Technical Reports Document Library

Note:  This report is included because of its possible historical value.  It has been replaced by more recent plans.

[From the Executive Summary]
The study area consists primarily of the Nueces River Basin, which covers an area of approximately 17,000 square miles in South Texas as shown in Figure ES-1. Several entities interested in the potential development of additional water supplies in the basin, along with the Texas Water Development Board (TWDB), have jointly participated in the performance of this study. These four entities are: Nueces River Authority (Authority); City of Corpus Christi; Edwards Underground Water District (EUWD); and South Texas Water Authority (STW A).

Over the past several decades, increasing water demands on the Edwards Aquifer have raised concerns about the ability of the aquifer to meet these demands without causing social, economic, and environmental problems. The headwaters of the Nueces River Basin contribute about 57 percent of the total volume of surface water recharge to the San Antonio portion of the Edwards Aquifer. Streams crossing the Edwards Aquifer recharge zone lose a significant portion of their flow through faults and solution cavities in the limestone formations. A large portion of the runoff from the headwater area, however, occurs during storms which exceed the capacity of the recharge zone. It has been suggested that, if recharge enhancement structures were constructed, aquifer water levels, well yields, and springflows would benefit.

The concept of building recharge structures is not new. In 1964, the U.S. Army Corps of Engineers (COE) identified numerous potential sites for recharge projects.[See Volumes 1-3 of Survey Report on Edwards Underground Reservoir Guadalupe, San Antonio and Nueces Rivers and Tributaries, Texas.) Since 1974, the Edwards Underground Water District has undertaken the construction of three small recharge projects in the basin. The locations of the EUWD recharge projects as well as the locations of those projects identified by the COE (and others) are shown in Figure ES-1.

Approximately 98 percent of the drainage area of the Nueces River basin is located upstream of the Choke Canyon Reservoir/Lake Corpus Christi System (CC/LCC System). The locations of these two reservoirs are shown in Figure ES-1. The CC/LCC System is operated by the City of Corpus Christi, with the majority of water being diverted from the system at the Calallen Diversion Dam located 35 miles downstream of Lake Corpus Christi.

At this location, the water is diverted from the river and distributed to various municipal and industrial users. The CC/LCC System is the primary source of municipal and industrial water supply for a significant portion of the Texas Coastal Bend. Reductions in the inflows to these two reservoirs that could result from the construction of additional recharge projects is an important consideration in the evaluation of any recharge program.

Ongoing studies of the Nueces Estuary, which include Nueces, Corpus Christi, Oso, and Redfish Bays and a portion of the Laguna Madre, by the Texas Water Development Board (and others) have shown that freshwater inflows play an important role in the productivity and viability of the estuary. Reduction of inflow to the Nueces Estuary that could result from the construction of additional recharge structures is also an important consideration.

The primary objectives of this study are listed below and were accomplished through the development and application of a computer model of the Nueces River Basin.

• Determine the potential for increasing artificial recharge to the Edwards Aquifer through construction of additional recharge structures in the Nueces River Basin;

• Calculate the firm yield of the Choke Canyon Reservoir /Lake Corpus Christi System with and without additional recharge structures; and

• Quantify the potential impacts of additional recharge structures on inflows to the Nueces Estuary.

Additional objectives of the study included:

• Independent evaluation of U.S. Geological Survey (USGS) estimates of historical natural recharge to the Edwards Aquifer from the Nueces River Basin;

• Estimation of future water demands for the Nueces River Basin through the year 2040 with emphasis on estimating future demands of the CC/LCC service area;

• Evaluation of the firm yield of the CC/LCC System with respect to its ability to meet future demands through the year 2040; and

• Development of recommendations for additional study.

…

Significant study findings and conclusions are as follows:

• Historical recharge to the Nueces River Basin portion of the Edwards Aquifer can be increased by an average of about 85,300 ac-ft per year if all seven Type 1 recharge structures are constructed and all water rights are honored. This represents an increase of about 26.3 percent in the historical average recharge to the Nueces River Basin portion of the Edwards Aquifer from surface water sources. Recharge during the 10-year drought period from 1947 through 1956 could be increased by about 19,100 ac-ft per year or 12.3 percent of the historical average during this 10-year period.

• Recharge with all twelve Type 2 recharge structures in place can be increased.on the average .by about 61,100 ac-ft per year or 18.9 percent if ·all water rights are honored. For the 1947-1956 drought period, recharge could be increased by about 24,100 ac-ft per year or 15.5 percent

• The recharge estimates in this report represent a theoretical maximum and are subject to significant reductions due to likely economic, environmental, structural, and political limitations on more detailed review.

• With no additional recharge structures in place, the firm yield of the CC/LCC System under Phase IV operating policy is 220,000 ac-ft per year for 1990 conditions and 197,500 ac-ft per year for 2040 conditions. These yields are based on existing water rights diverting at full authorization and do not consider the full release of 151,000 ac-ft per year to the Nueces Estuary. If system releases needed to insure 151,000 ac-ft of annual estuarine inflows are made, without abeyance provisions for drought conditions, then the 1990 firm yield is 166,300 ac-ft per year and the 2040 firm yield is 147,300 ac-ft per year.

• With no additional recharge structures in place, the firm yield of the CC/LCC System under Phase II operating policy is 187,800 ac-ft per year for 1990 conditions and 169,700 ac-ft per year for 2040 conditions. These ·yields are based on existing water rights diverting at full authorization and do not consider the full release of 151,000 ac-ft per year to the Nueces Estuary. If system releases needed to insure 151,000 ac-ft of annual estuarine inflows are made, without abeyance provisions for drought conditions, then the 1990 firm yield is 122,400 ac-ft per year and the 2040 firm yield is 107,100 ac-ft per year.

• The 1990 firm yield of the CC/LCC System would be reduced by up to 3,900 ac-ft ·per year with the implementation of all seven Type 1 recharge structures, if these structures were operated not to honor the water rights of the CC/LCC System.