FS_081-01.fm Page 1 Friday, June 4, 2004 4:17 PM In cooperation with the Texas Water Development Board Water Budget for the Nueces Estuary, Texas, May–October 1998 The Texas Water Development Board (TWDB), Texas stations previous to this study, and (3) evaporation and Parks and Wildlife Department (TPWD), and Texas Natural return-flow data obtained from other agencies. This fact Resource Conservation Commission (TNRCC) are charged sheet describes the data-collection methods and the results by the Texas Legislature with determining freshwater of the water-budget estimates for the Nueces estuary. inflows required to o o o o 97 37'30" maintain the ecologi- 28 97 30' 97 22'30" cal health of streams, SAN PATRICIO COUNTY bays, and estuaries in State 631 FM 1944 FM 1074 Texas. To determine US 77 required inflows, the US 181 I–37 three agencies collect State 631 data and conduct studies on the needs for freshwater inflows Calallen 08211503 to Texas estuaries. Dam US 181 Rincon Bayou FM 1074 08211500 The U.S. Geologi- 27o52'30" NUECES NUECES RIVER cal Survey (USGS), in RIVER DELTA cooperation with the TWDB, conducted a NUECES BAY 08211504 Nueces Bay study in the Nueces Causeway estuary (fig. 1) during Corpus Christi May–October 1998 to Corpus Christi Ship FM 1694 FM Channel provide water-budget I–37 CORPUS CHRISTI BAY data for calibration of US 77 State 44 a TWDB model that State will be used to esti- 358 mate the effects of State 286 NUECES COUNTY different freshwater 27o45' inflow volumes on 0 2 4 6 8 MILES circulation and salin- EXPLANATION ity in the estuary. The water budget (inflows Land subject to inundation and outflows) for the Marsh TEXAS Nueces estuary was 08211500 Streamflow-gaging station and estimated by using number Study (1) data collected Texas Agricultural Experiment area Station weather station Nueces during this study, estuary (2) data collected Power station discharge site at two upstream LOCATION MAP streamflow-gaging Figure 1. Location of Nueces estuary, Texas. U.S. Department of the Interior USGS Fact Sheet 081–01 U.S. Geological Survey December 2001 FS_081-01.fm Page 2 Friday, June 4, 2004 4:17 PM 3,000 Station 08211500 2,000 1,000 0 300 250 Station 08211503 200 150 (Positive discharge is flow into Rincon Bayou from the Nueces River; DISCHARGE, IN CUBIC FEET PER SECOND negative discharge is flow into the Nueces River from Rincon Bayou) 100 50 0 -50 May JuneJuly August September October Figure 2. Daily mean discharge at stations 08211500 Nueces River at Calallen and 08211503 Rincon Bayou Channel near Calallen, May–October 1998. Study Area About 1.6 miles (mi) downstream from the Calallen The Nueces estuary consists of two areas of nearly equal Dam is the Nueces overflow (Rincon Bayou) channel. This size: Nueces Bay and the Nueces River delta (fig. 1). Nueces channel was constructed by the Bureau of Reclamation in Bay is a shallow, 27-square-mile (mi2), secondary bay of 1995 to divert flow from the Nueces River into Rincon Corpus Christi Bay. Nueces Bay has a mean depth of about Bayou in the upper Nueces delta. Streamflow-gaging station 2.3 feet (ft) and a volume of about 39,700 acre-feet (acre-ft) 08211503, Rincon Bayou Channel near Calallen, was (Ward, 1997, p. 10). The Nueces delta in southern San Patri- installed to measure rainfall in the study area and stream- cio County is a 28-mi2 area of vegetated marshes, mud flats, flow in the Rincon Bayou channel. and open water formed where the Nueces River flows into Table 1. Rainfall and rainfall volumes for the Nueces Nueces Bay. estuary, May–October 1998 The Nueces estuary is located between a humid subtropi- Nueces delta Nueces Bay cal region to the northeast and a semiarid region to the west (17,920 acres) (17,280 acres) Month and southwest. Annual rainfall in the study area is about 30 Rainfall1 Rainfall volume Rainfall2 Rainfall volume inches (in.). Annual lake evaporation is about 57 in. (Texas (inches) (acre-feet) (inches) (acre-feet) Department of Water Resources, 1981). Prevailing winds May 0 0 0 0 are from the southeast. June .21 310 .07 101 July .76 1,130 1.30 1,870 Instrumentation and Data Collection Aug. 4.75 7,090 3.52 5,070 Streamflow-gaging station 08211500, Nueces River at Sept. 4.16 6,210 2.72 3,920 Calallen (fig. 1), is located at the Calallen Dam, a small Oct. 9.67 14,400 9.67 13,900 (crest altitude about 5 ft above sea level) rock-fill dam that Total3 19.55 29,100 17.28 24,900 serves as a barrier to saltwater intrusion from Nueces Bay. 1 Streamflow in the Nueces River at Calallen is regulated by At station 08211503. 2 At station 08211504. upstream reservoirs. 3 Total volumes are rounded. 