For a snapshot on salinity and diagnosis refer to the MyCrop salinity fact sheet. More detailed information is provided below.
Where does salt come from?
Soil salt comes from three main sources:
- salt in rainwater derived from the ocean
- the breakdown of parent rock – a very slow process
- prehistoric flooding by the oceans – only on discrete areas.
In most of Australia, rainfall is the source of stored salts responsible for salinisation. Salt in rainfall can range from more than 200 kilograms per hectare per year (kg/ha/yr) in coastal areas with high rainfall, to less than 20kg/ha/yr in northern inland areas with low rainfall (Hingston and Gailitis, 1976)
Before European settlement and over tens of thousands of years the perennial native vegetation used most of the rainfall, leaving behind the salt to accumulate in the subsoil and regolith (all materials between fresh air and fresh rock).
Stored salt – how much is down there?
Salt is stored in the landscape when input (through rainfall) exceeds loss through leaching or drainage from the catchment. Where potential evaporation is high and rainfall is low (semi-arid and arid zones), salt falls on the landscape but is not flushed out. It therefore accumulates below the root zone of vegetation.
Accumulation of salt is more noticeable in a flat landscape where there is no clear exit for groundwater or surface water to drain. It also accumulates in landscape sumps where most drainage is internal (i.e. groundwater cannot exit the catchment and moves to a low point such as a salt lake). This type of landscape describes most of the Western Australian wheatbelt (the zone of ancient drainage). Accumulation of saline surface water can also create similar features.
Where the regolith is deep, salt storage can rise to thousands of tonnes per surface hectare. With an input of 50kg/ha/yr of salt and limited flushing, 1000 tonnes per hectare could accumulate in 20 000 years. Western Australian landscapes have been stable for many times more than 20 000 years.
In the higher rainfall and more dissected land to the lower south west (zone of rejuvenation), leaching has removed more of the salt, and flushing of salt following clearing is an active process.
Where does the water come from and go to?
Water in a catchment is part of the hydrologic cycle, which is a continuous interchange of water between land, water bodies and the atmosphere. Annual rainfall provides nearly all of the water input to dryland catchments.
In some cases, localised flooding or inundation from rivers and streams flowing through the catchment will be important recharge sources. Irrigated catchments can have a large proportion of water, which is involved in salinisation and derived from irrigation supplies.
Water entering a catchment can:
- be evaporated from water storages, plant foliage or the soil surface
- run off the surface and exit the catchment
- add to surface water storage (lakes, dams)
- enter the soil and then be transpired by plants
- move deeper and recharge a groundwater system (add to storage) – seen as a rising watertable
- leave as groundwater flow and/or groundwater discharge.
Catchment water – balancing water in and water out
The concept of a catchment water balance is a simple accounting exercise – what comes in (rainfall) must be balanced by what goes out (run-off, evaporation and transpiration, groundwater flow and discharge) plus any change in storage (soil water, surface water and groundwater).
The change in groundwater storage is partly related to the amount of groundwater recharge, and assuming groundwater flow and the seasonal changes in surface and soil water storage are minimal in relation to the other components (run-off, evaporation and transpiration), then a more simplified water balance is where rainfall equals evaporation and transpiration, groundwater recharge and surface run-off.
Clearing native vegetation for agriculture has significantly altered the catchment water balance. Groundwater recharge and run-off have increased and plant water use (evapotranspiration) and interception have decreased (Table 1).
|Catchment type||Rainfall||Run-off||Evaporation & transpiration||Interception||Groundwater |
|Uncleared (native vegetation)|| |
|Cleared (crops and pastures)|| |
Recharge is the water that reaches the watertable (groundwater) and is usually measured in millimetres. In dryland catchments the recharge water comes from the component of rainfall that drains below the root zone and percolates to the watertable. Average recharge rates over dryland catchments are typically 3-10% of average rainfall, although this varies widely.
Two modes of recharge occur: matrix and preferential or preferred pathway recharge.
- Matrix recharge is where the water recharging the watertable flows vertically around soil particles, is relatively evenly distributed horizontally and infiltration occurs across a vast area.
- In contrast, preferential or preferred pathway recharge is where recharge water flows through discrete pathways that are not evenly distributed and infiltration occurs only at specific points within the soil profile or landscape.
