Soil Water Infiltration

Name ________________________

The infiltration rate refers to how fast water soaks into the soil. This 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. It helps determine how rainfall is divided between recharging the groundwater and running off over the surface as sheetwash and in streams as the following:

At the surface: Water In = Water Out

Or, in general: Rainfall = Runoff + Recharge + Evaporation

In general, High Infiltration is good because it reduces runoff and increases recharge,
and Low Infiltration is bad because it increases runoff (and erosion) and decreases recharge.

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, 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.

Materials needed:

1. large can (like a coffee can) with both top and bottom removed
2. a bucket and small can to hold water and measure water
3. digging tool, like a screwdriver or trowel
4. stop watch or something that measures seconds
5. ruler

Procedure for determining the Infiltration Rate

Embed the rim of the big coffee can to a depth of about 1 inch (2 cm) into the soil, so water does not leak out around the rim
Pour a full small can of water into the open coffee can
Use a clock or watch to time how long it takes the water to completely soak into the ground (stop if more than 3 minutes and estimate the depth the water level dropped)
Perform the measurements two times at each site
Use the average time to determine infiltration rate in millimeters per second (divide the depth of water in the can by the time it took to soak in)

CAREFUL! Do not let the water seep out from the rim of the can, try to ensure that it soaks into the ground

Measurement. Fill in the Table, taking two measurements at each location.

Location	Time 1 (seconds)	Time 2 (seconds)	Average (seconds)	Water Depth (mm)	Infiltration Rate (mm/second)
Gravel or Lava Rock				25
Loose Soil				25
Compacted Soil (path)				25
Grass (not used as a path)				25
Natural area under trees				25

For gravel or lava rock, use the gravel beside a building.
For loose soil, you should be able to push in can fairly easily, like open soil that has not been walked on.
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.
For grass, you may have to cut through the grass with the digging tool to embed the can and then press firmly on the can so water does not seep out the edges. Minimize the disturbance to the soil and do not remove the grass.
For natural area under trees, use an undisturbed area of small trees or bushes.

1. Compare infiltration rates.

Grain Size: Compare your measurements in gravel and loose soil giving measured values. Which had higher infiltration rates and by how much?

Compaction: Compare your measurements with compact soil and loose soil giving measured values. Which had higher infiltration rates and by how much?

Vegetation: Compare your measurements with grass, tree area, and loose soil. Which had higher infiltration rates and by how much?

2. Reason. Why do you think the infiltration rate varies between different surfaces? What is the most important factor that influences the infiltration rate?

3. Hawaii. List some areas of Hawaii that might be similar to the surfaces where you measured infiltration.

Gravel _______________________________________________________________________
Loose Soil ____________________________________________________________________
Compacted Soil ________________________________________________________________
Grass ________________________________________________________________________
Natural Area __________________________________________________________________

4. Calculations. Assume that it rains 1 mm per second. Calculate the amount of runoff (in mm/sec) for the following surfaces (show your work):

Runoff = Rainfall - Infiltration

Gravel _______________________________________________________________________
Loose Soil ____________________________________________________________________
Compacted Soil ________________________________________________________________
Grass ________________________________________________________________________
Natural Area __________________________________________________________________

If it rains steadily for 10 seconds, how what will be the total runoff (in mm). Multiply above numbers by 10 (maximum of 10 possible).

Gravel _______________________________________________________________________
Loose Soil ____________________________________________________________________
Compacted Soil ________________________________________________________________
Grass ________________________________________________________________________
Natural Area __________________________________________________________________

(NOTE: this exercise is simplied, but helps to demonstrate the basic relationships between water balance variables.)

5. Water Balance. For the scenarios below, determine in the amount of recharge. Fill in the runoff using values you calculated above. (remember: Rainfall = Runoff + Recharge + Evaporation )

	Rainfall (mm)	Evaporation	Runoff	Recharge
Forest	10	5
Farmland (loose soil)	10	3
Lava Rock (gravel)	10	1
Urban (compact soil)	10	2

Based on the table above, which areas of Hawaii provide the greatest recharge to the groundwater aquifers (name specific places)?

6. Land Use Scenarios. Based on your findings, speculate on how you think the following land use changes might effect 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 your reasoning.

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

Conversion of farmland (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 it maximum sustainable capacity. In fact, voluntary water rationing was recently implemented by the Board of Water Supply.

Suggest two ways to increase recharge on Oahu.

Why do you think that the forested mountain areas are protected from development in Hawaii?