Revegetation, using trees and other deep-rooted perennials such as lucerne, is one of the tools being employed by landholders to reduce groundwater levels and the impact of salinity.
As with any management system, it is important to evaluate progress by objectively measuring key indicators such as the area of salinity or the process driving salinity - the depth to watertable.
Land affected by salinity is difficult to measure objectively but groundwater levels are monitored easily and will show clearly the success or otherwise of the perennial system implemented.
Monitoring groundwater levels is a long-term undertaking so it is important to design a system to ensure adequate measurement of the full impacts of the perennial system.
A well-designed monitoring system can provide important information about changes to the groundwater system away from the perennials to define local trends and risks.
Installing a monitoring system prior to planting can provide valuable information on the soil and groundwater conditions that the vegetation will have to contend with e.g. watertable depth, salinity, depth to rock and presence of hardpans.
This will help to ‘fine tune’ the final extent of the vegetation, choice of species, placement or other management options.
Ways to monitor groundwater:
The most common and practical way to monitor groundwater is to use observation wells or piezometers (bores) installed into the watertable. Piezometers differ from observation wells in that they measure the pressure of water at a specific depth. Piezometers have a seal (usually bentonite) placed directly above the slotted section of the casing beneath the watertable.
Observation wells have a slotted section that extends from the bottom of the bore to above the watertable and have no seal. It is recommended that piezometers rather than observation wells are used in vegetated areas.
If observation bores are used in vegetated areas, particularly if the slotted section extends above the watertable, root invasion into the casing and eventual blockage may occur.
- installation of groundwater monitoring wells/ piezometers;
- monitoring procedure; and
- data collection, storage and analysis Placement and number of bores
Basic system:
The most basic system is a shallow bore placed in the middle of the planted area together with a shallow ‘control’ bore located in an adjacent, non-vegetated area (at least 100 m from the treatment) in similar landscape position. Ideally, bores should be installed before planting the perennials to set the baseline depth to watertable and groundwater salinity.
This design compares groundwater level trends between treatments to detect their impact. Addition of a deep piezometer at each site will provide important information about the depth to basement rock and define the underlying groundwater conditions that could be important in analysis of observed trends.
However this approach will not define off-site impacts of the perennials, nor will it provide early warning of groundwater trends upslope (which may impact on the treatment in the longer term).
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