estuarine and marine aquatic environments. There- 

 fore, key hydrographic indicators like water level 

 and salinity represent the combined effect of 

 hydro-meteorological forces, including tides, wind, 

 precipitation and local runoff, and river inflow. 



Water Level 



Water level variations in the Nueces Estuary are 

 generally the result of interactions between 

 (astronomical) tides, long-term secular excursions of 

 the Gulf of Mexico and meteorological forcing (winds) 

 (Ward 1997). 



Tides 



Of the many Fourier tidal components, four in 

 particular account very well for the obser\'ed variation 

 in the Nueces Estuary (\)C^ard 1997). The three 

 astronomical tides, which are generally short-term 

 influences, include a semidiurnal (12.4-hour) and 

 diurnal (24.8-hour) tide, each of which is then 

 modulated by a 27.2-day lunar tide (i.e., the variance of 

 water level resulting from variations in the declination 

 of the moon). The physical causes of fourth tidal 

 component, a long-term secular variation in the Gulf 

 of Mexico, are not well understood. In a "normal" 

 year, there are clear maxima in the spring and autumn, 

 and clear minima in winter and summer. The 

 semi-annual variation is generally considered to be 

 dominated by the winter minimum and autumn 

 maximum (Chew 1964). Ward (1997) has summarized 

 the featvures of this secular variation to include the 

 following characteristics: 1) there are considerable 

 year-to-year differences in the seasonal water-level 

 variation, 2) the autumn maximum is usually the 

 highest mean water elevation of the year and the winter 

 minimum is usually the lowest, 3) the summer 

 minimum in July and the autumn maximum in October 

 are the most consistent in terms of seasonal regularity, 

 4) both the winter (December to March) and spring 

 (April through June) extremes have a considerable 

 seasonal range in which they occur and can exhibit 

 multiple extremes during these periods, and 5) despite 

 reference to a semiannual period, the signal is not 

 harmonic, as both the autumn maximum and the 



summer minimum, especially the former, tend to be 

 more sharply focused in time and extend over two-six 

 weeks in duration. 



This semi-annual variation is a prominent component 

 of water level variation in the Nueces Estuary, and 

 becomes increasingly important, compared to the other 

 tidal components, with distance into the upper reaches 

 of the bays, especially in Nueces Bay and Delta. The 

 filtering properties of the inlets, shallow bays and 

 channels gready attenuate the shorter period 

 frequencies {e.g., semidiurnal and diurnal signals), but 

 pass the longer periods with virtually no attenuation. 

 As a result, water level variations caused by the cyclical 

 lunar tide and the semi-annual secular rise and fall of 

 water level in the Gulf represent the predominant long- 

 term mechanism of water exchange with the Nueces 

 Estuary. These two factors also account for most of 

 the (gready attenuated) tidal variation of water level 

 within the upper Nueces Bay and Delta. 



Wind 



On a shorter time scale, meteorological forcing (wind) 

 is a prominent mechanism for water-level variation. 

 The principal process is the response of a free surface 

 to an imposed stress, referred to more colloquially as 

 "setdown" or "setup" of water levels. Where there are 

 adjoining bay systems (such as Nueces and Corpus 

 Christi bays, or Corpus Christi Bay and the Gulf of 

 Mexico), winds cause a direct downwind set-up of 

 water levels across component bays. This difference in 

 water levels causes a direct and an indirect water 

 exchange between the water bodies. The direct effect 

 is that of wind stress on the bay itself The indirect 

 response of the same bay is that resulting from wind 

 stress on the adjacent water body, which forces water 

 into or out of the former. The area over which the 

 winds operate and their duration are both important in 

 the magnitude of the estuary's response. 



There are three types of meteorological forcing 

 elements common to the Nueces Estuar^': frontal 

 passages, sea breezes and storm surges. Frontal 

 passages produce water level variations and 

 accompanying transports of water. The primary 

 mechanism is the change in direct wind stress on the 



2-4 ♦ Study Area 



