Growing avocados – annual water requirements

Introduction

Avocado trees are quite sensitive to soil water availability. Even a mild moisture stress at critical stages can lead to fruit shed or leaf drop. This can lead to significant reduction in yield and impact negatively on orchard profitability.

Avocado trees have evolved under rainforest conditions which has resulted in them having:

• limited adaptation to hot dry climates as they have only partial control over moisture loss
• A relatively shallow root system, with up to 80% of the moisture being obtained from the top 30cm of soil. In many Western Australian orchards you will find the greater percentage of fine avocado roots in the top 15cm of soil. It is these fine roots that are the primary water and nutrient absorbers.
• None of the ultra-fine root hairs present in many other crops such as apples or peaches. As a result they are not as able to search for and extract water that is tightly held to soil particles.
• a low tolerance to salts, particularly chlorides

For these reasons, the correct application of water is a critical management factor for successful avocado production. In Western Australia (WA), which has a dry Mediterranean climate, correct water management is arguably the most important management tool in achieving high yields of quality fruit.

Correct application of water requires careful planning and execution. Consideration must be given to how much is required and the quality of the available water, as well as the timing and method of application.

Water is a limited asset with few orchardists having a plentiful supply of cheap and easy to access water that is surplus to their requirements.

How much water do I need?

To do this you need to estimate what the likely water use of your mature orchard will be on an annual basis. This gives an indication of the amount of water you will need to have available for your project. All orchards will need to either build a dam to catch and store this water, or get access to underground supplies. If you are sourcing underground supplies, your estimated orchard annual demand will provide a close approximation. If you are relying on dam supplies, you need to also consider evaporation losses from the dam. As water supplies are limited in WA, quite often it is your water supply that dictates the size of the orchard you can maintain.

Access to all underground water supplies and most surface water is subject to government regulation and in most cases requires a license from the Department of Water and Environmental Regulation.

Estimating annual water requirements for a mature orchard

This is essentially a calculation to estimate how much water the plant will use in a year, less any amount that is provided by rain. Some of the factors influencing water use include:

• temperature
• wind
• humidity
• sunlight
• tree size
• tree growth phase.

These issues will vary during the season and from year to year.

A simple method is to use long term mean monthly evapotranspiration figures for your region. For many locations, these can be obtained from the Department of Primary Industries and Regional Development (DPIRD) weather station pages, the Bureau of Meteorology, or figures may be available from your own private records. The monthly mean evapotranspiration (ETo) is multiplied by a monthly avocado crop coefficient (Kc) (Table 1) to calculate an estimated monthly water use in millimetres. By then subtracting any rainfall for the month (also from long term monthly means) an average monthly water requirement is achieved.

Example calculation of crop water use for Manjimup in November

ETo for November = 141 millimetres (mm)
Rainfall for November = 51mm
Avocado crop coefficient (Kc) for South-West in November = 0.8

November crop water use for avocados in Manjimup
= (monthly ETo multiplied by monthly Kc) less rainfall
= (141mm X 0.8) – 51mm
= 62mm

The crop water use figure (in millimetres) now needs to be converted to a water volume (litres) figure. To do this, we use the understanding that it takes one litre of water applied over one square metre to provide a 1mm application. Therefore to calculate the amount of water required per hectare, we simply multiply the monthly water needs in millimetres by 10 000 (there are 10 000 square metres in one hectare). So, in the above example there is a need for 560 000 litres (0.56 megalitres) per hectare for the month of November.

Crop water use must be calculated for each month and then totalled for the year, disregarding any months where rainfall exceeds evaporation. This calculation provides the basis for determining how much water is required to develop your planned operation, or the size of the operation you can develop if you already have a fixed water resource. There are more complex methods used to calculate this requirement that take into account other factors such as actual tree size (or tree spacing and number per hectare), the number of rainy days, sprinkler efficiencies, dry seasons and water quality, but this relatively simple method provides a reasonable estimate. Table 2 provides some examples from a range of locations.