Irrigation calculator

The Irrigation calculator provides broad guidance for water budgeting on commonly grown commercial crops in Western Australia. It should not be used for detailed farm irrigation scheduling. 

Historical average weather data is used to calculate water requirement for crops.  

Users need to allow for years that are hotter, drier, cooler, or wetter, extra water required for frost control, erosion and sandblasting prevention, crop cooling and the application of nutrients through irrigation, in addition to watering requirement, and as well as other farm uses of water, such as washing produce, packing sheds and winery operations. When planning water needs, these variables should be considered, and calculated separately. 

How the Irrigation calculator estimates water requirements

The calculation used by the Irrigation calculator to estimate the water requirement to produce commercial crops is:

Daily water use = Average effective daily evaporation x Crop factor x Irrigation proportion/Effective area of shade x Irrigation efficiency factor. 

The calculator does not consider extra water required for frost control, erosion and sandblasting prevention, crop cooling or nutrient applications through irrigation, in addition to watering requirement. These variables and other farm uses of water (such as washing produce, packing sheds and winery operations) should be considered when planning water needs and calculated separately.

Evaporation and rainfall

Evaporation and rainfall have been calculated for each month using data from the past 25 years (1990 to 2014), sourced from the Scientific information for landowners (SILO) weather data service. SILO is an enhanced climate database hosted by the Australian Government's Department of Science, Information Technology and Innovation. To calculate the most accurate data for your farm, first select the region and then the nearest station to you. 

Effective daily evaporation considers the evaporation, rainfall and storage capacity of soil. It is used to estimate the water required, in addition to rainfall, to supply crop demand. 

The following rules have been applied to rainfall when calculating effective daily evaporation. 

• 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 2 mm but not including the first 2 mm are considered effective. 

To calculate effective monthly rainfall, multiply the average depth of effective rainfall by the average number of days that rainfall occurred.  

To calculate effective daily evaporation, effective rainfall is subtracted from monthly evaporation, then divided by the number of days per month.  

For example, over the past 25 years, April data from the Perth metro weather station recorded an average of 2.9 days with rainfall greater than 3 mm. These events averaged 11.7 mm, so 3 mm is subtracted from 11.7 to get 8.7mm. 

Therefore, effective rainfall for April at the Perth metro station = 2.9 (average) x 8.7 (effective rainfall) or 25.2 mm. 

Average evaporation over April is 133.4 mm over 30 days. 

Effective daily evaporation is calculated as 133.4 mm – 25.2 mm = 108.2 / 30 (days) = 3.6 mm. 

Soil type and effective evaporation

Soil types have different capacities to store water. For example, loam and loamy clays hold more water, drain slower, and have lower surface evaporation losses than the coarse sandy soils of the Swan Coastal Plain. This affects the amount of rainfall that is considered effective, because rainfall greater than the holding capacity of the soil and root zone of the crop is not available for use by plants as it drains away. 

The following assumptions have been made when considering rainfall: 

• When rainfall is greater than average daily evaporation, no more than 6 mm is stored in sandy soils. 
• When daily effective rainfall is greater than daily evaporation, no more than 20 mm is stored in loam/clay soils. 
• In coarse sandy soils, it is assumed that rainfall will only reduce plant irrigation water requirement by a maximum of 6 mm on any rainy day. 
• In loam and clay soils with higher levels of clay than coarse sand, rainfall will only reduce plant irrigation water requirement by a maximum of 20 mm on any rainy day. 

While the rules above will not calculate the exact water requirements for your crops, the range in volumes of water required by sand and clay loam soils should be representative of crop requirements. 

Crops

An annual plant, such as vegetables, some pastures, wheat, and lupins, usually germinates, flowers, and dies in one year or season. True annual crops will only live longer than a year if they are prevented from setting seed.  

A perennial plant, such as fruit trees, lucerne and some other pasture species, lives for more than a year.  

Irrigation inefficiency factor

The Irrigation calculator estimates the total water required to produce a crop. As no irrigation system delivers water with 100% uniformity, an inefficiency factor is applied to calculate the extra water needed to adjust for the delivery efficiency of different irrigation systems.  

The inefficiency factor of your system can be calculated by carrying out a uniformity test that determines the distribution uniformity (DU) or emission uniformity (EU). 

The Inefficiency factor calculation is 100/DU or 100/EU. 

The minimum standards for irrigation uniformity are a distribution uniformity (DU) of more than 75% and a coefficient of uniformity (CU) of more than 85%. Ideally, irrigation uniformities should be higher to minimise the extra water required to compensate for inefficient application of irrigation water. 

While poor irrigation practices should not be the cause of a higher water allocation, it needs to be accounted for when budgeting irrigation and water storage requirements. Irrigation improvements should be made to achieve a high efficiency of irrigation. 

Table 1 shows the irrigation inefficiency factor and extra minutes’ run time required per hour of irrigation for different DU’s.  The calculator defaults to a standard 1.1 but users can adjust this based on irrigation performance testing. 

