The effects of waterponding on the reclamation of degraded scalds in the semi-arid rangelands of N.S.W.

Abstract

Soil erosion as a result of extensive overgrazing and the subsequent reduction in vegetation cover is seriously affecting the long term ecological and economic sustainability of the grazing industry. Where this occurs on saline-sodic soils, the loss of the Ai horizon and consequent exposure of the silty A2 horizon, results in the creation of an impervious crust over the clayey B horizon. The resultant scald is an extremely hostile environment for plants. Currently 10%, or the semi-arid rangelands of western NSW is affected by scalding. A lack of water in the soil profile is the primary factor restricting plant reestablishment on scalds. Although the A2 is thin, it reduces infiltration rates dramatically, so that water is quickly lost from the scald surface by runoff and evaporation. Waterponding, whereby U-shaped banks are formed on the scald surface to create shallow ponds approximately 0.5ha in area, is arguably the most effective method of scald reclamation. These ponds trap rain, allowing greater opportunity for infiltration and thus leaching of salts from the upper soil profile. The leaching reduces the structural stability of the upper soil profile and encourages shrink-swell processes to occur, with the subsequent formation of cracks. These cracks enable water to by pass the crust, thereby improving the water availability and likelihood of reestablishment of vegetation on the pond. This study was undertaken to investigate the role of spatial and temporal factors on the above soil processes and vegetation characteristics. The study area, covering approximately 1 800 km^, is located on the Bogan River/Marra Creek/Macquarie River flood plain, approximately 100 km north of Nyngan, NSW. Between September 1995 and March 1996, soil and vegetation data were collected at 9 sites from both ponds and their adjacent scalds. The ponds range in age from 6 months to 31 years. Changes in soil and vegetation properties after these different periods of waterponding were measured. The success of reclamation was assessed in terms of vegetation and soil response to ponding.