CHAPTER ONE 

 THE REGIONAL SETTING 



The unique characteristics of the 

 region and its marshes result froin the 

 interaction of three forces - the 

 subtropical climate, the oceanic regime, 

 and the river - all acting on the 

 physiographic template of the northern 

 gulf coast. The forces control the 

 geomorphic processes that have formed the 

 delta and also the biological 



characteristics of the delta marshes. 



For individual plants on the coastal 

 marsh these forces resolve into insola- 

 tion, tenperature, and water. Insola- 

 tion and temperature determine the poten- 

 tial and the rate, respectively, of biotic 

 productivity. Within the constraints set 

 up by these two parameters water is the 

 major controlling function which makes a 

 wetland wet and determines, directly or 

 indirectly, its characteristics. It is 

 also the most complex of the three parame- 

 ters. Insolation and tenperature are 

 determined primarily by latitude, with 

 only minor modification by local circum- 

 stances. But, the water available to 

 marshes, the depth and duration of flood- 

 ing, current velocity, and water quality 

 are complex functions of marine energy, 

 fluvial processes, rainfall, and evapora- 

 tion, operating over an irregular surface. 



THE CLIMATE, THE OCEAN, AND THE RIVER 



Insolation 



There is apparently no weather 

 station in the Mississippi Delta region 

 that routinely records insolation. 

 Existing records of this important 

 parameter are scattered and fragmentary. 

 However, the insolation reaching the top of 

 the atmosphere is a constant that varies 

 seasonally at a particular point on the 



earth's surface, depending on latitude. 

 Assuming an atmospheric transmission 

 coefficient of 0.7, Crowe (1971) showed 

 how insolation varied seasonally with 

 latitude (Figure 5). In the Mississippi 

 Delta region, at about 30° north latitude, 

 solar energy reaching the earth's surface 

 varies from about 200 cal/cm^/day during 

 the winter to a peak of nearly 600 

 cal/cm^/day in June and July. During the 

 summer insolation at this latitude is 

 higher than anywhere else on the globe; it 

 falls off both north toward the Arctic and 

 south toward the Equator. Therefore, 

 midsummer growth potential in terms of 

 solar energy is as high in the Mississippi 

 Delta as it is anywhere on earth. 



Cloud cover diminishes the potential 

 irradiance, and on the coast where daytime 

 seabreezes move moisture-laden gulf waters 

 inshore, there are clouds almost every day 

 during the hot summer. Consequently the 



CAL'Cm'/ oay 



eoo 



600 

 400 

 300 

 200 

 100 



JFMAMJ J I A I S I O I N I D 



J F m|a|m|j|j|a|s|o|n|d 



Figure 6. The seasonal variation of 



insolation at various latitudes. The 

 computation assumes a transmission 

 coefficient of 0.7 throughout (Copyright. 

 Reprinted from "Concepts of Climatology," 

 1971, by P.R. Crowe wi th permi ssion of 

 Longman Group Ltd., England). 



