\begin{table}%t1
\caption{IRIS duty cycles (\%). {\bf (a)} annual IRIS; {\bf (b)}  annual partial gap filled IRIS; {\bf (c)} 4-month summer IRIS ; {\bf (d)} 4-month summer partial gap filled IRIS. (NB: (1) Values for 1989 start the 1st July. For the~\mbox{4-month} duty cycle values, only July, August and September are used~/ (2) Values for 1999 ending 30 August. For the~\mbox{4-month} duty cycle values, only June, July and August are used.).}\label{tab1}
%\centering
\par
\begin{tabular}{c c c c c} 
\hline \hline
{\bf Years} & {\bf a} & {\bf b} & {\bf c}  & {\bf d}\\ \hline 
\bf 1989 & 34.5 & 58.0 & 50.6 & 77.6 \\ \hline
\bf 1990 & 33.9 & 60.6 & 52.6 & 84.1 \\ \hline
\bf 1991 & 41.0 & 70.3 & 57.3 & 88.1 \\ \hline
\bf 1992 & 38.9 & 68.0 & 54.4 & 85.8 \\ \hline
\bf 1993 & 33.6 & 62.4 & 41.7 & 74.7 \\ \hline
\bf 1994 & 43.2 & 70.3 & 64.7 & 89.9 \\ \hline
\bf 1995 & 47.4 & 75.8 & 67.7 & 92.6 \\ \hline
\bf 1996 & 41.3 & 71.1 & 53.6 & 84.8 \\ \hline
\bf 1997 & 35.4 & 61.8 & 44.5 & 71.7 \\ \hline
\bf 1998 & 28.9 & 53.8 & 33.9 & 57.5 \\ \hline
\bf 1999 & 17.1 & 33.4 & 19.9 & 40.5 \\ \hline 
\end{tabular}
\end{table}