Infiltration data from mini disks and bottomless buckets: postfire soil hydrologic and biogeochemical response and recovery in northern California, USA

California’s wildfires are setting historic records for length, size, and severity (Safford and others, 2022, Kreider and others, 2024). These wildfires are altering soil processes like infiltration rates and carbon cycling over increasingly large areas (Staley and others, 2017; Kelly and others, 2020; Baur and others, 2024; Dow and others, 2024). In northern California, it is unclear how wildfire might increase runoff hazards or limit ecosystem recovery. To understand postfire soil response and recovery, we characterized the short and long-term effects of wildfire on soil infiltration and biogeochemical cycles using burned and nearby unburned sites. We seasonally conducted field measurements or collected soils for lab measurements to quantify soil hydrological and biogeochemical recovery for at least 3 years following the fire. Understanding the interaction between rocks, microbes, and water is key to identifying areas of elevated risk postfire and prioritizing postfire mitigation. Here are hydraulic conductivity (Kfs) summary tables for mini disk (MD) and bottomless bucket (BB) measurements, including the time series from the MD tests, conducted at the 41 (31 burned, 10 unburned) soil hydrology monitoring sites across the 2020 LNU Lightning Complex, Walbridge, and Glass Fires and the 2021 Dixie Fire. NOTE: MD measurements from the 2019 Kincade Fire are also included. No BB measurements were collected from the Kincade Fire sites. 

We measured infiltration rates and calculated field-saturated hydraulic conductivity (Kfs) using two infiltration methods to understand water movement through the soil matrix and macropores. We measured infiltration capacity of the soil matrix using mini disk (MD) tension infiltrometers (METER, 2021), which are 90 ml-capacity plastic, 2-chamber cylindrical reservoirs. The water is held under tension connected to a 4.5‐cm‐diameter, 3‐mm‐thick sintered stainless-steel disk through which water flows into the soil. Infiltrating water under tension prevents the filling of the macropores and gives a field-saturated hydraulic conductivity (Kfs) characteristic of the soil matrix only. We cleared the test surface by gently removing any pebbles, clipping vegetation to the surface, and blowing away ash to ensure good contact without disturbing the soil. The device was held on the soil surface while volume readings were typically taken every 30 seconds for 15-16 minutes, or until the reservoir was empty, whichever came first. The MD tension, or suction head, was set at 1 cm for all measurements for consistency and comparability to other studies. For simplicity, we used the general method recommended by the instrument manufacturer (METER, 2021) to calculate Kfs as a function of the volume of water infiltrated as a function of the square root of time, suction head, and soil texture. The suction head and soil texture are included in the A parameter (METER, 2021). If the sorptivity value was negative, which is not valid or realistic, we changed the value to zero and re-fit the curve. The calculated Kfs values, as well as the A parameters and sorptivity (sorp) values, are reported in the 'Kfs-MD_summary.csv' file. The raw volume measurements from each individual test are zipped together by fire. See 'datadictionary_Kfs-MD.txt' for description of the zipped files. NOTE: This method can result in negative Kfs values, which are not valid or realistic, and should be removed from future analyses.
 
We measured the combined infiltration capacity of the soil matrix and macropores using falling-head, single-ring infiltrometers called bottomless buckets (BB). These metal rings, 10 cm in diameter and 13 cm in length, were pressed into the soil during initial site visits. The rings remained in-place for the duration of the study. The method for calculating the field-saturated hydraulic conductivity (Kfs) accounts for both variable falling head and subsurface radial spreading that unavoidably occurs with small ring size (Nimmo et al. 2009). The BB method allows for many measurements either over set time increments (typically 10-60 seconds) or set volumes of water (typically 500 mL). The geometric mean of each incremental water measurement was used to calculate Kfs for each time test, or the total volume and total time were used to calculate Kfs for the set volume test. Field tests were repeated until a steady Kfs was reached, and all Kfs values were averaged together and used as the reported Kfs in the 'Kfs-BB_summary.csv' file.
 
For more information on the site locations and associated hydrological and biogeochemical data, refer to the project landing page: https://www.sciencebase.gov/catalog/item/680fa722d4be022940539f4d

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References:
Baur, M.J., Friend, A.D., and Pellegrini, A.F.A., 2024, Widespread and systematic effects of fire on plant–soil water relations: Nature Geoscience, 17(11), 1115–1120. https://doi.org/10.1038/s41561-024-01563-6.

Dow, H.W., East, A.E., Sankey, J.B., Warrick, J.A., Kostelnik, J., Lindsay, D.N., and Kean, J.W., 2024, Postfire Sediment Mobilization and Its Downstream Implications Across California, 1984–2021: Journal of Geophysical Research: Earth Surface, 129(8). https://doi.org/10.1029/2024JF007725.

Kreider, M.R., Higuera, P.E., Parks, S.A., Rice, W.L., White, N., and Larson, A.J., 2024, Fire suppression makes wildfires more severe and accentuates impacts of climate change and fuel accumulation: Nature Communications,15(1). https://doi.org/10.1038/s41467-024-46702-0.

Kelly, L.T., Giljohann, K.M., Duane, A., Aquilué, N., Archibald, S., Batllori, E., Bennett, A.F., Buckland, S.T., Canelles, Q., Clarke, M.F., Fortin, M.-J., Hermoso, V., Herrando, S., Keane, R.E., and others, 2020, Fire and biodiversity in the Anthropocene: Science, 370(6519). https://doi.org/10.1126/science.abb0355.

Nimmo, J.R., Schmidt, K.M., Perkins, K.S., & Stock, J.D., 2009, Rapid measurement of field- saturated hydraulic conductivity for areal characterization. Vadose Zone Journal, 8(1), 142-149. https://doi.org/10.2136/vzj2007.0159.
 
METER, 2021, Mini Disk Infiltrometer Manual. Pullman, WA: METER Group, Inc. USA. Accessed 27 June 2025: https://publications.metergroup.com/Manuals/20421_Mini_Disk_Manual_Web…

Safford, H.D., Paulson, A.K., Steel, Z.L., Young, D.J.N., and Wayman, R.B., 2022, The 2020 California fire season: A year like no other, a return to the past or a harbinger of the future? Global Ecology and Biogeography, 31(10), 2005–2025. https://doi.org/10.1111/geb.13498.

Staley, D.M., Negri, J.A., Kean, J.W., Laber, J.L., Tillery, A.C., and Youberg, A.M., 2017, Prediction of spatially explicit rainfall intensity–duration thresholds for post-fire debris-flow generation in the western United States: Geomorphology, 278, 149–162. https://doi.org/10.1016/j.geomorph.2016.10.019.