Water drop penetration test
Passamani C. 1
1 DISAFA – University of Torino
Last updated: 24/09/2026
Recommended citation:
Passamani C. 2026. Water drop penetration test. In: Pere Casals, Davide Ascoli, Lena Vilà-Vilardell, Imma Oliveras, Madoka Krick (eds.) 2026. Methods handbook for characterizing fire use behavior, assessing fire effects, and monitoring soil and vegetation responses at the stand level. FIRE-ADAPT project (number 101086416), deliverable D1.1
Indicator
Ecosystem responses: Soil responses
Brief introduction
Soil hydrophobicity describes a condition where soil repels, rather than absorbs water, resulting in increased surface runoff. Fire affects soil properties and its hydrological response. Soil hydrophobicity induced by fire is a widely documented phenomenon that depends on the characteristics of both fire behaviour and the soil’s initial properties. The water drop penetration time (WDPT) test is an easy and rapid method to evaluate soil hydrophobicity directly in the field (Tinebra et al., 2019). The WDPT involves placing a water drop on a soil surface and recording the time taken for its complete penetration (Letey et al., 2000). This method is particularly useful for comparing different situations. In particular, it can be used to evaluate the effect of prescribed burning on soil by repeating the test before, soon after, and at various times after burning. Comparing the results allows to assess the effect of fire on water repellency.
Method specifications
Method type
Soft
Hard
Method timing
Before
fire
During
fire
After fire
(short-term)
After fire
(mid-term)
After fire
(long-term)
Field skills
Low
Medium
Hard
Lab skills
Low
Medium
Hard
Material cost
Low
Medium
Hard
Time cost
Low
Medium
Hard
Calculation expertise
Low
Medium
Hard
Equipment
- Pipette
- Pipette tips
- Spatula
- Stopwatch
- Distilled Water
Experimental design
In each plot, identify and locate systematically an adequate number of sampling microsites along a transect (6-8 microsites suggested by Hubbert et al., 2006). It is recommended to avoid steep terrains, non-representative situations (e.g. rocks, animal burrows), and to sample on dry soils (better if at least 5–6 days have passed since the last rainfall event). At each microsite, visualize a square grid of 4 x 4 cm2, with each point of the grid at a mutual distance of 1 cm, resulting in a total of 16 points. Each microsite can be marked adequately, or coordinates recorded with a sub-metric GPS for future samplings.
Methodology
- At each sampling microsite, gently expose the mineral soil surface with a spatula.
- For each point of the grid, place a droplet, applied with a micropipette from an approximately distance of 1 cm. It is better to apply a droplet with a standard volume, in order to assure comparable results (Tinebra et al. (2019) suggest a standard volume of 50 µL).
- Start timing with the stopwatch when the droplet first touches the soil surface.
- Stop the time once the droplet infiltrated into the soil, and record the time it has taken to infiltrate. If the infiltration time exceeds 36000 sec (Tinebra et al.2019)1, stop the time anyway.
- Write down the time that the drop has taken to infiltrate the soil; if > 36000 sec, write down > 36000 sec.
- For each droplet, based on the measured time, assign a class of hydrophobicity, according to Bisdom et al. (1993) classification2:
| Class | WDPT (s) |
|---|---|
| Wettbale | <5 |
| Very low repellency | 5 - 60 |
| Low repellency | 60 - 600 |
| Moderate repellency | 600 - 3600 |
| Extreme repellency | >3600 |
- Assess the repellency class of a sampling microsites using the modal class of the droplets applied at each site.
1 Different thresholds can be used, depending on the soil type, the required accuracy and the disposable amount of time. For low severity prescribed burning in South Californian chaparral, Hubbert et al. (2006) adopted 180s as maximum time. If a different maximum time is used, the repellency classification must be adequately adapted.
2 These classification times are essentially arbitrary; several threshold times to classify soil repellency were proposed from different authors. However, they are useful in summarizing and comparing different levels of hydrophobicity (for other classification see Doerr et al. 1998). Hubbert et al. (2006), adopted the following classification, adapted for the maximum recorded time of 180 sec.
i) Not repellent: < 1 sec; ii) Very low repellency: 1 – 5 sec; iii) Low repellency: 5 – 30 sec; iv) Moderate repellency: 30 -180 sec; v) Extreme repellency > 180 sec.
Relation with other methods
This method can be combined with other methods to assess the burn severity, or to characterize the soil properties.
Data templates
NA
Further information
Bisdom, E.B.A., Dekker, L.W., Schoute, J.F.T., 1993. Water repellency of sieve fractions from sandy soils and relationships with organic material and soil structure. Geoderma 56, 105–118.
Doerr, S.H. 1998. On standardizing the ‘Water Drop Penetration Time’ and the ‘Molarity of an Ethanol Droplet’ techniques to classify soil hydrophobicity: A case study using medium textured soils. Earth Surface Processes and Landforms, 23, 663-668.
Hubbert, K.R., Preisler, H.K., Wohlgemuth, P.M., Graham, R.C., Narog, M.G., 2006. Prescribed burning effects on soil physical properties and soil water repellency in a steep chaparral watershed, southern California USA. Geoderma, Volume 130, Issues 3–4, 284-298.
Letey, J. 1969. Measurement of Contact Angle, Water Drop Penetration Time, and Critical Surface Tension. Proceedings of a Symposium on Water Repellant Soils, Riverside, 6-10 May 1968, 43-47.
Tinebra, I., Alagna, V., Iovino, M., Bagarello, V., 2019. Comparing different application procedures of the water drop penetration time test to assess soil water repellency in a fire affected Sicilian area, CATENA, Volume 177, 41-48.