Year,Spring_num_tow,Winter_num_tow
1968,0.07,
1969,0.103,
1970,0.111,
1971,0.105,
1972,0.25,
1973,0.337,
1974,1.229,
1975,0.513,
1976,0.688,
1977,0.604,
1978,0.598,
1979,0.446,
1980,0.462,
1981,0.36,
1982,0.073,
1983,0.339,
1984,0.186,
1985,0.268,
1986,0.755,
1987,0.514,
1988,0.602,
1989,0.245,
1990,0.27,
1991,0.186,
1992,0.665,0.991
1993,0.201,0.951
1994,0.175,0.405
1995,0.314,0.847
1996,0.203,1.058
1997,0.542,0.422
1998,0.189,0.351
1999,0.537,0.612
2000,0.301,1.082
2001,0.792,1.866
2002,1.253,3.436
2003,1.614,3.16
2004,0.711,1.213
2005,0.727,0.558
2006,0.456,1.061