To guide irrigation, the following are suggested minimum DU and EU for different irrigation systems that allow the use of evaporation-based irrigation scheduling, used in combination with soil moisture or plant monitoring: 

• Overhead fixed irrigation 85% DU 
• Pivot systems: 90-95% DU Modified Hermann and Hein 
• Drip irrigation: >95 % EU 

Irrigation proportion/effective area of shade

Where an irrigation system wets 100% of the area, usually by overhead sprinklers in vegetable crops or under-tree sprinklers in perennial crops, the irrigated proportion is 1. 

For some drip irrigated crops, like melon and pumpkin, the ground area being irrigated may only be half the area but the canopy of the crops at maturity is extensive and will be close to full coverage. In this case also, a proportion of 1 should be used. 

When the area wet by irrigation is less than 100%, the area of canopy coverage should be used to estimate the reduction in water requirement. For example, if a mature citrus orchard, irrigated by drip irrigation, has a tree canopy occupying 70% of the ground area, the irrigated proportion is 0.7. 

For strawberries that are grown under plastic mulch with drip irrigation, the wetted area is only as wide as the bed area and irrigation proportion should be based on how much bed area is covered by the canopy of the crop, at maturity. Depending on spacing and vigour, this may be close to 1. When this approach is used, the bed area only, not including wheel tracks, should be entered in the area planted tab. 

For example, if a bed is 1.2 metres across its top and bed centres are spaced at 1.4 m, there is only 1.2 m / 1.4 m = 0.86 ha of bed area per full hectare of area used, and 0.86 ha should be added into the area planted tab. 

To calculate the irrigated area of trellised crops, use the effective area of shade (EAS) method (Goodwin, I. and O’Connell, M. 2004). EAS is determined from the measurement of the percentage of shade cast by the trees or vines at three times a day (3.5 hours before solar noon, at solar noon and 3.5 hours after solar noon).  

These measurements per day account for changes in the percentage of shade during the day due to the angle of the sun, foliage extent (that is, training system and tree/vine size), planting arrangement (such as row orientation and tree spacing) and leaf area density. To calculate EAS, average the three measurements. 

This procedure is explained by Goodwin I. (2009) Determining effective area of shade in orchards and vineyards to estimate crop water requirement. 

Gap fraction is estimated based on the percentage of light shining through the area shaded by the trees, with 0 being no shade and 1 being full shade. 

For example, an orchard has trees spaced at 5 m.  The percent shade measurements are made at 3.5 hours before solar noon, at solar noon and 3.5 hours after solar noon. 

To calculate percent shade: 

Percent shade 1 = [Length of shade (m)/ Row width (m)] x Gap fraction x 100 

                            = 4.45 / 5 x 0.65 x 100 

                            = 57.85%, rounded up to 58% 

Percent shade 2 = [Length of shade (m)/ Row width (m)] x Gap fraction x 100 

                            = 2 / 5 x 0.65 x 100 

                            = 26% 

Percent shade 3 = [Length of shade (m)/ Row width (m)] x Gap fraction x 100 

                            = 4.6 / 5 x  0.65 x 100 

                            = 59.8 %, rounded up to 60%.

To calculate the average percent EAS, add the percent shade in the morning (58%), solar noon (26%) and afternoon (60%), and divide by 3. 

EAS % = shade (am) + shade (midday) + shade (pm) / 3 
           =  58 + 26 + 60 /3 

           =  48% 

The irrigation proportion in this case is 0.48. 

Yearly assessments may need to be made as the crop matures. When calculating water required for licences, you should factor in fully mature tree sizes which, depending on training and spacing, may cover between 50% and 100% of the area being calculated.  

Crop stage

Water requirements have been estimated for annual crops, considering the different stages of plant growth described by Doorenbos and Pruitt (1977). 

• Initial stage covers germination and early growth when the soil surface is hardly covered by the crop (that is, when groundcover is less than10%) 
• Cop development covers the period from the end of initial stage to attainment of effective full groundcover (that is, when groundcover is at 70-80%). 
• Mid-growth stage covers the period from attainment of effective full groundcover to time of start of maturing. 
• Late growth stage covers the period from end of mid-growth stage until full maturity or harvest. 

Where further research has been done, more stages of growth may have been used. 

Crop duration (growing period) is based on the number of days from planting of a transplant or seed to harvest. Crop duration changes for each month of the year and for different regions of Western Australia.  

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, the crop factor increases as the crop stage progresses. For some crops, like potatoes, crop factors reduce as the plants approaches 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 crop, regardless of the area or season in which it was grown, the number of days in each growth stage was a similar proportion of the total time from planting to harvest. Data 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, DPIRD used information on crop growing time when planted throughout the year. 

Water requirements for perennial crops

Crop factors for most perennial crops vary for the stage of growth, maturity and leaf cover at different times of the year. 

All crop factors used for perennial crops are for mature crops. Users can adjust the factors by considering the leaf area index or effective area of shade, which influences water requirements.