The hydrological cycle refers to the continuous movement of water on, above and below the surface of the earth. It mainly involves the processes of evaporation, condensation, precipitation, infiltration, percolation, interception and runoff/overland flows. The main source of energy that drives the cycle is solar energy and therefore the main changes in energy involve changes in temperature. When surface water is heated, it gains energy and changes from liquid state to gaseous state. This results in a cooling effect to the surrounding region (Arctic Climatology and Meteorology) Transpiration also contributes in adding water vapor in the atmosphere; this is referred to as evapotranspiration.
Once the water is in the atmosphere, condensation, which is the reverse of evaporation, takes place. The condensation process involves release of energy to the surrounding in form of sensible heat. Further release of energy leads to freezing which is the solidification of liquid water which may eventually fall as snow. When vapor condenses, it precipitates. Precipitation involves all the processes through which water falls from the atmosphere to the land inform of rain, hail, fog, sleet, snow fall, mist etc. (Arctic climatology and meteorology). Some of the water is intercepted by plant foliage and thus does not reach the surface. When the water reaches the earth surface, some of it forms the surface run off which is the water that moves across the land confined in channels-streams and rivers (Keith Beven, Robert E. Horton's, 2004). This happens when the surface gets saturated with water such that it cannot infiltrate any more.
When the water reaches the surface, infiltration takes occur. Infiltration is the process through which water enters the soil by capillarity action and gravity. The infiltrated water forms underground water which is held in aquifers. Water table is the part of the soil where the pressure head is equal to the atmospheric pressure i.e. gauge pressure is zero. It is the subsurface part which is saturated with ground water. Saturation may extend above the water table as water is held in the pores by surface tension (Freeze, R. Allan; Cherry, John A. (1979). An artesian well develops when an artesian aquifer has a positive pressure. When the natural pressure is enough water may reach the surface. When water is pumped out of an aquifer, a cone of depression develops, water flows from the surrounding aquifers into the well. This is because the pressure head is reduced and hence the water table is lowered. Percolation takes place when water further enters the underground water storage.
The general water budget is where f is infiltration, B1 is boundary input, B0 boundary output, P is precipitation, T is transpiration, E is evaporation, ET is evapotranspiration, IA is initial abstraction, S is storage-retention and R is surface runoff. This process can be summarized diagrammatically as shown
Food chain refers to the monophagous paths in a food web from the producing species (producer) to the end species (decomposer). For example,
The tropical level forms the biome which mainly comprise of grass. The grass is therefore the primary producer. The grasshoppers feed on the grass and hence form the primary consumers. The rats feed on the grasshoppers and thus are the secondary consumers, the snake feeds on the rats and hence form the tertiary consumer and the hawk feeds on the snakes becoming the quaternary consumer.
A biogeographer focuses on the spatial relationships between the organisms and emphasize on aspects like the population, rates of population growth, and the environmental characteristics that govern the existence of each of the species among other aspects. A bio geographer seeks to know where each species exist and why? Quammen, David (1996). A biologist shall consider the flow of energy between the organisms in a food chain and not the spatial distribution of the organism.
Biome is defined as the continuous area with similar characteristics climatically on the surface of the earth like a continuous community of animals, plants and soil organisms (The World's Biomes). Oftenly Biomes are referred to as ecosystems. Biomes are determined using factors like plant structures which may include trees, grasses, shrubs; leaf types like needle leaves or broadleaved; plant spacing like forests, savanna, woodlands and climate.
A zoogeographic region is a region with organisms sharing similar historical background. The species in a zoogeographic region evolve in isolation for long period of time separated by a geographic feature like a vast desert, mountain ranges, oceans which are barriers to migration (Dinerstein, Eric; David Olson; Douglas J. Graham; et al.1995)
Hot Desert biome consists mainly of shrubs and short woody trees. The leaves are small with thick cuticles that store water, in some species the leaves are reduced to spines like cacti. The species are also thick-barked to store water. The soil is usually sandy and rocky with good drainage. The climatic conditions comprise of very low rainfall, high temperatures during the day and very low temperatures during the night. A grassland biome on the other hand consists of lands dominated by grass with moderate climatic conditions, i.e. moderate temperatures and rainfall.
According to Michael E. Ritter (2009) Soil formation is influenced by factors like parent material, topography, climate, time and living organism. Soils formed in the forested regions are more developed than those in steep slopes. This is because of mainly the abundance of the biomass in the forested region compared to the steep slopes. The grassland too have soils which are more developed than those found in the steep slopes due to high organic content and less erosion. The steep slopes experience high rates of soil erosion and lesser living organisms and hence the soil has little time to accumulate. Therefore the steep slopes have thin soils. The climatical conditions in the forested regions and the grass lands also lead to different soils.
Parent material in soil formation is the single most influential factor. The parent material determines the soil mineral characteristics. It also determines the texture of the soil and to great extend the nutrient levels of the soil. The parent materials are classified depending on how they are deposited. Transported parent materials are those which are transported either by water, wind, ice or gravity while residual materials are those which are weathered from primary bedrock while cumulous materials are those organic materials that have accumulated in place overtime.
Aeolian process leads to formation of loess which is about 60-90 % silt. Alluvial, lacustrine or marine are water transported parent materials which mainly comprise of sediments in the deposition areas. Parent materials moved by gravity lead to formation of talus cone at the base of slopes. In case of accumulation of a lot of organic materials, the parent material might be outweighed. A similar scenario occurs in regions of deposition where the primary rock in the area has little influence on the formation of the soil found in the area.
