1895  21.69
  1896  23.72
  1897  30.66
  1898  21.97
  1899  24.61
  1900  17.73
  1901  20.85
  1902  19.00
  1903  21.67
  1904  23.95
  1905  28.37
  1906  38.76
  1907  33.86
  1908  31.95
  1909  40.74
  1910  24.05
  1911  28.09
  1912  30.25
  1913  27.69
  1914  29.44
  1915  27.66
  1916  34.44
  1917  24.87
  1918  30.30
  1919  25.25
  1920  34.87
  1921  32.69
  1922  30.84
  1923  29.08
  1924  22.48
  1925  31.34
  1926  25.58
  1927  35.20
  1928  22.45
  1929  29.49
  1930  33.05
  1931  21.64
  1932  28.73
  1933  23.80
  1934  19.74
  1935  23.60
  1936  36.32
  1937  32.39
  1938  33.88
  1939  25.07
  1940  34.37
  1941  47.35
  1942  24.38
  1943  28.65
  1944  29.18
  1945  33.27
  1946  32.77
  1947  33.20
  1948  26.80
  1949  33.48
  1950  21.92
  1951  32.51
  1952  27.53
  1953  24.41
  1954  25.22
  1955  25.15
  1956  17.68
  1957  38.86
  1958  21.72
  1959  24.18
  1960  23.24
  1961  29.67
  1962  24.21
  1963  28.17
  1964  27.56
  1965  38.86
  1966  23.11
  1967  31.12
  1968  29.92
  1969  33.22
  1970  29.31
  1971  30.07
  1972  28.98
  1973  30.84
  1974  21.69
  1975  30.12
  1976  19.48
  1977  24.10
  1978  36.30
  1979  25.25
  1980  38.43
  1981  34.26
  1982  39.98
  1983  44.88
  1984  35.15
  1985  32.77
  1986  33.43
  1987  32.56
  1988  24.43
  1989  19.25
  1990  26.34
  1991  29.72
  1992  28.55
  1993  36.09
  1994  30.18
  1995  34.29
  1996  31.93
  1997  37.95
  1998  38.81
  1999  27.31
  2000  31.04
  2001  26.24
  2002  20.14
  2003  26.04
  2004  33.91
  2005  38.07
  2006  32.97
  2007  25.15 
  2008  25.43
  2009  26.16
  2010  31.45
%http://www.wrcc.dri.edu/cgi-bin/divplot1_form.pl?4203