Abe Springer, Department of
Geology, Northern Arizona University, Flagstaff, AZ 86011 abe.springer@nau.edu
Although
the focus on droughts is usually on a lack of precipitation, this decrease in
precipitation has consequences for hydrologic systems on and below the surface
of the Earth. Drought leads to a
decrease in recharge to aquifers through (1) areally distributed recharge, (2)
focused recharge, (3) septic return flow, (4) irrigation return flow, and from
(5) increased evaporation and transpiration.
The typical response of humans to a hydrologic drought is to (1) pump
more groundwater at existing wells to make up for loss of surface water, (2)
drill more boreholes to augment supply, (3) remove riparian vegetation to
decrease transpiration, (4) implement conservation practices, (5) develop
alternative water supplies.
Hydrologic
drought impacts soil water and ground water in many ways. It leads to a loss of groundwater and soil-water
storage. This causes water levels in
wells to be lowered and an increased depth to water in riparian areas with less
water available for the plants to transpire.
It also leads to a decrease in aquifer discharge. This will cause a decrease in baseflow in
streams and lakes, decreased spring discharges, and decreased wetted area
(aquatic habitat) in streams and lakes.
Other impacts are increases in the risks from land subsidence and
saltwater intrusion.
The
U.S. Geological Survey began a system of monitoring stream discharge in the
late 1800s on many streams in the U.S.
There are now continuous gages on over 7,000 streams, but a lack of
stations with records of more than 30 years and a growing loss of gages on the
smallest, unregulated streams, making it difficult to observe the impacts of
drought on baseflow. In response to the
drought of the 1930s, the USGS began monitoring water levels in a network of
wells. Generally, this program has a
more complete and extensive record in the Eastern U.S. than in the Western
U.S. Some states use these wells as
part of their drought management strategy.
Because
of the lack of direct hydrological measurements of the impacts of recent
droughts, it is useful to look at the hydrological impacts of land-use management
to understand potential impacts on hydrological systems. There are good examples of changing
agricultural practices and changing forest management practices to use as
surrogates to understand the changes we might anticipate from drought. Also, vegetative indicators, such as
springsnails, may be very useful to understand drought on longer time scales.
Case
studies will be provided from the Verde Valley of Arizona to show what is known
about baseflow of springs and streams with existing monitoring, and how we can
use this to interpret the impacts of the current and future droughts.