Jones C.E. (2000). Stipa Newsletter No. 14, pp. 6-11
Reprinted in the Holistic Management™ Australian Quarterly Newsletter, Sept/Dec 2000
Reprinted in The Australian Farm Journal, January 2001, pp. 58 - 61
The Great Salinity Debate: Part II
Why the recharge-discharge model is fundamentally flawed
The recharge-discharge model which has been used to describe the changes in water balance since European settlement is based on false assumptions concerning i) the nature of pre-European vegetation and ii) the way water moves in the landscape. The use of this flawed model as a basis for strategies to combat dryland salinity underpins the poor success rates achieved to date.
We hear a lot about the clearing of native vegetation in relation to dryland salinity. Most people assume that the words "native vegetation" mean "trees and shrubs". Contrary to popular opinion, the historical record clearly shows that in the early years of European settlement many of the higher rainfall areas of temperate Australia were grassy woodlands, that is, widely spaced trees with a grassy understorey.
The explorers and early surveyors described the richness and diversity of this vision splendid, with grasses frequently up to their horses' bellies. Many of the hills were recorded as being grassed to their summits, having only thinly scattered trees, or being treeless. The descriptions of the grassy vegetation were remarkably similar across the temperate parts of eastern, southern and south-western Australia, and the comment was invariably made that, unlike many parts of America where clearing was a pre-requisite, here most of the land was immediately ready for grazing or the plough.
Early settlers could not have anticipated the rapid deterioration in the quality and diversity of groundcover and the decline in soil quality that accompanied European style grazing and cultivation. In parallel with the loss of grassland habitat was the extinction of 20 previously common species of small marsupials and the near extinction of a myriad of others. The significant role that these native fauna played in soil enhancement is not widely recognised. In combination with the cessation in aboriginal burning and soil disturbance regimes, the widespread loss of the thousands of small animals that loosened soil, buried organic matter and consumed emerging tree seedlings, produced massive changes to the ecology of the Australian landscape. So much so that today's "remnant vegetation" probably bears little resemblance to the plant communities in existence 200 years ago.
Europeans were caught unawares by the sudden explosion in the numbers of trees and shrubs which followed settlement. In 1848, Thomas Mitchell, Surveyor General for NSW, described "thick forests of young trees, where, formerly, a man might gallop without impediment, and see whole miles before him". Observations of regrowth were reported many times thereafter by other observers across southern Australia. For example, Howitt (1890) described the tree regrowth in Victoria "…After some years of occupation, whole tracts of country became covered with forests of young saplings…and at present time these have so much increased, and grown so much, that it is difficult to ride over parts which one can see by the few scattered old giants were at one time open grassy country". Subsequent generations found it necessary to clear this regrowth in order for agricultural activities to proceed.
The changes in the quality and quantity of the groundcover since European settlement have had enormous implications for water balance in the Australian landscape. The diverse perennial grassland communities which proved so productive for early settlers could respond to rain at any time of the year. Furthermore, the soil organisms which proliferated in response to the high root biomass and the activities of the grassland fauna, produced humic materials and microbial gums which glued soil particles together, creating a crumb structure which resisted erosion. Soil microbes also produced plant growth hormones which stimulated root growth and enabled plant roots to penetrate clay subsoils. The many pore spaces in these healthy, living soils enabled them to hold large volumes of water.
The movement of water in the landscape
Dryland salinity is the result of a water cycle that is out of balance. The salt is an unwelcome fellow traveller with rising groundwater, and even though serious in its own right, salinity is merely an indicator of a more deep-seated problem. It is therefore extremely important that we look very carefully at what is happening at the landscape level, sooner rather than later.
In comparison with pre-European times, there is now LESS water entering aquifers in the HIGHER parts of the landscape (and hence LESS fresh groundwater available to feed springs and streams), MORE runoff and lateral subsurface flow on undulating country (which may be intercepted by dams and contour banks and may not necessarily reach rivers other than in periods of high rainfall) and MORE recharge to water tables in the LOWER parts of the landscape (Fig.1, Part B).
