USDA Forest Service Research Note RMRS-RN-33. 2008 1

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Note

RMRS-RN-33

March 2008

New Procedure for Sampling Infiltration to 

Assess Post-fire Soil Water Repellency

P. R. Robichaud, S. A. Lewis, and L. E. Ashmun

Robichaud, P. R.; Lewis, S. A.; Ashmun, L. E. 2008. New procedure for 

sampling infiltration to assess post-fire soil water repellency. Res. Note. 

RMRS-RN-33. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station. 14 p.

Abstract—The Mini-disk Infiltrometer has been adapted for use as a field test of post-fire infiltration and soil water repellency. Although the Water Drop Penetration Time (WDPT) test is the common field test for soil water repellency, the Mini-disk 

Infiltrometer (MDI) test takes less time, is less subjective, and provides a relative infiltration rate. For each test, the porous base 

plate of the MDI is placed on the soil surface and the amount of water that passes into the soil in one minute is measured.

Thousands of paired WDPT and MDI tests were applied at burned sites throughout the western United States, and the data 

were significantly correlated (r = –0.64). A classification tree analysis was used to group the MDI test results into “degree 

of soil water repellency” categories (strong, weak, and none) that correspond to similar categories established for the WDPT 

test. Fire-induced soil water repellency has high spatial variability and requires a valid sampling method if the data are to be 

credible. The MDI test protocol and sampling method described in this Research Note were developed for post-fire assessment, and provide a practical evaluation of burned soil infiltration characteristics in a limited time.

Introduction ____________________

After forest fires, the potential for flooding and erosion dramatically increases, partially due to decreased 

infiltration across the burned landscape (Pierson and 

others 2001, Robichaud 2000). Combustion of the surface organic material (litter and duff) can create water 

repellent layers within the mineral soil. When organic 

material burns, some of the volatilized material with 

hydrophobic properties moves downward into the soil 

profile and condenses on cooler soil particles beneath the 

surface (DeBano 1981). The coated soil particles form 

a non-continuous water repellent layer that is generally 

parallel to and within 5 cm of the mineral soil surface 

(Clothier and others 2000, DeBano 2000). On naturally 

water repellent soils, fire may destroy existing surface 

soil water repellency and create a stronger water repellent layer beneath the surface. Water repellent soils have 

decreased infiltration and increased runoff and erosion 

as compared to non-water repellent soils (BenavidesSolorio and MacDonald 2001). The combustion of surface organic material reduces the absorption and water 

storage capacity (“sponge” effect) above the mineral 

soil, which also contributes to post-fire decreases in 

water infiltration rates.

Soil water repellency, whether naturally occurring or 

fire-induced, is not a stable phenomenon. When the soil 

is wet after snow melt, hydrologic behavior is normal; 

however, when the soil dries, water repellent conditions 

reoccur (Dekker and Ritsema 2000). Consequently, 

soil water repellency is generally strongest in the drier 

summer months. Over time, as the soil is intermittently 

exposed to moisture, fire-induced soil water repellency 

slowly declines (DeBano 1981, Letey 2001). The persistence of fire-induced soil water repellency depends 

on many physical and biological factors that affect the 

breakdown of the hydrophobic chemicals that coat the soil 

particles, and as a result, is highly site-specific (Doerr 

and others 2000). The time needed for fire-induced 

soil water repellency to dissipate has been reported to 

be less than 1 year after the Bobcat Fire in Colorado 

(Huffman and others 2001) and as long as 6 years after 

a severe wildfire in a lodgepole pine forest in Oregon 

(Dyrness 1976). 

Estimating the reduced infiltration after a fire is 

essential for modeling post-fire hydrologic processes 

(Pierson and others 2001, Robichaud 2000). Because 

water repellent soils have a large impact on the soil 

infiltration characteristics, post-fire assessment of the 

extent and degree of fire-induced soil water repellency is 

used to estimate the reduction in infiltration (Robichaud 

and others 2000). This assessment is usually done within 

days after the wildfire is contained.

Of the two current field methods for assessing water 

repellent soil—the Molarity Ethanol Drop (MED) test 
2 USDA Forest Service Research Note RMRS-RN-33. 2008

(King 1981) and the Water Drop Penetration Time 

(WDPT) test (DeBano 1981)—the WDPT is the most 

commonly used for post-fire assessment. In the WDPT 

test, water drops are placed on an exposed surface of 

the mineral soil and the time to infiltrate into the soil 

is observed (fig. 1). The time to infiltrate is often averaged over a number of drops and the mean is used to 

determine the degree of soil water repellency at the 

sample location and depth. The two extreme conditions 

of soil water repellency are easily evaluated—when soil 

is strongly water repellent water drops “bead up” and sit 

on the soil surface for the total observation time (300 

seconds), and when soil has no water repellency water 

drops infiltrate within 5 seconds. However, in situations 

where water drops sit on the soil surface for a portion 

of the observation time and then slowly collapse and 

infiltrate the soil, more subjectivity is introduced. As 

water drops collapse on the soil surface, the observer 

must judge when infiltration has occurred. Given that 

water drops differ in size, may trap air within them, 

and tend to roll around soil particles and down slopes, 

determining the time to infiltration can be subjective 

and imprecise. In addition, some researchers feel that 

the time needed for a water drop to infiltrate measures 

the stability of soil water repellency rather than the 

degree of soil water repellency (Dekker and Ritsema 

1994, Letey 2001).

