Farm Table says:
Evaluating the impacts
What is the problem?
This research evaluated the impact of deep, open drains on groundwater levels, soil root zone salinity, crop productivity, drain design and effectiveness, and the environment.
This research delivered a detailed and comprehensive assessment of the on-farm deep, open drainage along with assessment of the quantity and quality of flow from the drainage system.
What did the research involve?
Six sites in WA (Deluis, Pini, Latham, Town in the drained areas and Bailey, John Deluise in the undrained areas (future drain) of the sub-catchment) were selected.
All sites, in the drained areas were set up with instruments during 2000. Partial set up of the Bailey site was carried out in 2000. Bailey and John Deluis sites were fully set up with instruments in early 2001.
The instrumentation included transects of shallow and deep piezometers for monitoring shallow and deep groundwater levels, stream gauging stations for monitoring flow rates, automatic water samplers for monitoring drain water quality, weather stations for rainfall, wind speed, radiation and other data.
Soil sampling was carried out biannually to monitor soil root zone salinity and soil moisture. In-situ soil physical properties were measured for modelling.
What were the key findings?
The study concluded that deep, open drains are effective in lowering groundwater levels. In order to be effective they should be more than 2m deep.
The study found the impact of drains on shallow and deep groundwater levels was significant, provided the drains were more than 2m deep and initial water levels were shallow.
The effect of drains often extended to distances (>200-300m) from the drain. Pre-drain soil root salinity in the shallow layers was high. Post-drainage salinity remained low (below thresholds for barley and wheat) at most sites throughout the monitoring period. At all sites, the crop productivity improved after drainage.
At drained sites, there was also evidence of an increase in the soil surface layer pH. The salinity of shallow and deep groundwater ranged between 4,000 milliSiemens per metre (mS/m) and 8,000mS/m.
Based on detailed scientific evaluation of the impact of deep, open drains on groundwater levels, soil salinity, crop productivity, drain design, quality and quantity of flow, critical guidelines for the drainage design and effectiveness were framed.