This "before it evaporates" is kind of confusing. Does it describe "human use and crop production" or the whole sentence of "rising temperatures...crop production"?
If it describes the whole sentence, then it's a contradiction that rainwater will diminish before evaporation, as it diminishes after evaporation. On the other hand, if it describes "human use and crop production", it doesn't seem a normal form.
ex)It is known that the sum of evaporation and transpiration of water through the leaves of plants will increase with temperature, so that in hot environments, rising temperatures will make rainwater less available for human use and crop productionbefore it evaporates. Rising temperatures will also accelerate the melting of glaciers and snow in the high mountains.....
Most of that rainwater will be hogged (held) by certain types of trees and plants. By the time the rainwater, kept in the possession of those greedy trees and plants, evaporates, too much time will have passed and we won't have as much water for human use and crop production as is necessary.
The amount of rainwater that is taken in by the leaves and released (through evaporation) increases as the temperature rises. So, less rainwater is available for human use and crop production in the short time before that rainwater evapaorates too.