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Dry-season soil water repellency affects Tahoe Basin infiltration rates

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Authors

Erin C. Rice, UC Davis
Mark E. Grismer, UC Davis

Publication Information

California Agriculture 64(3):141-148. https://doi.org/10.3733/ca.v064n03p141

Published July 01, 2010

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Author Affiliations show

Abstract

Lake Tahoe's declining clarity makes the identification of runoff and erosion sources and evaluation of control measures vitally important. We treated relatively undisturbed, native, forested sites of 10% to 15% slope with surfactant and used a rainfall simulator to investigate the effects of repellency. We compared infiltration measurements made by the simulator and a mini-disk infiltrometer (MDI). Runoff was produced by all plots with untreated water, but only two of 12 plots with surfactant. At volcanic soil sites, infiltration rates using surfactant exceeded those with water by only 20% when there was little litter cover, but with substantial litter the infiltration rates increased threefold. Similarly, at the granitic soil sites surfactant-enhanced infiltration rates were four times greater with scant litter, and eight times greater with substantial litter cover. Postsimulation soil moisture content and wetting depths were greater with the surfactant treatment. Excavations under surfactant treatments revealed that discontinuities in the soil's hydrophobic organic layer resulted in preferential infiltration zones in the mineral soils below.

Full text

Lake Tahoe is a spectacular, deep mountain lake of exceptional clarity, historically maintained due to low nutrient (ultraoligotrophic) conditions. Since continuous water-quality monitoring began in the early 1960s, algal growth has increased by more than 5% per year, with a corresponding 1-foot-per-year decline in water clarity.

Lake Tahoe's famed clarity is declining due to increases in sediments and nutrients, which coincide with nearly a half-century of urban growth in the region. Soil repellency may be a factor contributing to erosion and runoff into the lake.

Lake Tahoe's famed clarity is declining due to increases in sediments and nutrients, which coincide with nearly a half-century of urban growth in the region. Soil repellency may be a factor contributing to erosion and runoff into the lake.

The consensus among researchers is that: (1) the documented decline in clarity coincides with more than 40 years of growth in urban areas (now 10% of total land area), which contribute 72% of fine particles to the lake (TERC 2008); (2) lake-floor sediment accumulations correspond with periods of human activity (Heyvaert 1998); (3) annual phosphorus loading to the lake depends directly on sediment concentrations (Hatch 1997); and (4) fine (1 to 8 microns [???]) particles diminish the lake's clarity by transporting adsorbed nutrients and scattering light while in suspension (Swift et al. 2006). Understanding the sources, transport and means of controlling fine-particle delivery is essential to stem the water quality decline.

Infiltration, runoff and erosion near Tahoe have been studied extensively, yet knowledge of repellent (hydrophobic) soil conditions often remains anecdotal or oversimplified. Soil water repellency can be induced by fire (Doerr et al. 2010) and also occurs during late-season dry conditions. The commonly acknowledged paradigm that hydrophobicity is responsible for greater runoff and erosion after fires (Robichaud 2000), while accurate in some locales, has not always been verified (Doerr and Moody 2004).

Larsen et al. (2009) noted that high-severity fires alter the vegetative cover and characteristics of mineral soil, making it difficult to separate the effects of fire-induced soil water repellency from other changes in soil characteristics and surface cover. In the Western states, Pierson et al. (2008) found that repellency was greatest on unburned slopes and that dry-season variability had a more substantial impact than fire. Postfire hydrologic responses were not attributed to intensified repellency, but rather to the increased connectivity of runoff sources following the removal of vegetation and soil cover. Seasonal, non-fire-induced repellency has been considered a function of soil moisture, but its recurrence following wet periods appears to depend not only on soil drying, but also on input or the redistribution of hydrophobic substances (Doerr and Thomas 2000).

Researchers studied soil repellency with and without surfactant treatments at four sites in the Lake Tahoe Basin.

Researchers studied soil repellency with and without surfactant treatments at four sites in the Lake Tahoe Basin.

The realization that few studies actually isolated the hydrologic effects of repellency prompted new research directions. Leighton-Boyce et al. (2007) modified earlier methods developed in Southern California, in which surfactants were applied during rainstorms on burned slopes. Surfactants may be used as wetting agents to induce infiltration and mimic normal infiltration conditions. The 2007 study in Portugal used surfactant-treated water in a rainfall simulator to isolate hydrophobic effects.

