Wildfires can increase soil water repellency (SWR), reducing water infiltration and elevating risks of runoff, erosion, and flooding. This report describes methods and data from a study measuring relative soil water infiltration rates at 10 sites across nine recently burned areas in the Sierra Nevada range of California and Nevada, spanning a chronosequence of fires that occurred between 2015 and 2021. Field measurements, conducted in summer 2022, used a handheld tension infiltrometer to assess relative infiltration at the soil surface and at 3 cm depth along transects located in areas of moderate and high burn severity. In total, 80 individual infiltration measurements were collected per burned area, alongside supporting environmental data including soil moisture, temperature, aspect, slope, and vegetation type. Results show that infiltration rates are consistently lower at depth than at the surface.

Study sites

Nine recently burned areas were selected for analysis. One area from each fire was selected for the study except for the Dixie Fire, where two areas were selected, for a total of 10 study sites. All of the burned areas are in the Sierra Nevada range of Nevada and California, and the environmental variables were kept as constant as possible to allow for comparison among the sample locations. The fires occurred from 2015 to 2021, and all field measurements were completed in the summer of 2022.

Two transect sites were selected for each of the 10 study sites: one in a moderate burn severity area and one in a high burn severity area. These sites were selected using burn severity maps.

Infiltration and soil measurements

For both moderate and high burn severity, a 100 m transect was set up (two transects total for each study site). Transects were located within ~50 m of each other to maintain site similarities. Measurements were conducted at 0 m, 10 m, 30 m, 60 m, and 100 m along each transect (n = 5 for each transect, two transects per burned area). At each measurement location, a handheld tension infiltrometer (Mini Disk Infiltrometer, METER Group, Pullman, WA) was used to measure the relative infiltration rate at the surface and at a 3 cm depth. This method measures the amount of water that leaves the infiltrometer through its porous base and enters the soil over one minute at a specific tension (1 cm). For example, when the infiltrometer is placed on a highly water repellant soil, the water surface tension at the bottom of the infiltrometer is not broken and no water infiltrates the soil. The result is a measurement of the relative infiltration in mL min^-1 (for the example above, the result would be 0 mL min ^-1).

The measurement was repeated four times at the soil surface and four times at the 3 cm depth at each of the five locations along the transect. For consistency, any post-fire ash, organic material, or rocks were removed from the surface before measurements were taken. This amounts to 80 individual relative infiltration rate measurements per burned area. The four relative infiltration measurements were averaged for each location along the transect. The results were then classified as either strong water repellency (0 to < 3 mL min ^-1), weak water repellency (3 to < 8 mL min ^-1), or no water repellency (≥8 mL min ^-1).