DSI calculation

Hello,

I have an rainfall anomaly data prec(month,gridcell) where month =
252 and gridcell = 30960.
I am trying to calculate three-month drought severity index (DSI3)

The definition of DSI3

Rainfall anomaly = Xt @ month=t
If Xt < 0 and the rainfall in three-month period, Xt, Xt-1, Xt-2, is
lower than its three monthly mean i.e. (Xt+Xt-1+Xt-2)< (1/3)(Xt+Xt-1
+Xt-2)
Then DSI3(t)=-Xt

For time t+1, if it satisfies the previous conditions,
Then DSI3(t+1)=-(Xt+Xt+1)

But if rainfall anomaly is positive i.e. Xt+1> 0 and the rainfall in
three-month period, Xt+1, Xt, Xt-1, is lower than its three monthly
mean i.e. (Xt+1+Xt+Xt-1)< (1/3)(Xt+1+Xt+Xt-1), Then DSI3(t+1)= -(Xt-Xt
+1)

If, for time t+1, it does not stratify any previous conditions then
DSI3(t+1)= 0

I used do loops and if statements to calculate DSI but it took long
time. I guess I need to use “where” or “ind” to make it faster.

Any help to do these calculations using above mention function will
be appreciated

Thanks

Debasish

Debasish PaiMazumder, PhD
Hydrology & Climate Science
Postdoctoral Researcher at UQÀM/Ouranos
550, Sherbrooke west, 19th floor, west tower,
Montréal, QC, Canada H3A 1B9
Tel: (514) 282-6464-343
      (514) 937-0042 (R)
email: debasish_at_sca.uqam.ca

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Received on Wed Aug 12 2009 - 11:59:12 MDT