New  York.    MacMillan  Co.    1903.
29.  Wilson,  H.  M.    Irrigation  Engineering,  Chap.  V.,  Quantity  of  Water  Re-

quired.   New  York,  Wiley  &  Sons.    1903.
Digitized  by  VjOOQ IC


CHAPTER  VIII.
RUN-OFF.
87.  Run-Off. — That  portion  of  the  rainfall  that  is  not  absorbed
by  the  strata,  utilized  by  vegetation  or  lost  by  evaporation,  finds
its  way  into  streams  as  surface  flow  or  run-off.  The  demands  of
the  first  named  factors  are  always  first  supplied  and  the  run-off  is ,
therefore  the  overflow  or  excess  not  needed  to  supply  the  other
demands  on  the  rainfall.  The  run-off,  therefore,  while  a  direct  func-
tion of  the  rainfall,  is  not  found  to  increase  in  direct  proportion
thereto,  except  perhaps  in  seasons  such  as  early  spring  when  from
seasonal  conditions  the  demands  of  vegetation,  percolation  and
evaporation  are  not  active  and  all  or  most  all  of  the  rainfall  flows
away  on  the  surface.  The  remainder  of  the  year  the  run-off  may
be  said  to  increase  with  the  rainfall  but  usually  at  a  much  less
rapid  rate  and  in  many  cases  the  rainfall  is  entirely  absorbed  by
the  strata  or  vegetation,  and  does  not  influence  or  affect  the  run-off.
In  this  case  the  run-off  is  supplied  from  the  ground  water,  stored
from  previous  rainfalls,  and  is  entirely  or  largely  independent  of
the  immediate  rainfall  conditions.
