[R] Summing daily data for each year

[Santosh.Aryal at csiro.au]

Hello there
 
I have a DAILY data set (as shown below) with Date and Streamflow from April 1995
to Aug 2006.  I want a yearly sum the streamflow value (for each year - first and last incomplete year).  
Does anyone know how to do this in R?
 
I have tried everything I could think of, e.g. imported the streamflow
data as time series using:
 
flowdata <- ts(ZVo[,2],start = c(1995,4,1), frequency = 7) # perhaps frequency = 365 is the better one
and tried to aggregate using aggregate() without success

>ZVo
            V1      V2
1  1995-04-01 0.002766309
2  1995-04-02 0.002402973
3  1995-04-03 0.002254335
4  1995-04-04 0.002221305
5  1995-04-05 0.002180017
6  1995-04-06 0.002031379
7  1995-04-07 0.001957060
8  1995-04-08 0.001940545
9  1995-04-09 0.001924030
10 1995-04-10 0.001783650
.
.
.
4156 2006-08-16 0.02861272
4157 2006-08-17 0.03652353
4158 2006-08-18 0.05372419
4159 2006-08-19 0.05630058
4160 2006-08-20 0.06274154
4161 2006-08-21 0.06981833

Many thanks 

Santosh