Further Info

The program is designed to be used for general water budgeting and not detailed farm irrigation scheduling. Average weather data has been used to calculate water requirements and crops. Allowance will need to be made for years that are hotter, drier, cooler or wetter.

The program calculates the water required to grow the crop and does not take into account extra water required for frost control, preventing erosion, sandblasting, applying nutrients through irrigation or cooling the crop. These activities plus other farm uses of water such as washing produce, packing sheds and winery operation should be calculated separately and accounted for when planning water needs.

How the Calculator estimates water requirement

The following calculation is used:

Daily water use =
Average effective daily evaporation (soil type) * Crop factor *
Irrigation efficiency factor * Proportion of land area irrigated

Average daily evaporation

Average daily evaporation has been calculated by averaging 25 years weather data (1983 to 2008) for each location (Table 2) and subtracting average effective monthly rainfall from average monthly evaporation. The monthly figure is then divided by the number of days in the month to give 'average daily evaporation'.

When calculating effective rainfall the following rules have been applied;

  • In January, February, March, November and December, rain events greater than 4 mm but not including the first 4 mm are considered effective.
  • In April, May, September and October, rain events greater than 3 mm but not including the first 3 mm are considered effective.
  • In June, July and August, rain events greater than 2mm but not including the first 2 mm are considered effective.

When daily effective rainfall is greater than daily evaporation, no more than 6 mm is considered to be stored in sand. Therefore in sand, rainfall will only reduce plant irrigation water requirement by a maximum of 6mm on any rainy day.

When daily effective rainfall is greater than daily evaporation, no more than 20 mm is considered to be stored in loam/clay soils. Therefore in loam/clay soils, rainfall will only reduce plant irrigation water requirement by a maximum of 20 mm on any rainy day.

Soil Type

Different soil types have different capacities to hold and retain water. Loams and clays hold more water, drain slower and have lower evaporation losses than sandy soils.

Crop Factors

An annual plant is a plant that usually germinates, flowers, and dies in a year or season. Examples include vegetables, some pastures, wheat and lupins. True annuals will only live longer than a year if they are prevented from setting seed. A perennial plant is a plant that lives for more than two years. Fruit trees, lucerne and some other pasture species are perennial.

Annuals crops

Water requirements of annual crops are estimated using the Doorenbos and Pruitt (1977) model. This model uses different crop factors (based on Class A pan evaporation) for different stages of growth as shown in Figure 1.

Figure 1: Diagrammatic representation of a crop factor curve (after Doorenbos and Pruitt, 1977). Data for annual crops are provided for each growth stage:

  1. initial stage: germination and early growth when the soil surface is hardly covered by the crop(groundcover <10 %);
  2. crop development stage: from end of initial stage to attainment of effective full groundcover (groundcover at 70-80 %);
  3. mid-growth stage: from attainment of effective full groundcover to time of start of maturing and;
  4. late growth stage: from end of mid-growth stage until full maturity or harvest.

Crop duration (growing period) is based upon the number of days from planting to harvest, and changes for each month of the year and for different regions of W.A. The calculator only allows for the months that crops are commonly planted. As crop growth and crop factor data for new varieties becomes available it can be entered into the calculator.

The crop factor is the proportion of evaporation that must be replaced with irrigation for a crop to produce a commercial yield. For most crops, crop factors increase as crop stage increases. However for some crops e.g. potatoes, crop factors reduce as the plants approach senescence or harvest.

The proportion of time in each of the growth stages was estimated from data published for crops grown under a wide range of conditions by Doorenbos and Pruitt (1977). Although the published data could not be directly applied to Western Australia, it was noted that for any particular crop, regardless of the area or season in which it was grown, the number of days in each growth stage was a relatively constant proportion of the total time from planting to harvest. Data for Western Australia was developed by applying these proportions to the time taken from planting to harvest for the crops planted at different times of the year throughout Western Australia. Where data was missing Department staff contributed information on crop growth length when planted at different times throughout the year.

Perennial crops

The program uses effective evaporation and crop factors for each month to determine the water requirements of perennial crops. Crop factors for most perennials vary for the stage of growth or maturity of the crop at different times of the year.

Crop factors used for perennials are for mature crops. Paulin (1984) published a set of crop factors for fruit crops in the South West of Western Australia. Estimates exist for other crops such as, table grapes (Campbell-Clause, 1990), bananas (Luke, 1990) and avocados (Paulin, 1988).

