Env. Science (GEOL 1305/ENVR 1301) Name 41 Points (38 for this assignment, 3 for
ID: 291219 • Letter: E
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Env. Science (GEOL 1305/ENVR 1301) Name 41 Points (38 for this assignment, 3 for completing Teamwork Evaluation) SOIL EROSION IN THE UNITED STATES Introduction Soil erosion is one of the major environmental geological problems in the world today, and ranks with global warming, ozone depletion, forest destruction, habitat loss, and over-population as global environmental disasters. In many large areas on continents, land degradation (desertification) has led to vast destruction of arable land by topsoil erosion. Land degradation, whose main effect is soil erosion, is caused by human misuse of the land by removal of all the native vegetation for cropland, deforestation, over-cultivation and other destructive farming practices, over-grazing, and irrigation. Soils will erode normally without human-caused acceleration, but in this case, soil formation processes would be in equilibrium with erosion. When humans turn grasslands and forest into cropland and rangeland, however, they inevitably increase soil erosion to amounts above what is replaced naturally and would be sustainable. Such increased soil erosion is widespread throughout the world, but is also occurring in the United States at non sustainable rates. The question we would like to answer is the following: How much more soil is eroded from the contiguous United States as a consequence of European settlement and subsequent soil use? This question can be answered quantitatively if we are provided with present-day soil erosion data and we make a few simple assumptions. Table 1. Present-day contiguous U.S. soil erosion amounts by land type. Pay close attention to the units. Land Type U.S. AreaErosion Amount Percent of Calculated Erosion Rate (106 km2) (109 tons/year) Total Area (tons/km2lyear) Cropland 1.43 1.96 Pasture 0.47 0.37 Forest 2.38 0.42 Rangeland 2.61 31.6 LI 1mly 1.38 Desert i 1.38 Small; assume 0.0 16.7 ?L28AU?ronc?xmelyr Total 13.21 ?15 100Explanation / Answer
The erosion rate is calculated as:
(Erosion Amount) / (Land Area)
Note that the erosion amount is in tons/year and the area is in km2. So, the erosion rate will have units in tons/year/km2
1. The total erosion amount for the present-day contiguous USA is
(1.96 + 0.37 + 0.42 + 1.38) x 109 = 4.13 x 109 tons / year.
2. The erosion rate for different present-day land types are:
Cropland: Amount / Area = (1.96 x 109 ) / (1.43 x 106) = 1.37 x 103 tons/year/km2
Pasture: (0.37 x 109 ) / (0.47 x 106) = 0.78 x 103 tons/year/km2
Forest: (0.42 x 109 ) / (2.38 x 106) = 0.17 x 103 tons/year/km2
Rangeland: (1.38 x 109 ) / (2.61 x 106) = 0.52 x 103 tons/year/km2
Desert: (0.0) / (1.38 x 106) = 0 tons/year/km2
3. During the pre-European settlements,
Area of forests was one-half of the total area, i.e. 1/2 x (8.27 x 106) = 4.13 x 106 km2
Area of grasslands was one-third of the total area, i.e. 1/3 x (8.27 x 106) = 2.75 x 106 km2
Area of deserts was one-sixth of the total area, i.e. 1/6 x (8.27 x 106) = 1.38 x 106 km2
Thus, forests covered 50%, grasslands covered 33.3% and deserts covered 16.6% of the total land area.
Erosion rates were equal for forests (0.17 x 103 tons/ year/km2) and deserts (0 tons/ year/km2), and for grasslands it was equal to present-day rangeland (0.52 x 103 tons/ year/km2)
Thus, the erosion amount for forests was
Rate x Area = (0.17 x 103 (tons/year) / km2) x (4.13 x 106 km2) = 0.7 x 109 tons/year
the erosion amount for grasslands was
Rate x Area = (0.52 x 103 (tons/year) / km2) x (2.75 x 106 km2) = 1.43 x 109 tons/year
the erosion amount for deserts was
Rate x Area = (0.0 (tons/year) / km2) x (1.38 x 106 km2) = 0.0 tons/year
Total erosion amount was 0.7 x 109 + 1.43 x 109 = 1.5 x 109 tons/year, as compared to 4.13 x 109 after the Europeans settled in North America. So, Present-day erosion amount = 2.75 x Pre-European erosion amount
Thus, European settlements have increased the erosion amount. The amount has increased by a factor of 2.75 since the Pre-European time.
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