This is almost the opposite of the widely accepted recharge-discharge model on which most salinity "solutions" are based. The recharge-discharge model depicts MORE water entering deep drainage in the higher parts of the landscape with the removal of the original native vegetation, which is assumed to be trees, which in turn are assumed to be deep rooted. This excess water then apparently travels underground, collecting salt along the way, to emerge as discharge at the break of slope or in low-lying areas (Fig. 1, Part A). Although the model appears seductively simple, there are no biological or physical mechanisms by which these processes can occur at the landscape or regional scale.
"Recharge" in the upper catchment
Imagine that you're standing on the side of a fairly steep hill in the pouring rain. The hillside is completely bare. Where does the water go? Straight down the side of the hill, taking soil with it. Not directly into the soil and into "deep drainage" as the recharge model tells us will happen if there are no trees. Any water that does infiltrate will also run downslope on top of the subsoil as lateral flow, under the force of gravity. If there are rocky outcrops, some water will seep through cracks, but this will only account for a small percentage. The remaining water has no mechanism for becoming recharge until it reaches the lower parts of the catchment.
Now imagine that there are trees on the hill, but no grasses or other groundcover. Where does the water go? Again, straight down the side of the hill, perhaps a little more slowly. If there's leaf litter, at least some of the rain will infiltrate, but it will then also travel as lateral flow unless the soil is high in organic matter.
Finally, imagine that the hill is covered with dense tussocky perennial grasses which have deep, fibrous root systems. The soil is well mulched and you can't see any bare ground. Where does the water go? The V-shaped grass architecture, in combination with high levels of organic matter both in soil and on the friable soil surface, will facilitate the rapid infiltration and storage of rain as it falls. The chance of water moving downslope will be significantly reduced. The water held in pore spaces between soil aggregates in the root zone will be available for later use by the grassland plants and the soil community of invertebrates and microorganisms.
A small amount will slowly percolate through the subsoil (or enter cracks in the parent material) and provide clear, filtered water for springs and streams. It is extremely important for future generations that this process continues. When the water runs on the top of the ground instead, or on top of the subsoil, we get into the all too familiar flood/drought cycle, with rivers carrying either too much or too little water, while freshwater aquifers are shrinking.
Recharge in the lower catchment
The conventional recharge-discharge model has provided landholders in the lower parts of the landscape with a scapegoat for their own inappropriate (although unintentional) land management practices. Where there are annual crops or pastures, or where perennials are overgrazed, enormous amounts of water enter the groundwater below the break of slope. Despite this, the tendency has been to point the finger at others higher in the catchment and blame them for all the recharge.
Certainly, some water has travelled downslope, but the lower parts of the landscape normally account for the major portion of the total land area, as well as for most of the recharge if conventional cropping or conventional grazing are the major land uses. The fact that the eruptions of saline water are often at the break of slope doesn't necessarily mean that all of the water came from above - it simply means that the rising groundwater put backward pressure on any water moving downhill and there was nowhere else for it to go. This phenomenon can be demonstrated by placing a piezometer above the high water mark on the beach. As the tide comes in, the water level rises in the tube. If you were only observing the water level in the piezometer and couldn't "see" the tide coming in, it would be natural to assume that the water had moved downslope from the sand dunes behind.
In the lower parts of the landscape, fibrous-rooted perennial grasses and associated organic components will again hold most of the rainfall in the root zone, where it can increase the productivity of a wide range of enterprises. Remember, a pulsed grazed native pasture base will be more nutrient and water efficient than a high input introduced pasture and will complement, rather than compete with, pasture cropping, viticulture, horticulture or silviculture. If the main land use is grazing, a diversity of cool season (C3) and warm season (C4) perennial native grasses will provide year round productivity, stability and drought tolerance, provided the management is appropriate (refer Part I this series). A small amount of water will still go through to deep drainage, but that's what was happening 200 years ago.
The rate of movement of water in underground aquifers depends on many factors, but in most situations takes between 300 and 1000 years to travel one kilometre. For water to travel 50 km underground could take up to 50,000 years. If you have saline discharge on your property, the chances are that recharge also took place there. The good news with respect to this local hydrology scenario is that landholders can have some control over their own destiny where dryland salinity and other land degradation processes are concerned.
