What is hydrological cycle in short answer?

3.4 Hydrological Cycle

Hydrological cycle is also known as the “water cycle”; it is the normal water recycling system on Earth (Fig. 3.4). Due to solar radiation, water evaporates, generally from the sea, lakes, etc. Water also evaporates from plant leaves through the mechanism of transpiration. As the steam rises in the atmosphere, it is being cooled, condensed, and returned to the land and the sea as precipitation. Precipitation falls on the earth as surface water and shapes the surface, creating thus streams of water that result in lakes and rivers. A part of the water precipitating penetrates the ground and moves downward through the incisions, forming aquifers. Finally, a part of the surface and underground water leads to sea. During this trip, water is converted in all phases: gas, liquid, and solid. As mentioned above, water always changes states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.

The hydrological cycle is intimately linked with changes in the atmospheric temperature and radiation balance. Warming of the climate system in recent decades is unequivocal, as it is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising of the sea level globally.

It is expected that the hydrological cycle will be affected from global warming due to the enhanced greenhouse effect [10]. The hydrological cycle may be strengthened with more precipitation and more evaporation, but the extra precipitation will be unequally distributed around the globe. It is expected that some areas of the world may see significant reductions in precipitation or even more major variations in the timing of wet and dry seasons. Many aspects of the economy, environment, and society are dependent upon water resources, and changes in the hydrological resource base have the potential to severely impact upon environmental quality, economic development, and social well-being [11].

6.5.1 Hydrological Cycle

The hydrological cycle of the earth is the sum total of all processes in which water moves from the land and ocean surface to the atmosphere and back in form of precipitation. The hydrological cycle is dependent on various factors and is equally affected by oceans and land surfaces. In the case of the land surface, vegetation plays a vital role in the maintenance of the hydrologic budget (Pielke and Niyogi, 2009). The presence of vegetation increases the capacity of the land surface to retain moisture. Precipitation is then intercepted by plants and directly evaporated when captured by the canopy. The plants themselves transpire and aid in the creation of a major amount of water vapor through evapotranspiration processes. The surface runoff, in the case of bare ground, is much greater than in vegetated lands. As plants dominate the processes of energy, water vapor, and carbon exchange, their presence is critical to the functioning of the hydrological cycle.

Summary

The concept of hydrological cycle is elegantly simple. But, its importance in the functioning of the geological and biological Earth is profound, transcending water itself. It plays an overarching role in the cycling of solar energy, sediments, and chemical elements vital for the sustenance of life. Although it is clear that contemporary ecosystems reflect an evolutionary adaptation to the delicate linkages that exist among the various components of the hydrological cycle, it is also apparent that evolving life must have influenced the evolution of the hydrological cycle over geological time. Life, it appears, is simultaneously a product of the hydrological cycle and its cause.

Summary

The concept of the hydrological cycle is quite simple. But, its importance to life on earth is profound. The hydrological cycle plays an overarching role in the cycling of solar energy, sediments, and chemical elements vital for life. Although it is clear that contemporary ecosystems reflect an evolutionary adaptation to the delicate linkages that exist among the various components of the hydrological cycle, it is also apparent that evolving life has affected the evolution of the hydrological cycle over geological time. Life, it appears, is simultaneously a product of the hydrological cycle and a factor causing changes in the cycle.

