When I first arrived in Granada in 1987 there was a serious water shortage and the water was cut off every evening after 6pm.
Since then they have built lots of big reservoirs.
The biggest one is Rules which is on the road from Granada to the coast which has a capacity of 112 cubic hectometres.
A hectometres is 1 million cubic metres or a cube which is 100m x 100m x 100m.
I read that the RULES dam was the largest piece of concrete in Eurpoe at one time.
The winters of 2009/2010 and 2010/2011 were very wet and the the reservoirs were almost 100% full. The average in the last 10 years is 45%.
The last couple of years (2015 + 2016) have been dry and if it continues it could get worrying.
An excellent website with information about all the embalses (reservoirs) in Spain is this one.http://www.embalses.net/