Soils 456 Laboratory 5

Title : Soil Infiltration rate Experiment

Objective:To familiarize the students with the use of cylinders in determining soil infiltration rates and how the information may be used in designing irrigation systems.

Procedures

- At a site selected by the instructor, install infiltration cylinders in pairs to a depth of about six inches or sufficiently deep that water cannot leak around the lower end of the cylinders when filled with water
- Place a thin plastic sheet in the cylinders and pour water inside the plastic liner to a depth of about 8 inches. The infiltration test will commence by quickly pulling the plastic liner out of the cylinder. When the liner is pulled , the time till commence. Using a look gauge made with a brazing rod or coat hanger and a ruler, measure the water level below the top of the cylinder at each minute for the first five minutes after zero time. Record the initial depth of the water and all infiltration readings . Subsequent infiltration readings may be taken at five to ten- minute intervals. For the more permeable soils , it may be desirable to refill the cylinders several time to obtain sufficient data to extrapolate the infiltration curve to 1000 minutes.
- Infiltration Table.

Number of reading Time ( hr and minutes) Hook gage readings in inches or mm Accumulated time in Hr Accumulated infiltration (inches or mm) 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Problems:

- Plot a mass curve of the infiltration data using excel. Do a linear transformation and use the linear regression option and determine the average accumulative infiltration curve. Fit the power function

D=at

^{b}

^{Where}

- D is accumulative infiltration (inches or mm).
- t is time in hr.

- What is the maximum amount of water that can safely be applied to crops in level boarder irrigation on a soil with this infiltration rate if the crop will drown out when covered with irrigation longer that 15 hours.
- Differentiate the equation deriving the infiltration rate equation and plot the function
using calclpus

I=a*b t

^{b-1}I=c t

^{d}

- Discuss what the maximum application rat of sprinkler irrigation system should be for this soil under sprinkler sets of one hour.

4. Determine the soil type by the feel method and run Campbells model and compared to measured values.

5. Use solver to solve for a modified Kostiakiv Coefficients D(cm) = At^b + Ct (t in minutes): compare the two infiltration functions to determine if the modified equation fits the data better. Look at the sum of error squared to answer the question see example.

Discussion.

An simple estimate of c and d can be determined by assuming that d= -0.5 and solving for c based on 10 hr. of infiltration and assuming that this represents the saturation conductivity which approximates the final infiltration rate.

Example. Sand loam soil

Ksat= 1.05 inch/hr.

I=c t^{d
}1.05= c*10^-0.5

1.05= c*0 .32

c=3.28

D=at^{b}

If d= -0.5 then b= 0.5 and if c= 3.28 the a= 6.56 and after 10 hr D= 6.56*10^0.5=20
inches.

Example Clay loam

Ksat= 0.07 inch/hr

I=c t^{d}

0.07=c*10^-0.5

0.07= c*0.32

c=0.21

D=at^{b}

a=0.42 and D= 0.42*10^0.5=1.32 inches.