If the water runs off into streams and rivers and on into the ocean, then it can be said to be "wasted", but if it soaks back into the ground and rejoins the gr

If the water runs off into streams and rivers and on into the ocean, then it can be said to be "wasted", but if it soaks back into the ground and rejoins the groundwater, it is still available in the aquifers. This is the premise of the vast majority of groundwater recharge projects. The trick is to know how quickly that water infiltrates given soil composition and local geology. Evaporation is not a concern if the water infiltrates quickly. It's barely a concern as it is unless your surface-area-to-volume ratio is relatively high (i.e. wide shallow pools), which is why on-surface reservoirs are as popular as they are. (And anyway, local humidity is slightly elevated surrounding bodies of water so the partial pressure of the atmospheric water is already pretty high and evaporation happens slower.) The real cause of groundwater aquifer depletion is a law of conservation. That when the water goes into plants and animals and then those products are shipped away, there's nothing bringing the water back into the parched lands except for the rather slow and increasingly unpredictable patterns of the water cycle. California's Central Valley is facing this issue. The snowpack from last year made it abundantly clear how important it is to recharge groundwater using what's pouring off the mountains, rather than letting it flow to the ocean. Use it or lose it, indeed.