Hi

As according to thiesssen there is only one station that influences the dams watershed, the isohyetal method should not bring much information... you could try somethING simpler, for instance use the time series of the station that is representative for the watershed according to thiessen as 75 pent ans use 25 pent of the average of the other stations. This is much quicker to do...  then compare the two time series of the representative station alone with this weighted average . If the time series are really different then you might try to get the isohyetal method if it can bring something more precise.

Dear Gil Mahe

how i can use time series please help me thank you!!

you can use google the book , also can be download by the net (www.sciencereading.cn). If you use the net, you can't get the CD resource with the book. If you can't find it, give me your email.

Thank you all!

I solved my problem by using IDW METHOD.