6.2 Hydrological cycle and climate change

Hydrological cycle, also known as water cycle, is a continuous movement of water between hydrosphere, atmosphere, and lithosphere in a cyclic manner (Fig. 6.1). Movement of water occurs from one reservoir to another through physical processes such as evaporation, condensation, precipitation, infiltration, and surface runoff. Earth’s surface is covered by 70% water which amounts to 1.4×1018 m3. Out of this, 97% reside in oceans as saline water and 3% constitute the fresh water sources such as rivers, lakes, glaciers, permanent snow and groundwater aquifers (Shiklomanov and Rodda, 2003; Green, 2016). The solar insolation causes evaporation to transfer approximately 577×1012 m3 of water from surface of the Earth to the atmosphere of which 86% is contributed by oceans and remaining 14% by land (Shiklomanov, 1993; Pimentel et al., 2004). Evaporated water from Earth’s surface reaches the atmosphere where it is condensed to form water droplets and subsequently it reaches the land in the form of precipitation (rain and snow) accounting for almost 20% of world’s precipitation. The surplus water, thus received on land, returns to oceans through rivers and groundwater thereby completing the water cycle (Shiklomanov, 1993; Pimentel et al., 2004). Therefore, solar energy moves a significant amount of water from oceans to land via atmosphere every year, thus making the hydrologic cycle vital not only to human life and natural ecosystem, but also to agricultural and industrial production.

However, the hydrologic cycle has started to alter due to the adverse effects of climate change and rising global temperatures. The processes that are involved in the hydrologic cycle are highly dependent on temperature. It has been observed that global temperatures have steadily been rising over millions of years and directly influencing the precipitation patterns, monsoonal intensity, water vapor concentrations, cloud formation, seasonal changes and river flow patterns. The rising temperature and changing climate has partly intensified this cycle because rising global temperature evaporates more water from the ocean and land. The warm air holds more water vapor which in turn produces more intensified rainfall causing flooding of coastal regions. Warm air also increases rate of evaporation that intensifies the evaporative process on the land which causes soil moisture to evaporate over a time period and thus, intensifying the drought condition on the hinterland areas of the continent. Therefore, shifts in climatic patterns and rising global temperature speeds up the water cycle leading to changes in extreme climatic phenomenon of more intensified rainfall, cloud burst situations, frequent storms and drought conditions. This direct effect on hydrologic cycle will also lead to changes in water resources.

Abstract

The hydrological cycle in Afghanistan is one of the high altitude snow and rain, commonly torrential, which produces catastrophic downstream effects such as avalanches and floods. Most of the precipitation that drives the river-flow lifeblood of the country outward from the tops of the watersheds in Afghanistan diminishes toward the borders from its highs in the northeast of the nation. The main river systems, listed in a counterclockwise direction around Afghanistan. include the Amu Darya, Hari Rud–Murgab, Helmand–Arghandab, and the Kabul, each of which is discussed in more detail herein. Lakes in Afghanistan include glacial, landslide-dammed, carbonate-precipitate types, diastrophic (tectonic and volcanic) lakes, tectonic—mixed types, and multiple deflation-basin sorts of lakes, many of which are intermittently dry. Underground water in Afghanistan occurs in aquifer basins throughout the country, with the basin beneath Kabul City undergoing severe drawdown.

Abstract

The hydrological cycle in Central and Southwest Asia, of course, operates essentially the same as it does in the rest of the world, but it does have regional variations in character and timing of its phases, energy sources, winds, distributions, climate and topographic influences, and other controlling factors that need to be understood. The high mountains of the region serve as the water-tower catchments for the elusive moisture that passes over the dry lowlands, but fortunately for the people who live there, that moisture precipitates orographically in the mountains above them. Not so fortunately, however, the common devegetation and soil erosion that also occur so commonly in the region, end up despoiling the surficial environments and reducing water infiltration into the surficial soils so that the runoff is accelerated into flashfloods and is thereby wasted. In any case, multiple drainage basins have resulted, the development and use of which are the focus of this book.

15.2.3 Hydrology

The hydrological cycle is influenced by several factors such as solar influx, rotation of the earth, distance from the ocean, topography, and general atmospheric circulation patterns. In the Boreal Uplands, topography and distance from the ocean vary considerably among watercourses. In general, the mean annual precipitation is highest in the west and north with values exceeding 4000 mm. In the east and in inland areas of large fjords, the mean annual precipitation is <1000 mm. The maximum and minimum mean annual precipitation during 1961–1990 was 6944 and 128 mm, respectively.

