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. The following relationships apply:

At the surface: Water In = Water Out

In general (simplified): 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. So, typically, a low infiltration rate is bad and a high infiltration rate is good.

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.

Materials needed:

1. tin can with both top and bottom removed
2. measuring container, like a measuring cup or another can
3. bucket of water
4. cell phone or stop watch to measure time in seconds
5. digging tool, like a trowel, old knife, or screwdriver
6. ruler to measure water depth

Procedure for determining the Infiltration Rate

First, determine the initial depth of water in your can.

Pour one cup of water into a closed can and measure the depth with a ruler in millimeters. This value goes in column 4 below - Water Depth.

Then, measure the time in seconds required for the water to soak into the ground through the bottom of the can at each of the 5 test sites.

  1. Embed the rim of the open can to a depth of about 2 cm (about 1 inch) by pushing and digging as required. 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 water does not leak out around the rim, and restore any disturbed soil inside the can to its original state. If more than a small amount of water leaks out the side, the test is ruined. This is why we will be using the slowest (most seconds) measurement for our calculations.
  2. Pour one cup of water into the embedded, open can. Use same amount for each trial.
  3. Use your stop 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 1, in Table 1 below. (Note: if the trial takes more than four minutes, you can stop and estimate the depth that the water level dropped by measuring the depth of the remaining water and subtracting it from the initial depth you determined above. Enter this value under Water Depth in column 4 instead the initially measured depth.)
  4. Repeat step 3 and enter the time in seconds in column 2, Time 2, below.
  5. In column 3, enter the slowest time of your two trials (the larger number from columns 1 and 2).
  6. Calculate the infiltration rates in mm/sec by dividing column 4 by column 3 and enter in column 5.

CAREFUL! Do not let the water seep out from the rim 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 anything but gravel, water is leaking out - hold down firmly on the can and try again. That is why we are using the slowest value; the faster value usually includes seepage.

Measurement

First determine the initial depth of water in your can. Just pour water in a closed can and measure the depth with a ruler in millimeters. This value goes in each row of column 4 below - Water Depth. Then fill in the table below taking 2 measurements at each site.

TABLE 1: Infiltration Rate Measurement

 
1
2
3
4
5
Location

Time 1
(seconds)

Time 2
(seconds)
Slowest Time
(seconds)
Water Depth
(mm)
Infiltration Rate
(mm/second)
Gravel or Lava Rock      
 
Loose Soil      
 
Compacted Soil (path)  
 
 
 
Grass      
 
Natural Area  
 
 
 
  • 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.

Analysis

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?

Grain Size:

 

Compaction:

 

Vegetation:

 

In your opinion, what is the most important factor that influences the infiltration rate? Why?

 

 

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 rate and amount of runoff for the following surfaces. NO NEGATIVE VALUES, if your calculation comes out negative, enter zero (0).

Rainfall is 1 mm/sec, infiltration is from Table 1 above, calculate runoff using:

Runoff = Rainfall - Infiltration

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

If it rains steadily for 10 seconds, what will be the total runoff (in mm)? To answer, just 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, total Runoff (mm) = _____________________________
Loose Soil, total Runoff (mm) = ______________________________
Compacted Soil, total Runoff (mm) = ___________________________
Grass, rate of Runoff (mm/sec) = _____________________________
Natural Area, total Runoff (mm) = _______________________

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

 

5. Water Balance. Determine the amount of recharge for each land use scenario in the table 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).

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
Forest (natural area)
10
   
5
Farmland (loose soil)
10
   
3
Lava Rock (gravel)
10
   
1
Urban (compact soil)
10
 
2

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

 

 

Based on the table above, suggest which areas provide the LEAST recharge to groundwater aquifers (name specific areas)?

 

 

 

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?