Year,Spring_num_tow,Winter_num_tow
1968,0.085,
1969,0,
1970,0,
1971,0,
1972,0.143,
1973,0,
1974,0,
1975,0.061,
1976,0.557,
1977,0.163,
1978,0.148,
1979,0.017,
1980,0.482,
1981,0.045,
1982,0.003,
1983,0.009,
1984,0.007,
1985,0.085,
1986,0.149,
1987,0.03,
1988,0.232,
1989,0.07,
1990,0.171,
1991,0.499,
1992,0.164,0.705
1993,0.007,0.174
1994,0.011,0.223
1995,0.162,0.48
1996,0.063,1.909
1997,0.024,0.192
1998,0,0.261
1999,0.347,0.545
2000,0.661,3.82
2001,0.078,1.026
2002,0.554,3.954
2003,0.154,2.889
2004,0.08,0.478
2005,0.219,1.617
2006,0.054,0.896