Soil Water Infiltration

Geography 101 Lab

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, stream flow, and soil erosion.

Purpose: 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 over time (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, decreases dry season stream flow, and other problems.

Many things can affect the infiltration rate, including soil grain size, vegetation cover, air voids, biotic activity (like worms), and mulching. One of the greatest environmental problems resulting from deforestation has been a huge increase in soil 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. When the plants are gone, there is little to slow the water from flowing over the surface and carrying soil with it. Other problems include increased flash flood hazard, increased sediment in rivers, and reduced river flow during dry seasons.

Materials needed

1. a 14.5 ounce 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
2. a one cup (8 ounce) measuring container
3. a water source or bucket of water
4. a watch, stop watch, or cell phone that shows seconds
5. you may need a trowel, old knife, screwdriver or other tool to loosen soil, especially for compacted soil.
6. a ruler or tape measure to determine water depth for other compacted soil test

Procedure for determining the Infiltration Rate

One cup of water will a standard 14.5 ounce tin can to a depth of about 60 millimeters. This value has already been entered in Table 1 below.

Measure the time in seconds required for the water to soak into the ground at the first three test sites and the depth of water remaining in the can after 120 seconds at the fourth.

Embed the rim of the open can to a depth of about 1-2 cm (about 1/2 inch). You should be able to push the can into the soil in all but the compacted soil area. 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.
Pour one cup of water into the embedded can. For some of the areas, the water will soak in pretty quickly, this is normal.
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 the Time column in Table 1 below. If the trial takes more than two minutes (120 seconds), stop and estimate the depth that the water level dropped by measuring the depth of the remaining water with the ruler and subtracting it from the initial 60 mm. The compacted soil test will ALWAYS take more then 2 minutes.
Calculate the infiltration rates in mm/sec by dividing Depth by Time.

CAREFUL! Try not to let water seep out from under the edge of the can to ensure that it soaks into the soil and you get a true measurement. A very small amount of leakage is acceptable. Again, the water will infiltrate quickly in some, but not all, areas.

Questions to Answer:

Question 1. 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.

Date _____________________________
Time _____________________________
Location __________________________
Sky Conditions _____________________

Measurement. Fill in Table1 with infiltration times at the first 3 sites and the water depth drop after 120 seconds for the 4th, compacted soil. Then calculate the infiltration rates in millimeters per second. SHOW YOUR DATA for Table 1 in your writeup.

One cup of water will fill a standard 14.5 ounce can to a depth of about 60 mm, this value is shown in Column 1. Fill in the remaining blanks in the table.

TABLE 1: Infiltration Rate Measurement

	1	2	3 (Col 1 / Col 2)
Location	Water Depth (mm)	Time (seconds)	Infiltration Rate (mm/second)
Gravel or Lava Rock	60
Natural Area (like forest or deep mulch, NOT mowed grass)	60
Loose Soil	60
Compacted Soil	60 - ______ = _______	120
Concrete or Asphalt	0	120	0

For gravel or lava rock, use any gravelly area, like a road shoulder, side of a building or open lava rock.
For natural area, use an undisturbed, natural area of small trees or bushes with lots of organic material like leaves covering the ground, not a mowed grassy area!
For loose soil, you should be able to push in can fairly easily, like open soil that has not been walked on. You may have to dig up an area of soil first to loosen it.
For compact soil, use an area frequently walked on. You may 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 there is substantial leakage, repeat the measurement. If it takes less than 120 seconds, water leaked out around the rim and you need to repeat the measurement carefully. Measure the depth of water remaining after 120 seconds and subtract this value from 60 mm, as shown in the Water Depth column.
For concrete or asphalt, do NOT measure anything. The values are give for you.

Question 2. Compare infiltration rates.

A. Grain Size:What were the infiltration rates for gravel (large grain size) and loose soil (small grain size)? Which had higher infiltration rates and by how much (either in mm/sec or as a multiple).

B. 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).

C. 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).

Question 3. What affects the infiltration rate? Name two factors that could affect the infiltration rate and explain how and why they would affect it. (This is easier than you think, just use common sense. What would cause more water to soak into the soil and why?)

A. _______________________________________________________________________________________

B. _______________________________________________________________________________________

Question 4 . Hawaii. List some areas of Hawaii that might be similar to the surfaces you measured. Give at least two examples each.

       Gravel (lava rock) _______________________________________________________________
       Natural Area (forest) _____________________________________________________________
       Loose Soil (agriculture) ___________________________________________________________
       Compacted Soil ________________________________________________________________
       Concrete and Asphalt ____________________________________________________________

Question 5 . 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:

Runoff = Rainfall - Infiltration

For example, if the infiltration rate is 0.3 mm/sec, then Runoff = 1 - 0.3 = 0.7 mm/sec

       Gravel, rate of Runoff (mm/sec) = _____________________________
       Natural Area, rate of Runoff (mm/sec) = _______________________
       Loose Soil, rate of Runoff (mm/sec) = ______________________________
       Compacted Soil, rate of Runoff (mm/sec) = ___________________________
       Concrete/Asphalt, rate of Runoff (mm/sec) = _______________________

If it rains steadily for 10 seconds, what will be the total runoff (in mm)? To answer, multiply the above runoff rates (in mm/sec) by 10. Again, the answer cannot be negative, nor can it be more than 10 mm.

       Gravel, runoff total (mm) = _____________________________
       Natural Area, runoff total (mm)) = _______________________
       Loose Soil, runoff total (mm) = ______________________________
       Compacted Soil, runoff total (mm) = ___________________________
       Concrete/Asphalt, runoff total (mm) = _______________________

NOTE: this exercise is greatly simplified, but helps to demonstrate the basic relationships between water balance variables.

Question 6 . 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

Surface type	Rainfall (mm)	Runoff	Recharge	Evaporation

Lava Rock (gravel)	10			0
Forest (natural area)	10			3
Farmland (loose soil)	10			2
Compacted Soil	10			1
Urban (concrete/asphalt)	10			0

A. Based on Table 2 above, which surface types in Hawaii appear to provide the MOST recharge to the groundwater aquifers?

Name a couple of specific places as examples _________________________________

B. Based on Table 2 above, which surface types in Hawaii appear to provide the LEAST recharge to the groundwater aquifers?

Name a couple of specific places as examples _________________________________

Review your answers for #5 and #6. Are there any negative numbers or numbers greater than 10? If so, read carefully and recalculate.

Question 7. Land Use Scenarios. Based on the values you measured and calculated in Tables 1 and 2 , speculate on how you think the following land use changes might affect the soil infiltration rate and the balance between runoff and recharge in Hawaii, and what environmental consequences there might be. Environmental consequences are discussed as the beginning of the lab and you can also do an internet search.

A. Conversion of natural, forested land to farmland (loose soil)

How would this affect infiltration rate, runoff and recharge (look at your tables)?

Would be the likely environmental consequences of this conversion?

B. Conversion of farmland (loose soil) to urban (housing, asphalt, concrete, compacted soil)

How would this affect infiltration rate, runoff and recharge (look at your tables)?

Would be the likely environmental consequences of this conversion?

These are relevant issues, especially on Oahu where the aquifer is being pumped at near its maximum sustainable capacity.

C. Based on your findings, suggest two ways to increase recharge on Oahu.

D. Based on your experience in this lab, why do you think that the forested mountain areas are designated "Soil and Water Conservation Districts" and protected from development in Hawaii?

Question 8 . Problems. Please tell me about how long it took you to complete this lab and what significant problems you had.