The Mini-disk Infiltrometer

The Mini-disk Infiltrometer (MDI) (Decagon Devices, 

Inc., Pullman, WA) is a hand-held field instrument for 

rapidly assessing soil infiltration capacity (fig. 2). When 

the infiltrometer is placed on a wettable soil surface, 

the suction from the soil side of the porous disk is able 

to break the water surface tension across the disk and 

water passes from the infiltrometer into the soil. As water 

passes through the porous disk into the soil, bubbles rise 

in the main chamber and in the bubble chamber. When 

the MDI is placed on strongly water repellent soil, there 

is not enough suction to break the water surface tension across the porous disk and no water infiltrates the 

soil. The suction on the infiltrometer side of the disk is 

controlled by the “suction control tube” (0.5 to 7 cm) 

at the top of the infiltrometer. During field testing, we 

determined that 1 cm is the optimal suction setting for 

post-fire soil infiltration and water repellency field tests. 

Figure 1—The water drop penetration time (WDPT) test 

performed at 1 cm below the surface of the soil. The water 

drops inside the rectangle are beaded up on the surface while 

drops outside of the rectangle have infiltrated the soil.

Figure 2—Diagram of the Mini-disk Infiltrometer (from 

Decagon Devices, Inc., Pullman, WA) with parts labeled. 
USDA Forest Service Research Note RMRS-RN-33. 2008 3

The 1 cm setting provides enough suction to keep the 

MDI from dripping when held off the soil, but still allows the MDI to differentiate soils with strong, weak, 

and no water repellency. 

The MDI test measures the volume of water (mL) that 

passes from the infiltrometer into the soil in 1 minute. 

The 1-minute interval was arbitrarily chosen and has 

proven to be long enough to detect water repellent soil 

conditions, yet fast enough to be a useful assessment 

procedure for post-fire assessment teams. The MDI test 

provides a relative infiltration rate, which can be used 

to classify soil water repellency as well as to compare 

the infiltration capacities of tested sites. 

Comparing the Mini-disk Infiltrometer Test 

to the Water Drop Penetration Time Test

For several years, we made paired WDPT and MDI 

test measurements to evaluate the use of the MDI. Paired 

measurements (n = 2069) from eight wildfires in four 

states (CA, CO, ID, and MT) were analyzed using the 

non-parametric Spearman rank correlation analysis 

(SAS Institute Inc. 1999a). The tests were performed at 

recently burned sites that varied in soil burn severity. 

The mean of eight WDPT and of three MDI measurements from each sample location were correlated. The 

correlation of MDI to WDPT (r = –0.64) was significant 

(p < 0.0001) for all data (table 1). 

An in-depth study of the MDI was implemented at 

the James Creek site following the 2003 Hot Creek Fire 

(Boise National Forest) in Idaho. Soil water repellency 

characterization was done adjacent to research plots 

that were established to study post-wildfire infiltration, 

erosion, and recovery. Twenty-nine high burn severity 

plot sites were tested in August 2004 using the MDI 

and WDPT tests at six depths (0, 1, 2, 3, 4, and 5 cm). 

The James Creek study provided data used to correlate 

the WDPT and MDI tests (table 1) and to derive MDI 

test values that could be used to classify “degree of soil 

water repellency” that correspond to the established 

WDPT test classifications.

The correlations between the WDPT and MDI tests 

were highest where soil water repellency existed 

(table 2). The Spearman correlation coefficients were 

Table 1—Spearman correlation coefficients (r) for soil water repellency measurements made using the WDPT and MDI tests. All values are 

significant with p < 0.0001. 

Fire name Year Sample size WDPT—MDI

 (n) Correlation coefficient (r)

Hayman 2002 182 –0.68

Black Mountain 2003 170 –0.67

Cooney Ridge 2003 383 –0.59

Roberts 2003 410 –0.57

Simi 2003 358 –0.66

Old 2003 289 –0.60

Wedge 2003 103 –0.40

James Creek 2003a 174 –0.83

 All data 2069 –0.64

a

James Creek measurements were taken in 2004, 1 yr after the fire.

Table 2—Spearman correlation coefficients (r) 

for the WDPT–MDI tests done at 29 

James Creek burned plots (n = 174) and 

sorted by depth. Values in bold type are 

significant at the p < 0.05 level. 

