Grassland ecosystems key to watertable control
In the last issue of Ground Cover (no. 35), we started a forum on salinity policy and solutions. As promised, this issue offers for discussion the alternative model for understanding the causes and management of dryland salinity, developed by rangeland ecologist Christine Jones.
This summary of Dr Jones’ published articles was written by Brett Bartel, Revegetation Consultant, PIRSA Rural Solutions, SA Department of Primary Industries and Resources, and appeared first in The Long Run, Eyre Peninsula Landcare publication, South Australia.
For many years we have accepted the general recharge-discharge model for the management of dryland salinity.
Now through the work of Christine Jones comes an alternative view, opening our eyes to the fact that perhaps we have not thought about salinity management enough.
Dr Jones, of the NSW Department of Land and Water Conservation, explains that our poor success rates in combating dryland salinity are due to the use of a flawed recharge-discharge model. Dr Jones believes that “many incorrect assumptions have been made about dryland salinity, and therefore many ill-conceived and simplistic solutions have been proposed”.
Many people believe that the native vegetation of Australia consisted of wall-to-wall trees and shrubs. The landscape actually consisted of a variety of vegetation types and, contrary to popular opinion, much of it was widely spaced trees or shrubs with a grassy understorey. There were also areas of native grasslands with no trees.
It is difficult for many to understand this because the grassy ecosystems were easily transformed and were the first vegetation type to disappear. In addition, many of the native grassland species have been selectively grazed out from woodland habitats due to their high palatability.
Good-quality native grasslands now exist only in remnants on non-arable land that have not been subjected to continuous grazing, or in areas regenerated through pulse grazing.
Native grass root systems hold it all together
Many people working with the recharge-discharge model have overlooked the value of native grasses. Native grasses have extensive fibrous root systems and in healthy perennial grasslands the soil is well mulched with very little bare ground.
When there is an absence of good ground cover on sloping ground, water runs straight down the side of the hill and recharges the lower parts of the catchment.
The rainfall does not go directly into deep drainage at the tops of hills as the recharge-discharge model implies. Even when there are trees and shrubs planted on these hills, water still runs down the side of the hill due to a lack of surface cover (refer diagram).
In a healthy catchment the salt in the soil profile is slowly leached downwards and stored below the root zone. We therefore need a little bit of water to be flushed downwards in the soil profile to take the salts below the root zone, while not adding significantly to the groundwater recharge.
Dr Jones explains that in order to do this we need a permanent ground cover of plants that enhance the infiltration of rainfall, have fibrous root systems with high water-holding capacity but low water-use (thus holding water in the root zone for both plants and soil biota) and which facilitate the very slow percolation of some rainfall to groundwater.
Perennial native grasses fit this requirement perfectly. Given that they constituted most of the original ground cover, this explains why the water cycle was reasonably balanced at the time of settlement.
Trees alone not the answer
Planting tree-lots will not restore water balance. Recent research has shown that plantation tree-lots use the fresher water from the top 1-3 metres of the soils, leading to an upward movement of the saltier groundwater from below. In fact, high-water-use plants can over-dry soils. Rain simply runs off the surface of dry, poorly mulched soils or enters the watertable through deep cracks.
Dr Jones’ theory is that we don’t have to increase transpiration rates in order to reduce deep drainage. A better result can be achieved by increasing the waterholding capacity of the soil.
In summary, this line of thinking about salinity is highlighting the fact that we need to understand what the vegetation was like prior to European settlement and how the water was balanced. Bare ground, annual crops and pastures, and continuously grazed perennial pastures result in excessive groundwater recharge.
High-water-use plants such as trees can be useful for lowering watertables, but they concentrate salt in the root zone. What is required is permanent cover of native perennial grasses and associated organic matter to hold moisture in the root zone and allow its slow percolation to aquifers which feed springs and streams.
Better-quality native grasses have high potential for grazing as well as forming a living ground cover base for annual pasture cropping. Their reinstatement promises a productive future for agriculture and an important role in recharge management.