Lecture 3 Hydrologic Cycle

The Hydrologic Cycle (A, O, G, and U)

Earth’s Water Distribution

Land-Ocean Transfers fluxes in cm/yr (adjusted for area of land and ocean) Since the areas of land and ocean are different, the land-ocean water exchanges by atmospheric transport and river runoff have different values depending on the reference area, as indicated by the parentheses. The smaller values are those referenced to the larger oceanic area. Ocean transfers water to land in atmosphere Land returns this water in rivers Most precip over land (48/75=64%) is “recycled” water

Groundwater Creviced: crack Water table:地下水位

How rivers work: The role of groundwater http://www.groundwateruk.org/html/forum/forum_role.htm

The land that a river drains is called the catchment and a river of this size could collect the rain that falls on an area of several hundred square kilometers. 集水區(流域)為河川行水區域，一般河川行水區的大小約為數百平方公里，並匯集此區域內降下的所有雨水。

The fate of each drop of rain depends where and when it lands The fate of each drop of rain depends where and when it lands. Some of the rain evaporates back into the atmosphere from the surface of plants ,Water hitting the ground can also evaporate ,as well as soak into the soil .Once in the soil it can be used by plants before being passed back into the atmosphere. 每滴雨水的命運端看它從那裡及何時落至地面。有些雨水落在植物葉面上會經由蒸發而回到大氣中。而落於地面的雨水也會發生蒸發或滲入土壤的情形。一旦滲入土壤的水份在回到大氣之前可能被植物所吸收利用。

The amount of rainfall that becomes run-off depends on how much rain there is and when during the year it falls. It also depends on the type of land on which it falls. Ploughing for example, will reduce runoff by holding-up the water, allowing it soak into the ground . 多少的降雨形成地表水乃是依降雨量的多寡及降雨的季節而定，當然，也受降雨所落下的集水區土壤種類的影響。舉例來說，耕作會保持住地表水以減少地表逕流，增加地表水滲入地表土壤。

Runoff is one way for water to reach the river but if this was the only way then rivers would dry-up when the rain stopped. This doesn’t happen, so there must be some other source of water. To understand we need to follow the path of raindrops which reach the ground surface and pass into the soil. 逕流是水到達河川的一種方法；但是，若這是地表逕流到達河川的唯一方式，則河川將會在雨停時發生乾旱的情形。事實上，河川的水永不停止，因此，一定存在有其它的水源。為了確實了解此一情形，我們需要追踪雨水抵達地表和滲入到土壤的路徑。

What happens when water hits rock depends on the nature of the rock What happens when water hits rock depends on the nature of the rock. If it’s made up of grains, like this sandstone, with spaces in between which are connected, then water will pass through and the rock is said to be permeable. 當在土壤中的水遇到岩石時所發生的情形是受岩石的質地影響。如果岩石是顆粒狀構造（例：砂石），其中的孔隙均相互連通，則此時的水分可在孔隙中移動，稱此岩石是可透水的。

It’s not just sandstones that are permeable It’s not just sandstones that are permeable. Chalk, made up of the remains of countless tiny shells is also permeable mainly because there are cracks in the rock. 可透水的岩石不僅止砂岩。由無數微小甲殼軟體動物的化石所組成的石灰岩也是可滲透的，因為此種石灰岩中具有裂縫。

The percentage of the rock that is made up of spaces is known as the rock’s porosity. The porosity of sandstone can be 20% or more, which means that a cubic metre of rock say the size of a large fridge, can absorb 200 or more litres of water. 由岩石所含孔隙的總體積的比率叫作岩石的孔隙率。砂岩的孔隙率可能是20%或者更多，這就好比在一個像冰箱大小的一立方米岩石中，能夠吸收200或者更多公升的水。

Water, that has passed through the soil will continue to move down through the underlying rock until it reaches a level where all the pore spaces are full of water. This level is called the water table. Water moving in the rock, below the water table is called groundwater. Layers of rocks that are permeable and allow a significant amount of water to flow through them are called aquifers. 水向下滲透穿過了土壤及其下面的岩石，再繼續向下滲透直到達孔隙均充滿水分的地層為止。這個時候的水位叫做地下水位。在地下水位下的岩石中流動的水叫作地下水。岩石層為可透水的且有大量的水在其中移動者稱為含水層。

These are the principal aquifers of the UK These are the principal aquifers of the UK. Chalk and particular sandstones, make up the 2 most important aquifers. About a third of Britain’s public water supplies come from aquifers, but in some regions of the country it’s as much as three quarters. 英國有很多的主要含水層，其中，石灰岩和特定的砂岩組成了2個最重要的含水層。在英國，公共用水的三分之一水量其來源抽取含水層的地下水；但是在英國的某些地區其公共用水水源引用地下水水量高達四分之三的總公共用水量。