WDPT–MDI 

Correlation coefficient (r)

All data –0.83

by depth 

0 cm –0.24

1 cm –0.68

2 cm –0.79

3 cm –0.74

4 cm –0.78

5 cm –0.70
4 USDA Forest Service Research Note RMRS-RN-33. 2008

significant at the p < 0.05 level for all depths below 

the surface and for all depths combined (r = –0.83). 

However, the surface measurements generally indicated 

non-water repellent soil (typical of recently burned soil) 

and the Spearman correlation coefficient for the surface 

measurements (–0.24) was not significant (table 2). 

Data from post-fire WDPT field tests are most commonly used to classify soil into water repellency classes. 

The classes are arbitrarily assigned and vary between 

countries and agencies. In the United States, most postfire researchers categorize WDPT results using the four 

water repellency classes developed by DeBano (1981); 

however, Burned Area Emergency Response (BAER) 

teams use a different classification system (fig. 3). To 

facilitate classification of soil water repellency using MDI 

data, class breakpoints in MDI values were determined 

using classification tree analysis of the James Creek data. 

This analysis did not find a break point in the MDI test 

values that would indicate moderate soil water repellency. Thus, two classes—strong (0 to <3 mL min–1) 

and weak (3 to <8 mL min–1)—of soil water repellency 

were identified. MDI test values of 8 mL min–1

 or more 

indicate no water repellency. A similar set of MDI values 

were determined by the BAER soil science team on the 

2006 Derby Fire in the Bitterroot National Forest of 

Montana. The post-fire MDI field measurements ranged 

from 1 to 2 mL min–1 on the strongly water repellent 

soils and 7 to 11 mL min–1 on the soils with no water 

repellency (Sirucek and others 2006). 

The relative infiltration rates measured by the MDI 

test provide more information than the WDPT about the 

infiltration capacity of low and non-water repellent soils. 

For example, 51 test locations within the James Creek 

data set were classified “non-water repellent” by both 

the WDPT and MDI test values. The range of values 

was 0 to 4 seconds for the WDPT test and 8 to 25 mL 

min–1 for the MDI test. Thus, some non-water repellent 

locations had three times greater infiltration rates than 

others. This difference in infiltration capacity cannot be 

detected using the WDPT test. Relative infiltration rates 

may be useful for predicting the watershed response to 

future rain events. 

In our research, fire-induced soil water repellency 

has most often been detected at 1 to 3 cm below the 

surface. In burned areas, surface measurements (immediately below the ash layer where soil and ash mix) often 

indicate non-water repellent soil, making sub-surface 

testing necessary to determine the existence and extent 

of a fire-induced water repellent soil layer. The data 

from James Creek is typical of forested areas burned 

at high severity—no detectable soil water repellency at 

the surface with a strongly water repellent soil layer at 

1 to 2 cm below the surface (fig. 4). 

In general, if fire-induced water repellency is prevalent at 1 to 3 cm below surface and there is little or no 

cover on the soil, the risk for post-fire erosion is high. 

During a rain event, the thin, wettable soil layer on the 

surface quickly becomes saturated. The water, which is 

hindered by the sub-surface water repellent soil layer, 

can not infiltrate deeper into the soil and becomes excess overland flow that easily entrains and carries the 

saturated soil downslope (Doerr and others 2006). 

Figure 3—Soil water repellency classifications based on water 

drop penetration time (WDPT) as used by USDA-FS BAER 

(USDA Forest Service 1995), USDA-FS researchers (DeBano 

1983), and European researchers (Doerr and others 2006).
USDA Forest Service Research Note RMRS-RN-33. 2008 5

The MDI Test for Fire-induced Soil 

Water Repellency and Infiltration___

Field Test Materials (fig. 5)

•	 Mini-disk Infiltrometer

•	 1 L (or larger) water bottle to refill the infiltrometer 

as needed

•	 small trowel

•	 stopwatch

•	 small ruler to measure soil depth (or a ruled trowel 

blade)

•	 recording sheets (appendix A) 

•	 plastic wash bottle to rinse the porous disk after 

each test

Test Steps

1. Use the trowel to cut to the soil depth being tested 

and lift off the overlying ash, surface organic 

material, and mineral soil to expose the soil at 1 

or 3 cm depth. 

2. Fill the infiltrometer (fig. 2)

a. Remove the upper stopper and fill the bubble 

(upper) chamber. Once it is full, replace the 

upper stopper and slide the suction control 

tube all the way down so that it rests on the 

rubber gasket between the two chambers.

b. Invert the infiltrometer, remove the bottom 

elastomer with the porous disk, and fill the 

main (lower) chamber. Replace the bottom 

elastomer, ensuring the porous disk is firmly 

in place.

3. Turn the infiltrometer upright and adjust the suction to 1 cm by aligning the surface of the water 

in the bubble chamber with the 1 cm mark on the 

adjustable suction tube.

4. Hold the top of the infiltrometer so that the water 

surface in the main chamber is at eye level and 

record the start volume (mL). 

