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I undertook a 2 year project analyzing data from over 50 years of monitoring in the Northern Territory of Australia and it was one of the hardest things I've done. Monitoring groundwater levels and correlating with rainfall, recharge and extraction is incredibly difficult let alone the second step of then using that data to make predictions.

As already mentioned it is important to have decent length records of groundwater levels and rainfall, but that is only the start. Two key variables that can be hard to assess are the changes in weather over the years and the volume of water being extracted. Without really good data it is very difficult to accurately predict what will happen to groundwater levels in the future. In areas with shallow and regional aquifers it is essential to have information on the aquifer and where the wells are screened as you can end up monitoring the wrong system. Depending on the location you might be looking at aquifers that are recharged over centuries, and local weather patterns are almost irrelevant (as is the case in the central desert of Australia). As with many things groundwater, it's complicated!

I could go on and on about this, but I'll try not to. The short answer to your question is; Historical rainfall data is one of several variables that can be used in predicting groundwater levels, however the validity of the prediction will depend on factors including the completeness of the data set, the aquifer under investigation, the availability of groundwater level records, the extent and relative location of extraction, and an understanding of future resource use.

In my analysis of data I found it very illuminating to do simple hydrographs with groundwater level against rainfall data over time, where there was understood to be seasonal variation in groundwater levels related to rainfall (e.g. rapid recharge in wet season). It's simplistic but as a visual representation of data it can be very powerful. Trend analysis can then be done showing a few scenarios;

• groundwater level not changing over time
• groundwater level increasing over time
• groundwater level varies with rainfall (seasonal) - can use multiple year data sets to establish if there is an overall rising or falling trend
• groundwater level varies not related to rainfall = extraction influence, over extraction, recharge from location/time outside of monitoring area, bad monitoring data!