Ch. 5— Wetland Trends • 109 



Table 22.— Surface and Subsurface Drainage of Farmland, 1900-1980 



Farmland currently Acreage shares Annual change, past Undepreciated 



Year drained 5 years drainage^ 



Surface Subsurface Surface Subsurface Surface Subsurface Surface Subsurface 



drainage drainage drainage drainage drainage drainage drainage drainage 



systems systems systems systems systems systems systems systems 



(Millions of acres)'' (Percent) (Thousands acres per year)'' (Millions of acres) 



1900 5.271 1.024 837 1^3 — — 3.975 1.014 



1905 9.775 1.902 83.7 16.3 900 176 7.447 1.877 



1910 18.673 3.632 83.7 16.3 1,780 346 15.313 3.572 



1915 29.344 5.701 83.7 16.3 2,134 414 25.029 5.541 



1920 43.452 5.993 87.9 12.1 2,822 58 38.131 5.573 



1925 .. . 41.420 6.143 87.1 12.9 -406 30 41.412 6.143 



1930 42.676 6.687 86.5 13.5 251 109 38.514 6.010 



1935 38.606 7.244 84.2 15.8 -814 111 32.697 6.118 



1940 36.532 8.905 80.4 19.6 -415 332 19.298 4.711 



1945 40.769 9.555 81.0 19.0 847 130 15.800 3.291 



1950 57.980 11.949 82.9 17.1 3,442 479 22.849 5.394 



1955 64.995 13.670 82.7 17.3 1,443 344 29.172 6.510 



1960 70.784 15.823 81.7 18.3 1,117 431 34.252 7.550 



1965 76.013 17.630 81.2 18.8 1,046 361 35.244 9.048 



1970 79.753 19.331 80,5 19.5 748 340 21.773 10.426 



1975 82.583 20.817 79.9 20.1 566 297 17.588 11.912 



1980 84.715 22.768 7a8 21^2 427 390 13.931 13.863 



* "Undepreciated drainage" refers to surface drainage systems in place for less tfian 20 years, to tfiose subsurface systems in place for less than 30 years if installed 

 before 1940. or to those subsurface systems in place for less than 40 years if installed in 1940 or thereafter. Note that by 1980 surface and subsurface systems were 

 about equal in importance on an "undepreciated basis," even though surface systems are still in much wider use, as indicated by the acreages and percentage distributions 

 for current drainage (cols. 1 to A). Such a breakdown is useful as an overall indicator of general age and condition of farm drainage systems and was helpful for measur- 

 ing active gross capital stocks and net capital values. 

 Acreages for surface and subsurface drainage add to the overall net acreage drained. 



"Rates of increase or decrease for surface and subsurface drainage add to the overall change for all farm drainage. 



SOURCE: G. A, Pavelis, unpublished draft, "Farmland Drainage in the United States, 1900 to 1980: Acreage, Investment and Capital Values, 1982." 



Other States in the study region, clearing had 

 peaked between 1957 and 1967. The study notes 

 that "rates of acreage decreases in bottom land 

 hardwood forest area closely reflect the magnitude 

 of reduction in total hardwood forest area by State 

 (10)." Thus, although national drainage rates have 

 declined, wedand drainage probably is continuing 

 in some areas. 



How Wetlands Are Lost to Agriculture 



Wedands are lost to agriculture through two pri- 

 mary means: direct conversions by draining artd/or 

 clearing and indirect conversions associated with 

 normal agricultural activities. Direct conversions 

 of wetlands for the purpose of expanding agricul- 

 tural operations probably result in far more lost 

 wetland acreage than do the indirect conversions 

 on a nationwide basis. However, indirect conver- 

 sions may be the major factor associated with loss 

 of wetlands to agriculture in some regions of the 

 country. Conversion activities are summarized in 

 table 18. 



Examples of direct conversion of wetlands to ag- 

 riculture include drainage to expand crop acreage 

 in the prairie-pothole region, construction of irriga- 

 tion reuse pits to improve irrigation efficiency and 

 to drain wedands in the Rainwater Basin of Nebras- 

 ka, clearing and draining bottom land hardwoods 

 for soybean or rice production in the Lower Mis- 

 sissippi River Valley and for soybeans and other 

 crops in North Carolina, and the mowing-chop- 

 ping-seeding-grazing sequence for improving 

 Florida sawgrass for agriculture. 



Examples of indirect conversions of wetlands as- 

 sociated with normal agricultural activities include 

 the general lowering of the water table for irriga- 

 tion, which results in drying of "wet meadows," 

 making them suitable for crops in the Platte River 

 Valley and the Sandhills of Nebraska; changing 

 water-management practices associated with crop 

 changes in the Central Valley of California (i.e., 

 when ricefields are converted to orchards, water 

 from flooded ricefields is no longer available for 

 discharge to wetlands); clean farming techniques 



