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Leeward Community College *
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101
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Geography
Date
Dec 6, 2023
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docx
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Uploaded by JudgeScienceMouse33
Hydrology
Introduction
Soil infiltration refers to how fast water soaks into the soil. It turns out to be a very important property of soils that affects vegetation growth, recharge of aquifers, streamflow, and soil erosion. The purpose of this lab is to introduce the concepts of soil water infiltration and the soil water balance. Study the factors that affect infiltration, practice taking environmental measurements, understand effect of land use practices on recharge and runoff. In this lab you
will determine the approximate infiltration rate of water into soils under various conditions, determine how surface conditions affect the infiltration rate, and speculate how change in surface conditions might affect other hydrologic variables.
Background
The infiltration rate is an important component of the hydrologic cycle of watersheds (shown in the diagram). It helps determine how rainfall is divided between recharging the groundwater and running off as sheetwash and in streams. The following relationships apply:
At the surface: Water In = Water Out
In general (simplified): Rainfall = Runoff + Recharge + Evaporation
Also, in general, High Infiltration is Good because it reduces runoff and increases recharge,
and Low Infiltration is Bad because it increases runoff, increases erosion, decreases aquifer recharge, and decreases dry season stream flow.
In general, the lower the infiltration rate, the greater the surface runoff, and thus the greater the potential for soil erosion. A high infiltration rate lets most of the rain water soak into the soil and make its way downward to the aquifer.
Many things can affect the infiltration rate, including soil grain size, vegetation cover, compaction, and biological activity. One of the greatest environmental problems resulting from
deforestation has been a huge increase in erosion: not only do plants slow down runoff so that it has more time to soak into the soil, but the roots aerate the soil, giving water tiny tube pathways as infiltration channels. Also, with less recharge to renew the groundwater, the water table can drop leading to decreased stream flow during dry periods.
Materials needed
1.
a 15 ounce (approximately) tin can with both top and bottom removed. Try to find a can with a rim ridge around both the top and bottom or your can opener might not be able to open the bottom. If you use a different size, you will have to measure the initial
1 cup water depth and enter that value in Column 2 of Table 1 below.
2.
a one cup measuring container.
3.
a water source or bucket of water.
4.
a watch or cell phone that shows seconds.
5.
you may need a trowel, old knife, screwdriver or other tool to loosen soil.
6.
a ruler or tape measure.
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Procedure for determining the Infiltration Rate
If you have a 15 ounce can, the initial water depth will be about 60 millimeters. This value is shown in Column 2, Table 1. If you have a larger can, you will have to measure the initial depth that one cup of water fills the can using a ruler and enter that value in Column
2.
Measure the time in seconds required for the water to soak into the ground through the bottom of the can at each of the 4 test sites.
Embed the rim of the open can to a depth of about 1 cm (about 1/2 inch). Try not to disturb the soil in the center of the area, only dig the area where the rim will be embedded
as needed. After embedding, pack soil around the buried part of the can so that that water
does not leak out around the rim. It works best if you push down on the the top of the can after pouring in the water to prevent leakage. You want the water to soak into the soil so if more than a small amount leaks out underneath the side of the can, the test is ruined and you will have to repeat it. A small amount of leakage will not harm your results, however.
Pour one cup of water into the embedded can.
Use your watch or cell phone to measure the time in seconds required for the water to completely soak into the ground. Enter this value under Column 1, Time, in Table 1 below. If the trial takes more than three minutes (180 seconds), stop and estimate the depth that the water level dropped by measuring the depth of the remaining water and subtracting it from the initial 60 mm. The compacted soil test will ALWAYS take more then 3 minutes.
Calculate the infiltration rates in mm/sec by dividing Column 2 by Column 1.
CAREFUL! Do not let water seep out from under the edge of the can, try to ensure that it soaks into the ground. If you are only recording a few seconds for the water to infiltrate into the loose soil or compact soil, water is leaking out, hold down on the can firmly and try again. A very small amount of leakage is acceptable.
Questions to Answer:
Location and Measurement. Record the date, time, location, and sky conditions and fill in values for the table below. Include the completed table in your writeup.
1.
Date _____________________________
2.
Time _____________________________
3.
Location __________________________
4.
Sky Conditions _____________________
Measurement. Fill in Table1 with infiltration times at the first 3 sites and the water depth drop in 180 seconds for compacted soil. Then calculate the infiltration rates in millimeters per second. SHOW YOUR DATA for Table 1 in your writeup.
As note above, first determine the initial depth of water in your can. This is easiest if done before you cut the bottom out, but can be done afterwards as well. If after, seal the bottom, pour in the water, and measure the depth before it drains out. This value goes in Column 2 below - Water Depth.
TABLE 1: Infiltration Rate Measurement
1
2
3 (Col 2 / Col 1)
Location
Time (seconds)
Water Depth (mm)
Infiltration Rate (mm/seconds)
Gravel or Lava Rock
60
Natural Area (like forest, NOT mowed grass)
60
Loose Soil
60
Compacted Soil
180
60 - ______ = _______
Concrete or Asphalt
180
0
0
5.
For gravel or lava rock, use any gravelly area, like a road shoulder, side of a building or open lava rock.
6.
For natural area, use an undisturbed, natural area of small trees or bushes, not a mowed grassy area!
