Soil moisture





Days-106 yearsb

a Global average.

b Longer turnovers associated with large watershed areas.

a Global average.

b Longer turnovers associated with large watershed areas.

1978). Assuming an input flux equal to oceanic evaporation, this would give a turnover time of about 750 years. The turnover time analysis is not strictly correct since freshwater resides in a number of interconnected reservoirs; however, since freshwater volume is essentially equal to non-marine storage and net evaporation is the only output flux from the oceans, this may be taken as a reasonable estimation. For practical purposes, freshwater resources available to humans cycle more rapidly, but since 97% of freshwater is stored in ice, the global average turnover is much longer.

6.2.3 Fluxes

Robert Horton, an influential pioneer in the field of hydrology, developed one of the first comprehensive representations of the hydrologie cycle in 1931. His original diagram, Fig. 6-4, illustrates the processes by which water moves between the Earth's hydrologie reservoirs. Hydrologie fluxes can be summed up in four

Fig. 6-4 The fluxes of the hydrologic cycle, developed by Robert Horton (1931).

processes, shown around the outside of Hor-ton's wheel - precipitation, surface dissipation of precipitation, evaporation, and atmospheric moisture transport. This section will discuss precipitation, evaporation (and closely related transpiration), and runoff - the processes that link the oceans, atmosphere, and land surface. Atmospheric moisture is addressed in Chapter 7. These fluxes are highly variable over the Earth's surface in both space and time, which has extremely important implications for water resources; spatial and temporal variability is discussed in Section 6.3. Precipitation

The flow of water from the atmosphere to the ocean and land surfaces as rain, snow, and ice constitutes the atmospheric efflux in the hydro-logic cycle. Although most precipitation falls on the oceans (ca. 79% of the global total), precipitation onto land is much more hydrologically significant. On a global scale, nearly two-thirds of the land portion returns to the atmosphere via évapotranspiration (see below), while the remaining one-third contributes to groundwater and surface runoff. Precipitation is highly variable over the globe, with atmospheric circulation patterns concentrating it in the tropics and mid-latitudes.

An important component of precipitation on a regional scale comes from precipitation recycling; that is, a portion of the precipitation in a region comes from water vapor evaporated from within that region, with the remainder composed of atmospheric moisture advected into the region. The precipitation recycling ratio, the ratio of recycled precipitation to total precipitation, is then a function of evaporation and internal and external atmospheric moisture fluxes (Budyko, 1974; Eltahir and Bras, 1996). The settling of the Great Plains in the late 19th and early 20th centuries was in fact spurred by early (and largely unfounded) concepts of precipitation recycling. S. Aughey (cited in Holz-man, 1937) wrote of Nebraska in 1880 that increased evaporation from cultivated land would increase moisture and rainfall - i.e., that "rain follows the plow." Evapotranspiration

Water returns to the atmosphere via evaporation from the oceans and évapotranspiration from the land surface. Like precipitation, evaporation is largest over the oceans (88% of total) and is distributed non-uniformly around the globe. Evaporation requires a large input of energy to overcome the latent heat of vaporization, so global patterns are similar to radiation balance and temperature distributions, though anomalous local maxima and minima occur due to the effects of wind and water availability.

Evapotranspiration (ET) is the collective term for land surface evaporation and plant transpiration, which are difficult to isolate in practice. Transpiration refers to the process in which water is transported through plants and returned to the atmosphere through pores in the leaves called stomata, and is distinct from direct evaporation of intercepted precipitation from leaf surfaces. Some land surface processes and the roles of vegetation in the water and energy balances are illustrated in Fig. 6-5. Due to

Fig. 6-5 Evaporation and transpiration from vegetation are among the complex land surface interactions in the hydrologie cycle. (From Dickinson, 1984.)

the number of variables involved, ET can be extremely difficult to measure and is often determined by closing the water or energy balance calculated from better-known components. Runoif

The excess of evaporation from the oceans is made up for by runoff from the land. Although this flux is much smaller than precipitation and ET, it is a major link in many cycles and is of particular importance to humans in terms of water supply. Runoff can be broadly categorized into subsurface, or groundwater, flow and surface flow, consisting of overland runoff and river discharge. Subsurface runoff. When precipitation hits the land surface, the vast majority does not go directly into the network of streams and rivers; in fact, it may be cycled several times before ever reaching a river and the ocean. Instead, most precipitation that is not intercepted by the vegetation canopy and re-evaporated infiltrates into the soil, where it may reside as soil moisture, percolate down to groundwater, or be transpired by plants.

Very little groundwater is discharged directly to the oceans, but groundwater does provide a significant contribution to stream discharge in most areas. Subsurface flow is generally much slower than surface runoff, allowing groundwater to provide perennial baseflow to streams far into a dry season, long after surface storm runoff has been discharged. Figure 6-6 shows a typical storm hydrograph, with baseflow and stormflow components indicated. Groundwater flow velocities have been found to follow Dar-cy's law:

where v is velocity, K is soil hydraulic conductivity with units of (length/time), n is the dynamic (or actively available) porosity, and dh/dL is the hydraulic gradient.

Hydraulic conductivities vary over a range of 10"12 cm/s for unfractured igneous and meta-morphic rocks to 2 or 3 cm/s for porous (karst)

June 1927 July 1927

Fig. 6-6 Hydrograph showing the rapid contribution of surface runoff and more steady baseflow. Runoff in cubic feet per second, precipitaion in inches. (From Langbein and Wells, 1955.)

limestone and gravel; surface flow is typically on the order of a meter per second. Surface runoif. Hydrologists have identified two processes for generating surface runoff over land. The first, saturated overland flow (SOF), is generated when precipitation (or snow-melt) occurs over a saturated soil; since water has nowhere to infiltrate, it then runs off over land. SOF typically occurs only in humid environments or where the water table rises to intersect with a stream. Horton overland flow (HOF or infiltration-limited overland flow) occurs when precipitation intensity exceeds the infiltration capacity of the soil in a non-saturated environment. In this case, only the excess precipitation (that exceeding the infiltration capacity) runs off over the surface. Both types of overland runoff generate relatively rapid flows that constitute the surface water contribution to the hydrograph (Fig. 6-6).

Figure 6-7 illustrates the runoff paths for HOF and SOF, as well as for subsurface stormflow and groundwater flow. Subsurface stormflow is a moderately rapid runoff process in which water flows to a stream through highly permeable surface soil layers (without reaching the water table). Note in Fig. 6-7 that while HOF and subsurface stormflow may occur over a large fraction of an infiltration-limited hillslope, SOF occurs over a smaller portion adjacent to the stream.

Hillslopes occupy about 99% of the landscape

June 1927 July 1927

l' Runoff l' Runoff


Fig. 6-7 Vertical cross-section showing pathways for surface and subsurface runoff. Path 1: HOF; path 2: groundwater flow; path 3: subsurface stormflow; path 4: SOF. (From Dunne and Leopold, 1978.)

and provide stream channels with water supply, making hillslope processes extremely important on a local scale. However, the much more visible component of surface runoff comes from river discharge. Globally, rivers discharge roughly 45 000 km3 per year to the oceans (Shiklomanov and Sokolov, 1983). The 16 largest rivers account for more than one-third of total discharge, and over half of that contribution comes from the three largest. Table 6-5 lists the 10 largest rivers in the world in terms of average discharge rate (m3/s) and annual discharge volume (km3/yr) (Dingman, 1994).

While river discharge is the primary means of transferring water from the land to the oceans, its magnitude pales in comparison to circulation within the oceans themselves. The total average

Table 6-5 World's largest rivers


Discharge (m3/s)

Discharge (km3/yr)


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