Runoff is not evenly distributed throughout the year and can be divided into specific runoff regions (Gottschalk et al. 1979). In coastal areas, with a so-called Atlantic regime, the lowest runoff occurs during May–August and runoff is similar during the other months. The inland regime, situated between the Atlantic and the mountain regime, is characterized by low runoff in winter (January–March), a marked increase due to snow melt in April and May, and low values in summer that increase from August until winter begins. The geographical variation in precipitation is reflected in the flow regime of the rivers (Figure 15.2). In some rivers, the period of recording covers several years prior to and after development of hydropower schemes. Hydropower development has resulted in a significant reduction in the ratio between flood and minimum discharge.

The Global Water Cycle

The hydrological cycle describes the perpetual flux and exchange of water between different global reservoirs: the oceans, atmosphere, land surface, soils, groundwater systems, and the solid Earth (Figure 1). Most of the world’s water – approximately 96.3% – is in the world’s oceans, where water molecules have an average residence time of about 3300 years. Glaciers and ice sheets lock up more than half of the remaining water (Table 1), with 90% of this stored in the Antarctic Ice Sheet. Most of what remains lies below the surface, in groundwater aquifers, where vast reserves of water are saline or difficult to access.

Table 1. The global water inventory (km3)

Reproduced from Shiklomanov, I. (1993). World fresh water resources. In: Gleick, P.H. (eds.) Water in crisis: A guide to the world's fresh water resources. New York: Oxford University Press, with updates from other sources as indicated.

Freshwater in circulation, on which ecosystems and society so critically depend, therefore makes up only a tiny fraction of Earth’s total water supply. Surface water constitutes only 0.02% of the global inventory, distributed between rivers, lakes, wetlands, soils, and the biosphere. The United Nations Environmental Program (UNEP) estimates the global, accessible freshwater supply to be about 200 000 km3. This equates to about 29 million liters of water for each person on the planet. Global water supplies are bountiful, though not easily accessed or equitably distributed.

Fluxes of water between reservoirs are indicated in Figure 2 and are discussed in the Global Water Cycle section of the ESES module. There are high rates of turnover in the atmosphere, biosphere, soils, and rivers; the average lifetime of a water molecule in the atmosphere is 9.2 days, and considerably less than this in the world’s rain belts. Once on the land surface, water can be stored for extended periods in soils, lakes, groundwater aquifers, vegetation, and seasonal snowpacks. On an annual basis, however, discharge from the world’s rivers is in near-equilibrium with global precipitation, returning what the ocean gives up through evaporation.

15.2.4 Hydrology

The hydrological cycle is influenced by several factors such as solar influx, rotation of the earth, distance from the ocean, topography, and general atmospheric circulation patterns. In the Boreal Uplands, topography and distance from the ocean vary considerably among watercourses. In general, mean annual precipitation is highest in the west and north with values exceeding 4000 mm. In the east and in inland areas of large fjords, the mean annual precipitation is less than 1000 mm. The maximum and minimum mean annual runoff of these areas during 1961–90 was 6944 and 128 mm, respectively. The period 1961–90 is the latest standard climate period. The next period will be 1991–2020.

Runoff is unevenly distributed throughout the year and can be divided into specific runoff regions (Gottschalk et al., 1979). In coastal areas, with a so-called Atlantic regime, runoff intensity follows the precipitation distribution in time. Low runoff occurs normally during May–August, and increases during the autumn. The inland regime is characterized by low runoff in winter (January–March), followed by a marked increase due to snowmelt in April and May, and low values in summer that increase from August onward until winter begins. The geographical variation in precipitation is reflected in the flow regime of the rivers (Fig. 15.2). In some rivers, the period of recording covers several years prior to and after development of hydropower schemes. Hydropower development has resulted in a significant reduction in the ratio between flood and minimum discharge.

Climate change is expected to have substantial effects on precipitation and river discharge. Simulations show that the boreal rivers will have an increased winter and spring runoff by 20%–40% and an equal drop in runoff during summer and fall.

The total annual runoff is expected to increase by about 3%. With elevated emissions of CO2, an increase by approx. 10% is expected for the boreal rivers (Hanssen-Bauer et al., 2015).