The shape of the water table follows that of the ground surface The shape of the water table follows that of the ground surface. Groundwater flows from areas where the water-table is high, to areas where it’s low. When the water table meets the ground surface, water leaves the rock, either as a spring or by seeping directly into rivers. The time the groundwater takes to travel to the river will depend on how permeable the rock is and on whether it follows a shallow or a deep path. Either way it can be in the ground for a long time, from months, up to centuries in some cases. 地下水位的形狀分佈隨地面起伏而有所變化，地下水由水位高的區域其往水位低的地方流動。一旦地下水位高達地面時，地下水便以湧泉，或滲流方式穿過岩石直接流入河川中。而地下水由地底下流入河川所經的時間受岩石的可透水性及地下水在岩石中移動的路徑深或淺的影響。任何一種地下水移動的方式，其所需時間，短則數月，長則（在某些案例中）可達幾個世紀。

The reason the stream dries-up is because the water table that feeds it with groundwater falls below the bed of the river during the summer. As it drops, the point at which it curs the ground surface moves down the valley. There slopes are gentle, that might be kilometers away from the source of the stream. 河流乾涸是因為地下水位於夏季時低於河床所致 (指英國)。當水位下降時，與地面的交界處將降至山谷低處。如果河床坡度平緩，則此新的地下水位與地面交界點可能是離上游河川達數公里遠的下游處。

It’s not surprising that if millions of liters of water are being extracted daily from this borehole, that the pumping will have an effect on the water-table. The greatest drawdown in the water-table occurs around the borehole but the effects may be seen kilometer away. It is possible that the fall in the water-table can reduce the amount of groundwater that feeds into nearby rivers and streams so reducing their flow 如果每天從鑽孔抽取大量地下水，水位會有明顯改變。其中最大洩降發生在鑽孔處，但是帶來影響更可達數里之遙。因為地下水水位下降，造成地下水流入河川的水量減少，河川流量也因而減少(positive feedback)。

However, it’s often difficult to say how times in some rivers are due to pumping and how much they are due to natural changes in the water-table. Where it is recognized that groundwater pumping is having a bad effect on particularly important stretches of river, water together with the environment regulators to find ways of improving the flow. 可以確定的是，抽取地下水對河川蜿蜒結構影響頗鉅，目前水公司與環保當局正積極設法，改善河川流量問題。然而，判斷究竟是人為抽取或地下水位的自然改變所造成河川流量的減少則是一件困難的事情。

These are only a few examples but they highlight the challenges that we face to meet the ever-increasing demands of society while keeping the impacts on the environment to an acceptable level. This is particularly important given the impact that climate change is likely to have on the country’s water resources. 上述僅為一些例子，也正凸顯了隨著時代變遷，社會需求日新月異，努力做好環保，將傷害降至最低，一直是我們所面臨的挑戰。而氣候變遷對整個國家的水資源，也佔了相當重要角色。

Climate Hydrology Water Resources

Wind Rl Precip Temp Rs Climate SST

Snowmelt Hydrology Evapotranspiration Orography Vegetation Runoff Groundwater

Water Resources Recreation Water Supply Irrigation Hydropower Flood Control Navigation Fisheries

Atmospheric Water annual mean precipitatble water (mm) Mean ~ 25 mm (1 inch) Mean precip rate is about 2.6 mm/day Residence time ~ 9 days Near balance: E ~ P ~ 2.6 mm/day Averaging 1968-1996

Surface Water Balance Equation P = Q + E + ∆S P : precipitation Q : runoff E : evapotranspiration ΔS: change in storage (in soil or the bedrock)

Atmospheric Water Balance P-E = Df = fin - fout Net water imported by atmosphere Water vapor is imported into the tropics and midlatitudes Water vapor is exported from the subtropics

Potential Evaporation (PE) (or potential evapotranspiration) Definition: The rate of evapotranspiration that would occur if the surface was always wet Reason: to prevent the more difficult issues of soil moisture availability and physiological processes in living plants PE measures the effects of energy supply and air humidity on the actual evapotranspiration rate. Surface and air temperatures, insolation, and wind all affect PE. If estimated PE exceeds the actual evaporation (hence precip.), then a moisture deficit exists, causing a dry land, and vice versa. Thornthwaite: “the water loss which will occur if at no time there is a deficiency of water in the soil for use of vegetation”

Various hydroclimates “PE” tracks temp and radiation Winter rain/summer dry climates on the US West Coast Summer rain climates in tropics

Various hydroclimates Actual E is strongly limited by water availability in many places (E ~ P rather than PE) Some midlatitude locations (e.g., Boston) have little seasonality in P (why?), but strong seasonality in E Weird place?

4 basic types of water balance cycles Winter precip. maximum with dry summer (west coast of continent) Spring and summer precip. maximum followed by a drying period in late summer (interior of continent) Less seasonality of precip. but PE peaks in summer (east coast of continent) Winter precip. minimum but maximum in the warmest season (tropical-subtropical latitudes)