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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



action can be expected to result in more wide- 

 spread deterioration of soil surface aggregates 

 and structure {20) leading to an increase in net 

 surface runoff, R {28, 62). A reduction in surface 

 storage capacity, D, can also be expected, but 

 this is unlikely to be of as great importance. 



Soils in Australian rangelands vary consider- 

 ably in their infiltration rate (I, equation 2). 

 Rainfall intensities are such that runoff can be 

 expected from some soils, even when initially 

 dry, most of the time, while other soils, when 

 dry, are capable of absorbing most of the rain- 

 water received (fig. 3). Much variation in re- 

 sponse of soils, due to increased direct raindrop 

 action can therefore be expected, with soils (B 

 in figure 3) of intermediate infiltration rates 



60 

 TIME (min.) 



Figure 3. — Infiltration rate of water into rangeland soils 

 of different textures and the maximum expected rain- 

 fall for recurrence intervals of 1 to 200 years. The 

 cumulative infiltration curves refer to initially dry 

 soils, using data from Jackson (28), Stannard (60) 

 and Marshall (unpublished). The rainfall intensities 

 refer to the average maximum intensities expected in 

 the Western Division of New South Wales, using data 

 from Stewart (61) and Wiltshire (74). Soils (A) are 

 those capable of absorbing all of even the heaviest 

 rains, soils (B) will absorb or shed water depending 

 on rainfall intensity and soils (C) will shed water 

 for all rains of the intensities plotted. For initially 

 wet soils (data not plotted) most soils would, irrespec- 

 tive of texture, shed water from rainfall of intensities 

 in the range plotted. (After Marshall, (43).) 



relative to expected rainfall intensities probably 

 showing the greatest response. 



Modification Of Surface Soil Structure 



By breaking down soil aggregates and by com- 

 paction of the surface soil, trampling can locally 

 reduce the infiltration rate (I) of water into the 

 soil and so influence runoff where the relation- 

 ship between the two is 



R = P-(JTdt+AD) 



(2) 



Several other effects of trampling on surface soil 

 structure have been noted including a reduction 

 in pore space, soil penetrability, hydraulic con- 

 ductivity and air permeability, and an increase 

 in bulk density {3, 21, 24-, 6Ji). However, because 

 of the prime importance of infiltration rate in 

 the water balance of the soil a study was made 

 of infiltration in terms of sorptivity, S, in re- 

 lation to sheep tracks (fig. 4). Sorptivity and in- 

 filtration rate are related in the initial stages of 

 infiltration by 



I = St 1/2 (3) 



The results obtained from the application of the 

 ring infiltrometer method described by Talsma 

 {63) are shown in figure 5. 



The results in figure 5 show that sorptivity at 

 first falls rapidly, at least on the desert sandy 

 loam investigated, with an increase in trampling 

 index and thereafter remains relatively un- 

 changed. This implies that it is not so much the 

 intensity of trampling that is important, but 

 the proportion of the total area traversed by 

 tracks. 



The convergence of many tracks on watering 

 points is an obvious example of an increasing 

 proportion of trampled ground (fig. 6). An in- 

 dication of the proportion of ground covered by 

 tracks can be obtained using the track intensity 

 to distance from watering point regression of 

 Lange {35) and assuming a mean track width of 

 30 cm. when the tracks are found to occupy 16, 

 21, 24, and 25 percent of the ground at 1,600, 

 800, 200, and 100 m. respectively from the water- 

 ing point. 



Reduced infiltration due to trampling has also 

 been shown by Lusby {38) at Badger Wash on 

 the Colorado Plateau. There it was largely re- 

 sponsible for a 30-percent increase in runoff and 



