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WATER INFILTRATION TEST BY A SINGLE-RING INFILTROMETER

NATURAL RESOURCE CONSERVATION ENGINEERING – BSEN 3230

Group C Members: Bud Bliss, John Llorens, Miles Bateman, Ann Nunnelley, Holly Haber, Eric Vogt and Trey Colley

Date Performed: Monday, April 6, 2015 Date Submitted: Monday, April 13, 2015

ANN NUNNELLEY

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AIM/ OBJECTIVES: The main objective of this process was to measure the water infiltration rate using a single-ring infiltrometer in order to find coefficients of the Kostiakov’s infiltration equation. This equation is important because calculation of infiltration rates is applicable to problems including soil erosion, leaching and drainage, irrigation, runoff, pond seepage and evaluation of potential septic-tank disposal fields, as well as various others. Although the more widely used method for measuring infiltration rates is the double-ring infiltrometer, the single-ring is an accepted measurement system as well. METHOD/ PROCEDURE: Group C used a single-ring infiltrometer as opposed to a double-ring infiltrometer. This slightly altered the method of measuring the infiltration rates. This group measured the infiltration of an area with mulch and grass cover, low soil moisture and an ambient temperature of 73˚F. The procedure was as follows:

• Drove the ring 15cm into the ground in the area of interest using the driving cap, wooden block and sledgehammer.

• If the soil were disturbed more than 1/8 in from the wall of the ring, the ring would have been reset with less disturbance.

• When the soil was disturbed less than 1/8 in from the wall, the disturbed soil adjacent to the wall was tamped until it was firm.

• After covering the outer surface with splash guards, the ring was filled with water up to 5 cm below the top of the ring

• Immediately marked the starting point and time • Marked the water height in the ring every minute for 30 min • Did not let the water level drop more than 10 cm below the starting point to minimize the

effect of hydraulic pressure on infiltration rates. • When refilling was necessary, the new water height was marked and the time was

recorded. This “Refill” recording was the start of new measurement, but the time interval was not interrupted

• Used a tape measurer to calculate changes in water depth based on the markings over the 30 min

• Repeated the measurement process for Trial 2 slightly up-grade from the site of Trial 1 to ensure none of the water infiltrated previously will alter the measurements.

• Graphically and mathematically analyze the data to determine the Kostiakov coefficients. DIAGRAM:

FIGURE 1: SINGLE-RING INFILTROMETER DIAGRAM

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RESULTS: Trials 1 and 2 were completed consecutively in order to ensure nearly the exact same ambient and soil conditions. However, despite the fact that the two trials were run in the same area under the same conditions, the results of the infiltration measurements varied slightly. The tabular data shows information gathered from the trials and provides the simulated infiltration rates (in/hr) and cumulative infiltration (in) for the two sites. These values were calculated from graphical analysis of the infiltrometer data. The following process in Figure 2 demonstrates a sample of the calculations used to determine these values:

1. 𝑡 = 1𝑚𝑖𝑛

2. 𝑑! = 2.0𝑚𝑚× !!"!".!!!

= 0.0787𝑖𝑛 𝑑! = 3.0𝑚𝑚× !!"!".!!!

= 0.1181𝑖𝑛

3. 𝑑!"# =!.!"#"!!.!!"! !"

!= 0.0984𝑖𝑛

4. 𝑖! = !.!"#$!"

!"#× !"!"#

!= !.!"#!"

!

5. 𝐼! = 𝑑!"#,! + 𝑑!"#,! +⋯+ 𝑑!"#,! = 0.0984𝑖𝑛

6. 𝐼! = 𝑘𝑡! = 0.1074𝑡!.!!"# = 0.1074𝑖𝑛

7. 𝑖! = 𝑏𝑡! = 3.553𝑡!!.!!"# = !.!!"!"

!

8. 𝑏 = 60𝑘𝑎 = 60×0.15×0.55 = 3.553 𝑛 = 𝑎 − 1 = 0.5514 − 1 = −0.4486

where: t = time (min) d = depth change (mm or in) im = measured infiltration rate (in/h) Im = measured cumulative infiltration (in) Is = simulated cumulative infiltration (in) is = simulated infiltration rate (in/h) k and a = constants determined by graphical analysis

FIGURE 2: SAMPLE CALCULATION OF Is and is

The values of k and a are determined by graphing the measured cumulative infiltration against time and applying a power trendline to the plot. This method of calculations was used for each of the records for Trials 1 and 2. Figures 3 and 4 below are the graphs that were used in this analysis. Notice that the trendline equations were used directly to determine k and a.

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FIGURE 3: GRAPHICAL ANALYSIS OF TRIAL 1

FIGURE 4: GRAPHICAL ANALYSIS OF TRIAL 2

Tables 1 and 2 below give the corresponding data from Trials 1 and 2, respectively. The graphs above of the measured cumulative infiltration (in) and infiltration rates (in/hr) were used to determine the values of Kostiakov’s coefficients, k and a. For Trial 1, k = 0.1074 and a = 0.5514, and for Trial 2, k = 0.2703 and a = 0.9055.

