21 



vertical percolation. Efficient water use implies a permanently 

 expanded root system at all depths in the soil profile. Annual grasses 

 and forbs, like small grain crops, obtain their water only from the 

 upper soil levels; their root systems are shallow, expanding and 

 functioning only during the growing season. Perennial grasses and 

 shrubs, absent from most cropping systems, use water that escapes 

 from shallow- rooted plants into the subsoil; their root systems are 

 deeper, permanently expanded, and functional over a longer period. 



In nature, big sagebrush (Artemisia tridentata) occupies the 

 niche of a deep-rooted perennial shrub. A Department of Fish and 

 Game study in the Winnett area shows that a substantial amount of 

 moisture was released into the soil during the year sagebrush was treated 

 with 2,4-D (24). In this study, even perennial grasses could not 

 grow enough during the season of treatment to consume all the released 

 moisture. 



Precipitation that reaches the ground must either run off, 

 evaporate, or soak in. Water penetrating the ground surface is 

 infiltration, whereas water moving through the soil is percolation. 



Infiltration studies conducted on fallow and sod (native grass- 

 land) situations on the Highwood Bench show no appreciable difference 

 in infiltration rates but greater horizontal water movement with the 

 sod system. The reason for this greater horizontal movement in sod 

 is the humus layer at the soil surface. This pourous mulch, usually 

 over two inches thick, acts as a sponge, or soil moisture storage 