There exist two main forces that shape the surface of the earth. These force acts both on the surface and the interior of the earth. Regional isostasy of lithospheric flexure refers to the process by which the outer layer of the earth bend under action of the forces like weight or changes in ice thickness ( A. B. Watts). When weight is offloaded from a region, the internal forces push outward in response to the weight lost and at the same time, in the area of deposition the area sink in equal response (US Geological Survey Monograph). This maintains the equilibrium between the internal and external forces in shaping the earth.
The surface of the earth is shaped by weathering, erosion and mass wasting which are all called denudation. Weathering refers to the breakdown and disintegrations of rocks in situ (without movement) (Goudie, A.S. and Viles H. (2008). It is categorized as physical, chemical and organic depending on the agents causing it. Erosion is the removal of soil and rock from the surface of the earth caused by exogamic forces like wind, water or ice (Toy, Terrence J. et al (2002) whereas mass wasting is the downward movement of weathered materials under the influence of gravity, (Monroe, Wicander , 2005). These processes shapes the surface of earth by mainly moving materials from one location to another and at the same time the agents sculpture the surface forming different features.
The theory of plate tectonics superseded the original continental drift theory. The plate tectonic theory describes the large scale movement of the earth’s lithosphere. It is taken as a modernized continental drift theory. The theory explains that the continents are made up of solid layers called plate which are floating on asthenosphere and are moving toward, away or past each other. When the plates converge they form a convergent boundary, diverge they form a divergent boundary, and a transform if the move past each other ( Condie, K.C. 1997).
In 1965, Tuzo Wilson added the idea of transform fault, 1966, he introduced the Wilson cycle (Wilson 1966). In 1967, Jason Morgan proposed that the earth surface is covered by 12 plates and at the same year Xavier Le Pichon published 6 main plates. The plate tectonic explained how basins and mountain ranges are formed due to the apparent movement of the plates. In the previous year, geologist held the assumption that earths major features were fixed and most geological features like basins and mountain ranges were due to vertical movement of the earth. This was mainly described in the geosynclinals theory.
The boundaries of the plates are unstable and this explains why there are many earthquakes and volcanic activities associated with them. In the divergent boundaries, new material move from the interior of the earth leading to volcanic activity. In the convergent boundaries, the plates collide leading to breaking of rocks and consequently earth quakes.
There are several proofs that make the plate tectonic the most acceptable theory. The shapes of the continents indicate that they were once joined together. Considering the western coast of Africa and the Eastern coast of America, if the two are brought together they would fit into each other indicating that they were once joined together. The mid-Atlantic ridge is a clear indication that he plates are drifting away from each other and at the same time in zones of convergence mountains are been formed ( Lippsett 2001, Lippsett 2006). Fossils found at the edges of the plates are similar indicating that the continents where once joined.
The debate on glaciation has taken centre stage over the years. This is because the scientist differs on the basic considerations to determine or describe a period. Considering the fact that there exist large packs of glacier, then it would basically mean that the ice age is not yet over. The continuing melting of ice can also indicate that the ice age over. But there is that there is no any empirical data that can entirely back any one side on this issue.
References
Arctic Climatology and Meteorology. Evaporation. Retrieved on 2013-12-14.
Keith Beven, Robert E. Horton's(2004). perceptual model of infiltration processes, Hydrological
Processes, Wiley Intersciences DOI 10:1002 hyp 5740
Freeze, R. Allan; Cherry, John A. (1979). Groundwater. Englewood Cliffs, NJ: Prentice-Hall.
OCLC 252025686.
Quammen, David (1996). Song of the Dodo: Island Biogeography in an Age of Extinctions.
The World's Biomes, Retrieved August 19, 2008, from University of California Museum of Paleontology
Dinerstein, Eric; David Olson; Douglas J. Graham; et al. (1995). A Conservation Assessment of
the Terrestrial Ecoregions of Latin America and the Caribbean. World Bank, Washington
DC.
Michael E. Ritter. Factors Affecting Soil Development, Soil Systems, The Physical
Environment: an Introduction to Physical Geography, University of Wisconsin, Stevens
Point, 1 October 2009, retrieved 3 January 2012.
Isostasy and Flexure of the Lithosphere, A. B. Watts, University of Oxford. DOI:
10.2277/0521006007
"Lake Bonneville" US Geological Survey Monograph No. 1. 1890. 438 p.5
Goudie, A.S.; Viles H. (2008). Weathering Processses and Forms". In Burt T.P., Chorley R.J.,
Brunsden D., Cox N.J. & Goudie A.S. Quaternary and Recent Processes and Forms.
Landforms or the Development of Gemorphology 4. Geological Society. pp. 129–164
ISBN 1-86239-249-8.
Toy, Terrence J. et al (2002). Soil Erosion: Processes, Predicition, Measurement, and Control.
John Wiley & Sons. p. 1. ISBN 978-0-471-38369-7.
Monroe, Wicander (2005). The Changing Earth: Exploring Geology and Evolution. Thomson
Brooks/Cole. ISBN 0-495-01020-0.
Condie, K.C. (1997). Plate tectonics and crustal evolution (4th ed.). Butterworth-Heinemann.
p. 282. ISBN 978-0-7506-3386-4. Retrieved 2013-12-14.
Wilson, J. Tuzo (August 13, 1966). "Did the Atlantic close and then re-open?". Nature 211 (5050): 676–
681.
Wilson, J. Tuzo (December 1968). "A Revolution in Earth Science". Geotimes (Washington DC) 13 (10):
10–16.
Lippsett, Laurence (2001). "Maurice Ewing and the Lamont-Doherty Earth Observatory". Living Legacies. Retrieved 2013-12-14.
Lippsett, Laurence (2006). "Maurice Ewing and the Lamont-Doherty Earth Observatory". In
William Theodore De Bary, Jerry Kisslinger and Tom Mathewson. Living Legacies at
Columbia. Columbia University Press. pp. 277–297
