Water circulation

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hydrology A synthesis of many disciplines, hydrology is the science of water, its properties, its circulation, and its distribution in Earth and atmospheric systems. In the Earth sciences, hydrology is studied at various scales, ranging from hill slopes (10⁰–10³ m²) to drainage basins (10⁰–10⁶ km²). The hydrology of hill slopes is a combination of surface flow processes, which include infiltration–excess overland flow and saturation overland flow (see overland flow) and shallow subsurface flow processes (see subsurface flow and erosion) (Fig. la). Soil infiltration capacity, the rate at which soils absorb rainfall, determines whether water flows on the surface or through the subsurface. Infiltration–excess overland flow occurs when intensity of rainfall exceeds the capacity of the soil to absorb water. If rainfall intensity does not exceed infiltration capacity, then through flow or saturation overland flow occurs (Fig. la). Saturation overland flow is the combination of return flow (subsurface flow that returns to the surface downslope because of saturation) and direct rainfall on to areas of subsurface saturation. Throughflow travels entirely through the subsurface, typically along the interface of a less permeable soil horizon. For a given hill slope, the dominant run-off process is therefore regulated by infiltration capacity, which distributes rainfall between surface and subsurface hydrological systems. Infiltration capacity and the flow process that dominates on a slope are a function of several variables, including climate and vegetation, rainfall characteristics, soil thickness, slope morphology, and human disturbance.

The hydrology of drainage basins is a combination of hill-slope flow processes, groundwater flow processes, and channelized flow processes in surface streams (see drainage basins). Hydrographs, plots of either flow discharge or depth versus time, are the cumulative record of discharge by the various flow processes on the hill slopes that comprise a drainage basin (Fig. 1b). The discharge characteristics of a drainage basin, such as the peak and total discharge and the time-to-peak discharge, depend on the flow process as well as on the characteristics of the drainage basin such as the shape of the basin and the drainage density. The flow process is critical to the shape of the hydrograph because of differences in flow velocity between surface flow paths, including channelized and non-channelized flow, and subsurface flow paths. Hydrographs dominated by infiltration–excess overland flow tend to have higher peak discharges and faster times-to-peak discharge than hydrographs dominated by saturation overland flow. In addition, the recession limbs of saturation overland flow hydrographs are generally less steep and of longer duration because of the lag in response of the return flow or throughflow contribution to discharge.

Hydrologists' understanding of the conditions under which the different flow processes dominate and their characteristic hydrographs, has wide-ranging applications in the Earth sciences, from flood forecasting and routing to the evolution of drainage basins. Accurate flood forecasts require an understanding of the response of dominant flow processes to rainfall events of varying intensities and durations, superimposed on seasonal differences in soil moisture and vegetation, among other variables. If we consider the hydrograph characteristics and their relation to flow process, as described above, determinations of peak and total discharge and time-to-peak discharge from storm characteristics and basin parameters are especially critical for evaluating potential human impacts as well as the integrity of flood-control structures within the impacted drainage basin. Hydrological routing is used to predict downstream changes in the hydrograph.

The role of hydrology is being incorporated increasingly in models of landscape evolution. The frequency, magnitude, and spatial extent of surface and subsurface flow processes also affect the evolution of drainage basins. Drainage networks develop in response to surface flow processes and are therefore limited in extent to the portion of the basin that experiences overland flow. The role that flow process plays in drainage-basin evolution is best expressed by considering the hydrological response of a hill slope and drainage basin to climatic change. Change from a semi-arid to an arid climate is associated with increased dominance of desert scrub communities over grassland communities. In the scrub community, rainfall is preferentially infiltrated in the vicinity of scrub vegetation, while interscrub areas are more susceptible to soil erosion by infiltration–excess overland flow. Spatially heterogeneous soil moisture and soil degradation, associated with the semi-arid to arid climate change, further stress remnant grassland vegetation and decrease soil infiltration capacity over time. Consequently, the dominant flow process would shift from throughflow or saturation overland flow to infiltration–excess overland flow. Aggradation in alluvial and fluvial systems has been temporally linked to reductions in effective precipitation during the late Quaternary and explained as the geomorphological response to newly imposed run–off infiltration balances. The increased dominance of infiltration–excess overland flow results in increased peak run-off in the basin hill-slope and channel systems, exceeding stream power thresholds for sediment entrainment, and would result in increased sediment supply and delivery to downstream depositional sites.

J. B. Ritter

Bibliography

Dunne, T. (1978) Field studies of hillslope flow processes. In Kirkby M. J. (ed.) Hillslope hydrology, pp. 227–93. John Wiley and Sons, New York.
Gregory, K. J. and and Walling, D. E. (1973) Drainage basin form and process. Halsted Press, New York.

water balance The balance between intake and excretion of fluids. Average daily intakes are: as drinks, 1–1.5 L; as aqueous part of food, 0.5 L; and formed in the body by oxidation of foodstuffs (metabolic water), 300–500  mL; total 2–3 L. Losses from the lungs, 400–500 mL; through the skin 400–500 mL; in faeces 80–100 mL; in urine 1–1.8 L.

Total body water is 40–44 L, as: blood plasma, 2–3 L; extracellular fluid (between cells), 10 L; and intracellular fluid (within cells), 27–30 L. The kidneys control the volume of extracellular water by excreting water. Ingestion of sodium chloride (salt) raises the osmotic pressure of the extracellular water, causing thirst.

hot dry rock (HDR) Rocks, usually granite, which have abnormally high heat production as a result of the decay of radiogenic elements rather than merely residual heat. Potentially these are a source of geothermal energy. One method of exploiting the heat generated is to fracture the rocks at depth using small, downhole, explosive charges, and then initiate a water circulation system from the surface. When cold water is pumped down it returns considerably warmer, and this energy can be extracted by heat exchangers. See also GEOTHERMAL FIELD; and GEOTHERMAL GRADIENT.

hy·drol·o·gy / hīˈdräləjē/ • n. the branch of science concerned with the properties of the earth's water, esp. its movement in relation to land. DERIVATIVES: hy·dro·log·ic / ˌhīdrəˈläjik/ adj. hy·dro·log·i·cal / ˌhīdrəˈläjikəl/ adj. hy·dro·log·i·cal·ly / ˌhīdrəˈläjik(ə)lē/ adv. hy·drol·o·gist / -jist/ n.

hydrology Study of the Earth's waters, their sources, circulation, distribution, uses, and chemical and physical composition. The hydrological cycle is the Earth's natural water circulation system. Hydrologists concern themselves with the provision of freshwater, building dams and irrigation systems, and controlling floods and water pollution.

water balance (water budget) A method of assessing the size of future water resources in an aquifer, catchment area, or geographical region, which involves an evaluation of all the sources of supply or recharge in comparison with all known discharges or abstractions. See MOISTURE BALANCE.

hydrology The study of the hydrologic (water) cycle. While it involves aspects of geology, oceanography, and meteorology, it emphasizes the study of bodies of surface water on land and how they change with time. See HYDROGEOLOGY.

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