Recharge in a given year is strongly affected by rainfall distribution. Wetter than average years have a higher percentage of that rainfall going to recharge and drier years have a lower percentage. Heavy and out-of-season rainfall may also dramatically increase the percentage of rain going to recharge which means that recharge tends to be strongly influenced by the season and will often give 'step' rises in watertables over a run of years. In contrast, long dry spells may enable watertables to fall back to much lower levels.
As our understanding of hydrology of the agricultural areas has improved, the area thought to contribute recharge in WA has expanded from the coarse-textured, sandy areas to almost the entire catchment. In some areas of a catchment such as a valley floor, groundwater recharge can occur during winter and the same area can discharge groundwater during summer.
Groundwater moves by slowly percolating through gaps (pores) in soils and rocks. The groundwater system is often called an aquifer. An aquifer is very rarely present as an underground stream and only in very specific types of aquifer systems (e.g. limestone on the coastal plain) do underground streams exist.
The rate of groundwater movement depends on 3 main factors:
- Hydraulic conductivity. The ability of the soil or underlying materials to carry or transmit water. In well-sorted sands and gravels, hydraulic conductivity is high (water has potential to move metres a day), while in clay, it is generally very low (moves less than 1mm/d). Soil acts like sponge in a pipe.
- Hydraulic gradient or pressure. Groundwater moves from areas of high to low pressure, along paths of least resistance. Water in a saturated aquifer can be thought of as acting like water in a pipe - water added to the groundwater at an elevated point (recharge) can push water out the end of the flow system almost immediately. Actual groundwater movement is usually slow, limited by the aquifer materials' hydraulic conductivity and the groundwater gradient.
- The area available for flow, in relation to the thickness of underlying materials across the catchment to provide an area for the water to flow through. This cross-sectional area – the saturated thickness – is measured across the flowpath of the aquifer, from the bottom of the aquifer (usually the bedrock) to the top of the aquifer (the watertable).
Groundwater discharge is the process where groundwater exits an aquifer. When groundwater discharges, the watertable is at or very near the surface. Discharge areas are sometimes referred to as seeps or seepages. Most saline areas have a discharge area within or close to them.
Australian researchers developed a national framework to broadly classify salinity discharge and groundwater flow processes according to which of 15 hydrological models are operating, and whether they represent local, intermediate or regional groundwater flow systems. The classifications examine the groundwater processes related to salinisation and the scale relationships between groundwater recharge and discharge.
Groundwater systems and aquifer types
Groundwater flow systems
Groundwater processes causing salinity can be categorised according to their flow systems because the scale (local, intermediate or regional) of the flow systems reflects the ease with which salinisation can be managed. On the basis of size and responsiveness, aquifers have different classifications:
- Local flow systems – recharge and discharge are close to each other (within 1–3km) and groundwater levels equilibrate quickly (10–100 years) after disturbance such as clearing. Localised flow systems are discontinuous and commonly overlie an intermediate or regional flow system. Most of the hillside aquifers in the wheatbelt are local flow systems.
- Intermediate flow systems – recharge and discharge may have a horizontal extent of 5–10km and generally occur across the entire catchment. They have a higher storage capacity than local flow systems and take longer to equilibrate. In the wheatbelt, intermediate systems are uncommon, although may occur in areas with palaeochannels (buried, prehistoric drainage channels).
- Regional flow systems – recharge and discharge may be separated by many tens to hundreds of kilometres. Groundwater movement may be independent of local topography (sub-catchments), involve long flowpaths and are typical of large sedimentary basins. Groundwater levels outside the recharge area are slow to respond and equilibrium may take many hundreds of years. The Perth Basin contains regional aquifers. Regional aquifers often contain local and intermediate flow systems.
Depending on the regolith materials, aquifers can be:
- confined – groundwater is confined between two impermeable layers
- unconfined – groundwater is unrestricted and rises and falls freely
- semi-confined – groundwater is only partially restricted by low-permeability material, and can leak out
- perched – groundwater sits on top of an impermeable or less permeable layer and are often fresh.
Most aquifers in the wheatbelt are semi-confined to unconfined.
For more information
McFarlane, DJ, George, RJ and Farrington, P 1993, ‘Changes in the hydrologic cycle’, in RJ Hobbs and DA Saunders (eds), Reintegrating fragmented landscapes towards sustainable production and nature conservation, Springer-Verlag, New York.
Hingston, FJ and Gailitis, V 1976, The Geographic Variation of Salt Precipitated over Western Australia, Aust. J. Soil Res., 14, 319-35, viewed on 16 November 2016 http://www.publish.csiro.au/sr/pdf/SR9760319