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314 Chapter Eight

Table 8.3 Scenarios of climate and groundwater use change effects considered for the Edwards Aquifer, south-central Texas. After
Loáiciga (2003).
Scenario
I II III IV
Base Climate change effect Groundwater use change effect Total effects
Climate (recharge) R 1978–89 * R 1978–89 (2 × CO /1 × CO )† R 1978–89 * R 1978–89 (2 × CO /1 × CO )
2
2
2
2
Groundwater use 1978–1989 use‡ 1978–1989 use 2050 use§ 2050 use
3 −1
9
* Historical recharge (R) during 1978–1989 (mean = 0.949 × 10 m a ).
† Historical recharge scaled to 2 × CO climate conditions.
2
9
3 −1
‡ Average groundwater use between 1978 and 1989 = 0.567 × 10 m a .
9
3 −1
§ Groundwater use forecast for 2050 = 0.784 × 10 m a .
wet and unusually dry. Estimated recharge in the hydrology to provide future recharge values which
basin was found to be sensitive to climatic variability were then used as input to a numerical groundwater
in the historical record, especially the precipitation flow model. The scenarios and model results are
variability. For GCM-projected climate changes to shown in Tables 8.3 and 8.4 and summarize the
carbon dioxide doubling, the variability in the esti- effects on spring flows in the Edwards Aquifer. In the
mated annual recharge was less than that estimated Comal Springs and, to a lesser extent, the San Marcos
from the historic data. In addition, the median annual Springs, there is a simulated increase in spring flow
recharge, for the case of a climate scenario averaged under a doubling of CO for an unchanged ground-
2
from three different GCM simulation runs, was less water use condition (Scenario II). For the case of
than 75% of the median annual recharge for the his- Scenario III (base climate and increased groundwater
torical simulation. This large change reflects the use) there are negative impacts on spring flow.
potential importance of climate on groundwater Combining the effects of both climate change and
recharge. increased groundwater use (Scenario IV) simulates a
In a more recent study, Loáiciga et al. (2000) and serious depletion of 73% in spring flow for the Comal
Loáiciga (2003) generated climate change scenarios springs but only a marginal decrease (1%) for the
from scaling factors derived from several GCMs to San Marcos springs. This study highlights that the
assess the likely impacts of aquifer pumping on the primary threat to groundwater use in the Edwards
water resources of the Edwards Balcones Fault Zone Aquifer is from the potential rise in groundwater use
aquifer system in south-central Texas. This karst caused by population growth and not from climate
aquifer, formed in the Edwards limestone formation, change, although protracted droughts under climate
is one of the most productive regional aquifers in the change will accentuate the competition between
United States and is a primary source of agricultural human and ecological water uses. Hence, and as con-
and municipal water supplies. The Edwards Aquifer cluded by Loáiciga (2003), aquifer strategies in the
has been identified as one of the regional catchments Edwards Aquifer must be adapted to climate variabil-
most vulnerable to climate change impacts in the ity and climate change.
United States. Groundwater recharge to the aquifer Uncertainty is inherent in the development of cli-
takes place almost exclusively as stream seepage, mate change scenarios due to unknown future emis-
with groundwater discharge directed to major sions of ‘greenhouse’ gases and aerosols, uncertain
springs that support an important and threatened global climate sensitivity and the difficulty of simulat-
groundwater ecosystem. To explore the vulnerability ing the regional characteristics of climate change. It is
of these springs to combined pumping and climate because of these uncertainties that the regional
change impacts, several pumping scenarios were changes in climate derived from GCM experiments
combined with 2 × CO climate scenarios. The clim- are termed scenarios or projections and cannot be
2
ate change scenarios were linked to the surface considered predictions. Comprehensive reviews of