Infiltration Protocol

Purpose

To deternine the rate at which water soaks into the ground as a function of time, which will give the saturated hydraulic conductivity. Infiltration rates affect how flood-prone an area is and how easily water moves vertically through the soil.

Overview

Seasonal triplicate measurements of water flow rates (infiltration) into the soil will be made at several soil/water-related GLOBE sites. Two nested cans are pushed into the soil and water is added to both to maintain a depth of about 5 cm. The time it takes water to drop a fixed 2 cm interval is recorded, repeatedly, as a function of time.

Time Required

It should take about 45 minutes for the flowrate to approach the "steady-state" saturated rate.

1 class period to build and test the double-ring infiltrometer
1 class period/meas. (so 3 classes or 3 groups to measure one site)

Skill Level

All (4-12)

Measurement Frequency

Seasonal/site (multiple sites encouraged; new classes can repeat and verify a former measurement). Make measurements near the beginning, middle and end of your gravimetric soil moisture study period.

Key Concepts

Infiltration rate changes depending upon the level of soil saturation
If water is not stored in the ground, it must evaporate or runoff

Skills

Building, testing, organizing, observing, recording, analyzing
This requires a relatively dry site, water play, monitoring flow intervals ranging from seconds to minutes and basic data analysis skills.

Materials and Tools

two metal rings, id=10 to 20 cm, od=id+(5 to 10) cm (Coffee cans work!)
two plastic paint buckets or milk/water jugs (~4 L each)
ruler
waterproof marker
stop watch or watch with second hand
Block of wood and hammer

Introduction

The rate at which precipitation soaks or infiltrates into the ground affects two important components of the hydrologic cycle (runoff and storage) and varies (decreases) as a function of time. Thus, understanding the water balance of an area depends on estimates of infiltration. Although soil texture is an important factor, macro structures due to plant roots and burrowing animals are equally important and are very difficult to estimate without careful ground measurements.

The primary objective of this study is to characterize typical soil infiltration rates for the area selected. It is useful as well to have the students investigate infiltration rates in atypical areas such as where the soil is compacted or where burrowing or roots, rather than soil properties, might determine infiltration rates.

Talk to your class about "rates". Infiltration rate is determined by measuring the time it takes a water level to drop a fixed distance. This rate changes with time as the soil pore space fills with water and reaches a steady rate, characteristic of saturated water flow through your soil. There are three flow regimes you might encounter:

Unsaturated flow - the initial flow rate is high as the dry soil pores fill with water.
Saturated flow - the flow rate is steady and water moves into the soil at a rate determined by soil texture and structure.
Ponding - the flow rate approaches zero when the ground becomes totally saturated and is no longer able to conduct water through its pores.

Site selection

Select a site within 2-5 m of the soil moisture or soil characterization site. Be careful that you do not leave a hose runnning where the water will flow over your soil moisture sampling points! Observations at other sites, such as the biometry site, can be made as additional activities. Follow the general site selection criteria given under the Overview of Soil Moisture Measurements in the GLOBE Teacher's Guide.

Preparation

Spend at least one class period doing the "Water Race" activity.
Cut the bottom out of your cans
Use a permanent waterproof marker to partially shade a ring on the inside of the smaller can to use as a timing reference mark. It should be 20-30 mm wide and about 9 cm from the bottom of the can. Many cans have impressed ribs that make good reference marks but it is still necessary to mark them for good visibility.
Measure and record the vertical width of your reference band (in mm) and the height these bands are above ground level when the cans are pressed into your soil.
Measure and record the widths of your inner and outer rings.
Timing considerations: You can use either a stop watch or a watch with a second hand. When using a stop watch, start it as water is first poured into the inner ring and read begin/end times from it for each subsequent interval (see example below).
Demonstrate or have the students practice the technique in a sandy area prior to making measurements at your site - have plenty of water available!
While one student team is working on this infiltration protocol, another team should be making a regular soil moisture observation nearby.

What to do and how to do it

Clip any vegetation (grass) to the surface and remove all loose organic cover over an area just larger than your largest can. Try not to disturb the soil.
Starting with the smaller can, twist or hammer the cans 2-5 cm into the soil or as far as it will go easily. A block of wood must be used to distribute the force of hammering - do not hammer so hard that the can crumples.
As quickly as possible, do the following, using a team of 3-4 students:
- Pour water into the outer ring, and maintain a level approximately equal to the level in the inner ring.
- Pour water into the inner ring, to just above the reference mark,
- Start the stopwatch or note the time to the second,
The outer ring should not be leaking water to the surface around its rim. If it is, start over in another location, push the outer ring in deeper and/or pack mud around its base.
As the water level in the inner can reaches the upper reference mark,
- record the start time
- During the timing interval, maintain the water level in the outer ring approximately equal to (+/- 4 cm) the inner ring, but be careful not to pour water into the inner ring or to let it go dry.
As the water level in the inner can reaches the lower reference mark,
- Record the end time and figure the interval time difference,
- Pour water into the inner ring, just above the reference mark,
- Maintain the water level in the outer ring.
Continue repeating steps 5-6 for 45 minutes or until two consecutive interval times are within 10 sec. of each other.
Some clays and compacted soils will be impervious to water infiltration and your water levels will hardly drop at all within this time period. In that case just record the depth of water change (to the nearest mm), if any, and the length of time over which this change occured. Your run will consist of a single data interval.
Remove the rings
Measure the near-surface saturated value of soil moisture from the spot you just removed the rings. Follow the Gravimetric Soil Moisture protocol.
Make two other infiltration measurements within a 5 m diameter area, either at the same time using other groups or over several days (if it does not rain and change near-surface soil water content).

Data Presentation

Use the "Infiltration Data Worksheet" to record and help calculate the values needed to plot your results. The flow rate we observe for each timing interval is really the average value during that interval. It is best to record that flow rate at the midpoint of the interval times. Infiltration should decrease with time and it is important that you keep track of the cummulative time since water was first poured into the inner ring. Look over the table and graph below and make sure that you can use the formulas on the Data Worksheet to calculate these values before analyzing your own data.

**Infiltration into Jim's Garden**
Time							Water level	Flow
Start		End		Interval	Midpoint	Cummulative	Change	Rate
[min]	[sec]	[min]	[sec]	[min]	[min]	[min]	[mm]	[mm/min]
31	00	32	00	1.00	31.50	0.50	20	20.0
32	30	34	15	1.75	33.38	2.38	20	11.43
34	30	36	45	2.25	35.62	4.62	20	8.89
37	15	40	00	2.75	38.62	7.62	20	7.27
40	45	44	00	3.25	42.38	11.38	20	6.15
44	15	47	45	3.50	46.00	15.00	20	5.71
48	15	52	00	3.75	50.12	19.12	20	5.33
52	15	56	15	4.00	54.25	23.25	20	5.00
56	30	00	30	4.00	58.50	27.50	20	5.00

Last updated: 5/19/97; Comments? globe@hwr.arizona.edu