Groundwater Modelling

The environmental problems faced by people exist because we exploit resources beyond their limits. Land is no exception; when it is used beyond its capability, problems will occur. Over-clearing, for example, usually leads to problems such as increased erosion and salinity of soil and water.

A majority of farmers are now convinced of the potential of trees and shrubs to lower watertables and therefore reduce soil salinity. However, they aren't always sure of the best place or how many they need or what groundwater control strategies will work on their properties. Cooperative farm planning is designed to help farmers develop a catchment scale plan to lower levels of the watertable and salinity. A computer model of groundwater flow of the catchment allows landholders to predict the depth of the watertable under different management options. This allows farmers and catchment groups to see the effects before they choose the best option for themselves.

High Water Use Farming Trial

In 1991, Land and Water Resources Research and Development Corporation (LWRRDC) funded a project to examine water use under different farming regimes. The area chosen was a distinct 90 ha catchment with salt affected areas in the valley floor and some hillside seepage areas.

The soil type was classed as gravelly loams (jarrah) in the upper and mid slopes with sand over clay (duplex soils) in the valley floor. A magnetometer survey of the catchment showed at least two dolerite dykes running across the catchment with associated saline areas. Mid slopes had bedrock at a depth of 20m and in the valleys bedrock appeared at 10m.

Contour banks were put in to control water flow into the head of the gully and 10 ha around the saline area was fenced and planted on the contour with a mixture of trees and perennial grasses.

In setting up the trial, Roclea and Agriculture Western Australia chose to test a mixture of commercial trees and perennial pastures. Some non-commercial trees were chosen to protect a dam and link remnant vegetation. Watertable levels and growth rates were measured.

Results

Since 1991, watertable levels during summer have decreased 1.3m with the tree population and have risen 30cm on the slope above. Annual changes in the ground reflect seasonal rainfall. Ground waters rose rapidly in 1991-1992 and fell in 1994-1995 and are likely to jump again in high rainfall years.

Results show that the tree-planting program may have stabilised the valley, but has done nothing to reduce the rising watertable in upper slopes. Under this regime, there is a likelihood that more hillside seeps will develop.

Commercial growth rates in bluegums have occurred in fresher areas; some parts of the plantation have grown 40 cubic metres of wood by year 4. In saltier areas, the plantings are not economically viable.

Salt tolerant clones of swamp sheoaks (Casuarina glauca and Casuarina obesa) are growing well in bare salt scalds and swamp mallet (Eucalyptus spathulata) and E. raveretiana in patchy barley grass areas.

Perennial pastures on trial include phalaris (Phalaris aquatica), cocksfoot (Dactylis glomerata), fescue (Festuca arundinacea), tall wheat-grass (Thinopyrum elongatum) and puccinellia (Puccinellia ciliata). The best performers are tall wheat-grass and puccinellia (Puccinellia ciliata). The best performers are tall wheat-grass and puccinellia.