FIGURE 3.13

Comparison of a multivariate index of the strength of several El Niño events from 1957 to 1998.

SOURCE: National Oceanic and Atmospheric Administration, 1998.

domestic and industrial water supply from local and regional aquifers, many of which are restored (recharged) at rates often less than 0.2 percent per year by volume (Pimentel et al., 1997). Almost all rural homes and many cities depend heavily on ground water. As a consequence, aquifers have been thoroughly mapped and are extensively mined, especially for irrigation supplies (Figure 3.15). Throughout the western and midwestern states, large aquifers have declined substantially because rates of extraction exceed recharge rates (e.g., Ogalala Aquifer).

While ground water basins do not always coincide with watersheds, there are important interactions between surface water and ground water. In most watersheds, surface water and ground water flow paths are interconnected (Gibert et al., 1994a). The recharge areas may be wetlands or simply areas in the watershed where there are very permeable soils and sometimes a shallow water table. While the regional flow of water in aquifers generally reflects the general surface topography above, ground water resources in one watershed may also be fed by surface



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