Hydrological cycle or the water cycle is a continuous circulation of water in the earth and its atmosphere. Processes of the water cycle include evaporation of water from the surface of oceans, condensation of water vapor to form clouds, precipitation, transpiration, and runoff (Marsalek et al., 2014) . The water cycle begins with the evaporation of warm surface water from lakes, oceans, and rivers. Water molecules rise to the atmosphere through a vapor or gaseous state. The lifted moist air, in its vapor state, cools and transitions to a liquid state through condensation. Transpiration, on the other hand, occurs when water evaporates into the atmosphere through the stomata in the leaves of plants. The evaporation of water from water surfaces and vegetation is known as evapotranspiration. Once the condensation process has occurred, the water returns to the earth’s surface in the form of precipitation. Some of the water that reaches the ground evaporates back into the atmosphere. Some percolate into the soil by infiltration to form groundwater. Water that remains on the earth's surface flows and empties into lakes, oceans, and streams as surface runoff.
Water is an essential natural resource for people. Human beings can barely live for a week without water. However, most of the water covering the earth's surface is saline and therefore unfit for human use and for irrigation. Clean and fresh water suitable for drinking is limited and can only be fetched from lakes, rivers, and groundwater. The process of evaporation and precipitation helps in the constant renewal and replenishment of the freshwater supply. Evaporation acts as a distillation process of the saline water to produce pure freshwater that lacks dissolved ions. Dissolved ions in saline water from oceans and seas remain in their liquid states as water changes from liquid to gas (evaporation). Condensation of the vaporized water leads to the formation of clouds and sometimes precipitation. The resultant rain flows into water bodies such as rivers and lakes as surface runoff. Some of the water seeps into the ground to form groundwater. Rainwater that falls onto land is fresh and is used f for drinking and irrigation. Though the hydrological cycle is a natural process, it can be affected significantly by human activities. Modification of the water cycle by humans is damaging and causes a lot of negative consequences. Example of human activities that affect the hydrological cycle includes the creation of impervious surfaces through urban development, wetland destruction, and the construction of dams.
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Urbanization and industrialization have led to the modification of landscapes and the destruction of wetlands. Urban development has altered the natural and physical pathways of the hydrological cycle. Urbanization has resulted in the removal of native vegetation, drainage patterns, loss of natural depressions which temporarily store surface water, and creation of impervious surfaces (e.g., roads, sidewalks, rooftops). Research findings suggest that volumes of runoff in urban areas range from 60-70 percent of the average annual rainfall while that of rural areas ranges between 10-20 percent. Highly impervious surfaces lower the rainfall absorbing capacity of the soil, lowers evapotranspiration, increases the frequency of surface runoff, and reduce groundwater infiltration and recharge. Urbanization, therefore, destabilizes processes of the hydrological cycle. Heavy rains in urban areas lead to an increased runoff which in turn causes intense local flooding. Destruction of wetlands has led to the interference of the regular and continuous flow of water in Earth’s hydrosphere. Wetlands are an essential part of the water ecosystem. Wetlands offer flood protection and control shoreline erosion. Wetlands trap excess runoff water and act as natural sponges that store and slowly release flood waters. Wetland vegetation such as trees and root mats slow down the speed of flood waters. The braking action of wetland vegetation lowers flood heights by ensuring that flood water is distributed slowly over a floodplain (Acreman, & Holden, 2013). Destruction of wetlands makes it difficult to counteract the increased rate and volume of surface water runoff in urban areas. It, therefore, becomes impossible to control floods in urban areas because the water-holding capacity of wetlands has been compromised. Increased cases of floods have made people build levees on river banks to prevent water from overflowing. Levees have proven to be advantageous to people. The building of levees has helped keep lakes and rivers at bay. Levees, therefore, act as a form of flood protection for people who live by oceans or river banks. In addition to offering protection, the flow of water in rivers is kept in check. People living near rivers are therefore in a position to successfully engage in crop cultivation and use the river water for irrigation.
Besides such advantages, levee construction comes with many disadvantages. They prevent floods in one area while diverting the adverse effects of the flood waters in another area (the downstream). Crops in downstream can drown thereby causing problems to farmers. Another con is that levees can also fail by either slumping, underflow, or erosion (Heyer, & Stamm, 2013). Severe flooding occurs whenever they fail thereby causing damage to property and loss of lives on an unsuspecting population. Additionally, levee construction reduces the amount of groundwater recharge because a river becomes disconnected from its natural floodplain. However, levee construction is done in the best interest of society besides the existing challenges. They prevent many suburban towns from flooding thereby increasing the population of these areas. Levees also protect people and cities during a hurricane. Whenever hurricanes happen, levees act as a shield that prevents water from overflowing and ruining the habitat of the whole community. The construction of dams influences the hydrological cycle in many ways. The surface area of a reservoir is greater than that of a river. More water vapor will, therefore, evaporate in the atmosphere which in turn translates to more rain (Jobin, 2014). Dams also store excess water, especially from floods. During a dry season, seasonal rivers and streams may dry up thereby affecting the availability of water in an area. The hydrological cycle in such regions may be altered significantly in an event where water reservoirs are unavailable. Construction of dams in upstream is essential, especially in areas that are prone to flooding. Dams help in harvesting water from rivers thus controlling the flow of water and flooding downstream. The stored water also ensures that an area has a continuous and stable water supply even during dry seasons. Farmers are also able to access water for irrigation at any time of the year. An increase in the human population has led to increased demand for food. Rainfall has become an insufficient and unreliable source of water to grow food crops. Artificial irrigation of crops from dam water has enabled farmers to meet increased food demand . It is not a secret that crop irrigation has enabled the human population to meet its food demand successfully. However, irrigation has its downside issues. One common problem associated with surface irrigation is waterlogging and salinization of soils. Over-irrigation leads to waterlogging which in turn causes alkalization (the build-up of sodium in the soil). Furthermore, water-borne diseases such as malaria, river blindness, and bilharzia are also associated with irrigation. The vectors of these diseases proliferate in irrigation waters. The risks of contracting such diseases increase because irrigation canals are unlined and have unchecked vegetation growth, soil drainage is poor in irrigation areas, and irrigation burrow pits are left with stagnant water (Jobin, 2014) . Irrigation also leads to the over-extraction of groundwater. The recharge rate of groundwater is usually lower compared to the extraction rate. The water table becomes low, and the water quality decreases. Groundwater is unsustainable, and aquifer exploitation leads to its depletion. The use of groundwater has become unsustainable because the use recharge rate is slower compared to the extraction rate.
References
Marsalek, J., Karamouz, M., Cisneros, B. J., Malmquist, P. A., Goldenfum, J. A., & Chocat, B. (2014). Urban Water Cycle Processes and Interactions: Urban Water Series-UNESCO-IHP . CRC Press.
Acreman, M., & Holden, J. (2013). How wetlands affect floods. Wetlands , 33 (5), 773-786.
Heyer, T., & Stamm, J. (2013). Levee reliability analysis using logistic regression models–abilities, limitations and practical considerations. Georisk: assessment and management of risk for engineered systems and geohazards , 7 (2), 77-87.
Jobin, W. (2014). Dams and disease: ecological design and health impacts of large dams, canals and irrigation systems . CRC Press.
