Seasonal Water Allocations for Horticulture

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Seasonal Water Allocations for Horticulture

The most important factor within any season and in particular dry seasons is your final allocation. Your opening allocation is generally set at the start of the irrigation season and may be adjusted during the course of the season as the storage's fill. It is for this reason that you may need to calculate your budget as frequently as new information becomes available.

In most years other than drought years, irrigators will receive an allocation that is equal to their water right, although in a drought, final allocation may be less than a water right.

Water Budget
There are three components necessary for developing a water budget:

Your estimated final allocation;
your predicted water use for the year on a monthly basis;
The volume of water that you have used to date in comparison to your predicted water use.

How to estimate a water budget for this season
To develop the budget you need to know:

what your water right is;
wow much water you used last season;
how much extra water you used this season during the very dry period in August to the end of September this season;
what percentage of Water Right is likely to be allocated this season.

The estimated amount of water needed this season can be worked out in the following way:

Amount of water used last year
(usually a dry year as well)

Plus

The amount of extra water you used this season during the very dry period in August to the end of September.

Minus

Amount of water allocated by your water Authority this season.

As an example if you used a total of 320 ML (Water Right - 300ML - plus 20 ML extra bought last year) and used 50 ML to the end of September this year, it is likely you will need a total of 370 ML to get through this season.

Eg. 300 ML water right + 20 ML sales + 50 ML applied very early (Aug. + Sept.) = 370 ML needed this year.

However, if it is unlikely that 100% water right will be allocated you may find the following table useful to estimate how much extra water the example orchard above would need to purchase based on 50, 55 or 70% water delivery. It is clearly difficult to predict exactly how the season will turn out and it is up to you to decide what level of risk you will work out your figures from.

Your estimate of percentage water right this season.	Amount of water is normal water right is 300 ML	Estimated Irrigation water needed this year for above example.	Need to buy extra irrigation water.
50%	150 ML	370 ML	220 ML (370-150)
55%	165 ML	370 ML	205 ML (370-165)
70%	210 ML	370 ML	160 ML (370-210)

From the above table, if you think this years water allocation for the above farm is 50% of water right, then this example orchard needs to buy (or save) 220 ML of water.

Use the table below to estimate your own water requirements based on the above example:

Water Right plus sales from last year	This years extra irrigation (Aug. + Sept.)	Total needed this season	Anticipated amount of water for this season	Amount of extra water needed to buy or save
Example orchard: 300 + 20 = 320	50	320 + 50 = 70	50% of 300 = 150	370 - 150 = 220
Your orchard:

From early December onwards there is little chance of saving water by various management practices. This is already close to the rapid fruit fill stage for most of the fruit trees and full irrigations are required to make sure fruit reaches commercial size.

Trees should have been thinned to appropriate crop loads and some form of irrigation monitoring should be in place to ensure trees are not under or over irrigated.

(Source: DPI, Drought Preparation and Survival Guide - Horticulture, 2002)

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