176 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



source. One of the effects of perennials is to re- 

 duce wind velocity. The smoother the surface, the 

 lower the value of the roughness parameter, z , 

 and the higher the wind velocity, u, at a compara- 

 ble height, given the same geostrophic velocity 

 (table 3). Another effect of vegetation on the 

 wind is to reduce the drag force on the interven- 

 ing soil surface and so reduce soil particle move- 

 ment. In table 3, drag, expressed as the shearing 

 stress for the entire surface, t , is 6 times greater 

 over the scrub covered ground than over bare 

 ground. However, this is partitioned between the 

 surface roughness and the intervening ground 

 surface. In the example in table 3 the proportion 

 of the total drag acting at the ground surface 

 is negligible in the presence of the scrub covering 



Table 2. — Distribution of mineralizable nitrogen 

 and available phosphorus with depth from 

 Atriplex vesicaria communities in western 



N.S.W. 





Mineralizable 



Available 



Depth 



nitrogen ' 



phosphorus 2 



Cm. 



P.p.m. 



P.p.m. 



- 2.5 



316 



17.3 



2.5 - 5 



200 



16.3 



5 - 10 



144 



8.0 



10 - 15 



49 



3.7 



15 - 22.5 



38 



2.3 



22.5 - 30 



29 



2.0 



40 - 45 



26 



1.7 



1 Mineral nitrogen released in 9-wk. incubation. 

 2 NaHC0 3 extraction. 

 Source: From Charley and Cowling (15). 



Table 3. — Wind parameters over bare soil and 

 scrub ' 



Surface 



Bare soil 



Scrub 



Roughness parameter z (cm.) 

 Friction velocity u* (cm./sec.) 

 Surface shearing stress t (dyne/cm. s ) 

 Wind velocity at 10 m. u w (m./sec. ) 



0.1 



100 



57 



138 



4 



24 



13 



7 



while over the bare ground surface it is all acting 

 at the ground surface. 



The proportion of the total drag exerted at the 

 ground surface has been shown to depend both 

 on the vegetation elements and their density (no./ 

 unit area) (40)- An expression for appraisal of 

 perennial plant cover for erosion prediction 

 makes use of the product of the unobstructed 

 drag coefficient of the vegetation elements, C f (z), 

 and lateral cover, L c , that is, the average frontal 

 area of the elements/average surface area occu- 

 pied per element. The product L c -C f is related to 

 a constant for the condition at which the ground 

 surface shearing stress, t , becomes negligible. 

 Thus when 



L e -Cf(z) ^0.0147 3 



(4) 



no net soil loss from the area occupied by the 

 vegetation elements is expected (4-0). The refer- 

 ence height, Z, here is 1.6H where H is the mean 

 roughness element height. Field tests and com- 

 parison of predictions of erosion hazard for shrub 

 steppe communities with the field assessments of 

 Jessup (29) show good correspondence when a 

 value of C f = 0.4 is assumed for the vegetation 

 elements (43) . 



The results of prediction of erosion hazard in 

 the field can be presented in terms of the value 

 of L c -C f relative to the threshold value, or as 

 the maximum average distance tolerable between 

 shrubs (D) in terms of their height (H) # . A typi- 

 cal value of maximum tolerable D/H for Austra- 

 lian shrub rangelands appears to be about 3.5 

 (43) where H is the average of the maximum 

 heights of the vegetation elements. Caution is re- 

 quired in extrapolation of this value to other sit- 

 uations, however, as it depends on the value of the 

 unobstructed element drag coefficient, the open- 

 ness of the vegetation elements, their general 

 outline, their diameter to height ratio, and their 

 behavior at different wind velocities. 



' Calculated for geostrophic wind velocity Uo — 25 m./ 

 sec; latitude 43°. From Deacon (17). 



*A more accurate expression is 

 Cf(u lH )=b/ v /Lc-|-a where, 



for 1.6H, b=0.1014, a =—0.0005; 

 2.36H. b = 0.0918, a =—0.046; 

 3.94H, b=0.0799, a =—0.066, 

 the derivation of which is given by Marshall (42). For 

 the values of C t expected for vegetation elements, how- 

 ever, the simpler expression is of sufficient accuracy for 

 practical use. 



