ENGINEERING SOLUTIONS that remove surface or subsurface water are an option that some growers have used with success in situations where trees, lucerne or plants would have little or no short-term impact.
Removing or avoiding surface water (run-off and puddles), or shallow subsurface water, is one way of reducing waterlogging on duplex soils in high- to medium-rainfall areas in WA. Successful strategies to do so have included graded banks, interceptor drains and raised beds. These strategies appear to cost-effectively increase crop yields.
The impact of surface drains has been only small in most areas. Although moving the water off-site might reduce recharge in some areas, interceptor drains can serve as local recharge points to groundwater, and even if tree belts are planted down-slope they may not entirely intercept the water.
Raised beds have met with more success and they have been more extensively researched. Trials and models in Victoria and WA have shown that raised beds prevent waterlogging by:
- increasing run-off or lateral drainage — higher infiltration on the beds and very low infiltration in the furrows, especially during high-intensity rainstorms
- eliminating perched watertables
- reducing root-zone soil water content
- reducing deep drainage beneath beds
- increasing evapo-transpiration and reducing soil evaporation through greater dry matter production, earlier canopy closure and larger leaf area.
The amount of nutrients in run-off and their careful management are the subject of ongoing research. Concerns about the potential for increased recharge down-slope from the extra surface water also warrant further investigation.
Mole drains — both conventional and gravel-filled — have a track record of reducing waterlogging to a depth 2-3 times that achieved by raised beds. However, they are suitable only for waterlogging-prone soils with at least 30 per cent subsoil clay and they must be installed at approximately 45-50 cm depth (the active root zone) under relatively wet (plastic limit) conditions — usually in late spring, which can present problems for cropping systems.
Trial results have shown that mole drains rapidly removed excess water within two days when the top 20 cm of soil became saturated. In the wet winter and spring of 1996, gravel moles reduced the number of days of waterlogging in the top 20 cm from 23 to 5. Canola yields were increased by over 30 per cent as a result of lowering of transient watertables and alleviating waterlogging.
Deep drains for groundwater pump out?
Removing subsurface water is already an option that many growers have considered. In WA alone, there are thousands of kilometres of deep open groundwater drains, and a large number of groundwater pumps, relief wells and siphons. Although they might work locally, there were many examples of groundwater drains, pumps and wells simply transferring a salinity and waterlogging problem downstream into another paddock or another property. Further research and development are needed to determine how best to manage such drainage to avoid off-site impacts.