spreads northward affecting freshwater 

 organisms, especially wetland plants, 

 that are unaccustomed to osmotic stress. 



The slope of land and its vegeta- 

 tive cover affects the rate of surface 

 water runoff. Porosity of soils and 

 their moisture deficits affect the rate 

 at which water percolates through sur- 

 face layers. Within the MDPR, the 

 average surface gradient is low (about 1 

 cm/km or 0.6 in/mi) and the soil is 

 saturated during most of the year. Much 

 of the surface soil overlies a rela- 

 tively impermeable clay layer. Although 

 the dense vegetative cover that charac- 

 terizes the MDPR retards water runoff, 

 it also enhances water loss by evapo- 

 transpiration. 



One of the most important regula- 

 tors of hydrologic conditions within the 

 MDPR, and also of the biological commu- 

 nities that characterize the region, is 

 maritime climate (Wax 1977). Tempera- 

 ture, precipitation, solar energy, rel- 

 ative humidity, winds, and barometric 

 pressure are all directly involved in 

 the regulation of water level and water 

 flow, salinity gradients, evapotranspi- 

 ration, erosion of sediments, and bio- 

 logical productivity. The climate of 

 the MDPR can be classified as moist 

 subtropical with essentially no water 

 deficit (Figure 11), in contrast to the 

 semi-arid conditions generally present 

 at 30 degrees north or south Latitude 

 (Figure 11). Precipitation in the MDPR, 

 which arises largely from evaporation in 

 the Gulf of Mexico, overrides this lati- 

 tudinal semi-arid tendency. 



The subtropical location of the 

 MDPR and its proximity to the Gulf of 

 Mexico ensure a long growing season (236 

 to 240 frost-free days) and relatively 

 small annual temperature ranges (Wax 

 1977). The moderating effect of the 

 gulf is dramatically demonstrated by the 

 presence of a thin fringe of black man- 

 groves ( Avicennia germinans ) along the 

 inshore borders of the barrier islands 

 of Louisiana. 



Evapotranspiration affects not only 

 the availability of water, but also the 



rate of primary production. Rates of 

 evapotranspiration, precipitation, and 

 runoff have been estimated for the MDPR 

 using water balance calculations by 

 Sklar (1980). Actual evapotranspiration 

 averages about 100 cm/yr (40 in/yr), 

 while precipitation averages near 160 

 cm/yr (63 in/yr), resulting in a large 

 volume of surplus water available for 

 runoff (about 60 cm/yr or 23 in/yr) . 



Muller (1977) described the climate 

 of south Louisiana in terms of eight 

 synoptic weather types. During the 

 warmer months, tropical weather types 

 (those associated with the Gulf of 

 Mexico) dominate; in cooler months, 

 continental weather types are more com- 

 mon. Winds are generally weakest in the 

 summer, except during tropical storms 

 and briefly during thunderstorms. The 

 strongest winter winds are associated 

 with the passage of cold fronts. 



Few occurrences in the MDPR are as 

 dramatic as the approach of a hurricane. 

 Such an event captures the attention of 

 the news media for days, and stories of 

 damaged property are always accompanied 

 by economic loss estimates in six or 

 seven figures. Usually there is ample 

 warning time to prevent extensive loss 

 of life, but in the case of Hurricane 

 Camille in 1969, 171 people were killed 

 in the area of Biloxi, Mississippi 

 (USACE 1970) when they ignored warnings 

 to evacuate. A recent study indicated 

 that New Orleans is extremely vulnerable 

 to hurricanes (USACE 1981). A level 5 

 hurricane (like Camille) could inundate 

 major portions of the city with water 

 7.6 m (25 ft) deep (Baton Rouge Sunday 

 Advocate 1982). 



Hurricanes are considered natural 

 calamities, but they produce beneficial 

 effects on natural coastal ecosystems. 

 For example, the extreme wind and wave 

 energies that accompany major storms 

 represent disordering processes that 

 send large amounts of sediment and nu- 

 trients into coastal estuaries. This 

 results in both short-term and long-term 

 increases in primary productivity, al- 

 though some animal populations may 

 decline initially (Baumann et al. in 



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