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TABLE  1:  SINGLE-­‐RING  INFILTROMETER  TRIAL  1  

TIME  ELAPSED  (min)  

AVG.  DEPTH  CHANGE  

(in)  

MEASURED  CUMULATIVE  INFILTRATION  

(in)  

INFILTRATION  RATE  (in/hr)  

SIMULATED  CUMULATIVE  INFILTRATION  

(in)  

SIMULATED  INFILTRATION  RATE  (in/hr)  

1   0.098   0.098   5.906   0.107   3.553  2   0.079   0.177   4.724   0.157   2.604  3   0.049   0.226   2.953   0.197   2.171  4   0.019   0.246   1.159   0.231   1.908  5   0.019   0.265   1.159   0.261   1.726  6   0.019   0.284   1.159   0.288   1.591  7   0.019   0.304   1.159   0.314   1.484  8   0.019   0.323   1.159   0.338   1.398  9   0.019   0.342   1.159   0.361   1.326  10   0.019   0.362   1.159   0.382   1.265  11   0.019   0.381   1.159   0.403   1.212  12   0.019   0.400   1.159   0.423   1.165  13   0.019   0.420   1.159   0.442   1.124  14   0.019   0.439   1.159   0.460   1.088  15   0.019   0.458   1.159   0.478   1.054  16   0.019   0.478   1.159   0.495   1.024  17   0.019   0.497   1.159   0.512   0.997  18   0.019   0.516   1.159   0.529   0.972  19   0.019   0.536   1.159   0.545   0.948  20   0.019   0.555   1.159   0.560   0.927  21   0.019   0.574   1.159   0.576   0.907  22   0.019   0.593   1.159   0.590   0.888  23   0.019   0.613   1.159   0.605   0.870  24   0.019   0.632   1.159   0.620   0.854  25   0.019   0.651   1.159   0.634   0.839  26   0.019   0.671   1.159   0.647   0.824  27   0.019   0.690   1.159   0.661   0.810  28   0.019   0.709   1.159   0.674   0.797  29   0.019   0.729   1.159   0.688   0.784  30   0.019   0.748   1.159   0.701   0.773  

The measurements from Trial 1, are evenly spaced after the first few data points because the pencil that was used to record the water height wasn’t precise enough for Group C to be able to distinguish between the lines, since all of the data points were in such a small space. Therefore, in order to estimate the change in water depth after each minute, the known measurements were subtracted from the total and the remainder was divided evenly among the rest of the records. This method was not used as extensively for the data from Trial 2 because the

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water infiltrated more quickly in this trial and it was much easier to determine one marking from the next.

TABLE  2:  SINGLE-­‐RING  INFILTROMETER  TRIAL  2  

TIME  ELAPSED  (min)  

AVG.  DEPTH  CHANGE  

(in)  

MEASURED  CUMULATIVE  INFILTRATION  

(in)  

INFILTRATION  RATE  (in/hr)  

SIMULATED  CUMULATIVE  INFILTRATION  

(in)  

SIMULATED  INFILTRATION  RATE  (in/hr)  

1   0.217   0.217   12.992   0.270   14.685  2   0.157   0.374   9.449   0.506   13.754  3   0.433   0.807   25.984   0.731   13.237  4   0.276   1.083   16.535   0.948   12.882  5   0.118   1.201   7.087   1.161   12.613  6   0.394   1.594   23.622   1.369   12.398  7   0.098   1.693   5.906   1.574   12.219  8   0.236   1.929   14.173   1.777   12.065  9   0.354   2.283   21.260   1.977   11.932  10   0.138   2.421   8.268   2.174   11.814  11   0.157   2.579   9.449   2.370   11.708  12   0.177   2.756   10.630   2.565   11.612  13   0.098   2.854   5.906   2.758   11.524  14   0.236   3.091   14.173   2.949   11.444  15   0.256   3.346   15.354   3.139   11.370  16   0.098   3.445   5.906   3.328   11.300  17   0.171   3.616   10.276   3.516   11.236  18   0.114   3.730   6.850   3.703   11.175  19   0.256   3.986   15.354   3.888   11.118  20   0.118   4.104   7.087   4.073   11.065  21   0.236   4.341   14.173   4.257   11.014  22   0.098   4.439   5.906   4.440   10.965  23   0.098   4.537   5.906   4.623   10.919  24   0.197   4.734   11.811   4.804   10.876  25   0.030   4.764   1.772   4.985   10.834  26   0.030   4.793   1.772   5.165   10.794  27   0.030   4.823   1.772   5.345   10.755  28   0.030   4.852   1.772   5.524   10.718  29   0.030   4.882   1.772   5.702   10.683  30   0.030   4.911   1.772   5.880   10.649  

DISCUSSION: Many factors affect infiltration rates in addition to soil structure. These could include the chemical and physical makeup of the soil and water, the moisture content of the soil, the ambient

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conditions including humidity and temperature, the ground cover on the soil, and various other considerations. For this reason, even tests made at the same site may not give identical results. This is the situation with Group C’s area of interest. Although many of the factors affecting infiltration were the same in this procedure, the results varied between the two sites, which were no more than 15ft apart. Apart from varying conditions within the soil, water and environment, technical differences between the two trials may have resulted in experimental error, which can be expected when using basic infiltrometers.

