Land Cover and Land Use

3.5.3 Land Cover and Land Use

Because of the time lag between surface water processes, including infiltration, and the actual groundwater recharge arriving at the water table, it is very important to take into consideration historic land use and land cover changes when estimating representative recharge rates. It is equally important to consider future land use changes when making predictions or when modeling groundwater availability.

Three main trends in land use and the associated human-induced changes in land cover have been taking place worldwide and disrupting natural hydrologic cycles: (1) conversion of forests into agricultural land, mostly a practice still taking place in un- developed countries; (2) rapid urbanization in undeveloped and developing countries converting all other land uses into urban land; and (3) rapid decentralization of cities, par- ticularly in the United States, where urban sprawl has changed the American landscape and resulted in deeply entrenched social and environmental problems. Urban devel- opment and the creation of impervious surfaces beyond a city core inevitably result in increase of runoff and soil erosion. In turn, this reduces infiltration potential and ground- water recharge. Increased sediment load carried by surface streams often results in the

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formation of fine-sediment deposits along stream channels, which reduces hydraulic con- nectivity and exchange between surface water and groundwater in river flood plains. Clear-cutting of forests also alters the hydrologic cycle and results in increased erosion and sediment loading to surface streams. Conversion of low-lying forests into agricul- tural land may increase groundwater recharge, especially if it is followed by irrigation. However, it is important to remember that often this additional recharge is irrigation return, and the origin of water may be the underlying aquifer in question. In such case, irrigation return would be a small fraction of the groundwater originally pumped. In con- trast, cutting of forests in areas with steeper slopes will generally decrease groundwater recharge because of the increased runoff, except in cases of very permeable bedrock, such as karstified limestone at or near land surface.

The evolution of land use in the United States follows a typical pattern outlined by Taylor and Acevedo (2006). During the eighteenth and nineteenth centuries, natural for- est or grassland habitat was extensively converted to agricultural use. The industrial revolution of the late nineteenth and early twentieth centuries spurred urban develop- ment and massive migration to city centers. Agricultural land was extensively reforested during this time period, either naturally or artificially. Following World War II, devel- opment patterns took on a more “modern” approach, as people moved out of cities into peripheral suburban communities. During this phase, which is still taking place, both agricultural and forested lands are converted to residential, commercial, and industrial areas, as jobs tend to rapidly follow Americans to the suburbs. Figure 3.17 shows land use changes in central and southern Maryland, an area experiencing dramatic sprawl due to expansion from Washington, DC, and Baltimore metropolitan areas.

Using the SCS CN method, one can approximate changes in runoff over time in the study area. Data from the Cub Run Watershed in Northern Virginia illustrate how land use changes impact runoff. A weighted approach to the SCS method was applied to derive one CN for the entire watershed. The basic procedure is multiplying the percent of the watershed comprising a certain land use by the CN corresponding to that land use and then calculating the summation of all such products. The study in the Cub Run Watershed begins in 1990, well after the transition from agricultural land to forest land.

F IGURE 3.17 Land use changes in Central and Southern Maryland, 1850–1992. (From Taylor and Acevedo, 2006.)

GroundwaterRecharge

60 Forest/agriculture land use

e 50

Urban land use

chang 40 cent 30 Per 20

F IGURE 3.18 Runoff changes in Cub Run watershed, northern Virginia.

Between 1990 and 2000, land was rapidly converted to urban use, consisting of higher density residential and commercial developments (Dougherty et al., 2004). The effects of these changes result in an approximate 15 percent increase in runoff from the watershed, as shown in Fig. 3.18. The increase in runoff means less groundwater recharge in the watershed and more erosion of streambanks and impairment of water quality.

A similar analysis for watersheds in California reveals the same trend, namely, that rapid urbanization leads to exponential growth in runoff (Warrick and Orzech, 2006). Figure 3.19 shows the annual average discharge normalized by precipitation for four rivers in Southern California from 1920 to 2000. The construction of dams for flood control purposes on the Santa Ana and Los Angeles rivers only temporarily delayed dramatic increases in river discharge. A significant portion of this runoff was once groundwater recharge, placing further strain on over-allocated aquifers in the region. Figure 3.20 shows how the cumulative sediment discharge of the Santa Ana River also increased between 1970 and 2000 as a consequence of urbanization and the increase in runoff. This and many other similar studies show that city planners and water managers must promote infiltration in urban and suburban environments, both to reduce runoff and erosion and to sustain groundwater resources.

As previously discussed, urban development often causes decreases in infiltration rates and increases in surface runoff because of the increasing area of various imper- vious surfaces (rooftops, asphalt, and concrete). However, Table 3.2 illustrates that the infiltration rate varies significantly within an urban area based on actual land use. This is particularly important when evaluating fate and transport of contaminant plumes, including development of groundwater models for such diverse areas. For example, a contaminant plume may originate at an industrial facility, with high percentage of im- pervious surfaces resulting in hardly any actual infiltration, and then migrate toward a residential area where infiltration rates may be rather high because of the open space (yards) and irrigation (watering of lawns).

Agricultural activities have had direct and indirect effects on the rates and com- positions of groundwater recharge and aquifer biogeochemistry. Direct effects include dissolution and transport of excess quantities of fertilizers and associated materials and hydrologic alterations related to irrigation and drainage. Some indirect effects include

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Santa Ana River

Los Angeles River

0.4 m n

0.6 Da c tio Post- o

is char

e 3 /s/cm)

Trabuco Creek av

ag er

(m

San Diego Creek

1960 1980 2000 F IGURE 3.19 The time history of the relationship between river discharge and precipitation for

southern California rivers showing increases in discharge with respect to precipitation. All records have been normalized by the annual precipitation measured at Santa Ana, CA. Solid lines show 10-year means; shadings are 1 standard deviation about the means. (From Warrick and Orzech, 2006.)

changes in water-rock reactions in soils and aquifers caused by increased concentrations of dissolved oxidants, protons, and major ions. Agricultural activities have directly or indirectly affected the concentrations of a large number of inorganic chemicals in ground- − water, such as NO +

3 ,N 2 , Cl, SO 4 2− ,H , P, C, K, Mg, Ca, Sr, Ba, Ra, and As, as well as a

wide variety of pesticides and other organic compounds (B ¨ohlke, 2002).