Water budget key to irrigation decision-makingTechnology & Data
The most common method used for scheduling orchard irrigation is ‘experience’. While this can be reasonably accurate in delivering tree water requirements, it is not so useful when it comes to creating a ‘water budget’ for a season. By using a standard method of calculating orchard evapotranspiration (ETc) and historical weather data, growers can estimate monthly water requirements for the blocks of an orchard. This water budget can then be used to plan water management strategies under different scenarios. For example, what is the irrigation requirement if deficit irrigation or post-harvest cut-off are used? If allocations are low, can I maintain irrigation levels to some priority blocks? A computer spreadsheet is available from Agriculture Victoria to aid growers in creating seasonal water budgets and this article describes the basis of the calculations therein.
The water budget, and for that matter day-to-day irrigation scheduling, is based on the standardised method of computing ETc as detailed in FAO56:
where, ETo is reference crop evapotranspiration, Kcb is the basal crop coefficient and Ke is the soil evaporation coefficient. ETo is equivalent to water use of well-watered grass and is calculated from measurements of wind speed, solar radiation, temperature and humidity by the Bureau of Meteorology (contact the authors for historical ETo information relevant to your location). In orchards, Kcb is a factor to convert ETo to tree water use and Ke is a factor to convert ETo to understorey water use (the combination of soil evaporation and cover crop water use).
FAO 56 addresses the calculation of Ke in detail for a range of circumstances, but a good rule-of-thumb is that Ke is equal to the fraction of orchard floor that is wet after an irrigation event (although in spring, when there is consistent rainfall events, Ke can be estimated from the fraction of the orchard floor that is sunlit). Thus, Ke for the majority of the season is largely determined by the irrigation method, with microjet systems creating a larger wetted surface area and therefore requiring a greater Ke value than drip systems.
Sunlit leaf area is the major determinant of Kcb and hence tree water use. Sunlit leaf area will depend on canopy extent, planting arrangement, leaf area density and solar position.
Research has shown that sunlit leaf area can be estimated from effective area of shade (EAS, expressed as a fraction of the orchard floor):
where EAS is measured in the morning, at midday and in the afternoon to account for changes in sunlit leaf area over the daytime and k is a specific water use coefficient for genotype (e.g. crop type/cultivar) and environment (e.g. advection associated with discontinuous orchard canopies). Studies on peach and pear at Tatura have determined k to approximate 1.1, while a k of 1.3 is recommended for apple crops. EAS of new plantings may only reach 0.1 by the end of their first season whereas mature blocks can reach their seasonal maximum by the end of October. EAS of mature blocks in the Goulburn Valley commonly range from 0.3 to 0.7, meaning that there would be commensurate (and substantial) differences in water requirements between blocks. It is unusual for EAS of a mature block to exceed 0.7. Guidelines for estimating EAS can be found at: http://agriculture.vic.gov.au/agriculture/horticulture/fruit-and-nuts/orchard-management/determining-effective-area-of-shade-in-orchards-and-vineyards-to-estimate-crop-water-requirement.
An example water budget for an apple orchard in the Goulburn Valley, generated using the computer spreadsheet (available from the authors), is shown above. Grey columns are calculated within the spreadsheet, while the remaining columns can be altered by the user. For instance, the user could decrease ‘average rainfall’ to investigate the impact of dry conditions.
In this case, irrigation requirement was estimated for a typical orchard with 0.45 mid-season EAS. The example orchard had a root-zone depth of 600 mm, a sandy loam overlying a clay loam soil, adequate winter rainfall to wet up the entire root-zone and average within season rainfall. Effective rainfall was set to 75 per cent. In other words, 75 per cent of rain during the season was captured in the root zone. The orchard was drip irrigated and water use from the understorey (i.e. soil evaporation and cover crop water use) was set to 0.5 of ETo in October due to an active inter-row sward and 0.1 for the rest of the season to account for direct evaporation from the wetting pattern. Deficit irrigation was not used. The example shows that irrigation requirement commenced in October and increased to a maximum in January of approximately 106 mm per week (1.06 ML/ha). Total seasonal irrigation requirement was 453 mm (4.5 ML/ha).
The ‘stress coefficient’ is equal to 1 when irrigation is managed to meet maximum water use. A stress coefficient less than 1 can be entered to quantify potential water savings from deficit irrigation or post-harvest cut-off.
The seasonal water budget provides the basis for scheduling plans within the season. Once the typical water requirements are known, growers can predict the intervals between irrigation events. Of course, adjustments will need to be made dependent on actual weather conditions. Irrigation system and block information (emitter spacing and rate, tree and row spacing) are entered in the spreadsheet and an irrigation interval is calculated for each month. The interval between irrigation events for the example orchard block decreases from four days in October to three days in summer.
An alternative to altering the irrigation frequency is to alter the run-time. Decisions regarding run-time should be based on the water-holding capacity of the soil, the wetting pattern and the depth of the fibrous root zone. Run-times that are too long will waste water due to drainage below the root zone, while run-times that are too short risk increased understorey evapotranspiration and subsequent inadequate wetting of the root zone.
Maximum irrigation runtime (h) can be calculated assuming a cylinder-shaped wetting pattern from:
where R (m) is the radius of the wetting pattern, D (m) is the depth of the fibrous root zone, Q is the emitter discharge rate (L/h) and RAW (m3/m3) is the readily available water. RAW values of 0.08 and 0.06 m3/m3 can be used for loam and sandy soils, respectively.
The value of assessing your irrigation system performance should not be forgotten. The benefits of careful water budgeting and accurate calculation of water requirements will be compromised if your irrigation system is not delivering the water you think it is. In short, clean your filters, flush your lines, check your pressures and calculate your distribution uniformity. Agriculture Victoria offers training in understanding your irrigation system design, irrigation system maintenance and evaluation. Contact Jeremy Giddings (T: 03 5051 4566, M: 0427 102 285, firstname.lastname@example.org) for more information.
About the authors:
Lexie McClymont, Ian Goodwin, and Mark O’Connell
Agriculture Victoria Department of Jobs, Precincts and Regions (DJPR)