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土地和水通過水循環互相密切地聯繫在一起。太陽的能量推動著這種水循環以及集水流域內的其他自然循環。氣候是一個地區在長時期內所具有的天氣類型，它決定了有多少水量通過季節循環流入集水流域。降雨量和氣溫變化的季節性變更方式控制著河水流量和水的開採量。雖然每年的降雨量各不相同，但地球的總水量是有限的，不能夠有所增加。
有些降雨會滲透入土壤之中，並經過可滲透的岩石滲濾到稱作蓄水層的地下儲水庫中去。天然的地下水成為許多河流的主要水源。
工業、灌溉或家庭生活用途從地下蓄水層中抽水會減少它的儲水量。這樣下去，蓄水層總有一天會乾枯，除非減少抽取的水量，或者增加地下水的重新注入量。乾枯的蓄水層會由於覆蓋在它上面的泥土沈積而塌陷。塌陷了的地下蓄水層不再具備原先那樣大的吸水和貯水容量，因泥土發生沈積並凝縮會導致蓄水層貯水量的下降。重新往蓄水層裏注水是困難的，因那裏的容積減少了，回收量也會大大減少。曾經依靠地下水位供水的泉水也會因過度抽水而乾涸。
氣候既會影響集水區的流失水量，也會影響對它的供水量。如果天氣炎熱，乾燥，或者風大，無遮蔽的土壤和（江，河，湖等）水面的蒸發損失將會很大。這些使得蒸發量增加。氣候因素同樣也會增加植物的蒸騰作用。與水的蒸發相比，植物的蒸騰從更深層的土壤中吸收水分，因植物的根可以伸到更深的地方，找到可以汲取的水分。在植物的生長季節，蒸騰作用最強烈；而當天氣較寒冷時，蒸騰作用變得最薄弱，這時候大多數植物處於相對休眠狀態。
版權歸屬 國際河網(IRN)
環境信託基金會(陶俊 譯，吳國玢、陳維立審校)
中英對照全文：http://news.ngo.org.tw/issue/water/issue-water-irn00101601.htm
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Land and water are closely linked through the water cycle. Energy from the sun drives this and other natural cycles in the catchment. Climate - the type of weather a region has over a long period of time - determines how much water comes to the catchment through seasonal cycles. The seasonal pattern of precipitation and temperature variation control streamflow and water production.Although the amount of precipitation can vary from year to year, the earth has a finite amount of water which cannot be increased.
Some precipitation infiltrates the soil and percolates through permeable rock into groundwater storage called aquifers. Natural groundwater discharge is a major source of water for many streams.
Pumping water from an aquifer for industrial, irrigation, or domestic use reduces the aquifer's volume. Unless withdrawals are modified or groundwater recharge is increased, the aquifer will eventually be depleted. A drained aquifer can collapse from the settling of overlying lands. Collapsed underground aquifers no longer have as much capacity to accept and hold water, because the soil settles and condenses, resulting in less volume to hold water. Recharge is difficult, volume is less, and yields are considerably reduced. Springs once fed from the water table also dry up.
Climate affects water loss from a catchment as well as providing water. In hot, dry, or windy weather, evaporation loss from bare soil and from water surfaces is high. The same climatic influences that increase evaporation also increase transpiration from plants. Transpiration draws on soil moisture from a greater depth than evaporation because plant roots may reach deeper into the available moisture supply. Transpiration is greatest during the growing season and least during colder weather, when most plants are relatively dormant.
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