Water sustainability and organic agriculture

Most Australians are wasteful of water and treat access to plentiful water as a right. We think little about our water consumption and less about reducing our use. Despite recent improvements in awareness of water sustainability, there is still very little recycling of grey water (water used in the home, excluding water used in the toilet, which is black water). But in reality the fresh water resources of Planet Earth are a rare and enormously valuable resource.

Of all the water on earth, 97% is in the oceans, 2% is contained in glaciers and only 1% is on land. Of the 1% on land, 97% is below the surface, as groundwater, and more than half of that is out of reach.

Of the global reserves of fresh water, 0.06% is in soil moisture, 0.3% is in lakes, 0.03% is in rivers and 0.03% is in the atmosphere as water vapour. Only 11.39% of global freshwater is accessible, non-saline water.

Of the liquid fresh water found at the surface, 30% is in lakes in Africa, 25% in lakes in North America, 18% is in Lake Baykal (Russia) and 27% is in smaller lakes and rivers elsewhere in the world. Rivers are really insignificant in global terms. They carry only 1,200 km3 compared with 125,000km3 in fresh water lakes and inland seas.

Human requirement for water

The human body is 60% water. The required daily intake to maintain life and health is about 2.25litres, or 1 litre per day just to stay alive. Household use, which includes washing, food preparation and cooking, waste disposal and sewage, but not garden use, varies greatly according to availability and cultural expectation. In Karachi it is around 90litres per day, in London 435litres, 635litres in the USA and 170litres in Adelaide (or 62,000litres per year).

Crop production opportunities increase dramatically as access to water improves. For instance it takes nearly 500litres of water to produce 1 kg of dry, non-irrigated wheat, 16000- 2,200litres to produce 1 kg of rice and 4,000litres to produce 1 litre of milk.

A pig with a body weight of 60 kg requires 25 litres of water per day and a lactating Jersey cow requires 45 litres per day to produce 3-12 kg of milk. A sheep on very good pasture needs hardly any water, but on dry range needs between 3 and 6 litres per day, or 8 litres if it is eating salty browse.

A large paper mill uses as much water as a city of 50,000 people and a power station uses 600 tonnes of water for every tonne of coal burnt.

Water in Australia

It is often said that Australia is the driest continent. This is because the land surface in Australia receives a lower average rainfall than any other continent, other than Antarctica, where most precipitation falls as snow. Average rainfall in Australia is 460mm, compared with the world average of 660mm. Also, in Australia we have a hot, dry climate with plenty of sunlight, which results in 87% of precipitation evaporating. For instance, SA receives 7.1% of the precipitation falling on the continent but produces only 3.4% of the runoff, however almost all of this is from the River Murray and the Glenelg River (i,e, the water originates outside of SA). The runoff from all other streams combined is only 0.43%. Of the remaining 13%, 10.6% is discharged from rivers and 2.2% ends up in groundwater. A remarkable 50% of surface runoff is flow to the oceans from Tasmania, or into the Gulf of Carpentaria or the Timor Sea.

Potential evaporation (i.e. the amount that would evaporate from an exposed water surface) in central Australia is 4,500mm, or more than 20 times the rainfall. Three quarters of the continent has evaporation rates higher than 2,500mm.

Underground water

Much of the underground water is on its way back to the ocean. It seeps into the soil profile after precipitation, or from streams and lakes, and begins a slow journey through soil and porous rocks, back to the sea. Some is trapped by impervious layers, and forms a basin. A little very old water was trapped in sedimentary beds during formation, and this is called ‘connate’ water.

When it first infiltrated into the soil, a considerable amount of water is retained in the spaces between soil particles. This portion also seeps gradually downwards, under the influence of gravity, and some eventually becomes groundwater. Some is evaporated from the soil surface or used, and eventually transpired, by plants. Even when soils look dry, they have ‘hydroscopic water’ – a very thin layer around individual soil particles, held there by surface tension forces (remember the meniscus in the glass, from school science lessons). This water cannot be squeezed out of soil, and to get soil completely dry (say for laboratory analysis), it is necessary to bake soils in an oven. Hydroscopic water is very important for plants. They can extract it, with their fine root hairs.

Permeability of soil and bedrock depends upon porosity, or the total amount of pore space and the ‘connectedness’ of pores, to allow water to flow through the medium. Soft rocks like sandstone can be 30% or more porous, and therefore they permit good water storage and flow.

Soil and water storage

The best place to store water for crops is in the soil. Good soil is full of shrinkage cracks, wormholes, root holes, spaces between crumbs (or lumps) of soil and very fine spaces between the individual particles. The actual mineral part of a good soil is only around 45%. Organic mater accounts for 3-5% of soil. The remainder is air spaces or pores. After a saturating rain these pores are filled with water. If there are plenty of larger particles (i.e. sand) and aggregates (or lumps) and there is good connectivity between the pores, some water will quickly infiltrate into the deeper layers under the influence of gravity, leaving about half the soil pores (or 25% of soil) as soil air. Ideally there will be some smaller particles (i.e. silt and clay) to hold onto or trap soil in the smaller spaces between them, and on the surface of particles (hydroscopic water).

In normal conditions sandy soils drain quickly, because they have larger particles, that stack together with larger and better connected pores between them, and they require more frequent water applications. Clay soils trap and hold more water for a longer time, and require fewer applications. Clay particles hold onto much more water due to the very small spaces between them (small channels impede gravitational flow) and the very large surface area of particles that have hydroscopic water.

