APPENDIX D 



Recent Trends in Precipitation 

 Occurring on the Nueces River 

 Watershed of South Texas 



Jonnie G. Medina Meteorologist, U.S. Department of the Interior, Bureau of Reclamation, P. O. Box 

 25007, Code D-3720, Denver, CO 80225. 



ABSTRACT: Precipitation from the Nueces River watershed was analyzed for recent trends as part of the 

 Rincon Bayou Demonstration Project (Rincon Project) conducted by the United States Bureau of Reclamation. 

 Of interest were comparisons of precipitation across the three periods, 1940-57, 1958-81, and 1982-99, of 

 interest to the Rincon Project, a demonstration and study of impacts on the ecology of the upper Nueces delta 

 Bay from enhanced inundations of the marsh. The role is discussed of the EI Nino-Southem Oscillation 

 (ENSO) and other climate variables to tropical cyclone occurrence in the Atlantic basin and in particular, the 

 Gulf of Mexico. The potential modulation by ENSO of watershed precipitation was investigated. A 

 multivariate comparison was conducted of daily precipitation sampling distributions across project study 

 periods. The occurrence of large precipitation events was of particular interest because they could lead to 

 natural inundations of the marsh. Analyses indicated no particular watershed precipitation trend unless the 

 basis of comparison was the extreme drought period of the 1950s. The La Nina (El Nino) phase does appear 

 to lead to more (less) tropical cyclone impacts in the western Gulf of Mexico than baseline years. Of 21 years 

 (since 1948) with large precipitation events (=> 4 inches per day) impacting Corpus Christi, 6 were El Nino 

 years, 8 were La Nina years and 7 were baseline years. Tropical cyclones occurred in 7 of the 21 large-event 

 years; 5 were La Nina years and 2 were baseline years. Of 7 years with annual precipitation exceeding 

 40 inches, 5 years were baseline years, 1 year was La Niiia, and the largest-amount year was El Nino. Two of 

 the seven highest years did not have a high daily event. The multivariate comparison revealed the sampling 

 daily precipitation of the drought years differed from the other two study periods, otherwise no differences. 



INTRODUCTION 



The United States Bureau of Reclamation (Reclamation) is sponsoring and partnering in the Rincon Bayou 

 Demonstration Project (Rincon Project) aimed at determining marsh response to increased freshwater 

 inundation caused by construction of an overflow channel on the Nueces that flows into Nueces Bay just 

 north of Corpus Christi, Texas (see Figure 1). The occurrence of natural over-banking events from the Nueces 

 River into the Nueces Delta was reduced by the construction in 1958 of Lake Corpus Christi on the Nueces 

 River and in 1982 by Choke Canyon Reservoir on the Frio River (Irlbeck and Ward 2000). In addition to 

 regulated flows into the Nueces River, precipitation in the Frio and Nueces watersheds, and locally in the delta, 

 also impact the delta ecosystem. The question arises as to how much, if any, of the observed decreases in the 

 magnitude, frequency, duration and timing of delta inundation events since 1958 (Idbeck and Ward 2000) has 

 been caused by variations in climate. 



The weather systems impacting south Texas migrate from the temperate zone to the north or the tropics. Some 

 disturbances originate in southwestedy or westerly flow over Mexico. Tropical disturbances approach the 

 Nueces watershed generally from the southeast or east. Except in circumstances of deep continental flow, the 

 troposphere over south Texas generally contains ample moisture for precipitation, but often lacks a trigger 

 mechanism to build storm clouds. Perturbations moving toward the Rincon area generally provide the triggers. 

 For example, a slow-moving front or tropical wave can trigger the development of clouds that can produce 

 small amounts to several inches of precipitation, often in isolated patterns rather than general coverage. So, a 

 weather system may produce littie precipitation at a location, but a short distance away can produce three inches 



Appendix D ♦ D-1 



