﻿HYDROLOGY 
  OF 
  NEW 
  YORK 
  

  

  119 
  

  

  many 
  reports 
  and 
  papers 
  it 
  is 
  assumed 
  that 
  averages 
  of 
  a 
  series 
  

   of 
  percentages 
  can 
  be 
  safely 
  taken. 
  The 
  following 
  illustration, 
  

   with 
  five 
  cases 
  drawn 
  from 
  observation, 
  may 
  be 
  taken 
  to 
  show 
  

   that 
  this 
  is 
  erroneous 
  : 
  

  

  Runoff, 
  Per 
  Cent 
  of 
  Rainfall 
  

  

  Case 
  

  

  Rainfall 
  

  

  Runoff 
  

  

  Per 
  cent 
  

  

  1 
  

  

  Inches 
  

   44.3 
  

   62.5 
  

   16.2 
  

   24.3 
  

   '40.4 
  

  

  Inches 
  

  

  20.1 
  

  

  35.0 
  

  

  1.2 
  

  

  2.5 
  

  

  13.1 
  

  

  45.37 
  

  

  

  

  56.00 
  

  

  3 
  

  

  7.41 
  

  

  4 
  

  

  10.29 
  

  

  5 
  

  

  32.42 
  

  

  

  

  

  187.7 
  

  

  71.9 
  

  

  5)151.49 
  

  

  30.29 
  

   ^38.31 
  

  

  Runoff 
  _ 
  71.9 
  

   Ilamfall~187.7 
  = 
  _ 
  

  

  Difference= 
  8.02 
  

  

  Runoff 
  coefficient 
  misleading. 
  As 
  a 
  corollary 
  to 
  the 
  preceding 
  

   proposition, 
  it 
  follows 
  that 
  the 
  ratio 
  between 
  annual 
  rainfall 
  and 
  

   runoff, 
  known 
  as 
  the 
  " 
  runoff 
  coefficient 
  or 
  factor," 
  is 
  essentially 
  

   misleading. 
  A 
  realization 
  of 
  this 
  fact 
  has 
  led 
  the 
  writer, 
  in 
  his 
  

   report 
  to 
  the 
  United 
  States 
  Board 
  of 
  Engineers 
  on 
  Deep 
  Water- 
  

   ways, 
  to 
  practically 
  expurgate 
  this 
  statement, 
  or 
  anything 
  ap- 
  

   proximating 
  to 
  it, 
  from 
  his 
  report. 
  The 
  expressions, 
  " 
  average 
  

   runoff 
  " 
  and 
  " 
  percentage 
  of 
  the 
  rainfall," 
  do 
  not 
  appear. 
  

  

  Relation 
  oeticeen 
  total 
  runoff 
  and 
  runoff 
  of 
  storage 
  period. 
  

   Attention 
  may 
  also 
  be 
  again 
  directed 
  to 
  the 
  fact 
  that 
  the 
  total 
  

   runoff 
  of 
  a 
  stream 
  in 
  any 
  given 
  year 
  depends 
  very 
  largely 
  on 
  the 
  

   runoff 
  of 
  what 
  may 
  be 
  termed 
  the 
  " 
  storage 
  period." 
  Usually 
  

   about 
  0.55 
  to 
  0.75 
  of 
  the 
  total 
  rainfall 
  of 
  this 
  period 
  appears 
  as 
  

   runoff 
  in 
  the 
  stream, 
  while 
  for 
  the 
  summer, 
  or 
  growing 
  period, 
  

   not 
  more 
  than 
  about 
  0.1 
  of 
  the 
  rainfall 
  appears. 
  This 
  great 
  dif- 
  

   ference 
  is 
  due 
  to 
  greater 
  evaporation, 
  as 
  well 
  as 
  to 
  the 
  absorption 
  

   of 
  water 
  by 
  plants 
  during 
  this 
  period. 
  The 
  total 
  amount 
  for 
  

   the 
  year 
  which 
  will 
  appear 
  as 
  runoff 
  in 
  the 
  stream 
  will 
  depend, 
  

  

  