Determine the $4-$hour unit hydrograph using the following data for a watershed having a drainage area of $200$ km$2,$ assuming a constant rainfall abstraction rate and a constant baseflow of $20$ m$3/$s$.$

Four$-$hour period $1234567891011$

Rainfall $($cm$)1\mathrm{.}02\mathrm{.}54\mathrm{.}02\mathrm{.}0$

Storm flow $($m$3/$s$)20306095130170195175702520$