5. Place the infiltrometer porous disk flat against the 

soil with the infiltrometer held perpendicular to 

the surface. Start the timer when the infiltrometer 

disk and soil come into contact. (On steep slopes 

[50 to 60 percent or more], you may observe water 

from inside the tube seeping from the side of the 

infiltration disk and running downslope along the 

soil surface and not infiltrating. If this happens, 

use the trowel to cut a level “shelf” as close as possible to the depth being tested within the mineral 

soil. Set the infiltrometer perpendicular to the cut 

surface rather than the hillslope.)

6. Continue to hold the infiltrometer against the soil 

surface so that the entire infiltration disk is in 

contact with the soil for an uninterrupted minute. 

The infiltrometer needs to be held against the soil, 

but it does not need to be pushed into the soil with 

any force. 

7. At the end of 1 minute, remove the infiltrometer 

from the soil and hold the top of the tube so that 

the water is at eye level. Record the end volume. 

Figure 4—Mean Mini-disk Infiltrometer (MDI) test values at 

the surface and at five depths (1 to 5 cm) below the surface 

for burned sites.

Figure 5—Using the Mini-disk Infiltrometer in the field.
6 USDA Forest Service Research Note RMRS-RN-33. 2008

8. Record the amount of water that has infiltrated the 

soil during the 1-minute test. 

9. Rinse the porous disk to remove any soil particles 

that cling to the disk.

10. Refill the infiltrometer as needed.

11. Repeat steps 4 through 10 for each test.

Sampling a Burned Area__________

Post-fire assessments of soil water repellency and 

reduced infiltration are needed within days after fire 

containment. This short time frame for sampling necessitates a sampling scheme that 1) focuses on areas where 

soil water repellency and reduced infiltration are most 

likely; 2) provides a logical method for extrapolation of 

sample results to burned areas that are not sampled; and 

3) attaches a level of confidence to the measurements. 

Fire-induced soil water repellency has high spatial 

variability and may vary at the 10-cm scale (Lewis and 

others 2006). MDI test measurements usually reflect 

this high spatial variability, and if the sample size is 

small, the results have low statistical power and may 

not accurately reflect the average soil water repellency 

of the area. The classification of the burned area based 

on variables related to soil water repellency that have 

spatial structure (in other words, are not random) can 

increase statistical power and at the same time reduce 

sample size (Klironomos and others 1999). Regardless 

of the sampling method used, it is recommended that a 

minimum of three MDI tests be done in close proximity 

(immediately adjacent to, but not on top of or beneath, 

a previous test) at each sample location to compensate 

for measurement variability.

The sampling method described in appendix B is 

applied to a burned area that has similar soil (e.g., granitic or volcanic) and pre-fire vegetation (e.g., forest or 

grass). If the post-fire assessment includes more than 

one general soil or vegetation type, a separate evaluation of infiltration and soil water repellency should be 

done in each area. To determine where to sample, the 

burned area is divided using two characteristics—burn 

severity and slope aspect. Slope aspect has been highly 

correlated to burn severity, and soil burn severity is 

the factor most highly correlated to post-fire soil water 

repellency (Lewis and others 2006). Areas burned at 

high severity often have strongly water repellent soils, 

while areas burned at low severity have weak or no 

fire-induced soil water repellency. Moderately burned 

areas are typically a mosaic of high and low soil burn 

severity areas and generally have soils with a mix of both 

strong and weak water repellency. Hillslopes with north 

aspects tend to be wetter and more densely vegetated, 

while slopes with south aspects are generally dryer and 

have less fuel. Fire reacts to these differences in slope, soil 

moisture, and vegetation, often spreading more quickly on 

a slope with a south aspect, but sometimes with less fire 

residence time and, therefore, less soil heating. Although 

the hillslopes in an area may all be designated as high 

or moderate burn severity, the soil water repellency may 

vary by slope aspect. In addition, because fire spread 

and duration also vary from the base to the top of a 

hillslope, soil water repellency may vary by hillslope 

position. For this reason, samples from both the upper 

and lower portion of the hillslope are needed.

With any sampling scheme, the goal is to estimate the 

true variability and mean of a specific trait within the 

whole population by measuring that trait in a subset of 

the population. If too few samples are measured, the 

results will have a low confidence level (a measure of 

the certainty that the sample data represents the entire 

population). However, there is always a trade off between 

confidence level and time and resources spent on the 

sampling process.

Adequacy of sample size is often evaluated in terms 

of statistical power—the probability that the true mean 

of the data is within a pre-determined precision (SAS 

Institute Inc. 1999b). When determining post-fire infiltration and soil water repellency using the MDI, statistical 

power is dependent on the inherent variability of the 

data and the selected precision—the pre-determined 

limit that the measured mean can vary from the true 

mean MDI test value. The real statistical power of any 

data set can only be calculated after-the-fact, when the 

true variability (standard deviation) can be calculated. 

