Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 83

become disconnected during Stage III if streamflow cannot provide enough recharge to
maintain water levels in the alluvial aquifer.
Groundwater in the region is important in sustaining unique desert ecosystems and
in allowing humans to live and thrive in arid and semiarid areas with insufficient or no
surface water. During the latter part of the twentieth century, the region experienced the
most rapid population growth within the nation. Continued population growth and reli-
ance on groundwater for public supply will require careful planning and management of
resources to minimize undesired consequences of groundwater pumping.
4.7 Groundwater-Surface Water Connection
In the desert Southwest, most water rights for surface and groundwater are set by state law.
In states such as Arizona and California, surface water and groundwater are considered
legally to be distinct entities despite the hydrologic fact that they are intimately linked. 32,33
As discussed previously, surface water interacts in a complex way with geomorphology
and groundwater in the Southwest. Discharge from bedrock or alluvial aquifers creates
perennial or intermittent streams, and subsurface geologic structures or bedrock can force
groundwater levels to the surface. Abrupt termination of such features allows surface
water to infiltrate back into the aquifer. Finally, channel downcutting can reduce bed levels
below the water table of the alluvial aquifer, releasing stored groundwater; conversely,
aggradation can increase the distance between channel bed and the water table.
Continued groundwater pumping in alluvial basins can create a water table that slopes
away from connected streams. Instead of effluent flow conditions, where groundwater
discharges to a channel, influent conditions creates what is known as a groundwater
mound immediately beneath the channel (Figure 4.5). This type of water table is common
beneath channels with artificial flow, such as wastewater effluent released into natural
channels. New riparian ecosystems may be able to take advantage of this limited-extent
alluvial aquifer depending upon surface-water discharge and the extent of groundwater
level declines from pumping in the vicinity.
Groundwater recharge beneath alluvial channels can be substantial during floods. For
example, following the floods of December 1978, water levels rose up to 82 ft in wells in
alluvial aquifers in southeastern Arizona. 34 Similar rises occurred following runoff in 1979,
1980, and 1983. Streamflow measurements during a February 1978 flood on the Gila River
indicated that 112,000 ac ft (17%) of the inflow recharged the alluvial aquifer; an earlier
measurement in January 1966 indicated that 175,000 ac ft (29%) of inflow was recharged. 35
Groundwater rises attributed to flood discharges can also be a problem. In 1979, rises in
groundwater levels owing to flood-related dam releases to the lower Gila River caused
waterlogging in agricultural lands, temporarily removing them from production. 34
4.8 Water Quality
Water quality is extremely important in determining the suitability of water for domestic
supplies, irrigation, or industrial use. Dissolved and suspended constituents vary considerably
across the desert Southwest and depend in large part on geologic and soil characteristics in the
headwaters of watersheds, where water is recharged into aquifers, and/or the characteristics