Interaction between trees and groundwater in the Ord River Irrigation Area

[摘要] EXECUTIVE SUMMARYThis report describes a study of the interaction between trees and groundwater in the Ord River Irrigation Area (ORIA) in northern Australia. The research was initiated in response to growing interest in potential hydrological impacts of commercial tree planting on groundwater conditions in the irrigation area. The number of plantations of predominantly Indian sandalwood (Santalum album) has grown rapidly since the late 1990s and trees now occupy around one-quarter (3,046 hectares) of the irrigated area.Tree plantings in the ORIA might be beneficial if they help to lower the watertable and reduce the risk of groundwater associated salinity. The perennial growth habit of trees and the deep root systems they can develop may result in less irrigation water and less rain water reaching the watertable than beneath shallow-rooted annual crops. Trees grown over a shallow watertable might also use groundwater directly, thereby assisting maintaining watertables at depth and also reducing the need for irrigation.The principle aim of this study was to investigate the potential role of tree plantings in preventing rising watertables. This was achieved by measuring the water use of sandalwood and host plantations, and comparing the measurements to rainfall and soil moisture changes to estimate groundwater use by trees. We also analysed historical groundwater data to assess the impact of plantations on groundwater depth.MethodField data on groundwater levels, soil moisture content and tree water use were collected during 2006 to 2008 at three tree plantations on Ivanhoe Plain (Figure 1 and Table 1). Two monitoring sites were established within African mahogany (Khaya senegalensis) blocks at Kimberley Tree Corporation (KTC) plantation and single monitoring sites were established within sandalwood blocks at Scott Tree Farm and Mock Farm.A simple water balance assessment was undertaken for each field site to provide estimates of rainfall, runoff, effective irrigation, canopy interception, soil evaporation and tree water use. Modelling of observed watertable decline beneath the KTC plantation provided an independent estimate of groundwater use by trees at that site.ResultsMahogany trees at the KTC plantation have been using groundwater directly for around 10 years. Young trees probably developed tap roots into the then shallow (2 m depth) watertable and then developed deeper root systems which followed the watertable as it fell. The aquifer beneath the KTC plantation has a low transmissivity that restricts lateral flow and movement of groundwater. Uptake of the groundwater by the trees induced a watertable drawdown beneath the plantation of 7–8 metres, substantially lower than surrounding fields. Groundwater levels have since stabilised, indicating that the groundwater uptake by the trees is now balanced by the inflow from shallower watertables in surrounding areas. Scott Tree Farm and Mock Farm are more typical of sandalwood plantations on Ivanhoe Plain. The trees were planted over relatively deep (6–8 m depth) watertables and would have been unlikely to have access to groundwater when newly planted. Annual and daily water balances for Scott Tree Farm suggested that direct groundwater use by trees was unlikely but the results for Mock Farm suggest that although groundwater did not contribute significantly to the annual water balance it may have been used at times when soil water contents were low. The annual water balance for Mock Farm suggested that the total of annual tree water use, canopy interception and soil evaporation was approximately equivalent to rainfall and irrigation minus runoff, with no net groundwater use. However, the daily water balance indicated shorter periods of water deficit in shallow soils; at these times trees may have accessed deeper (2 m depth) soil or groundwater.Evapotranspiration rates were relatively low (600 to 1,000 mm yr-1) in comparison to FAO56 reference evapotranspira

[发布日期] 2010-09-28 [发布机构] CSIRO

[学科分类] 地球科学(综合)