400 



300 



(0 



o 200 



w 



TJ 

 C 

 CO 

 (0 



3 



o 



100 



(0 



o 



Area of harvested cropland 



Yield of rice 



X"- 



5000 



.4000 



3000 £ 



Q. 



2000 



1000 



.w . 



1929 



1939 



1949 

 Year 



1959 



1969 



Figure 3-5. Change in area of harvested cropland and yield of rice in coastal Louisiana (Corty 1972, Fielder and 

 Guy 1975). 



Agricultural Drainage Canals. About 3,450 km 

 (2,144 mi) of agricultural canals (40% of the total 

 length of all canals in the Chenier Plain) were dredged 

 primarily for agricultural drainage or access (table 

 3.12). Canal density in individual basins varies from 

 0.37 to 0.56 km/km^ (average of 0.46 km/km^). 

 About half of these canals drain upland areas. The 

 rest flow through wetlands adjacent to the uplands 

 (plates 5A and 5B). The agricultural drainage canals 

 form a gridlike network along the northern parts of 

 the Chenier Plain. The impacts of these canals on the 

 natural system are discussed in parts 3.3, 3.4, and 3.5. 



Agricultural Runoff. Canals are dredged in the 

 low-lying agricultural lands of the Chenier Plain 

 primarily to drain the land and not for irrigation, 

 since much of the water for flooding rice land comes 

 from wells (see the following section about Surface- 

 and Ground-water Use). The accelerated runoff 

 increases erosion and the leaching of fertilizers and 

 manure from the soil. The U.S. Department of 

 Agriculture (1975) reports that an average of 0.38 

 ± 0.04 tonnes fertilizer is used on each fertilized acre 

 in the Chenier Plain parishes. This amount and the 



Table 3.12. Length and density of agricultural drainage and access canals in the Cheneir Plain basins^. 



Basin 



Upland 



(km) 



Wetland 

 (km) 



Total 



(km) 



Density" 

 (km/km^) 



167.4 

 488.3 

 329.2 

 300.0 

 83.9 

 212.2 

 296.1 

 108.1 

 1,689.1 



233.6 

 1,156.5 

 329.2 

 517.5 

 142.4 

 765.7 

 908.1 

 305.3 

 3,450.2 



0.41 

 0.56 

 0.37 

 0.55 



0.37 

 0.56 

 0.46 



Wetland drainage plus canals associated with access roads. 



b 2 



Total canal length (km) -rarea (km ) of wetlands and uplands in basin. 



36 