However, we derived the inputs needed for a power 

analysis, including the standard deviation, from the portion of our existing post-fire MDI test data that could 

be classified by slope and aspect (n > 1000). We used 

these inputs to calculate a range of sample sizes with 

corresponding confidence levels. We then estimated 

sample sizes corresponding to 70, 80, 90, and 95 percent 

confidence levels and applied them to the sampling method 

presented in appendix B. Like most statistical analyses, the 

more measurements taken, the higher the confidence level 

assigned to the conclusions.

The short time available for post-fire assessment often 

restricts the number of samples that can be obtained 

and results may have less-than-desired confidence levels. Also, the results obtained from a relatively small 

sample within a category will, by necessity, be applied 

to other burned areas of the same category. Although 

these minimal sampling guidelines may not be adequate 

for science research purposes, they do provide practical guidance for making the most of the limited time 

available for post-fire assessment. 
USDA Forest Service Research Note RMRS-RN-33. 2008 7

Management Implications_________

Soil water repellency and infiltration assessments 

are needed for post-fire emergency stabilization and 

rehabilitation planning. The Mini-disk Infiltrometer 

has been adapted for use as a field test of fire-induced 

soil water repellency. The MDI test is an effective 

technique to quickly assess the existence and degree of 

water repellent soil as well as the relative infiltration 

capacity. In addition to the development of a MDI test 

protocol, the MDI test values have been correlated to 

the current soil water repellency classifications based 

on the WDPT test. Thus, MDI results can be used for 

reporting the degree and extent of soil water repellency 

in customary terms. 

The sampling method described in appendix B uses 

burn severity and slope aspect to classify the burned 

area into hillslopes that are likely to have similar soil 

water repellency and infiltration characteristics. This 

classification provides a way to systematically sample 

the burned area and have reasonable confidence that 

soil water repellency and infiltration assessments can 

be applied to non-sampled areas with similar characteristics. The sampling method provided in appendix B 

may challenge the time and resources available to many 

BAER teams; however, it does describe the minimum 

evaluation needed for various levels of statistical power 

and assessment confidence.

The Authors ____________________

Peter R. Robichaud is a Research Engineer and Sarah A. 

Lewis and Louise E. Ashmun are Civil Engineers with the 

Air, Water and Aquatic Environments Science Program 

of the Rocky Mountain Research Station located at the 

Forestry Sciences Laboratory in Moscow, Idaho.

References _____________________

Benavides-Solorio, J.D.; MacDonald, L.H. 2001. Post-fire runoff and 

erosion from simulated rainfall on small plots, Colorado Front 

Range. Hydrological Processes. 15(15): 2931-2952.

Clothier, B.E.; Vogeler, I.; Magesan, G.N. 2000. The breakdown 

of water repellency and solute transport through a hydrophobic 

soil. Journal of Hydrology. 231-232: 255-264.

DeBano, L.F. 1981. Water repellent soils: a state of art. Gen. Tech. 

Rpt. PSW-46. Berkeley, CA: U.S. Department of Agriculture, 

Forest Service, Pacific Southwest Forest and Range Experiment 

Station. 21 p.

DeBano, L.F. 2000. Fire-induced water repellency: an erosional factor in wildland environments. In: Proceedings of Land stewardship 

in the 21st century: the contributions of watershed management. 

Tucson, AZ, 13-16 March 2000. USDA Forest Service, Rocky 

Mountain Research Station, Fort Collins, CO: 307-310.

Decagon Devices Inc. (2005). Infiltrometer Product Guide, [Online]. 

Available: http://www.decagon.com/infiltrometer/ [17 January 

2007].

Dekker, L.W.; Ritsema, C.J. 1994. How water moves in a water 

repellent sandy soil. 1. Potential and actual water repellency. 

Water Resources Research. 30: 2507-2517.

Dekker, L.W.; Ritsema, C.J. 2000. Wetting patterns and moisture 

variability in water repellent Dutch soils. Journal of Hydrology. 

231-232: 148-164.

Doerr, S.H.; Shakesby, R.A.; Walsh, R.P.D. 2000. Soil water repellency: its causes, characteristics and hydro-geomorphological 

significance. Earth-Science Review. 51: 33-65.

Doerr, S.H.; Shakesby, R.A.; Blake, W.H.; Chafer, C.J.; Humphreys, 

G.S.; Walbrink, P.J. 2006. Effects of differing wildfire severities 

on soil wettability and implications for hydrological response. 

Journal of Hydrology. 319: 295-311.

Dyrness, C.T. 1976. Effect of wildfire on soil wettability in the high 

Cascades of Oregon. Res. Pap. PNW-202. Corvallis OR: U.S. 

Department of Agriculature, Forest Service, Pacific Northwest 

Research Station. 19 p.

