If the upper reaches of the rivers are verdant forests, the soil and vegetation will absorb great amounts of rain and will be able to support plentiful groundwater that acts as a natural "green reservoir."
河流的上游若是森林茂密，降雨被大量地吸附於植物、土壤中，可以涵養豐沛地下水源，也是一座天然的「綠色水庫」。