(A) Basin hydrologic processes, or water storage 

 and flow through a basin; 



(B) The natural resource productivity of a 

 basin, or its capacity to support wildlife 

 and fishery species, and to perform other 

 work services for man, such as the purifi- 

 cation and storage of fresh water; 



(C) Land modifying processes, particularly 

 those which result in loss of natural wet- 

 land; and 



(D) Basin-level socioeconomic processes, or 

 those human activities and management 

 decisions that impinge directly on natural 

 processes in a basin. 



HYDROLOGY (A) 



The hydrologic regime at any specific site within 

 a Chenier Plain basin is ultimately responsible for 

 determining the kind of habitat that develops at 

 that site. Basin hydrology results from interactions 

 among three modules (fig. 10); water storage in a 

 basin (Aj^); upstream riverine and rainfall inputs of 

 water and sediment (A2); and downstream water 

 with accompanying salts and sediments and tidal 

 and oceanic storm forces (A3). 



The role of hydrology in determining habitat 

 type is primarily mediated via water levels and 

 durations, and salinity levels and durations. Water 

 levels are controlled by the pressure head between 

 water level at a given site, and upstream and down- 

 stream water levels. If rainfall raises water levels 

 upstream, water flows toward the Gulf; likewise, if 

 tidal stage or a southerly wind raises sea level at the 

 Gulf, a wave proceeds upstream, gradually diminish- 

 ing as it goes. 



Mean salinity and salinity range at a given site 

 in the basin are determined by mixing, over time, 

 of upstream and downstream inputs, and by the 

 relative volumes of fresh and saline water inputs. 

 Sediments are carried into a basin by the currents 

 produced by salinity (density) and pressure 

 gradients. Sediment deposition is a function of cur- 

 rent speed, sediment load, salinity, and in some 

 cases, biological activity. 



In summary, the hydrologic submodel sym- 

 bolizes the complex physiographic configuration of 

 a basin, which, together with upstream and down- 

 stream water mputs, determines water level, water 

 flow, salinity, and sediment regimes at any point in 

 a basin. These parameters, in turn, constrain the 



type of habitat that can develop at any site in ques- 

 tion. For example, if water level is always below 

 the land surface, then the habitat is terrestrial. If 

 the water level is always above the land surface, 

 then the habitat is aquatic. If water level alternates 

 above and below the land surface, the habitat is 

 wetland. Salinity dynamics determine whether a 

 habitat will be fresh or saline, and sediment 

 dynamics (either gain or loss) can change one 

 habitat to another. Man's activity is an important 

 factor affecting water, salinity, and sediment 

 cycles. 



NATURAL RESOURCE PRODUCTIVITY (B) 



Submodel B (fig. 10) represents the natural 

 work services of a basin; that is, the quality of a 

 basin with respect to its ability to do such things as 

 support important fishery and wildlife species, and 

 to "purify" and store water, all at no cost to man. 

 "Quality" refers to both the particular blend of 

 habitats that comprise one basin, and to the fact 

 that two areas having similar habitat types can vary 

 greatly in their abUity to support consumer or- 

 ganisms. For example, the open water habitat can 

 be in a balanced state with respect to nutrient 

 input and use, or it can be degraded (by excess 

 nutrient loading) into various degrees of eutrophica- 

 tion. 



The natural resource productivity (NRP) sub- 

 model consists of four components (fig. 10): pro- 

 ducers (Bj), consumers (B2), a refugium (B3\ ^nd 

 a water storage module (B4). Bj and B2 represent 

 the species that occur naturally in all wetlands, 

 water bodies, and ridges in a basin. A particular 

 habitat can be characterized by its carrying ca- 

 pacity for these species; as its quality diminishes, 

 so does its carrying capacity. Diminishing quality 

 may also lead to changes in community structure 

 such as the proliferation of undesirable fish species 

 in eutrophic waters. 



Wetlands are natural water reservoirs. Fresh 

 wetlands and water bodies are especially valuable 

 for storing surface water, which is often used by 

 man. For example, much of the irrigation water for 

 rice in Louisiana and Texas is stored in fresh 

 marshes. Ground water often extends beyond basin 

 boundaries, becoming a regional resource. 



As water flows over wetlands, many chemical 

 transformations take place. Inorganic nutrients, 

 which could encourage eutrophic conditions in 

 aquatic habitats, undergo important changes. The 



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