Hydrological hazard assessment for irrigated agriculture in the Irwin focus area

Abstract

The Midlands groundwater and land assessment project aimed to identify 2000–3000 hectare precincts suitable to develop intensive irrigated horticulture. The primary focus area was at Irwin, where the Department of Water and Environmental Regulation investigated groundwater resources and the Department of Primary Industries and Regional Development undertook a multi-faceted site assessment. This report describes the hydrological hazards assessment for the Irwin focus area. The Irwin focus area is located on fertile loam and clay flats associated with the Irwin River. In the east, it encompasses the Irwin River valley floor and the western boundary loops to the south of the Irwin River to capture an area of alluvial clays. The Gingin Scarp forms a boundary between the east and west parts of the focus area. We used groundwater data from resource condition monitoring in the Arrowsmith Hydrozone to assess the hydrological hazards in the Irwin focus area and guide more intensive field investigations. We undertook a shallow drilling program to investigate the profile and to sample and monitor the watertable in the western part of the Irwin focus area. Monitoring bores were established at 13 sites on the alluvial clay flats. The shallow drilling program was complemented with ground-based electromagnetic surveys. A vehicle-mounted system was developed to record electrical conductivity measurements from Geonics™ EM38 and EM31 instruments. Historical groundwater level monitoring in the Irwin River valley indicates consistently rising groundwater levels east of the Gingin Scarp in the Arrowsmith Hydrozone. This trend and shallow depth to groundwater poses a significant risk of dryland salinity developing in this landscape. This existing hydrological hazard, rising groundwater and salinity, makes the eastern part of the focus area unsuitable for irrigated horticulture. West of the Gingin Scarp, the soil profile under the alluvial flats that extend south of the Irwin River is dominated by stiff, moist, grey clay that becomes red-brown or mottled brown and pale grey clay with depth. While the surface soils are not salt-affected, there is significant salt storage at depth, starting from about 3m to about 7–10m. Since groundwater is not rising in this area the regolith salt storage is not a hazard for dryland agriculture. However, if irrigation water is applied and groundwater rises, it will become a significant hazard. The hydraulic properties of the alluvial clays could not be closely observed during the investigation because of the absence of recharge due to low rainfall. However, the drill cuttings of heavy, moist clay indicate that there is low hydraulic conductivity or permeability. If the surface soils were to become saline from irrigation, they would likely remain saline because of the limited leachability of the clays