The information in this section is the key to earning points in… The information in this section is the key to earning points in calculating rates of soil erosion in different stock pond watersheds.?o please go slowly.Note #1: even if you just skimmed over the informational sections below, you may want to take it much more slowly at this point. The reason is that you will go through the same steps for points when this example is over.?ote #2: you will be given 4 stock ponds to analyze. The stock ponds will be randomly selected from the 17 remaining stock ponds. Thus, you will have to analyze and then answer a question similar to the one at the end of this example four times. Each of these stock pond analyses will be worth a lot of points, so take your time.?EXAMPLE STOCK POND: Cigar Tank?AND-USE HISTORY:?Either given, or you will have to do research using Google Earth historical imagery?when the change is particularly clear such as building a subdivision, or a wildfire). In other cases, like this, you are given local insight that the watershed became?n active area for off-road motorcycle and 4-wheel drive use starting in 2005 until 2009.???n each sample tank, you are also given information about the mean annual precipitation and the number of days with intense rainfall in the form of graphs below.?he first thing you will do is figure out how many years under what land use: look carefully at the graph and you’ll figure out 14 years under cattle grazing and then 5 years where there was a lot of off-road vehicle activity.Image transcription textCigar (1990-2009) 1 km 1997: Cattle grazing 2005: Year 19901991 1992 1993 1994 1995 1996 1997 1998 1999 2000 20012002 2003 2004 2005 2006 2007 2008 2009… Show more… Show more?You are given information about the?ATURAL BACKGROUND EROSION RATE: (catchment-wide denudation rate, called CWDR): Using the cosmogenic isotope beryllium-10, it is possible to measure the long-term erosion rate of a drainage basin.????The CWDR method relies on the observation that cosmic rays interact with Earth’s surface, producing?0Be, an otherwise exceptionally rare isotope, in quartz minerals. The production of?0Be occurs predominantly within a few meters of Earth’s surface and decreases exponentially with depth. Thus, the concentration of?0Be in outcropping rock or in river sediment reflects near-surface residence time.?ampling river sand occurs right at the mouth of each stock pond, and presumes that the networks of dry washes mix and deliver sediment from the entire basin.????r. Ara Jeong measured CWDR in all of the stock ponds that you will be studying.???he natural background erosion rate (CWDR) of the Cigar watershed is 4.48 ?0.05 millimeters per thousand years or rounded off to?.5 mm/ka.?n average, over the last 120,000 years, the ground surface eroded just 4.5 mm/ka (millimeters every thousand years).?Yes. This is a slow rate of erosion, in part, because the slope of the watershed is very low, just 0.36?.??You will also be given the area of the pond accumulating the sediment (6000 square meters) and the thickness of the sediment in the pond (0.288 m for the cattle grazing period and 0.329 m for the off-road vehicle period).THIS IS THE END OF THE INFORMATION THAT IS GIVEN TO YOU. NOW, YOU HAVE TO COLLECT INFORMATION AND THEN DO SOME CALCULATIONS:?Start by opening Google Earth Pro, and you need to find the area of the stock pond Cigar Tank found in the KMZ folder you downloaded. By right clicking (or control-click for Mac) on Cigar Tank and selecting GET INFO.?hen, you look for the measurements tab, and switch to meters and square meters. The square meters will be somewhere around 2,650,00 Because Google Earth’s measurements are not that precise, please round to the nearest thousand square meters or 2,654,000.?Do not be bothered about being wrong at this stage. You will be able to get the “correct answers” by being within a few tens of thousands of square meters.]Image transcription textSoarch 0: 15213 (0> Google Earth -Edit Polygon CigarTank Style, ColorView Altitude Measureme… Show more… Show moreFor the period from 1990-2004 (14 years), sampling in the 6000 square meter stock pond showed that 0.288 meters of sediment accumulated.?ultiplying 6000 m2?.288m = 1728 m3 for the volume of sediment.?rea times thickness is a volume.Dividing 1728 m3 by the 2,654,000 m2 watershed means that 0.000651 meters eroded (on average) in 14 years.?(This is like taking a jar of peanut butter (volume) and trying to spread it evenly over a football field. If you could, the peanut butter would be incredibly thin.)Since the thickness is so small and since we need to get the erosion rate into units of millimeters per thousand years (mm/ka), 0.000651 multiplied by 1000 converts to 0.651 millimeters.Next, we have to divide the millimeters eroded off by the number of years of erosion (14), so 0.651/14 = 0.0465 mm/year.?hen you multiply this by 1000 to get to mm/ka, it comes to 46.5 mm/ka.?ow comes the comparison. Remember, the natural background rate of erosion for the Cigar Tank watershed is 4.5 mm/ka. Thus, the rate of erosion during the 14 years of cattle grazing has been 46.5/4.5= or 10.3 times greater than natural background.?f you like to see this in an excel file that you can modify in working on other watersheds, you are welcome to download this:?rbanizationLab_ExcelFile_Example.xlsx?download,?nd this is what the excel file looks like pasted:TankCigarTime period1990-2004Year in Land use14Stock Pond Watershed Drainage Area (m2)2654000Thickness sediment (m)0.288Area of Pond (m2)6000Stock Pond Volume (m3)1728.0Meters eroded (volume/area)0.000651mm eroded0.6511mm/yr0.0465mm/ka46.5Background Erosion (mm/ka)4.5Ratio comparison to Natural Erosion10.3?hen, you do the same thing all over again for the period(s) with a different land use, in this case increased off-road vehicle activity from 2005-2009 (a 5 year period).?he only other difference (other than the 5 year period of record) would be the thickness