Huffman, E.L.; MacDonald, L.H.; Stednick, J.D. 2001. Strength 

and persistence of fire-induced soil hydrophobicity under ponderosa and lodgepole pine, Colorado Front Range. Hydrological 

Processes. 15: 2877-2892.

King, P.M. 1981.Comparison of methods for measuring severity of 

water repellence of sandy soils and assessment of some factors 

that affect its measurement. Australian Journal of Soil Research. 

19: 275-285.

Klironomos, J.N.; Rillig, M.C.; Allen, M.F. 1999. Designing belowground field experiments with the help of semi-variance and 

power analyses. Applied Soil Ecology. 12: 227-238.

Letey, J. 2001. Causes and consequences of fire-induced soil water 

repellency. Hydrological Processes. 15: 2867-2875.

Lewis, S.A.; Wu, J.Q.; Robichaud, P.R. 2006. Assessing burn severity and comparing soil water repellency, Hayman Fire, Colorado. 

Hydrological Processes. 20: 1-16.

Pierson, F.B.; Robichaud, P.R.; Spaeth, K.E. 2001. Spatial and temporal effects of wildfire on the hydrology of a steep rangeland 

watershed. Hydrological Processes. 15: 2905-2916.

Robichaud, P.R. 2000. Fire effects on infiltration rates after prescribed fire in Northern Rocky Mountain forests, USA. Journal 

of Hydrology. 231-232: 220-229.

Robichaud, P.R.; Beyers, J.L.; Neary, D.G. 2000. Evaluating the effectiveness of postfire rehabilitation treatments. Gen. Tech. Rep. 

RMRS-GTR-63. Fort Collins, CO: US Department of Agriculture, 

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Version 8.2. Cary, NC: Statistical Analysis Systems (SAS) 

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SAS Institute, Inc. 1999b. The Analysts Application, First Edition. Cary, NC: Statistical Analysis Systems (SAS) Institute 

Inc. 476 pp. 

Sirucek, D.; Rolfes, T; Rone, G. 2006. Soil Resource—Derby Mountain Fire, U.S. In: Department of Agriculture, Forest Service, 

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Fire BAER 2500-8). Unpublished report on file at: U.S. Department of Agriculture, Forest Service, Gallatin National Forest, 

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Rehabilitation Handbook, Section FSH 2509.13 of the Forest 

Service Handbook. Washington D.C.
8 USDA Forest Service Research Note RMRS-RN-33. 2008

Appendix A—Data Recording Sheet _____________________________________

Soil Burn Severity: High Moderate Site: Slope Aspect: North South

Date: Sampler: Slope Position: Upper Lower

For each test, record the MDI water level at the start, place the MDI on the soil for 1 min, and record the

MDI water level at the end. Subtract the two readings to obtain “water infiltrating (mL)”

0 m Transect meter mark

Trial 1 Trial 2 Trial 3 

Depth (cm) 

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

1 

3 

10 m Transect meter mark

Trial 1 Trial 2 Trial 3 

Depth (cm) 

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

1 

3 

30 m Transect meter mark

Trial 1 Trial 2 Trial 3 

Depth (cm) 

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

1 

3 

60 m Transect meter mark

Trial 1 Trial 2 Trial 3 

Depth (cm) 

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

1 

3 

100 m Transect meter mark

Trial 1 Trial 2 Trial 3 

Depth (cm) 

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

Start

reading

(mL) 

End

reading

(mL)

Water

infiltrating

(mL)

1 

3 
USDA Forest Service Research Note RMRS-RN-33. 2008 9

Classification of the Burned Area

The burned area is divided into areas of similar characteristics based on the factors that correlate strongly 

with post-fire soil water repellency—burn severity and 

slope aspect. MDI tests are done along transects located 

on selected hillslopes (upper and lower slope positions) 

of each class or category (fig. B1). The results from the 

sampled hillslopes will be extrapolated and applied to 

other burned hillslopes of the same class (in other words, 

with the same burn severity and aspect). However, if the 

burned area covers more than one general soil type (for 

example, course granitic and fine ash cap) or vegetation 

type (for example, conifer forest and chaparral), a separate 

post-fire assessment of soil water repellency should be 

completed in each soil and vegetation type. 

Soil Burn Severity—The initial Burned Area Reflectance Classification (BARC) map provides the first 

post-fire burn severity designations (unburned, low, 

moderate, or high). Because ground verification of this 

map is often done simultaneously with soil water repellency testing, it is necessary to first check the soil burn 

severity classification of each slope chosen for soil water 

repellency testing (Note: in the descriptions below, the 

canopy information is included as “confirmation” of the 

degree of fire effects on the hillslope).

•	 High soil burn severity will have extensive exposed 

charred (orange, gray, or black) mineral soil and ash 

with very little litter or other organic cover. Tree 

canopies are mostly burned and/or black. 

•	 Moderate soil burn severity is characterized by 

a mix of charred soil, ash, and some remaining 

vegetation cover. There will often be charred or 

scorched needles remaining in the trees or on the 

ground as a post-fire litter cover. 