Humus particles are also very good at storing soil moisture. They have an open, lattice structure that traps water, and organic matter also holds hydroscopic water. Small improvements in soil organic matter can result in a very large increase in water storage (see the table below).

Water use on organic farms

There is ample anecdotal evidence and some experimental evidence that organic farms use less water than conventional farms. Anecdotal evidence from many growers, especially irrigation users, indicates that organic growers apply water less often than their non-organic neighbours. It is reasonable to assume that this is largely due to increased soil moisture storage in soils well supplied with organic matter, and to common cultural practices on organic farms, such as use of mulch. Mulch may directly limit evaporation from the soil surface and prevents soil from forming a water-resistant crust, but also allows plant roots to more effectively use the top five centimetres of soil, by keeping them cool and protected from bright light. Better water use on organic farms may also be due in part to greater awareness of water management issues by organic farmers, including plant requirements and irrigation programming, and a greater willingness to directly observe soil moisture, or to use remote sensing devices to help manage irrigation.

There is some experimental evidence from surveys of organic farms, such as the one that compared conventional and biodynamic dairy farmers in northern Victoria, conducted by the Victorian Department of Agriculture in the early 1990s. Total water use on biodynamic farms averaged 323 Ml/ha/year, compared to 410 Ml/ha/year for the conventional farms. Although total production on BD farms was less (3,436 litres/cow/year, compared to 4,585) and herds were slightly smaller, irrigations were spaced considerably further apart, at 15 days rather than 9.

We may also reasonably speculate that avoidance of very-soluble fertilisers and long-lasting pesticides on organic farms causes organic growers to contribute less to off-farm water quality issues than their conventional neighbours. Again we could assume that significant awareness of environmental impacts, combined with (at least) annual inspection of environmental performance indicators on certified farms will ensure that the most unsustainable practices do not occur on organic farms and that most organic growers are not causing major water quality issues downstream.

Water use by plants

Vegetation transpires at least 100 times more water per annum than is present in the plant as biological water. This figure varies greatly with different plant types, according to their specific adaptations for preserving water. For instance, drought-adapted plants have a much greater capacity to close down stomata (pores on the leaf surface) to limit transpiration. Plants are therefore a type of water pump. They move large volumes of water because they use it as a medium to extract dissolved nutrients, and as an evaporative cooling mechanism – a room with plants will always feel cooler. A plant with a large leaf area, such as a mature apple tree, is therefore capable of removing many tonnes of soil water during the growing season.

Plants have many different adaptations to help them obtain, and conserve moisture. A grape vine, for instance, may put down water-seeking roots to a depth of 40metres. In Princess Margaret Rose Caves, on the Glenelg River, there are roots 50mm across nearly 50metres below the surface. Presumably they are seeking groundwater at the level of the river or below, still nearly 10 metres lower, but the trees on the surface are less than 15metres tall.

While plants may have deep roots for survival, they will generally prefer to seek water at or near the soil surface, where air and nutrients are readily available. If there is not a continual input of water into soil, plant roots will dry out the soil around their main feeding roots. Some of this water can be replaced by capillary action (water seeping upwards through the soil pores), but there is always a tension between the two forces of gravity (pulling water down) and capillarity (pulling water up).

Irrigation management

Flood irrigation is the oldest and still the most common type of irrigation. Flood irrigation has two advantages. It is easy to manage (low technology) and most of, or the entire root zone, of the plant is watered. However flood irrigation is wasteful of water and is the major cause of environmental damage in the Murray-Darling Basin (the main irrigation areas of Australia are within the MDB). The reason for the wastefulness and damage caused by flood irrigation is the same. Unless the irrigation bays are very short and the delivery time for water is also very short (i.e. there is very good pressure) the water at the start of the irrigation bay will have infiltrated below the main root zone of the plant (and therefore be effectively wasted, or unrecoverable by the crop) before water has reached the end of the bay. Flood irrigation can be slightly improved by delivering water in surges, so that the wetting front has time to cause soil to expand and pore spaces seal, thereby minimising soakage at the start of the bay.

Sprinkler systems can be much more efficient, depending on irrigation timing and management. Short irrigation cycles, especially during the hot part of the day, result in 80% of water being lost to evaporation or by blowing away on the wind. It is probable that 40% of all water applied to lawns in the Adelaide suburbs is lost to evaporation, even accounting for nighttime irrigation. Very long irrigations cause water to soak past the root zone. Remember too, that even after the system is turned off, water will continue to soak through the soil profile.

Drip irrigation is now considered much more efficient than the systems mentioned above. One significant advantage is restriction of water to weeds in the inter-row area. The main limitation on efficiency of drippers is the narrow zone of distribution of water, which results in plants having very restricted root zones. There may also be increased salinity at the margin of the wetted rim. Drippers are also sometimes mismanaged, by having a long duration of operation and inadequate frequency of irrigation.

Subsurface irrigation is increasing in popularity and has several theoretical and practical advantages, especially now that new pipe designs have been developed, with holes that do not easily become blocked. It is possible to operate subsurface irrigation for very short intervals, because the pipe is already located within the root zone, so effective delivery of water starts immediately. Other advantages are: there are no surface pipes to tangle equipment, there are no evaporation losses, no water is present at the surface to encourage weeds and irrigation can occur simultaneously with other management activities on the surface (eg pruning, harvesting etc). The main disadvantage of this system is that it is prone to mismanagement, as it occurs almost entirely out of view of the operator. Water is most effectively delivered in subsurface irrigation if it is delivered at high flow rates, in short bursts, to encourage lateral distribution rather than deep penetration.

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