2 FS_081-01.fm Page 3 Friday, June 4, 2004 4:17 PM Table 2. Monthly inflow to the Nueces estuary and discharge into Rincon Bayou, May–October 1998 [In acre-feet] Station no. Station name May June July Aug. Sept. Oct. Total 08211500 Nueces River at Calallen1 76 28 44 4,920 55,900 69,200 130,000 08211503 Rincon Bayou Channel near Calallen 8 2-1 2-10 13 640 2,280 2,930 1 Total measured inflow. 2 Flow from Rincon Bayou into Nueces River. Streamflow-gaging station 08211504, Nueces Bay at Rincon Bayou, depending on tidal conditions. However, Nueces Bay Causeway, was installed at the entrance to during August–October most of the flow into Rincon Bayou Nueces Bay to measure tidal exchange between Nueces and was freshwater inflow resulting from higher discharges in Corpus Christi Bays. This station measured rainfall, water- the Nueces River. surface elevation (stage), and water-current velocity at 15- minute intervals. Discharge measurements also were made Unmeasured Inflow at this site under various tidal conditions and correlated with Periods of heavy rainfall result in unmeasured inflow station records of stage and velocity to develop a discharge into the estuary and Nueces Bay from about 148 mi2 of rating at the site. From this rating, continuous (15-minute ungaged area (not including 28 mi2 of Nueces delta). Most interval) discharge estimates were computed. of the unmeasured inflow enters Nueces Bay directly, but some of the inflow enters through the Nueces River. Several Water-Budget Components rainfall events during August–October resulted in runoff in Components of the Nueces estuary water budget consid- the ungaged area. Runoff volumes from the ungaged area ered in this study were rainfall, measured inflow, unmea- were estimated from unit-runoff computations using data sured inflow, return flow, evaporation, and net tidal flow. from streamflow-gaging station 08189700, Aransas River Gains from and losses to ground water were not included. near Skidmore, about 27 mi north of the Rincon Bayou Withdrawals from Nueces Bay were negligible. channel station. Hydrologic conditions and land use in the 247-mi2 Aransas River watershed are similar to the ungaged Rainfall area adjacent to Nueces Bay. Monthly runoff coefficients— Rainfall volumes onto the Nueces delta and Nueces Bay ratios of runoff volume to rainfall volume—were computed were computed as the product of area and rainfall from the for the Aransas River near Skidmore station. The rainfall Rincon Bayou and Nueces Bay rain gages, respectively volumes used for these computations were estimated using (table 1). rainfall data from the National Oceanic and Atmospheric Administration rain gage at the Welder Wildlife Refuge in Measured Inflow San Patricio County (about 18 mi north of Nueces Bay). Inflow from the Nueces River provides most of the The computed coefficients were then applied to the rainfall freshwater inflow to the Nueces estuary, and the station at measured at the Rincon Bayou channel station to estimate Calallen accounts for all of the measured inflow to the unmeasured inflow (table 3). estuarine system. The Rincon Bayou channel station does not measure additional inflow to the system but only mea- sures the flow that is diverted from the Nueces River to the Table 3. Estimated unmeasured inflow to the Nueces upper Nueces estuary through the Rincon Bayou channel. estuary, May–October 1998 Monthly inflows measured at the two stations are listed in [--, not computed] table 2. Hydrographs of daily mean discharge at the Calallen Runoff coefficient Runoff and Rincon Bayou channel stations (fig. 2) indicate that Month (acre-feet per inch-square mile) (acre-feet) freshwater inflow to the Nueces estuary during May–July was low compared with inflow during August–September. May -- 0 Because of tidal influence, discharge through the Rincon June -- 0 Bayou channel is not always a result of freshwater inflow July -- 0 into the Nueces River. During high tides, saltwater from Aug. 1.08 760 Nueces Bay can move up the Nueces River into Rincon Sept. 1.41 870 Bayou through the Rincon Bayou channel. Also, during Oct. 3.84 5,500 low-to-moderate discharges at the Calallen station, flow at the Rincon Bayou channel station might be into or out of Total -- 7,130 3 FS_081-01.fm Page 4 Friday, June 4, 2004 4:17 PM 1.0 0.5 Sea level INSTANTANEOUS STAGE, IN FEET STAGE, INSTANTANEOUS -0.5 40 20 0 -20 INSTANTANEOUS TIDAL FLOW, TIDAL INSTANTANEOUS -40 IN THOUSANDS OF CUBIC FEET PER SECOND IN 18 20 22 24 26 28 30 JULY 1998 Figure 3. Instantaneous stage at Nueces Bay Causeway and tidal flow into and out of Nueces Bay, July 17–30, 1998. Return Flow relatively small and consist primarily of wastewater dis- The primary return flow to Nueces Bay is cooling water charges.