•	 Low soil burn severity has little charred soil or ash, 

and usually has a majority of remaining litter cover. 

Tree canopies are mostly unscorched and green. 

Areas of high and moderate soil burn severity should 

be tested for water repellency, but it is not essential to test 

low soil burn severity and unburned areas when under 

the time constraints of post-fire emergency assessment. 

Moderately burned areas are typically a mosaic of highly 

and lightly burned areas. Efforts should be made to place 

the sampling transects through a representative area that 

captures the average burn severity of the hillslope.

Figure B1—Diagram of the transect layout and Mini-disk Infiltrometer sampling points (note: 

diagram not to scale).

Appendix B—Sampling Method for Post-fire Assessment of Soil Water 

Repellency ______________________________________________________
10 USDA Forest Service Research Note RMRS-RN-33. 2008

Slope Aspect—A simple dichotomous division of aspect 

into north (270 to 360 and 1 to 89 compass degrees) 

and south (90 to 269 compass degrees) is adequate for 

post-fire assessment of soil water repellency. Aspect is 

measured facing downhill in the direction water would 

run if poured on the slope. Both north and south aspects 

within areas burned at high and moderate severity need 

to be sampled. 

Sampling Along Transects

After a forest fire, typical hillslopes of high and moderate soil burn severity with north and south aspects are 

selected for sampling. Two slope positions—upper slope 

and lower slope—are sampled. A 100-m transect is laid 

along the hillslope contour and MDI tests are performed 

at the 0, 10, 30, 60, and 100 meter marks (fig. B1). At 

each test location, three MDI tests are done at both 1 and 

3 cm below the mineral soil surface. (Testing below the 

3-cm depth may be necessary if unusually high soil heating is suspected, such as under slash piles, blowdown, 

or other large accumulations of materials.)

Determining the Number of Transects or 

Sample Size

Power analysis was performed to estimate the number 

of sample transects needed to confidently draw conclusions about post-fire soil water repellency and infiltration. A statistical power of 80 percent and a precision 

of 2 mL min–1 (±1 mL min–1) were used as inputs in 

the power analysis. This means that the power analysis 

was completed with the assumption that “an 80 percent 

probability that the true mean is within ±1 mL min–1

of the sampled mean” would be adequate power for the 

post-fire assessment of soil water repellency.

The results of the power analysis provide a range of 

sample size prescriptions correlated to an alpha, which 

is a measure of the certainty that the sample data represents the value of the entire population (in our case, all 

the burned hillslopes of the class). The alpha represents 

the risk of being wrong, and its counterpart, confidence 

level ([1-α] ×100 percent) is the probability of being correct. Table B1 presents the prescribed sample sizes for 

confidence levels of approximately 70 percent (α = 0.3), 

80 percent (α = 0.2), 90 percent (α = 0.1), and 95 percent 

(α = 0.05). The number of samples needed based on the 

power analysis is listed in the second column of table B1; 

however, the prescribed number of transects is rounded 

to the nearest multiple of 10, or a full transect amount of 

samples. At the 70 percent confidence level, the sample 

size (23) is rounded down to 20 to fit two full transects, 

which will have a confidence level that is slightly less 

than 70 percent. At 80, 90, and 95 percent confidence 

levels, the sample size is rounded up to fit the transect 

sampling method, which increases the confidence levels 

as compared to the values listed in the first column of 

table B1. As a check on this methodology, we used a 

stratified random sampling of our data to test the ability of 20 samples from a class to approximate the class 

mean from the full data set and calculated an average 

difference of 6 percent. 

Selecting a confidence level is a balance between not 

wanting to incorrectly assess the soil water repellency 

or reduced infiltration (especially as results are likely to 

be applied to other similar, but not sampled hillslopes) 

and the time and expense of post-fire MDI testing. In 

most post-fire assessments, setting the confidence level 

will not be the criterion for sample size selection, rather 

time and availability of human resources will dictate 

the maximum amount of sampling that can occur. The 

time needed for MDI testing has been estimated for 

four sampling prescriptions (table B2). Based on our 

experience using the MDI, a person can complete the 

six tests (three tests at two depths) at a sampling point in 

Table B1—Sample size prescriptions based on power analysis. The selected inputs for this 

analysis were a) 80 percent power and b) ± 1 mL min–1 precision. The other input, a 

standard deviation of 4 mL min–1, was calculated from over 1000 postfire measurements (Note: 2 depths × 5 locations = 10 samples per transect).

Power analysis Sampling prescription

Confidence level Samples Transects Samples Approximate

(%) [α] per class per class per class confidence level (%)

70 [α = 0.3] 23 2 20 <70

80 [α = 0.2] 34 4 40 >80

90 [α = 0.1] 53 6 60 >90

95 [α = 0.05] 73 8 80 >95
USDA Forest Service Research Note RMRS-RN-33. 2008 11

Table B2—The number of person-days, samples, and transects needed to estimate post-fire soil water 

repellency and reduced infiltration at four confidence levels—70, 80, 90, and 95 percent. 

