Hello Nataraj,
the conversion relation is
1 W = 1 J s-1.
So, if you want to get MJ m-2 day-1
(SRAD in DSSAT) you will need to
multiply your hourly values (Wm-2)
by 0.0036 and sum these values
over the whole day (integration).
Likewise, you could also sum up the
10 minute records using a conversion
factor of 0.0006.
Good luck.
Matthias