5 E 



 DEFICIT 



LJ SURPLUS ,»». 



5 E ^"^ 



2 "> 



a o 



u a. 



Ill < 



B > 



a uj 



PE 



'M 



'"^^1 , 



1 rn r 



"1 I r 



J J A 



MONTH 



T~r 



Figure 9. Average water budget for the 

 upper Barataria basin, 1914-1978 (Sklar 

 1983). P=precipi tation, PE=potential 



evapotranspiration, AE=actual evapotran- 

 spiration. 



Evapotranspiration and rainfall sur - 

 plus. The effect of precipitation depends 

 not so much on the absolute amount but on 

 the relationship between rainfall and 

 evaporation from water and plant surfaces. 

 Although apparently no one has recorded 

 evapotranspiration directly in the delta 

 marshes, water balances have been calcu- 

 lated from equations developed by Thorn- 

 thwaite and Mather (1955), These show 

 that water surpluses occur during the 

 winter months, but during the summer 

 precipitation and evaporation tend to be 

 fairly closely balanced, with occasional 

 deficits in May through August (Figure 9). 

 Annual rainfall surplus is about 60 cm 

 along the northern edge of the delta 

 marshes (Gagliano et al. 1973), decreas- 

 ing to about 40 cm on the coast. This 

 surplus is important in the total water 

 balance of the marshes that includes 

 riverine inputs and gulf marine water, as 

 will be discussed in the following 

 sections. 



Upstream freshwater inflows. The 



largest source of freshwater to delta 

 marshes is the Mississippi River and its 

 major distributary, the Atchafalaya River. 

 The combined annual flow of these two 

 rivers averages about 15,400 cumecs. The 

 flow is strongly seasonal, peaking in late 



spring, fed by melting snow and spring 

 rains in the upper Mississippi watershed 

 (Figure 10). River flow can be nearly 

 independent of local rainfall because of 

 the size of the Mississippi River 

 watershed, but often spring rains along 

 the coast reinforce the river flow. 



The older basins of the delta are 

 isolated from direct riverine input by 

 natural and manmade levees. Therefore the 

 rivers debouch through the Balize and 

 Atchafalaya hydrologic units and in 

 extreme floods through the Bonnet Carre 

 control structure into Lake Pontchartrain. 

 Their waters flow on out into the gulf and 

 are carried westward along the coast, 

 freshening the tidal water that moves in 

 and out of the Barataria, Terrebonne, and 

 Vermilion basins. Thus, while these 



three basins have almost no direct 

 freshwater inflow except from local 

 runoff, the salt marshes are never 

 strongly saline because of the moderated 

 salinities offshore. 



In addition to the Mississippi and 

 Atchafalaya Rivers, smaller rivers also 

 feed freshwater into the coastal marshes 

 (Figure 10). The Pearl River delivers its 

 water to the mouth of the Pontchartrain 

 basin, freshening the Lake Borgne marshes 

 and through tidal action the lower Lake 

 Pontchartrain marshes. Other small 



rivers flow into the northern edge of Lake 

 Pontchartrain. The other basins receive 

 negligible stream flow, however, the 

 interior marshes are maintained as fresh 

 marshes by the precipitation surplus. 



Water fluxes in 



marshes are driven by the water 



differences across the estuary. 



change in three time scales: long 



seasonal, and daily. Since the 



reached its approximate present 

 about 7,000 years ago, it has been 



relative to the land at a rate 



Marine processes 



delta 



level 



These 



term, 



ocean 



level 



rising 



measured in centimeters per century. The 



term "coastal submergence" is used to 



long-term process, which is 



to true sea- level rise but 



subsidence as discussed in 



section on geoiTiorphology. 



identify this 

 due not only 

 also to land 

 the following 



In the last 20 years the rate of 

 submergence has accelerated. Presently 

 in delta marshes it averages about a 



11 