The four classes are indicated as HS = High burn severity-South aspect; HN = High burn 

severity-North aspect; MS = Moderate burn severity-South aspect; MN = Moderate burn 

severity-North aspect.

1 person-day 2 person-day 3 person-day 4 person-day

Number of

samples – transects 80–8 160–16 240–24 320–32

Number of 2/HS, 2/HN, 4/HS, 4/HN 6/HS, 6/HN 8/HS, 8/HN

transects/class 2/MS, 2/MN 4/MS, 4/MN 6/MS, 6/MN 8/MS, 8/MN

Number of 

hillslopes (sites) 4 8 12 16

Confidence level 70% 80% 90% 95%

Description A B C D

A. one person would spend one day: 

 2 transects (upper/lower) at 4 sites (hillslopes) = 2 transects per class 

B. one person would spend two days or two people would spend one day: 

 2 transects (upper/lower) at 8 sites (hillslopes) = 4 transects per class

C. one, two, or three people sample to total 3 person-days: 

 2 transects (upper/lower) at 12 sites (hillslopes) = 6 transects per class

D. one to four people sample to total 4 person-days:

 2 transects (upper/lower) at 16 sites (hillslopes) = 8 transects per class 

under 15 minutes and, with five sampling points along 

a transect, the time needed for one transect is less than 

11⁄4 hours. Given two transects per hillslope, it would 

likely take one person about 21⁄2 hours to sample a 

hillslope site. 

Interpreting Results

At each transect test location (0, 10, 30, 60, and 100 

m), six individual MDI tests (three tests at each of two 

depths) are performed (fig. B1). The mean of the three 

individual MDI readings is the MDI value at that sample 

location and depth. The MDI value determines the degree 

of soil water repellency (Strong, Weak, or None) at each 

sample location, and for each hillslope, the proportion 

•	 Moderate soil burn severity, North aspect: Strong-35 percent; Weak-40 percent; None-5 percent

•	 Moderate soil burn severity, South aspect: Strong-75 percent; Weak-20 percent; None-5 percent

•	 High soil burn severity, North aspect: Strong-75 percent; Weak-15 percent; None-10 percent

•	 High soil burn severity, South aspect: Strong-90 percent; Weak-10 percent; None-0 percent

of MDI values (percent) that indicate Strong, Weak, and 

No soil water repellency are calculated. These percentages are used to describe the degree and extent of soil 

water repellency on the assessed hillslope. 

Example

Table B3 presents a spreadsheet format and analysis 

for a soil water repellency sample data set that could 

have been collected in 1 day by a single person (this 

spreadsheet is available for downloading at http://

forest.moscowfsl.wsu.edu/BAERTOOLS). Based on 

these sample data, a post-fire assessment team would 

assume that soil water repellency in the burned area is 

distributed as follows: 
12 USDA Forest Service Research Note RMRS-RN-33. 2008

Because only two transects on a single hillslope in 

each sampling class were assessed (table B1), these 

results have a fairly low confidence level (slightly less 

than 70 percent). Additional sampling within a class 

would improve the statistical confidence level for that 

class. In this example, the BAER team may decide that 

additional sampling should be done on moderate soil 

burn severity hillslopes to increase confidence that the 

post-fire soil water repellency and infiltration differences 

between north and south aspect slopes is valid. 

The soil water repellency data is specific to a class of 

soil burn severity and slope aspect (M/N; M/S; H/N; or 

H/S as described in table B2), and the assessment from 

the sampled slopes is applied to the unsampled slopes 

of the same soil burn severity and aspect. This can be 

useful in prioritizing areas for post-fire stabilization treatments. In the example (table B3), stabilization treatments 

may not be needed on moderate soil burn severity-north 

aspect slopes (only 35 percent Strong soil water repellency), especially if fallen needle cover (needle cast) 

may provide substantial natural protection from erosion. 

However, stabilization treatments may be necessary on 

the moderate soil burn severity-south aspect slopes (75 

percent Strong soil water repellency) and the high soil 

burn severity slopes (75 and 90 percent Strong soil water 

repellency) in areas where downstream values are at risk 

from increased flooding and sedimentation. 
USDA Forest Service Research Note RMRS-RN-33. 2008 13 Table B3. Example spreadsheet of MDI test data analysis. In the column headers, ID = site identification code; TM = transect mark (0, 10, 30, 60, or 100); ml = mL min–1; and SBS = soil burn severity.
14 USDA Forest Service Research Note RMRS-RN-33. 2008 Table B3 continued—Example spreadsheet of MDI test data analysis. In the column headers, ID = site identification code; TM = transect mark (0, 10, 30, 60, or 100); ml = mL min–1; and SBS = soil burn severity. 
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