Page 282 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

246 part II The Water, Weather, and Climate Systems
▼Figure 9.5 Pathways for precipitation on Earth’s surface. The principal pathways for precipitation include interception by plants; throughfall to the ground; collection on the surface and overland flow to streams; transpiration and evaporation from plants; evaporation from land and water; and gravitational water moving to subsurface groundwater.
precipitation over land infil- trates the subsurface, and about 85% of this water returns to the atmosphere either by evapora- tion from soil or transpiration from plants.
If the soil is saturated, then any water surplus within the soil body becomes gravitational water, percolating downward into the deeper groundwater. The latter defines the zone of saturation, where the soil spaces are completely filled with water. The top of this zone is known as the water table. At the point where the water table intersects a stream chan- nel, water naturally discharges at the surface, producing base flow, which refers to the por- tion of streamflow that consists of groundwater.
Under natural conditions, streams and groundwater ulti- mately flow into oceans, thus continuing movement through the hydrologic cycle. In some cases, streams flow into closed lake basins, where water evap-
Precipitation +
–
+ indicates inputs to soil-moisture zone
– indicates outputs away from soil-moisture zone
Transpiration from plants
+ – waterRunoff – – – to streams –
Evaporation from land and
Gravitational water to groundwater
–
Overland flow
the passage of liquids), then the water will begin to flow downslope as overland flow, also known as surface runoff. Overland flow will also occur if the soil has been infil- trated to full capacity and is saturated. Excess water may remain in place on the surface in puddles or ponds, or may flow until it forms channels—at this point it becomes streamflow, a term that describes surface water flow in streams, rivers, and other channels.
Figure 9.4 shows that 8% of the water in the cycle is moving on or through land. Most of this movement— about 95%—comes from surface waters that wash across land as overland flow and streamflow. Only 5% of water movement is slow-moving subsurface groundwater. Although only a small percentage of water is in rivers and streams, this portion is dynamic and fast-moving com- pared to its sluggish, subsurface counterpart.
Water in the Subsurface
Water that infiltrates the subsurface moves downward into soil or rock by percolation, the slow passage of water through a porous substance (shown in Figure 9.5). The soil-moisture zone contains the volume of subsurface water stored in the soil that is accessible to plant roots. Within this zone, some water is bound to soil so that it is not available to plants—this depends on the soil tex- ture (discussed in Chapter 18). An estimated 76% of
orates or soaks into the ground. Many streams flow into reservoirs behind dams, where water is stored until it evaporates or is released into the channel downstream. Groundwater flows slowly toward the sea, intersecting the surface or seeping from underground after reach- ing the coast, sometimes mixing with seawater in coastal wetlands and estuaries (bodies of water near the mouths of rivers). Groundwater is discussed later in the chapter.
Water Budgets
and Resource Analysis
An effective method for assessing portions of the water cycle as they apply to water resources is to establish a water budget for any area of Earth’s surface—a conti- nent, country, region, field, or front yard. A water bud- get is derived from measuring the input of precipitation and its distribution and the outputs of evapotranspira- tion, including evaporation from ground surfaces and transpiration from plants, and surface runoff. Also in- cluded in this budget is moisture that is stored in the soil-moisture zone. Such a budget can cover any time frame, from minutes to years.
A water budget functions like a money budget: Precipi- tation is the income that must balance against expenditures for evaporation, transpiration, and runoff. Soil-moisture
Infiltration Percolation
Notebook
Chapter 9, Table 9.1, Types of Precipitation
Soil-moisture zone
Groundwater zone
Water table
Zone of saturation