﻿HYDROLOGY 
  OF 
  NEW 
  YORK 
  191 
  

  

  conclusions 
  from 
  data 
  about 
  which 
  there 
  is 
  considerable 
  doubt. 
  

   The 
  rainfall 
  differs 
  by 
  4.33 
  inches 
  and 
  the 
  evaporation 
  by 
  4.89 
  

   inches, 
  or 
  from 
  20 
  per 
  cent 
  to 
  25 
  per 
  cent. 
  

  

  In 
  preparing 
  these 
  tables 
  the 
  figures 
  of 
  table 
  No. 
  26 
  of 
  Mr. 
  Free- 
  

   man's 
  report 
  have 
  been 
  used. 
  This 
  table 
  is 
  in 
  million 
  gallon® 
  per 
  

   24-hour 
  day, 
  and 
  has 
  been 
  reduced 
  to 
  inches 
  per. 
  month 
  on 
  the 
  

   catchment 
  area 
  of 
  338.8 
  square 
  miles. 
  The 
  following 
  gives 
  the 
  

   water 
  surfaces 
  exposed 
  to 
  evaporation 
  at 
  different 
  periodls 
  : 
  

  

  Per 
  cent 
  

  

  5.8 
  square 
  miles, 
  1868-1873, 
  =1.73 
  

   6.2 
  square 
  miles, 
  1873-October, 
  1878, 
  =1.83 
  

  

  6.9 
  square 
  miles, 
  1878-1891, 
  =2.03 
  

  

  8.4 
  square 
  miles, 
  1891-1893, 
  =2.48 
  

  

  9.5 
  square 
  miles, 
  1893-1895, 
  =2.82 
  

   11.0 
  square 
  miles, 
  1895-1897, 
  =3.28 
  

   12 
  . 
  square 
  miles, 
  1897-1900, 
  = 
  3 
  . 
  56 
  

  

  It 
  may 
  at 
  first 
  thought 
  be 
  imagined 
  that 
  these 
  large 
  water 
  sur- 
  

   faces 
  exposed 
  to 
  evaporation' 
  have 
  considerably 
  increased 
  the 
  

   ground 
  evaporation 
  over 
  the 
  entire 
  catchment. 
  When, 
  however, 
  

   one 
  considers 
  that 
  it 
  is 
  only 
  the 
  difference 
  between 
  what 
  a 
  water- 
  

   surface 
  evaporation 
  and 
  what 
  a 
  ground-surface 
  evaporation 
  would 
  

   be, 
  the 
  difference 
  is 
  seen 
  to 
  be 
  not 
  very 
  much. 
  For 
  instance, 
  

   assuming 
  the 
  water-surface 
  evaporation 
  at 
  36 
  inches 
  per 
  year 
  and 
  

   the 
  ground 
  surface 
  evaporation 
  at 
  27 
  inches 
  per 
  year, 
  the 
  differ- 
  

   ence 
  becomes 
  9 
  inches. 
  With 
  12 
  square 
  miles 
  of 
  water 
  surface 
  in 
  

   1900, 
  giving 
  3.56 
  per 
  cent 
  of 
  the 
  whole, 
  the 
  excess 
  of 
  water-surface 
  

   evaporation 
  over 
  ground-surface 
  evaporation 
  is 
  0.32 
  of 
  an 
  inch, 
  a 
  

   quantity 
  which 
  is 
  so 
  far 
  within 
  the 
  limit 
  of 
  possible 
  error 
  in 
  other 
  

   directions 
  as 
  to 
  be 
  negligible. 
  At 
  the 
  most, 
  taking 
  the 
  catchment 
  

   area 
  at 
  338.8 
  square 
  miles, 
  it 
  would 
  only 
  reduce 
  the 
  evaporation 
  

   from 
  26.5 
  inches 
  to 
  26.2 
  inches. 
  

  

  The 
  minimum 
  year 
  in 
  this 
  table 
  is 
  seen 
  to 
  be 
  1880, 
  when 
  only 
  

   13.71 
  inches 
  Tan 
  off. 
  In 
  1883 
  the 
  runoff 
  was 
  also 
  very 
  low, 
  being 
  

   only 
  13.74 
  inches. 
  

  

  Table 
  61 
  gives 
  the 
  rainfall, 
  runoff, 
  and 
  evaporation 
  of 
  the 
  stor- 
  

   age, 
  growing, 
  and 
  replenishing 
  periods 
  for 
  the 
  Hudson 
  river 
  area 
  

   for 
  a 
  period 
  of 
  14 
  years, 
  from 
  1888-1901, 
  inclusive. 
  The 
  minimum 
  

  

  