In some places freshwater aquifers are drying up while saline water tables are expanding. How could those two things be happening at the same time? It can be explained quite easily if the recharge-discharge model is in fact upside down. The conventional model states that recharge occurs high in the catchment and discharge occurs lower down. The available evidence suggests that there is very little true recharge at the top (albeit too much lateral flow, which adds to the discharge at the bottom) and that both recharge and discharge are occurring in the lower parts of the landscape. Unfortunately this has resulted in some of the freshwater aquifers beginning to backfill from enlarging saline aquifers below.
The current situation
The recharge-discharge model as shown in Fig.1 (Part A) is being taught in schools across Australia today. A whole generation of children will grow up believing that it is their duty to plant trees in the upper parts of the landscape to "prevent recharge". Meanwhile, dryland salinity will continue unabated.
Furthermore, our children are being led to believe that all trees have deep tap roots, as depicted in salinity models. The tap root of the seedling tree degenerates over time, and although some fine roots may occasionally follow rock fissures, most mature trees of the species commonly found on hillsides do NOT have a tap root. More usually, up to 90% of the root mass is concentrated in the top 50 cm of the soil profile. Once the water has run off a hillside covered in trees, there is no way the trees can get it back.
The recognition of urban salinity as a mostly local hydrological phenomenon has clearly demonstrated that we don't need a fool on the hill, or even a hill, or even an agricultural landscape, to encounter water balance problems. In the urban context, dryland salinity results from the combined effects of activities such as watering shallow rooted lawns (all short grasses are shallow rooted) and rain falling on impermeable structures such as rooftops, paths, driveways and roads, and becoming runoff. That is, urban salinity is the result of excessive runoff added to excessive recharge in situ.
I fail to see much difference between this and the expression of dryland salinity in agricultural landscapes. Planting trees on a hill 20 km away will do little to resolve the problem in either the agricultural or the urban context. Trees and shrubs form an integral and ecologically valuable component of grassy woodland vegetation and I am by no means dismissing their importance. My concern is with the promotion of broadscale tree planting (mostly same-age monocultures) as a panacea, not only for dryland salinity, but for all land degradation problems. In a healthy perennial grassland soil, there may be 50 tonnes of biomass (roots, soil organisms and humic materials) below ground for every tonne of biomass above ground. In forests, there is far more organic material above ground than below. The fact that we can only see the biomass above ground may explain the distorted image many people have of these respective plant communities.
We certainly do have to mimic the native vegetation to restore hydrological balance, but let's get the facts right. The vegetation of the temperate zone was almost exclusively perennial 200 years ago, but Australia was not a forest. The majority of aboriginal people were not forest dwellers. Neither do we have to be. How many rural communities will be lost in this mad rush to return Australia to a land of trees we never had?
The aboriginal people lived in a diverse and dynamic grassy ecosystem. So can we. Grasslands produce more food than forests and the intuitive response would be to manage the landscape to favour grassland species. To refer to the pre-European vegetation as "natural" or "pristine" totally ignores thousands of years of prior habitation, exceptional observational skills and active management to achieve desired outcomes. Australia has been mismanaged for the last 200 years. Now it's crunch time.
In our low and variable rainfall environment, the increasing reliance on high water use plants or engineering solutions to "dewater" soils makes neither ecological nor economic sense. We can restore water balance and improve soil health, nutrient cycling and productivity if current agricultural and horticultural activities are conducted in an appropriately managed perennial groundcover base. That's the topic for the next issue.
The compilation of this article would not have been possible without the vision, critical thinking, historical perspective and encouragement provided by many people. While all errors in interpretation are mine, I am particularly indebted to Allan Savory, Darryl Cluff, Greg Martin, Stephen Hailstone, Wal Whalley, Pam McGregor and Bruce and Suzie Ward.
A: the widely promoted recharge-discharge model
(Thanks to Australian Farm Journal for use of these diagrams)
Prior to Settlement Today
Deep rooted trees USE large amounts of water and keep water table in balance. Tree removal results in water table recharge in upper catchment and discharge in lower catchment.
B: a more realistic model
Prior to Settlement Today
Healthy perennial groundcover, fibrous root mass and associated humic materials HOLD water where it falls with some percolation to groundwater. Loss of healthy groundcover in both upper and lower catchments results in more runoff, lateral flow, recharge and discharge. Replenishment of elevated aquifers may be reduced.