CONCLUSIONS 



Within the geographical area covered by this study (and probably in many similar 

 areas) water repellency is a major limiting factor in the capacity of granitic soils to 

 absorb high-intensity summer rainfall. Other limiting factors are: (1) Inadequate 

 moisture storage capacity due to thin soil, (2) surface sealing caused by raindrop 

 impact on soil surfaces unprotected by litter and vegetative cover, and (3) low porosity 

 due to compaction caused mainly by human activity. Although this report is concerned 

 primarily with water repellency, it is not meant to imply that the other limiting 

 factors listed are unimportant. On the contrary, they cause serious problems on steep, 

 poorly-vegetated slopes and on heavily-used areas. However, water repellency can cur- 

 tail infiltration into soils that have sufficient depth, porosity, and cover to absorb 

 high- intensity rain as fast as it falls; and it can further reduce infiltration of water 

 into soils where one or more of the other limiting factors are present. 



At times, even though no water repellency was found at the surface of bare soil, 

 severe repellency was found below the surface. Some repellency may be residual from 

 past fires, but much of it is believed to be caused by fungal activity on plant roots 

 that frequently occupy the soil under bare openings. If the subsurface repellent layer 

 is continuous and unbroken, infiltration is limited to the storage capacity of the 

 wettable surface layer and severe runoff and erosion will occur during high-intensity 

 storms. The best control in such openings would be obtained by establishing plants in 

 them. Establishment of plants on such sites is difficult, but could be facilitated by 

 destroying the live roots already present under the bare openings. 



Water repellency does not appear to be a problem under shrub cover because it is 

 broken up by numerous roots, rodent burrows, and by other means not yet identified. 



13 



