In a localized sense, the loss is of the water that's readily available for use in that region. I suppose you could think of it as a sort of "debt," where you measure the loss in terms of energy or dollars required to restore the water at the rate it's being consumed, for instance through desalination.
I suspect it's just simpler to call it "loss of water."
Another way to think about it, is that farming has historically occurred in places where there are decent natural growing conditions: Climate, soil, and water. If you let the water go somewhere else, through evaporation and rainfall, then you might end up having to maintain growing conditions artificially by bringing the water to where there is decent climate and soil. And again, you could measure the cost of doing that as a "debt."
I suspect it's just simpler to call it "loss of water."
Another way to think about it, is that farming has historically occurred in places where there are decent natural growing conditions: Climate, soil, and water. If you let the water go somewhere else, through evaporation and rainfall, then you might end up having to maintain growing conditions artificially by bringing the water to where there is decent climate and soil. And again, you could measure the cost of doing that as a "debt."