Year	mm per year
1961	18.1
1962	18.1
1963	18.1
1964	18.1
1965	18.1
1966	18.1
1967	18.1
1968	18.1
1969	18.1
1970	18.1
1971	18.1
1972	18.1
1973	18.1
1974	18.1
1975	18.1
1976	18.1
1977	18.1
1978	18.1
1979	18.1
1980	18.1
1981	18.1
1982	18.1
1983	18.1
1984	18.1
1985	18.1
1986	18.1
1987	18.1
1988	18.1
1989	18.1
1990	18.1
1991	18.1
1992	18.1
1993	18.1
1994	18.1
1995	18.1
1996	18.1
1997	18.1
1998	18.1
1999	18.1
2000	18.1
2001	18.1
2002	18.1
2003	18.1
2004	18.1
2005	18.1
2006	18.1
2007	18.1
2008	18.1
2009	18.1
2010	18.1
2011	18.1
2012	18.1
2013	18.1
2014	18.1
2015	18.1
2016	18.1
2017	18.1
2018	18.1
2019	18.1
2020	18.1