of the sediment for those 5 years was 0.329 meters, and this will be given in the stock ponds you will work on for points.??Seen in the excel file, the calculations for both periods look like this:TankCigarCigarTime period1990-20042005-2009Year in Land use145Stock Pond Area (m2)26540002654000Thickness sediment (m)0.2880.329Area of Pond (m2)60006000Stock Pond Volume (m3)1728.01974.0Meters eroded (volume/area)0.0006510.000744mm eroded0.65110.7438mm/yr0.04650.1488mm/ka46.5148.8Background Erosion mm/ka4.54.5Ratio Comparison to Natural Erosion10.333.1??f you take the erosion rate for the off-road vehicle period (148.8 mm/ka) and divide it by the grazing period (46.5), the ratio is 3.2??ONCLUSION: The off-road vehicle activity period experienced 33x faster erosion than natural rates and 3.2x faster than grazing rates.?he period of grazing experienced about 10.3x faster erosion than natural rates.??n science, data like these are not the end of the story. Observations are just the starting point, where observations lead to some generalization (hypothesis) that can help predict events in the future.??n this case, Wolman’s model of a “spike” of soil erosion from exposure of bare ground during urbanization seems to be supported by these data. The vegetation in the cattle grazing phase did protect the soil from erosion (but not as much as natural conditions), and there was a big spike from off-road vehicle driving (land use associated with bare ground exposure at the expanding urban margin).?+++++QUESTION: Which is the best answer for your analysis of the impact of humans on soil erosion in this watershed?+++++?xample of the Correct Answer:?he period of Cattle Grazing at Cigar Tank increased erosion from the natural erosion rate of 4.5 mm/ka to 46.5 mm/ka, or a 10x increase. Then, the period of increased off-road vehicle use increased erosion to ~149 mm/ka or an increase of about 33x. Thus, the bare ground exposure associated with driving off road on the margins of Phoenix greatly increased soil erosion.?Example of one incorrect answer:?he natural rate of erosion at Cigar Tank was 4.5 mm/ka. The period of grazing had a slightly higher erosion rate of 10 mm/ka or about double, and the period of off-road vehicle use tripled the natural erosion rate to about 13 mm/ka.??xample of another incorrect answer:?he period of Cattle Grazing at Cigar Tank increased erosion from the natural erosion rate of 4.5 mm/ka to 90 mm/ka, or a 20x increase. Then, the period of increased off-road vehicle use increased erosion to ~186 mm/ka or an increase of about 41x. Thus, the bare ground exposure associated with driving off road on the margins of Phoenix greatly increased soil erosion.?OTE: it is not fair to have potential answers too close together. The reason is that Google Earth’s measurements for drainage area will vary depending on the version you downloaded, your laptop/desktop, and also updated databases. Thus,?o not stress when your calculations are slightly different from one of the potential answers.?ust pick the closest answer.?UESTION: Which is the best answer for your analysis of the impact of humans on soil erosion in this watershed?GIVEN INFORMATION:??or the? years of cattle grazing, 0.031 m?f sediment collected in the pond with an area of?800 square meters.??hen, a wildfire burned the entire watershed. For a six year period after the wildfire called wildfire1, sedimentation in the pond was?.180 m?n the same area of 2800 square meters.?hen, for a 5 year period called wildfire2, sedimentation in the pond was?.119 m?n the same area of 2800 square meters.?Then, for a 6 year period called wildfire3, sedimentation in the pond was?.065 m?n the same area of 2800 square meters.?The reason for measuring erosion in so many different periods is to understand how a fire scar recovery (revegetation) influences soil erosion.?The natural background rate of erosion in the watershed is?9 mm/ka, and this CWDR (catchment wide denudation rate) is averaged over the last 21,000 years.?Please follow the example for how you make the calculations to answer the question.?mage transcription textRock (1989-2009) 1 km 1 km 1 km 1992: Cattlegrazing 2002: Wildfire II 2007: Wildfire Ill Year 19891990 1991 1992 1993 1994 1995 1996 … Show more… Show moreANSWER CHOICES: The natural background rate of erosion of 29 mm/ka doubled to 55 mm/ka during the cattle grazing period. Then, in the first 6 years after the wildfire, the erosion rate jumped to 159 mm/ka (or about 5.5x background). Then, for wildfire2 over 5 years, the erosion rate dropped to 140 mm/ka (or about 5x background). The watershed then dropped to an erosion rate of about?15 mm/ka (or about 4x background). ?This indicates that the wildfire impacted soil erosion for the 17 years of study afterwards, and soil erosion rates still have not returned to the pre-wildfire condition.?he natural background rate of erosion of 29 mm/ka doubled to 55 mm/ka during the cattle grazing period. Then, in the first 6 years after the wildfire, the erosion rate jumped to 140 mm/ka.?hen, for wildfire2 over 5 years, the erosion rate dropped to 65 mm/ka. The watershed then eroded at 52 mm/ka in the last period of monitoring (wildfire 3). ??his indicates that it took about 6 years for the watershed to return to soil erosion rates close to the pre-wildfire condition..?he natural background rate of erosion of 29 mm/ka doubled to 55 mm/ka during the cattle grazing period. Then, in the first 6 years after the wildfire, the erosion rate jumped to 159 mm/ka (or about 5.5x background). Then, for wildfire2 over 5 years, the erosion rate dropped to 126 mm/ka (or about 4.3x background). The watershed then returned to the erosion rate of the cattle grazing period in wildfire3 at about 57 mm/ka.??This indicates that it took about a decade for the watershed to return to the pre-wildfire condition — at least in terms of soil erosion rates.?Earth ScienceScienceGeographyGPH MISC
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