How Much Water is in the Guadalupe? A Preliminary Data Analysis and Gap Analysis
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The headwaters of the Guadalupe River originate in the Edwards Plateau province of central Texas and flow approximately 480 miles to San Antonio Bay in the Gulf of Mexico. The Blanco/San Marcos River system and the San Antonio River are the largest tributaries of the Guadalupe River. Surface water/groundwater interactions are dominated by the flow contribution of several major springs including the Plateau Edwards headwaters spring system, Comal Springs, San Marcos Springs, and Hueco Springs. There are unquantified gains due to shallow groundwater inflow along the river, but the major springs provide the majority of base flow to the river. There are no major documented natural losing reaches on the river, though localized losses occur.
The United States Geological Survey (USGS) maintains twenty-one stream gages on the main channel of the river as well as tributary and spring gages. Long term historic trends in flow in the main channel are difficult to assess due to the varying lengths of the period of record (POR) for many of the USGS gages. For gages with PORs over 70 years in length, such as Hunt, Spring Branch, and Victoria, discharge trends are generally flat. All gages, including major springs, with PORs since 2000 indicate decreasing discharge trends. The cause of the declines may be from increased withdrawals and/or climate change due to increasing temperatures and decreasing precipitation. Researchers have predicted continuing declines in the discharge from major springs due to climate change. In addition, population and water demand are projected to double by 2070. Additional permanent stream gages are necessary to capture discharge trends at significant points along the river.
The percentage total major spring discharge (Comal, San Marcos, and Hueco Springs) to river discharge at Victoria ranged from one percent during Hurricane Harvey in 2017 to over 190 percent during the drought of 2011. Any percentage over 100 percent represents water losses (diversions, losses to groundwater, and evapotranspiration) between the springs and Victoria. The average contribution is 62 percent of the discharge measured at Victoria. Spring flow is greater than discharge at Victoria eleven percent of the time from 2003 to 2017.
There are over 600 assigned water rights (diversions) on the Guadalupe River totaling over six million acre-feet of water per year, or over 8,700 cubic feet per second (cfs). This volume of water is significantly greater that the average of the mean daily discharge as measured at Victoria for the years 1934 to 2017 of 2,113 cfs, indicating the river is oversubscribed. The actual use is likely lower than the total assigned diversions. Rights held for hydroelectric power generation are the largest class of right holders, followed by industries and agriculture. The largest number of individual water right holders occur near headwater springs in Kerr County. The largest volume of water held by water right holders occurs in Guadalupe County.