Whatever happens to our climate it is clear that we will all have to rethink our water use, both in the house and in the garden.
By growing our own food we use far less water than commercial growers, but we still need to make the most of whatever comes on to our land.
All gardens need extra, or supplementary, water during dry periods. Installing water tanks to catch whatever comes from the heavens is an obvious step. There are a myriad of designs to fit even in the most confined space, and that suit every roof size. Many water tanks are available as slimline models – a 3,000 litre slimline tank measures approx 2.8m by 2.25m and 700mm wide so it can be slipped under house eaves, or against the fence.
To calculate how much water you can harvest from the house, shed or garage roof, you need to know the surface area of the roof. A small garden shed of 2 m by 3m (with corresponding roof size) has a total area of 6m². Multiply your roof area in metres by your rainfall in millimetres to calculate your possible yield, and deduct 10% to allow for losses. Melbourne has an average rainfall of 650mm so a garden shed in Melbourne can harvest 3,510 litres – that is 6m² x 650mm = 3,900 litres, less 10%, equals 3,510 litres. When you consider that the Department of Primary Industry calculates that it takes only 100 to 700* litres to irrigate 1m² of garden per year (when using drippers or micro-sprays) we can all have enough water to grow our own food. The pressure required to run a weepy hose is 70kPa or 10psi. This pressure is not attainable by using gravity feed (you would need a tank stand 70m high), so a pump is essential. There are electric, diesel, or solar powered pumps readily available. Of course, there are always watering cans and buckets.
Re-using grey water can help maintain your ornamental plants and to irrigate your fruiting trees and shrubs – but never use it on your vegetable garden. Only use grey water from baths, showers and washing machines, preferably the final rinse. Water from the kitchen and dishwasher contain so many chemicals and fats that it will damage your plants and your soil.
It is imperative that you use the most environmentally responsible detergents as possible. Choose detergents that are low in sodium where possible, as this element is the most problematic for both soils and plants. Visit the website www.lanfaxlabs.com.au for precise information on specific detergents, and www.epa.vic.gov.au for health and pollution details. Always rotate your grey water around the garden making sure that it is not concentrated in one spot. Not only will the chemicals eventually damage the plants, your soil will also be badly affected. Overuse of grey water can cause the complete break down of your soil structure, destroying soil aggregates that would otherwise enhance the soils' water holding capacity.
When, water is applied to the soil, either through rainfall or irrigation, we want it to infiltrate quickly rather than run off the surface. A mulched garden bed has no crust to prevent infiltration, and is capable of absorbing over three times the amount of water than un-mulched beds. Mulch also minimises the evaporation of water from the soil by over 50%. In winter, or times of expected high rainfall, mulch need not be as dense, so that rainfall can infiltrate the surface quickly. Green manures are great to grow from autumn to winter when they cover the soil when the summer mulch is thinning. They also provide masses of organic matter, to be dug in by late Winter.
Once the water has got into the soil we want to keep it there for as long as possible. Water and air are essential for plant growth, and they are contained in spaces or pores between soil particles or aggregates. Generally sandy soils have large pores or spaces between the soil particles, these allow water to enter the soil quickly, but also drain quickly . Heavy clays with many micro-pores or small spaces between the soil particles, hold water well but have low levels of air, making water infiltration and root penetration difficult. The size of soil pores are largely due to the specific texture of your soil, be it sand, loam, or clay. However all soils can be improved with organic matter.
The more decomposed organic matter in the soil, the more water it can store and still provide plenty of large spaces for air. By adding organic matter your soil will start to form stable soil aggregates with the meso, or medium sized pores that hold water. Stable soil aggregates are held together with decayed organic matter and have complex surface areas that carry a film of water. This water can be easily accessed by the plants over a long period when compared than soils with low levels of organic matter.
So grow green manures, dig in plenty of compost and well rotted manures, or simply layer them under the mulch for the worms to dig in for you.
Overhead sprinklers, once such a common sight in every backyard, now look like dinosaurs. Yes, they were fantastic to play under, but so wasteful. Sprinklers deliver water so that is simulates natural rainfall. As each water droplet flies through the air, on a hot day, up to 45% of it can be evaporated before it reaches the ground. Even on relatively still days sprinklers mounted on tall stands do not deliver an even amount of water to the garden bed – much is blown onto paths, or just one part of the garden by the wind.
Therefore it makes sense that the closest we can apply the water to the soil, the more efficient our water use will be.
Dripper systems and micro-sprays have been around for a while. Micro-sprays are really just sprinklers in miniature, and are extremely wasteful due to their fine spray. Drippers and leaky/weepy hoses are the most efficient. Drippers that are placed above the garden bed compact the soil.
Drip systems and certainly weepy hoses should be laid on the surface of the garden bed and covered by mulch. This way evaporation is minimised, as is the likelihood of soil blocking up your system.
*The variation between litres required, for supplemetary watering per square metre, are influenced by soil type, levels of organic matter in the soil, evaporation rates and annual rainfall. Be sure to provide windbreaks, add organic matter to the soil and mulch well to maximise the soil’s water holding capacity. Always supply water directly to the soil underneath the mulch, preferably in the evening or early morning.