The use of long-term rotation studies to determine the effect of fallowing on potential groundwater recharge

M.G. O'Connell, G.J. O'Leary and M. Incerti

Department of Agriculture, Victorian Institute for Dryland Agriculture Mallee Research Station, Walpeup VIC 3507; 
Private Bag 260, Horsham VIC 3400

Numerous studies within the Murray Darling Basin have been conducted comparing groundwater recharge under native vegetation and agricultural land (1). However, little is known on the effect of different types of agricultural land use on recharge. The traditional agronomic practice of including long winter fallows in dryland cropping rotations has been identified as having the potential to significantly contribute to groundwater recharge.

Methods

The method of determining recharge by steady-state mass balance techniques requires long periods of time for equilibration and sensitivity to enable measurement in an arid environment. Long-term rotation experiments located in North-western Victoria, in the Wimmera at Dooen (established in 1917) and the Malice at Walpeup (established in 1981), were used to measure soil water, matric potential and chloride profiles to estimate the effect of fallowing on recharge. Two distinct soil types were investigated; a Wimmera grey cracking clay (Ug 5.2) and a Malice sandy loam (Gc 1.2) (2).

Results and discussion

Significant differences in soil water content, matric potential and chloride were observed in replicate profiles to 5 m in the Wimmera and 9 m in the Malice between non-fallow and fallow cropping rotations. Estimates of potential groundwater recharge over a 72-year period show around 7 mm/yr additional recharge due to fallowing on the Wimmera grey clay soils. In the Malice over the 8-year period fallowing increased potential recharge on the sandy loam soil by between 25 and 33 mm/yr.

It was not possible with our experimental design to measure the actual recharge under the fallow or non-fallow cropping systems. Nevertheless, a reduction in recharge by 25 to 33 mm/yr brought about by non-fallowing on the Mallee soils will have a significant impact on a regional basis. Despite a lower average rainfall, the higher recharge rates in the Malice are most likely due to the lower clay contents of approximately 14% (0-10 cm) increasing gradually with depth to 40% at 100 cm compared to around 60% for the Wimmera grey clay. Recharge rates less than 10 mm/yr are commonly reported for agricultural soils with clay contents greater than 20% (050 cm).

The maintenance of long-term experiments beyond their original life often causes much debate amongst agronomists as to their future value, particularly if the experiment has changed over the years. In this case, at Dooen, the experiment had only undergone one major change since its inception, but it did not affect our investigation. Our quest for measurements of recharge have been aptly met and the technique could be applied to other long-term experiments throughout Australia to provide rapid and economic measurements of potential recharge not easily obtained by other means.

References

1. Jolly, I.D., Cook, P.G., Allison, G.B. and Hughes, M.W. 1989. J. Hydrol. 111, 391-396.

2. Northcote, K.H., Hubble, G.D., Isbell, R.F., Thompson, C.H. and Bettenay, E. 1975. A Description of Australian Soils. (CSIRO Publications: Melbourne). 170 pp.