Processes That Increase Rtver Runoff Groundwater Mining

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Groundwater mining or overdraft (G) is defined as the withdrawal or removal of groundwater in excess of natural recharge from infiltration and from underground inflow. Several parts of the world are already experiencing groundwater overdraft. This overexploitation of water resources is expected to increase as rising demand, spurred by population growth and development, encounters a diminishing supply, due to the increased evaporation, reduced precipitation, and less soil moisture projected for certain regions (e.g., the Mediterranean), from climate change (Houghton et al., 1996). In the United States, major water withdrawals from the High Plains aquifer (formerly called the Ogallala aquifer) for irrigation have resulted in an average lowering of the regional water table by around 3.7 m since 1940 (Dugan et al., 1994). This aquifer covers portions of South Dakota, Wyoming, Nebraska, Kansas, Colorado, Texas, Oklahoma, and New Mexico over an areal extent of 450,800 km2. Southern California, Arizona, and New Mexico are other states where groundwater is mined (Table 5.3). Elsewhere, groundwater mining is a problem in arid countries of North Africa, the Sahel, and the Middle East (Table 5.3). Groundwater withdrawals are already very close to natural recharge rates in several other countries, such as Tunisia, Portugal, Lebanon, Syria, and Israel (WRI, 1998).

The total amount of water withdrawn (both surface and underground) and the fraction used consumptively (i.e., evaporated, transpired, or otherwise consumed by humans, animals, or plants, and not returned to streamflow) are documented in global water balance inventories (e.g., Shiklomanov, 1997; WRI 1998; Solley et al., 1998, for the U.S.). The partitioning of water resources between surface and groundwater flow by continent is also known (Zekster and Loaiciga, 1993). Although groundwater mining data are lacking on a global level, a reasonable estimate can be made by extrapolating information on water usage, as follows: The volume of groundwater mined per year (Vgm) in selected countries or regions (Table 5.3, column 1) is listed in column 2. The total volume of water (both surface and underground) withdrawn annually (V«™, in these countries or regions is given in column 3. The volume of groundwater withdrawn annually (Vgw col. 5) in each of these areas is estimated by multiplying the total volume of water withdrawn col. 3) by the geographically corresponding fraction of the total water withdrawn that comes

Table 53

Estimated Contribution of Groundwater Mining to Sea Level Rise

Table 53

Estimated Contribution of Groundwater Mining to Sea Level Rise

Country/Region

(km3/yr)

' gw' * WW

' gm' ' gw

High Plains

4.9

SW U.S.A.

10.0

California

13.0

U.S.A.

27.9

471.7

0.224

105.7

0.26

Egypt

1.7

55.1

0.345

19.0

0.09

Libya

1.5

4.6

0.345

1.6

0.94

Mauritania

0.7

1.6

0.345

0.6

(1.1)

Algeria

0.3

4.5

0.345

1.6

0.19

Morocco

0.03

10.9

0.345

3.8

0.01

Africa

4.23

76.7

0.345

26.6

0.16

Greece

1.7

5.0

0.275

1.4

(1.2)

Italy

0.4

56.2

0.275

15.5

0.03

Spain

0.7

30.8

0.275

8.5

0.08

Europe

2.8

92.0

0.275

25.4

0.11

India

20.3

380.0

0.275

104.5

0.19

Saudi Arabia

5.2*

17.0

0.75*

12.8

0.41

Iran

0.2

70.0

0.275

19.3

0.01

Israel

0.2

2.0

0.275

0.6

0.33

Asia

25.9

469.0

0.275

137.2

0.19

Australia

0.04

14.6

0.149

2.2

0.02

TOTAL

60.9

1124

0.306

297.1

0.20

Data sources: Column 2: Dugan et al., 1994; Shiklomanov, 1997; UN, 1983; Marinos and Diamandis, 1992: Beretta etal., 1992; Singh, 1992; Jellali etal., 1992; Lopen-Camacho etal., 1992; Al-Ibrahim, 1991; Hosseinipour and Ghobackian, 1990, WRI, 1998. Column 3: WRI, 1998; Solley et al., 1998 (U.S.A.).

Column 4: Solley et al., 1998 (U.S.A.); Zektser and Loaiciga, 1993 (elsewhere). Column 5: col. 3 x col. 4; Column 6: col. 1 -=- col. 5; * from Al-Ibrahim, 1991.

Data sources: Column 2: Dugan et al., 1994; Shiklomanov, 1997; UN, 1983; Marinos and Diamandis, 1992: Beretta etal., 1992; Singh, 1992; Jellali etal., 1992; Lopen-Camacho etal., 1992; Al-Ibrahim, 1991; Hosseinipour and Ghobackian, 1990, WRI, 1998. Column 3: WRI, 1998; Solley et al., 1998 (U.S.A.).

Column 4: Solley et al., 1998 (U.S.A.); Zektser and Loaiciga, 1993 (elsewhere). Column 5: col. 3 x col. 4; Column 6: col. 1 -=- col. 5; * from Al-Ibrahim, 1991.

from groundwater (VgW/VW, col. 4). The ratio (Kgm/Fgw) of the volume of groundwater mined (col. 1) to the volume of groundwater withdrawn (col. 5) in each region is then listed in column 6.

