Availability of Freshwater in 2000
Average River Flows and Groundwater Recharge
local water budget changes from place to place within a continent. Therefore, we need to define a control volume, as defined in Equation 15.1, much smaller for local areas than those for the continents in order to analyze the local water budget qualitatively.
Any volume of water stored in a particular area (i.e., any size of area) can be used in Equation 15.1 as the control volume; however, in most cases, we use a watershed as the fundamental unit to analyze the local water budget. A watershed is defined as the land area where rainfall (and/or snow melt) drains into a stream and contributes runoff to one specific delivery point. Large watersheds may consist of several smaller watersheds, each of which contributes runoff to different locations that ultimately combine at a common delivery point. Any number of watersheds can be defined for a particular stream corresponding to any location along the stream.
Inflow and Outflow Components Constitute Water Balance at a Watershed
A watershed water balance consists of natural inflow and outflow components, which are described in Equation 15.2, as well as human-induced flow components (Figure 15.3).
Outflow components consist of natural outflow components, including evapotranspiration and surface/subsurface discharge, and human-induced outflow components. The main human-induced outflow component is surface/subsurface water withdrawal used for municipalities, industries, and agriculture. Such water returns to rivers and groundwater (i.e., the water body stored in the watershed) as input flow to the watershed after it has been used for human activities. This type of input includes (a) recycled sewage water, which is treated at wastewater treatment facilities and returns to rivers in many municipalities and industries, and (b) return water from irrigation applications in agricultural fields.
Some watersheds require more water than they can receive in the form of precipitation and transport water through aqueducts from other watersheds and/or desalinated water. Furthermore, artificial recharge to surface/subsurface is conducted in many municipalities to increase water availability during dry seasons. This water, in turn, constitutes human-induced input components.
These human-induced inflow and outflow components are strongly connected to the physical necessities of sustainabilities, including energy, land and nonenergy resources. Details of these connections and their impacts are described in the following sections.
Case Study: Carson Valley, Nevada and California
Using an example of a local-scale water budget analysis, we can quantitatively evaluate the effects of human activities, such as land use changes and water transportation, through observed changes in the water budget over time.
Rapid population growth and urban development in the United States has caused concern and raised issues over the continued availability of water resources needed to sustain such growth in the future. To address these issues, Maurer and Berger (2006) investigated the water budgets of Carson Valley, in west-central Nevada. Water demand increases with population growth, and the groundwater supply has been used to satisfy demand in the area. In addition, land presently used for agriculture is expected to be converted for urban use. The effects of these changes on groundwater recharge and discharge are not well understood, and these changes may affect the outflow of the Carson
Surface/subsurface water withdrawal
Surface/subsurface water withdrawal
Surface/subsurface discharge r
— Recycled sewage water
Figure 15.3 Water balance at a watershed showing input and output flow components. Human-induced flow components play important roles in the water balance.
River and, in turn, water users downstream of Carson Valley, who depend on sustained river flow (Figure 15.4).
Maurer and Berger (2006) calculated the water budgets for two time periods: 1941-1970 and 1990-2005. Water years 1941-1970 represent conditions prior to increased population growth and groundwater pumping, when the area did not import effluent from the neighboring watersheds. By contrast, water years 1990-2005 represent conditions under increased population growth, which caused changes in land and water use, increased groundwater pumping, and the application of effluent for irrigation. The effluent was imported from the Lake Tahoe watershed.
Based on water flow components ofthis area and the observed small contribution from groundwater outflow, Equation 15.3 will be modified accordingly:
where p is the average annual precipitation rate (L3T-1), and is and ig are the infiltration of streamflow and groundwater inflow (L3T-1), respectively. The term, i, represents the effluent imported from the Lake Tahoe watershed; T and Tg are the average annual surface water and net groundwater pumping rates
(L3T 1), respectively. The term, et, represents the annual average évapotranspiration rate (L3T-1). Table 15.3 illustrates water budgets for water years 1941-1970 and 1990-2005.
Although there is a decrease in streamflow between the years 1941-1970 and 1990-2005, an increase in imported effluent, in which most of water (11.7 x 106 m3/yr) was used for irrigation, sets off this decrease, and the overall total for both periods are similar (Table 15.3). As can be seen in Table 15.3, both evapotranspiration and streamflow decrease. Net groundwater pumping, however, greatly increases.
Distinct differences in the water budget for the two periods were:
• an increase in net groundwater pumping,
• a decrease in streamflow input,
• a significant increase in the use of effluent for irrigation, and
• a decrease in evapotranspiration, caused by land use changes in which agricultural and native vegetation were replaced with residential or commercial land.
Because net groundwater pumping increased significantly and stream inflow into the area decreased, we could expect an extensive decrease (31.3-34.5 x 106 m3/yr) in stream outflow when we consider the water balance equation. However, stream outflow from this area decreased by 18.5 x 106 m3/yr.
This abrupt change in stream outfl ow was caused by the combination of increased imported effluent for irrigation and decreased evapotranspiration caused by land use changes. Combining an evapotranspiration change with the application of 11.7 x 106 m3/yr of effluent for irrigation resulted in an overall
Water budget component
Estimated volumes (
x 106 m3/yr)
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