Proportion of area irrigated

Plant water use is related to the area of transpiring foliage. For perennial crops the crop factors used in this program are for mature trees. When crop canopy covers a large percentage of the ground area an irrigated proportion of 1 can be used. The irrigated proportion function allows the irrigation requirement to be reduced to account for crops with wide spacing or for young perennial crops when canopy cover is only a fraction of the area planted.

When overhead sprinklers are used on crops such as vegetables, where the whole area cropped is watered, the irrigation proportion is 1.

For under tree or drip irrigation systems the area the plant canopy occupies or the wetted area is generally used as the irrigated proportion. Common values for drip range from 0.4 to 0.8.

Example 1:
A mature citrus orchard irrigated with drippers has rows spaced at 5.5 m between rows and trees touching within rows. The farmer is able to drive his tracker with 2 m wheel centers down the rows without touching the trees. This suggests that around 2.5 m is not covered by the canopy. To calculate the irrigated proportion, simply divide the covered area by the whole area.

i.e. 5.5 - 2.5 = 3 m covered. Therefore the irrigated proportion is 3/5.5 = 0.55.

Example 2:
The trees in a newly planted olive orchard have a radius of one metre and the under tree sprinklers have a radius of throw of 2 metres. Area of wetting pattern can be calculated using πr2, 3.14 x 2 x 2 = 12.6 m2. There are 250 sprinklers per hectare.

Total wetted area = 12.6 m2 x 250 = 3150 m2
The irrigation proportion = 3150 m2/10000 m2 = 0.3

To determine the irrigation proportion you will need to know the coverage of the irrigation system and/or the spacing and canopy cover of perennial crops. If you require assistance please contact the Department of Agriculture and Food.

Irrigation efficiency factor

The program calculates an estimate of the total water required to produce a crop. However no irrigation system delivers water with 100% uniformity and an efficiency factor is applied to calculate the extra water needed to adjust for the inefficiency of different systems. The efficiency factor of your system can be calculated by carrying out a catch can sprinkler test and calculating the co-efficient of Uniformity (CU) or distribution uniformity (DU).

Efficiency factor = 100/DU

The minimum standards for sprinkler uniformity are a DU of greater than 75% and a CU of greater than 85% . Ideally uniformities should be higher.

Over head fixed irrigation: 80% DU & 90% CU
Pivot systems: 90-95% DU hh-mod
Dripper's: 90-95 % emission uniformity (EU)

Table 1: Conversion of DU to efficiency factors
% DU Efficiency Factor Extra minutes required
per hour of irrigation
70 1.45 27
75 1.33 20
80 1.25 15
85 1.18 11
90 1.11 7
95 1.05 3

Irrigating during the day, under strong drying wind conditions, operating the system at excessive pressure, poor irrigation design or lack of maintenance all contribute to system inefficiency.

Table 2: Locations of weather station used to calculate effective evaporation.
Town Latitude Longitude
Albany WA -35.0097 117.8836
Armadale WA -32.1306 116.0042
Carnarvon Airport WA -24.8819 113.6708
Geraldton Airport WA -28.7961 114.6961
Gingin WA -31.3481 115.9022
Kununurra WA -15.7828 128.7353
Manjimup WA -34.2556 116.1428
Medina Research Centre WA -32.2222 115.8067
Mount Barker WA -34.6256 117.6344
West Swan WA -31.8511 115.9942
Wanneroo WA -31.7333 115.7917
Wokalup WA -33.1319 115.8806
Margaret River Post Office WA -33.9525 115.0747
Broome Airport WA -17.9492 122.2336


Campbell-Clause J. (1990). "Irrigating table grapes". W.A.D.A. Farmnote No. 99/90.

Doorenbos J. and Pruitt W.O. (1977). "Guidelines for predicting crop water requirements". F.A.O. Irrigation and Drainage Paper No. 24.

George P.R. and Cripps J.E.L. (1985). "Watering requirements of vegetables grown on sandy soils". W.A.D.A. Farmnote no. 102/85.

Hoffman H. and Muller T. (1990). "Efficient irrigation for determinate tomatoes in the Gascoyne River area". W.A.D.A. Farmnote No. 27/90.

Luke G.J., Burke K.L. and O'Brien T.M. (1988). "Evaporation data for Western Australia". D.R.M. Technical Report No. 65. Western Australian Department of Agriculture.