Stats Question

#1
Hi,

I'm a farmer that is working on a crop model to forecast our crop yields. I know very little about stats so I'm sure this following question is very simple for all of you!

I am trying to work out how much of our summer rainfall is converted to available moisture for the crops to use.

We use the rule of thumb that around 25% of summer rainfall is converted to available crop moisture, however I would like to find out more accurately what this figure is. 100% of winter rainfall is used by the crop. Is there a way statistically that using our historical rainfall and yields data set to determine what % of summer rainfall is converted to available moisture? E.g. for year 2000 125mm*?% + 328mm = 3.06

I have attached the data set.

Thanks for your help