7.
For loose soil, you should be able to push in can fairly easily, like open soil that has not been walked on.
8.
For compact soil, use a well worn path or area driven on. You will have to dig around the edge a bit to embed the can. Press hard on the top of the can so that water does not leak out around the rim. If it does, repeat the measurement. If it takes less than 180 seconds, water leaked out around the rim and you need to repeat the measurement carefully. Subtract the depth of water at 180 seconds from 60 mm as shown above in Column 2.
Compare infiltration rates.
9.
Grain Size: What were the infiltration rates for gravel and loose soil? Which had higher infiltration rates and by how much (either in mm/sec or as a multiple).
10. Compaction: What were the infiltration rates for compacted soil and loose soil? Which had
higher infiltration rates and by how much (either in mm/sec or as a multiple).
11. Vegetation: What were the infiltration rates for natural area and loose soil? Which had higher infiltration rates and by how much (either in mm/sec or as a multiple).
12. Why do you think the infiltration rate varies between different surfaces?
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a.
Grain Size:
b.
Compaction:
c.
Vegetation:
13. In your opinion, what is the most important factor that influences the infiltration rate? Why?
14. List some areas of Hawaii that might be similar to the surfaces where you measured infiltration.
a.
Gravel _________________________________________________________________
_____
b.
Natural Area _________________________________________________________________
c.
Loose Soil _________________________________________________________________
__
d.
Compacted Soil _______________________________________________________________
e.
Concrete and Asphalt __________________________________________________________
15. Calculations. Assume that it rains 1 mm per second. Calculate the rate and amount of runoff using the simple equation below. NO NEGATIVE VALUES, if your calculation comes
out negative, enter zero (0). (show your work).
Rainfall is 1 mm/sec, Infiltration is from Table 1 above, calculate Runoff
using:
1
2
3 (Col 2 / Col 1)
Location
Time (seconds)
Water Depth (mm)
Infiltration Rate (mm/seconds)
Gravel or Lava Rock
60
Natural Area (like forest, NOT mowed grass)
60
Loose Soil
60
Compacted Soil
180
60 - ______ = _______
Concrete or Asphalt
180
0
0
Runoff = Rainfall - Infiltration
a.
Gravel, rate of Runoff (mm/sec) = _____________________________
b.
Natural Area, rate of Runoff (mm/sec) = ___________________________ c.
Loose Soil, rate of Runoff (mm/sec) = _____________________________
d.
Compacted Soil, rate of Runoff (mm/sec) = ________________________
e.
Concrete/Asphalt, rate of Runoff (mm/sec) = _______________________
16. If it rains steadily for 10 seconds, what will be the total runoff (in mm)? To answer, just multiply above runoff rates (mm/sec) by 10. Again, the answer cannot be negative, nor can it be more than 10 mm.
a.
Gravel, rate of Runoff (mm/sec) = _____________________________
b.
Natural Area, rate of Runoff (mm/sec) = _______________________
c.
Loose Soil, rate of Runoff (mm/sec) = ______________________________
d.
Compacted Soil, rate of Runoff (mm/sec) = ___________________________
e.
Concrete/Asphalt, rate of Runoff (mm/sec) = _______________________
NOTE: this exercise is greatly simplified, but helps to demonstrate the basic relationships between
water balance variables.
17. Water Balance. Determine the amount of Recharge for each land use scenario in Table 2 below. For the runoff, fill in the Runoff column using values you calculated above for 10 seconds of rain. Again, NO NEGATIVE VALUES, if your calculation comes out negative, enter zero (0). Also, if Evaporation + Runoff exceeds 10 mm, then reduce Runoff accordingly.
Use the equation given at the beginning of this lab: Rainfall = Runoff + Recharge + Evaporation
Table 2: Water Balance of Different Land Use Areas
Rainfall (mm)
Runoff
Recharge
Evaporation
Lava Rock (gravel)
10
1
Forest (natural area)
10
4
Farmland (loose soil)
10
3
Compacted Soil
10
1
Urban (concrete/asphalt)
10
0
18. Based on Table 2 above, suggest which areas of Hawaii provide the MOST recharge to the groundwater aquifers (name specific areas)?
19. Based on Table 2 above, suggest which areas provide the LEAST recharge to groundwater aquifers (name specific areas)?
20. Review your answers for #15,#16,#17. Are there any negative numbers? If so, change them to a value of zero (e.g. -3 becomes 0).
21. Land Use Scenarios. Based on your findings in Tables 1 and 2, speculate on how you think the following land use changes might affect the soil infiltration rate, and thus the balance between runoff and recharge, in Hawaii and what environmental consequences there might be. Once again, there are no wrong answers, use your imagination. Explain
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your reasoning.
a.
Conversion of forested land (natural area) to farmland (loose soil)
b.
Conversion of farmland (loose soil, like sugar cane) to urban (housing, asphalt, concrete, compacted soil)
These are relevant issues, especially on Oahu where the aquifer is being pumped at near its maximum sustainable capacity. In fact, voluntary water rationing was recently implemented by the Board of Water Supply.
22. Based on your findings and answers given throughout this lab, suggest two ways to increase recharge on Oahu.
23. Why do you think that the forested mountain areas are protected from development in Hawaii?