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1、Underground water Of all the earths water, 97% is found in the oceans, 2% in glaciers and only 1% on land. Of this 1% almost all (97%) is found beneath the surface and called sub-surface or underground water. Most of this water eventually finds its way back to the sea either by underground movement,
2、 or by rising into surface streams and lakes.These vast underground water deposits provide much needed moisture for dry areas and irrigated districts. Underground water acts in similar ways to surface water, also performing geomorphic work as an agent of gradation.Even though man has been aware of s
3、ub-surface water sinceearliest times, its nature, occurrence, movement and geomorphic significance have remained obscure. Recently, however, some answers have been found to the perplexing questions about underground waters relationship to the hydrological cycle.1.Source of Underground Water Since th
4、e days of Vitruvius at the time of Christ, many theories have been presented to explain the large volume of water underneath the earths surface. One theory was that only the sea could provide such large quantities, the water moving underground from coastal areas. Vitruvius was the first to recognize
5、 that precipitation provided the main source of sub-surface water, although his explanations of the mechanics involved were not very scientific. His theory, now firmly established, is termed the infiltration theory, and states that underground water is the result of water seeping downwards from the
6、surface, either directly from precipatation or indirectly from streams and lakes. This form of water is termed meteoric. A very small proportion of the total volume of sub-surface water is derived from other sources. Connate water is that which is trapped in sedimentary beds during their time of for
7、mation. Juvenile water is water added to the crust bydiastrophic causes at a considerable depth, an example being volcanicwater.2 Distribution of Sub-surface Water During precipitation water infiltrates into the ground. Under the influence of gravity, this water travels downwards through the minute
8、pore spaces between the mit particles until it reaches a layer of impervious bedrock, through which it cannot penetrate. The excess moisture draining downwards then fills up all the pore spacesbetween the soil particles, displacing the soil air. During times of excessive rainfall such saturated soil
9、 may be found throughout the soil profile, while during periods of drought it may be non-existent Normally the upper limit of saturated mil, termed the water table, is a meter or so below the surface, the height depending on soil characteristics and rainfall supply. According to the degree of water-
10、occupied pore space, sub- surface moisture is divided into two zones: the zone of aeration and the zone of saturation. (a) Zone of Aeration This area extends from the surface down to the upper level 0f saturation-the water table. With respect to the occurrence and circulation of the water contained
11、in it, this zone can be further divided into three belts: the soil water belt, theintermediate bell and the capillary fringe.(1) Soil Water Belt Assuming that the soil is dry, initial rainfall allows water to infiltrate, the amount of infiltration depending on the soil structure. Soils composed main
12、ly of large particles, with large pore spaces between each particle, normally experience a more rapid rate of infiltration than do soils composed ofminute particles. No matter what the soil is composed of some water is held on the mil particles as a surface film by molecular attraction, resisting gr
13、avitational movement downwards. The water held in this manner is referred to as hygroscopic water. Even though it is not affected by gravity it can be evaporated, though not normally taken up by plants.(2) Intermediate Belt This belt occurs during dry periodswhen the water table is at a considerable
