PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



177 



Breakdown Of Soil Aggregates And Disturbance 



Trampling influences soil stability in two ways. 

 First, by breaking down soil aggregates, it pro- 

 vides a supply of particles more susceptible to 

 movement by wind. Secondly, both rainfall and 

 trampling are disturbing agents, with the latter 

 particularly effective in mixing the heavier resid- 

 ual particles with underlying lighter particles 

 thereby recreating a surface layer of erodible 

 material. 



Runoff And Water Erosion 



The low percentage cover of vegetation in semi- 

 arid and arid rangelands for much of the year 

 makes their soils particularly susceptible to water 

 erosion with the combination of direct raindrop 

 impact and largely unimpeded runoff (43). Lack 

 of rainfall itself becomes limiting in terms of 



sediment yield produced from catchments cov- 

 ered by seminatural vegetation at the more arid 

 extreme of rangelands. However, in semiarid 

 rangelands, a combination of sufficient rain and 

 inadequate vegetation cover has been shown to 

 produce, for American conditions at least, a peak 

 in sediment yields in an annual rainfall regime 

 of between 10 and 20 inches (34)- Those effects 

 of grazing and trampling on the water balance 

 component which increase runoff (see Water Bal- 

 ance, p. 169) further aggravate this already nat- 

 urally severe problem of erosion of soils by water. 

 Some of the worst gullies arise initially from 

 cross-contour tracks, often leading to water holes 

 (fig. 8). These tracks, acting both as areas of low- 

 ered infiltration rate and as channels for water 

 shed from adjacent soil surfaces, become rapidly 

 eroded. 



Figure 8. — Gully erosion originating from cross-contour sheep and vehicle tracks leading to a watering point in 

 black and pearl bluebush (Kochia pyramidata and K. sedifolia) country 64 km. N.N.W. of Balranald, N.S.W. 