The global volume of groundwater withdrawn (VGW) ranges between 991.4 km3/yr circa 1990 (3240 km3/yr [WRI, 1998] X 0.306 [the global VgJ Fww ratio, Zekster and Loaiciga, 1993]) and 1150.6 km3 in 1995 (3760 km3/yr X 0.306, after Shiklomanov, 1997). The global volume of groundwater mined (VGm) can be extrapolated,

where (2l/„m/2 Vgw) is the ratio of the sum of the volumes of groundwater mined in the countries or regions (col. 2) to the sum of the volumes of

Table 5.4

Anthropogenic Contributions to Sea Level Rise

Equivalent sea level rise (mm/yr)

Equivalent sea level rise (mm/yr)

Table 5.4

Anthropogenic Contributions to Sea Level Rise

Process

Low

Mid

High

Groundwater mining (G)

0.10

0.20

0.30

Urban runoff (U)

0.30

0.34

0.38

Water released by oxidation of fossil fuels,

-0.06

0.010

0.07

vegetation, and other sinks (C)

Deforestation-induced runoff (D )

0.08

0.09

0.11

Wetlands loss (W)

0.001

0.0015

0.002

Reservoirs and dams (R)

Storage

-0.33

-0.30

-0.27

Infiltration

-0.81

-0.68

-0.56

Evaporation

-0.010

-0.008

-0.005

Irrigation (I)

Infiltration

-0.49

-0.44

-0.40

Evapotranspiration

-0.15

-0.12

-0.10

Total

-1.37

-0.91

-0.47

groundwater withdrawn in these areas (col. 5), or 0.20 (60.9/297.1). Thus, FGM ranges between 198.3 km3 (0.20 X 991.4) and 230.1 km3 (0.20 X 1150.6), equivalent to 0.55 to 0.64 mm/yr SLR, respectively.3

Because country or regional ratios of VgmIVgvl vary widely due to major differences in groundwater extraction practices, a more meaningful measure is the mean of the regional means of Vgm/Vgw> or 0.15 ± 0.09. For the stated range in Vgm/VgW (Eq. (3), V(,m varies between 59.5 and 237.9 km3/yr (after WRI, 1998), which is equivalent to 0.17-0.66 mm/yr SLR, or 69.0 to

276.1 km3/yr (after Shiklomanov, 1997), equivalent to 0.19-0.77 mm/yr.

Around 61% of the water withdrawn globally in 1995 has been used consumptively (Shiklomanov, 1997), thereby leaving only 39% to run off. Therefore, the amount of mined groundwater that contributes to runoff, and ultimately to sea level, may only come to between 0.1 and 0.3 mm/yr to SLR, if all of this water were to flow into the ocean (see Table 5.4). However, in the future groundwater mining may well increase along with economic and agricultural development.

5.2.2 Urbanization

Although urban centers only occupy around 1% of the earth's total land area at present, they are home to a rapidly increasing proportion of the world's population (WRI, 1996). In spite of the relatively small land areas involved, urbanization (U) exerts a strong impact on hydrology as natural vegetation

3 360 km3 is equivalent to 1 mm of sea level rise.

is replaced with impermeable structures. Concrete and other artificial surfaces sharply reduce évapotranspiration and infiltration rates, contributing to falling water table levels, and also increases in surface runoff (Walling, 1987). Excessive groundwater extraction for municipal and industrial use has lowered water tables in many urban areas, such as Mexico City, Tucson, Houston, Bangkok, Venice, Shanghai, Tokyo, and Calcutta, causing land subsidence (and noteworthy for coastal cities, creating unusually high rates of relative sea level rise and salt water intrusion—see Chapter 8). By contrast, water tables have risen in other cities (e.g., London, Cairo, Riyadh, Saudi Arabia and other Persian Gulf cities, and Barcelona, Spain), due to reductions in pumping for public or industrial water supply, or to leakage of sewers or pipes (Chilton et al., 1997). However, quantitative data on the global extent of groundwater changes accompanying urban water uses is lacking, except for specific case studies.

A significant consequence of urbanization is a net increase in total runoff due to the expansion of areas of impermeable pavements, buildings, and other covered surfaces, which reduce évapotranspiration and impede the infiltration of rainwater. Total urban runoff volumes may increase by factors of 2-2.5 (Jones, 1997, p. 226). The global increase in runoff associated with urban growth has been estimated to be around 137 km3/yr (or 0.38 mm/yr SLR) in the 1980s (L'vovich and White, 1990). This figure probably represents an upper bound, in that it is based on relationships between changes in the extent of impermeable surfaces and runoff for several Russian cities, which may be higher than average for urban areas. Another complicating factor is that not all urban runoff necessarily feeds directly into rivers downstream. Overexploitation of groundwater in an aquifer hydraulically connected to the river will tend to reduce discharge downstream. The net effects of urban hydrologie changes are complex and difficult to predict on a worldwide basis. The uncertainty associated with projecting such changes suggests a plausible sea level change in the range of 0.30 to 0.38 mm/yr.

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