14、 depth below the surface. It is similar to the soil water belt in that the water is held on the soil particles by molecular attraction, but differs in that the films of moisture are not available for transpiration or for evaporation back to the atmosphere. In humid areas, with a fairly reliable rain
15、fall, this belt may be non-existent or very shallow. Through it, gravitational or vadose water drips downwards to the zone of saturation.(3) Capillary Fringe Immediately above the water table is a very shallow zone of water which has been drawn upwards from the ground-water reservoir below by capill
16、ary force. The depth of this zone depends entirely on soil texture, soils with minute pore spaces being able to attract more water from below than soils with large pore spaces. In the latter types of soil the molecular forces are notabie to span the gaps between soil particles. Thus, sandy ils seldo
17、m exhibit an extensive capillary fringe, merging from soil water through to the zone of saturation. (b) Zone of SaturationThe zone of saturation is the area of soil and rock whose pore spaces are completely filled with water, and which is entirely devoid of soil air. This zone is technically termed
18、ground water even thoughthe term broadly includes water in the zone of aeration. The upper limit of the zone of saturation is the water table or phreatic surface. It is difficult to know how deep the ground-water zone extends.Although most ground water is found in the upper three km of the crust, po
19、re spaces capable of water retention extend to a depth of 16 km. This appears to be the upper limit Of the zone o rock flowage where pressures are so great that they close any interstitial spaces. The upper level of the saturated zone can be completely plotted by digging wells at various places. Stu
20、dies suggest two quite interesting points. (i) The water table level is highest under the highest parts of the surface, and lowest under the lowest parts of the surface. Hills and mountains have a higher-level phreatie surface than valleys andlakes. The reason for this is that water continually perc
21、olating through the zone of aeration lifts the water table, while seepage from the ground-water zone into creeks and lakes lowers the level. (2) The depth of the Water table Deiow the land surface is greatest in upland areas where the water moves quite freely downhill under gravity. Close to streams
22、, lakes, lakes and swamps tlne water table is close to, if not at, the surface, as water from the higher areas builds it up.译文：地球上的总水量中，95在海洋，2在冰川中，只有1在陆地上。陆地上的水几乎全部(97)埋藏在地面以下，称为地下水。大部分地下水或通过地下流动，回到海洋；或先进入河流和湖泊，最终又回到海洋。 这些广阔的地下含水层为干旱地区和灌溉区域提供了迫切需要的水分。地下水的作用与地表水的作用相似，也以均夷作用塑造着地貌。尽管人类自古以来就知道地下水，但对它的特
23、性、发生、运动和地貌意义还不清楚。然而，近来关于地下水与水文循环关系的这些错综复杂的问题已找到了一些答案。 1地下水的来源 自从公元维特鲁维亚时代以来，为了解释在地面以下存在大量的地下水，已经提出了许多理论。一种理论认为：只有海洋能提供大量的地下水，地下运动的水来自海岸带。维特鲁维亚是第一个认识到降水是地下水的主要来源，尽管他对所涉及的力学方面的解释很不科学。 现在，他的理论已经确立，称作渗透理论：认为地下水是水从地表渗入到地下的结果，或者直接来自降雨或者间接来自河流和湖泊。这种水称作天落水。地下水总量中的很小一部分来自其他水源。原生水是在沉积岩形成时滞留在其中的水。岩浆水是由于在相当深处的地
24、壳运动对地壳所添增的水。 2地下水的分布 ， 在降雨期间，水渗入地下。在重力的影响下，这种水通过土壤颗粒间的孔隙向下流动直至水流到达不透水层为止。向下移动的过量水分，充满了土壤颗粒之间的孔隙，挤出了土壤中的空气。在雨水过多时，整个土壤剖面达到饱和状态，而在干旱时期就不存在饱和土壤。通常，饱和土壤的上部界限称为地下水位，约在地面以下1米，其高度取决于土壤特性和降雨量。根据水充满孔隙的程度，地下水分成两层：包气带和饱水带，21 包气带 这一区域从地面向下延伸到饱和水面地下水位。根据该层中所含水的形成和运行情况，可进一步划为3个带：土壤水分带、中间带和毛细饱和带。 (1)土壤水分带。假设土壤是干燥的
25、，初期降雨量就向土壤中人渗，入渗量取决于土壤结构。主要由大颗粒构成的土壤，在每个颗粒之间有很大的孔隙，在正常情况下入渗速度要比在小颗粒组成的土壤中快得多。不论土壤由什么构成，土壤颗粒表面由于分子引力吸附着一些水，就形成一层水膜，阻止重力水向下运动。以这种方式保持的水称为吸湿水。即使它不受重力的作用，它也能蒸发，但是通常不能被植物吸收。 (2)中间带。中间带发生在干旱季节，地下水位在地面以下相当深的地方。中间带与土壤水分带相似，也是通过分子引力把水分保持在土壤颗粒上。但差别在于：中间带的水膜不能通过蒸腾或蒸发回到大气层。在湿润地区，降雨相当可观，中间带可能不存在或者非常浅。重力水即渗漏水通过中间
26、带向下渗漏到饱水带。 (3)毛细饱和带。这带位于地下水位以上，是个很浅的含水带，这些水是由毛管力从地下水库吸引上来的。这一层的深度完全取决于土壤的结构，含有微小孔隙的土壤比含有大孔隙的土壤能从下面吸取更多的水。在后一种类型的土壤中，分子引力不能跨越土壤颗粒间的空隙。因此，砂质土很少出现大面积的毛细饱和带，而土壤水分带和饱和水直接相通。 22 饱水带 饱水带是孔隙完全被水充满、没有一点空气的土石层。这一层的技术术语叫做地下水，尽管这一术语广义上也包含包气带的水。饱水带的上部边界是地下水位或地下水面。要了解地下水层的深度是困难的，虽然大多数地下水是在地壳表层三千米以内发现的，但是能够保存水的微小孔隙可以延伸到16千米的深度，这似乎就是贮水岩层的上限。那里的压力大到足以充满任何孔隙。在不同的地方挖探测井就能完全标绘出饱和层的上界面。通过研究提出了两个十分有趣的论点: (1)在地面最高的地方，地下水位也最高；而在地面最低的地方，地下水位也最低。丘陵和山脉的地下水位比峡谷和湖泊的地下水位高。这是因为水通过包气带不断地渗透提高了地下水位；地下水层的渗流进入河沟和湖泊，从而降低了地下水位。 (2)在山区高地，地下水位的埋藏深度最大，在重力作用下，那里的地下水非常顺畅的向下流动，靠近河流、湖泊和沼泽地区时，地下水位即使未达到地表，也接近地表，因为水从高的地方汇集而来时地下水位升高。