface, and transpiration through the plants.

The total water in the root zone on a particular day can be represented by the water-balance formula:

TW_T     =          TW_T-1 + Irr + Rain - ET_C - DEEP - Runoff + FLUX_net

where:             
TW_T      =   total water in the root zone on day T
TW_T-1    =   total water in the root zone on the previous day (T-1)
Irr           =    irrigation water applied
Rain        =    rainfall
ET_C         =   evapotranspiration (soil evaporation plus plant use)
DEEP      =   drainage or percolation below the root zone
Runoff      =   runoff
FLUX_net  =   any change in total water in the root zone from underground water movement (e.g. high water table or water moving laterally in the ground).

The water-balance approach to irrigation scheduling chooses a starting point total soil water in the root zone. Then the water-balance equation is solved on a daily basis, considering the amounts of water that move into and out of the root zone for that day.

The basic steps for water-balance irrigation scheduling of a paddock are:

1. Determine the depth of the effective root zone

The effective root zone (ERZ) of the crop is the depth of soil where you as the irrigator want to control soil moisture. It may or may not be the full depth of the plant roots. Table 1 shows the root depth at effective cover (when the crop has reached maximum ET_c and maximum rooting depth). The effective root zone where fully irrigated crops draw most of their water is usually between 60 cm and 1 m. Although roots may be found below this depth, but the bulk of the water extracted from the soil by an irrigated crop will come from the top 1 m of soil.

Source: Allen, R.G. et al (1998) Crop evapotranspiration: guidelines for computing crop water requirements, FAO Irrigation and Drainage Paper 56
1.  The larger values for maximum root depth are for soils having no significant layering or other characteristics that can restrict root growth. The smaller values should be used for irrigation scheduling and the larger values for soil water stress or raingrown conditions.
2.  The Depletion Fraction values apply for ET_C» 5 mm/day. The value for the Depletion Fraction can be adjusted for different ET_C conditions using the formula DF = DF_{Table 1} + 0.04 (5 - ET_C)

2. Determine the Total Available Water

Available water is the amount of soil water in the effective root zone that is available to plants. Following heavy rainfall or irrigation, water will drain out of the root zone until the Drained Upper Limit (DUL) is reached (also known as Field Capacity) - this is the amount of water that the soil can hold against gravitational forces. Crops will use this water and lower the water content in the absence of further rain or irrigation. As the water content falls, the remaining water is held by the soil with greater force and it becomes more difficult for the plant to extract it. Eventually a soil water level is reached where the crop can no longer extract water - the Crop Lower Limit (CLL).

The difference between the water level at DLU and CLL is referred to as the Plant Available Water Content (PAWC) - it is measured in mm water per metre of soil depth. Different soils hold different amounts of PAWC. Table 2 shows the typical PAWCs for a range of soil types.

The Total Available Water (TAW) in the root zone is found by multiplying the PAWC by the depth of the effective root zone. For example, a maize crop with an effective root zone of 0.8 m growing on a clay soil with a PAWC of 200 mm has a TAW of 160 mm (200 mm/m x 0.8 m).

3. Determine your Readily Available Water level

As the soil water level falls it becomes more tightly held by the soil and it is more difficult for plants to extract. Once the level falls below a threshold value, soil water cannot be transported quickly enough to crop roots to meet the demand of transpiration and the crop begins to stress. The fraction of TAW that a crop can extract from the root zone without suffering water stress is referred to as readily available water (RAW). It is found by multiplying TAW by the depletion fraction (DF) - the fraction of PAWC that can be depleted from the effective root zone before irrigation is necessary to minimise yield loss. The depletion fraction changes with crop and at different stages in crop growth. Depletion fractions for a range of full-grown irrigated crops are given in Table 1.

4. Determine your Refill Point

The Refill Point (RP) is the total soil water balance in the effective root zone at which irrigation is required. It is found by subtracting the readily available water (RAW) from the total soil water at the Drained Upper Limit in the effective root zone (TW_DUL).

5. Determine the starting point for total soil water in the Effective Root Zone

A starting point for soil water in the effective root zone (ERZ) is needed before beginning to schedule irrigations. It can be established before or after crop emergence by direct measurement (gravimetric soil water sampling as described in Soil Matters: monitoring soil water and nutrients in dryland farming) or a calibrated soil-monitoring device such as a neutron moisture probe or a capacitance probe (for example an EnviroSCAN, Gopher or Diviner). Alternatively you can estimate it using software such as HOWWET. In furrow irrigated cropping systems it is often assumed that a pre-irrigation will fill the effective root zone to TW_DUL, but direct measurement will be more accurate. Once the starting soil water content is known it is possible to estimate it on successive days using the water-balance formula.

6. Quantify water movement into and from the Effective Root Zone

Measure rain using rain gauges. Irrigation depth is calculated from the duration and rate of application of the irrigation system, or by dividing the total net amount of water applied by the irrigated area (this allows for the efficiency of the irrigation system, as none is 100% efficient). For an accurate estimate of irrigation depth you must measure its operational efficiency.

If the depth of rain or irrigation exceeds the depth of soil water depleted from the effective root zone the difference is considered to be deep drainage and/or runoff (the DEEP and RUNOFF terms in the water-balance formula). The FLUX_net is usually considered negligible although it can be significant where a perched water table exists.

The crop evapotranspiration (ET_C) term is the daily withdrawal figure from the soil water balance in the effective root zone. It is estimated from weather and crop information. The formula for estimating ET_C is:

ET_C      =          K_C x ET_O

where:    
K_C     =  the crop coefficient which expresses the difference in evapotranspiration between the cropped and a reference grass surface.
ET_O   =  a grass reference crop evapotranspiration (mm per day).

ET_O is calculated using the Penman-Monteith method and requires radiation, air temperature, air humidity and wind speed data. A number of automatic weather stations with sensors for these measurements calculate ET_O using this method. Class A Pans are no longer considered adequate for estimating ET_O owing to poor siting and maintenance.

The crop coefficient (K_C) integrates the effect of characteristics that distinguish a typical field crop from the grass reference, which has a constant appearance and a complete ground cover. Thus different crops have different K_C coefficients. It also changes over the growing season with changes in crop development and with changes affecting soil evaporation. Estimates of K_C values for the major irrigated crops are presented in Table 3.

Source: Allen, R.G. et al (1998) Crop evapotranspiration: guidelines for computing crop water requirements, FAO Irrigation and Drainage Paper 56.

The crop stages used to select a K_C value are:

1. Initial stage - planting until 10% ground cover.
2. Crop development stage - 10% to effective groundcover (around 70-80%).
3. Mid-season stage - 70-80% groundcover to the start of maturity.
4. Late season stage - the start of maturity until harvest.

The steps in constructing a crop coefficient curve similar to that in Figure 2 are:

1. Divide the growing period into the four crop stages above, determine their length and identify the corresponding K_C values from Table 3.
2. Adjust K_C values for frequent irrigation or rainfall events, humidity and wind speed.
3. Construct the curve by connecting straight lines through each of the growth stages as shown in Figure 2.