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Water logging may reduce crop yield by up to 1 bale/ha with yields affected before symptoms are noticed.
Palæochannels are common features in much of the irrigated landscape of the Murray Darling Basin. Exensive research has been carried out on palæochannels in the Namoi and Murrumbidgee River Basins and has indicated that these features are associated with irrigation water loss due to their sandier textures. While these features have been identified as potential sources of deep drainage, little is known about the pathways and movement of water after infiltration and how changes in soil properties and sedimentary layering govern this movement. This is particularly the case in the Gwydir River Basin, where palæochannels are less understood due to the expensive and invasive nature of direct physical measurement and the extreme variability of hydraulic properties in these structues. Previous research in this region has been aimed at identifying the characteristics of these structures through ancillary data, such as electromagnetic induction, but has generally been limited to one or two dimensions.
This study uses traditional measurements to identify the geomorphological and hydrological characteristics of a palæochannel in the Gwydir River Basin, where palæochannels are thought to affect water-use efficiency on farms relying on surface irrigation techniques. To improve on the information gained from a limited number of direct observations, the conceptual model is further refined through the use of geophysical information. Depth information was derived from the electromagnetic induction data by inverting bulk electrical conductivity readings from various combinations of electromagnetic measurements and using a regularisation process to stabilise the solution of the inverse problem. Four different inversion algorithms and three conceptually-different scaling relationships are subsequently used to derive saturated conductivity fields based on data from pedotransfer functions.
To test the utilitity of the geophyhsical data, two interpolation procedures are used to distribute this information in three dimensions. Three-dimensional ordinary kriging was used to interpolate the limited soil measurements, and the scaling factorderived saturated conductivity predictions. A more sophisticated method, regression kriging, incorporates the electrical conductivity data into the interpolation of the direct observations, providing the maximum amount of information.
This study finds that the palæochannel is morphologically-different from those previously described in this area. This channel contains coarse-textured bedload sediment of variable thickness along the length of the channel. A thin, but hydrologically-significant clay layer separates the coarse sediments from the unconfined aquifer below. The hydrological measurements indicate that significant pulses of water are being channelled through the structures from a neighbouring irrigation channel. The fate of this water remains unknown, but it is likely contributing to groundwater recharge in the underlying unconfined aquifer.
The electromagnetic induction measurements delineated the palæochannel, but the vertical predictions were highly dependent on the inversion method. Regularisation of the inversion process was a necessary step, as the unregularised profiles were significantly affected by instrument noise. This was attributed to the use of several different instruments to construct the conductivity profile. Furthermore, the relationship between the predicted EC and the various soil properties depended on the regularisation order. While 0th order Tikhonov regularisation provided the best fit to forward-modelled ECa, clay and saturated hydraulic conductivity predictions from the pedotransfer functions, 2nd order Tikhonov regularisation was most strongly correlated with ECe. In cases where a significant relationship existed between EC and saturated conductivity, the scaling factors approach provided more support for the palæochannel presence than was obtained using measured properties. This will likely translate to more realistic input for future groundwater model parameters. However, the relationship between saturated hydraulic conductivity and electrical conductivity, as a function of the inversion algorithm and the scaling factor multiplication procedure, needs to be improved before this information can be incorporated into a realistic groundwater model of the surrounding area.
