Year,Spring_num_tow
1982,
1983,0
1984,0
1985,0
1986,0
1987,0
1988,0
1989,0
1990,0
1991,0.235
1992,0.237
1993,0.481
1994,1.394
1995,0.952
1996,0.602
1997,1.182
1998,0.729
1999,0.448
2000,1.274
2001,0.623
2002,0.981
2003,0.774
2004,0.335
2005,0.404