A review of hydrology research within the open eucalypt woodlands of tropical semi-arid Australia: a possible source of baseline information for the West African Sahel

During the period 1975-1990, several research campaigns led by CSIRO with various partners were undertaken in the transect between Charters Towers and Torrens Creek of tropical semi-arid, central-north Queensland, focused on the hydrological and the landscape ecology impacts of the beef cattle industry. This work offers baseline hydrological information which can possibly be transposed to comparable areas of the West Africa Sahel, where such information is lacking. The landscape has a climate not too dissimilar from the African Sahel, viz, 400-500 mm annual rainfall, but against a backdrop of marked climate variability (linked with ENSO) produces considerable variability in yearly rainfall totals. The soils are very old dating back to the Tertiary and the Red Earths (Oxisols or Kandosols) can exceed 36 m depth and occupy ancient valleys; whereas the more shallow Yellow Earths (Oxisols or Kandosols) cover the ancient intervening ridges and so are less deep. The former are well-drained, whereas the Yellow Earths have temporary water tables during average to above average rainy seasons which rest on the weathered rock below. The vegetation is open eucalypt woodland with a floor cover of grass tussocks and bare soils patches, and apart from cattle ranching, there are no other human occupance pressures. The landscape is principally of very low relief (called in Australia “smooth plainlands”) with slope angle commonly 2 degrees. Thus, the conventional research basin approach was not practical and instead, the water balance was assessed and runoff-erosion was determined along transects. Despite the very low slope angles, the soils are very fragile from any disturbance which can lead to severe erosion in areas which have been subjected to over-grazing, tree clearing and cropping. Further disturbance can change the water balance from one where Hortonian overland flows are largely re-distributed and infiltrate across the landscape to one where the landscape loses water and sediment in runoff consequently placing recharge to groundwater systems under threat. In semi-arid tropical landscapes this can result in degradation of both the land and water resource.