Despite differences in the records collected, certain trends existed in the graphs and tabular data that are to be expected from an infiltration test. The first trend was that as time increased, the infiltration rate in each trial tended to decrease and then level out. The discussion of infiltration curves in Chapter 5 of Soil and Water Conservation Engineering mentions this trend when it says that initial infiltration rates typically exceed the rate of water application. However, when pores begin to fill with water, the rate of infiltration steadily decreases until it approaches a constant value know as the final infiltration rate. For Trial 1 the final infiltration rate is around 0.77 in/h, while that of Trial 2 is about 10.6 in/hr. Again, this is a significant difference in infiltration rates for two trials that were so close in proximity, however experimental errors and slight variations in factors affecting infiltration make such differences expected. The biggest error may have been caused by the way that the ring was driven into the ground for the second trial. Much more effort was put into making the top of the ring level, which consequently led to a much greater disturbance of the soil adjacent to the wall of the ring. This disturbance could be mostly responsible for the dramatic difference in final infiltration rates.

The second trend that was observed from the graphical analysis of each trial was that as time increased, the slope of the cumulative infiltration curves became slightly more gradual. Although this is a separate curve from the infiltration rate curve, its tendency to have a decreasingly steep, positive slope is thanks to the first trend, since the data points for each curve are mathematically linked (See Figure 2). PRECAUTIONS: This procedure is generally straightforward, however there are a couple of aspects that require greater attention to minimize error. As mentioned in the discussion above, when driving the ring into the ground, it is crucial to cause the least amount of disturbance to the soil as possible. If a significant amount of disturbance occurs, it is best to remove the ring and reset it in a new location to avoid possibly large experimental errors due to seepage. Another precaution is to ensure that the method of marking the water level is precise enough to be able to distinguish one mark from the next when the measurements are being recorded. Group C found that the best way to do this if a pencil is being used to mark the wall is to make marks in a stair-step pattern instead of one right below the next. This allows the recorder to distinguish clearly between each mark and measure the changes in depth accurately even if they are very small. It is also important to pay attention to the water level as time goes on. If the water drops too much, changes in hydraulic pressure will effect the infiltration rate. SOURCES OF ERROR: This experiment, although conducted with care, had a number of possible sources of error. Some of these include:

• Inconsistency in mulch depth may have affected infiltration rates

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• A slight rain began around 5 min in to Trial 2. Although the ring remained covered with the driving cap, this may have had a small impact on the measurements

• Disturbance of soil adjacent to the wall of the ring: For the sake of time, Group C did not reset the ring during Trial 2 even though there was significant disturbance of the soil around the ring from driving it into the ground (~1/2 in). Instead, the space was tamped slightly and filled with paper scraps to absorb the water that escaped

• Lack of precision when marking water level: The pencil used to mark the water level at each minute did not allow Group C to distinguish between the marks at all 30 measurements. Figure 1 below depicts this issue.

• Markings being smudged or washed away before they were recorded: After the refill at minute 18 of Trial 2, some of the pencil marks were smudged or washed away by the water before they could be recorded.

• Inconsistency in hydraulic pressure: This may be only a small factor, however if water is allowed to drop too far, changes in pressure could significantly affect the data.

• Human error: The markings may not have been perfectly level with the surface of the water at every minute, or taken immediately on the minute.

It is possible that these sources of error resulted in significant impacts on the data collected during this procedure. However, measures can be taken to minimize the effect of these sources on the records. This could be using a more precise and permanent method of marking the water level so that it is easier to read and not able to be washed away after a refill. Driving the ring into the ground more carefully or resetting the ring so that the soil adjacent to the wall is not disturbed more than 1/8 in. Putting an umbrella or tarp over the ring so that even if it rains, the data will not be affected. Redistributing the mulch so that depth is consistent throughout the area of interest. These changes would minimize error and provide for a more reliable infiltration records in the area. CONCLUSIONS: The final results of the two infiltration tests for an area with mulch and grass cover and low soil moisture varied significantly due to differences in physical conditions and experimental error. Trial 1 had a final infiltration rate of 0.77in/hr, while that of Trial 2 leveled out at 10.6in/hr. Graphical and mathematical analysis of the data collected for each trial was used to determine the coefficients of Kostiakov’s infiltration equation. For Trial 1 k = 0.1074 and a = 0.5514, and for Trial 2, k = 0.2703 and a = 0.9055. This information could potentially be used to help determine important information regarding the management of similar land, because infiltration rates are applicable to many different issues including erosion, leaching, drainage, runoff and irrigation.

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REFERENCES: Huffman, Rodney L. P.E., Delmar D. Fangmeier, P.E., William J. Elliot, P.E., Stephen R.

Workman, P.E. Soil and Water Conservation Engineering. 7th ed. ASABE. p. 86.