Upon review of the adopted Regional Water Plans submitted by the Lower Colorado (Region K) and the South Central Texas (Region L) Regional Water Planning Groups (RWPGs), the Texas Water Development Board (TWDB) has expressed concern with potential projected “over-allocation” of groundwater from the Carrizo-Wilcox (Simsboro) Aquifer in Bastrop County in the years beyond 2030. This potential projected over-allocation is summarized in Table 1 which compares decadal estimates of supply from and drought demand upon the Carrizo-Wilcox (Simsboro) Aquifer in Bastrop County. The South Central Texas RWPG expects that groundwater supplies from Bastrop County will be used in Bexar County throughout the planning period and in Comal and Guadalupe Counties after 2030. The TWDB’s concern arises over the potential conflict between recommendations for meeting demands from Bastrop County groundwater in the Region K and Region L Regional Water Plans. If such a conflict were to occur, the TWDB would have to resolve it.
As
a matter of policy, the Lower Colorado RWPG (Region K) has limited groundwater
availability to estimated long-term average recharge and has based such
recharge on recent studies completed by the Bureau of Economic Geology[1]. Hence, the Lower Colorado RWPG has adopted a
Bastrop County groundwater supply from the Carrizo-Wilcox (Simsboro) Aquifer
averaging 21,950 acft/yr. Subsequent
TWDB consultation with the Bureau of Economic Geology has indicated that an
additional 2,455 acft/yr could be added to this supply to account for recharge
to the Wilcox formation. The Bastrop
County groundwater supply from the Carrizo-Wilcox (Simsboro) Aquifer is
therefore estimated to average 24,405 acft/yr pursuant to adoption of this
revision by the Lower Colorado RWPG.
The Region K groundwater policy explicitly allows for withdrawals in
excess of this average rate during drought since withdrawals will be reduced
during wet periods resulting in long-term average withdrawals equal to or
perhaps less than recharge.
The South Central Texas
RWPG, on the other hand, has relied upon estimates of groundwater supply
provided to all planning areas by the TWDB in July 1998 and upon additional
guidance provided by the Evergreen and Gonzales County Underground Water
Conservation Districts during the regional planning process. It is noted that the estimates of
groundwater supply from the Carrizo-Wilcox Aquifer in Bastrop County provided
by the TWDB in 1998 range from an average recharge rate of 33,391 acft/yr to a
“mining” rate in excess of 72,000 acft/yr which could be recovered from storage
over a 50-year planning period.
It is clear that discussions
regarding groundwater availability in Bastrop County and the authority of an
RWPG to set groundwater policy will continue among the Lower Colorado and South
Central Texas RWPGs, the TWDB, and other interested parties. In recognition of this fact, the RWPGs agree
that discussions will be more productive with additional scientific information
to be available upon completion of the Groundwater Availability Models
(GAMs). The development of a GAM for
this portion of the Carrizo-Wilcox (Simsboro) Aquifer is in progress with
scheduled completion by 2003. In order
to comply with TWDB interpretation of statutory provisions, however, the Lower
Colorado and South Central Texas RWPGs and the TWDB now conditionally agree to
the assessment of Bastrop County groundwater supply and demand summarized in
Table 2. All interests remain committed
to the pursuit of additional scientific information and recognize that the
assessment in Table 2 may be revised at any time by action of the RWPGs, the
TWDB, and/or over-riding legal authority.
In the regional water
planning process, calculation of water needs has been based on comparison of
dependable supplies and projected demands during drought (below normal
rainfall) conditions. Simulations of
the potential cumulative effects of long-term aquifer pumpage have also been
based upon full utilization of projected drought demands in each and every
year, often resulting in over-estimation of withdrawals from aquifers because
municipal water demands subject to average climatological conditions (normal
rainfall) are substantially less than drought demands. Review of TWDB municipal water demand
projections for Comal, Guadalupe, and Bastrop Counties indicates that water
demand for normal (average) rainfall is at least 14 percent, and up to 20
percent, less than that for below normal (drought) rainfall. It is expected that municipal water
suppliers in Comal and Guadalupe Counties will rely primarily upon nearby
sources of supply such as the Guadalupe River, Canyon Reservoir, the Edwards
Aquifer, the Schertz-Seguin Water Supply Project, and other groundwater
available in Gonzales County. These
municipal water suppliers are expected to use more distant sources such as
Bastrop County groundwater only when absolutely necessary during drought. Hence, as summarized in Table 2, Bastrop
County groundwater supplies from the Carrizo-Wilcox (Simsboro) Aquifer are
calculated to be sufficient to meet projected demands throughout the planning
horizon.
Comparison of the decadal
Surplus/(Deficit) values in Table 2 with those in Table 1 gives an indication
of the frequency with which drought withdrawals may occur while maintaining a
long-term average withdrawal rate consistent with Region K policy. As the Surplus at 2040 under “normal
rainfall” conditions (Table 2) exceeds the Deficit under “below normal
rainfall” conditions (Table 1) (3,210 acft/yr is greater than 2,556 acft/yr),
drought withdrawals could occur every other year without exceeding the average
Supply identified by the Lower Colorado RWPG.
The Surplus at 2050 under “normal rainfall” conditions (Table 2) is
about one fifth of the Deficit under “below normal rainfall” conditions (Table
1) (2,550 acft/yr is about one fifth of 11,818 acft/yr). Hence, 2050 drought withdrawals could occur
about one year in six without exceeding the average Supply identified by the
Lower Colorado RWPG.
[1] Dutton, A.R., “Groundwater Availability in the Carrizo-Wilcox Aquifer in Central Texas – Numerical Simulations of 2000 through 2050 Withdrawal Projections,” Report of Investigations No. 256, Bureau of Economic Geology, University of Texas at Austin, Texas Water Development Board Contract No. 99-483-279, Austin, Texas, 1999.