We investigated plot-scale hydrologic responses — including infiltration, runoff and sediment yield — due to seasonal hydrophobicity at four relatively undisturbed, native, forested sites in the Tahoe Basin (fig. 1). (The entire basin was logged in the 1850s, and partially again in the 1920s.) We present baseline hydrologic responses to repellency that may be used for comparison to similar data gathered at disturbed sites targeted for erosion-control measures. Data from two infiltration measurement devices, a rainfall simulator and the more readily deployed mini-disk infiltrometer (MDI), was also compared and evaluated.

Tahoe Basin site map. Source: relief map, mytopo.com.

Fig. 1. Tahoe Basin site map. Source: relief map, mytopo.com.

Lake Tahoe study areas

The Truckee and Blackwood Canyon sites had volcanic soil, and Bliss State Park and Meyers road cut were granitic. The sites were similar in slope (10% to 15%), and rainfall simulations had been conducted previously at all four under similar conditions to those considered here (Grismer et al. 2009) (table 1).

Surfactants reduce water surface tension, are commonly used (e.g., detergents) and generally nontoxic. We modified methods presented by Leighton-Boyce et al. (2007), and used Pro-Spreader Activator surfactant (Target Specialty Products, Fresno, Calif.) mixed with available groundwater to a concentration of 0.25%, the upper limit of the manufacturer's recommendation. Initial field tests showed that this concentration was suitable to induce infiltration through repellent soils in the Tahoe Basin.

After plots were established at each site, we measured initial soil moisture (Campbell Scientific TDR moisture meter) and soil strength (cone penetrometer depth to refusal, 350 pounds per square inch).

Following the artificial rainfall test to determine infiltration and runoff rates, moisture and density (using cone penetrometer depths as an index) were again measured along with litter depth and composition, which was visually estimated. Measurements were taken with an MDI in areas adjacent to the rainfall-simulator plot frames at each site. Soils were hand-excavated to 10 inches, to observe wetting patterns and depths.

The rainfall-simulator tests were also slightly modified from the description by Grismer and Hogan (2004). Without foreknowledge of the treatment to be applied, 6.9-square-foot (0.64-square-meter) plot frames were installed, and simulated rainfall was applied at 4.7 inches per hour (120 millimeters per hour) for the duration necessary to produce steady runoff and fill sequential 6-ounce (175-milliliter) sample bottles. This sometimes took more than 70 minutes.

Rainfall simulations. Infiltration rates were calculated as the difference between the applied rainfall and runoff rates, and were assumed to be greater than the application rate when no runoff occurred. Three replicates of each treatment were conducted at each site for a total of 24 rainfall simulations. Following a series of surfactant treatment simulations, all equipment was cleaned with a mild bleach solution prior to the untreated-water simulations. Collected runoff samples were filtered in the lab (Whatman #541 and 0.45-micron filters). The sediment samples were oven-dried at 221°F (105°C) and then combusted at 806°F (430°C) to determine organic matter content (Grismer et al. 2008).

MDI. An MDI (Decagon Devices, Pullman, Wash.) was also used at each site to determine infiltration rates. These devices have been deployed by the U.S. Forest Service to evaluate hydrophobicity. Water held in a chamber resembling a graduated cylinder infiltrates when suction is sufficient to break surface tension across a porous disk at the base (Robichaud et al. 2008). The constant-head (water level) adjustment was set at 0.79 inch (20 millimeters), and measurements were taken for 1 minute with the difference in volume used to calculate the infiltration rate. At each site surfactant and untreated water were each replicated 10 times.

Site information and soil classifi cations

TABLE 1. Site information and soil classifi cations

Smaller sequential samples were also collected for texture analysis. The Coulter LS-230 particle-size analyzer uses laser-light scattering to produce particle-size distributions by volume. A revised version of the protocol developed by Eshel et al. (2004) was used to process the runoff samples. In the field, we collected 48 runoff samples from both volcanic and granitic sites and made composites for each site as needed for the analyses.

Statistics. Factorial analyses were conducted to test for significant interactions between site and treatment effects for rainfall simulation, MDI and particle-size distribution. For the rainfall simulation results this interaction was nonsignificant, providing the rationale to use a randomized design. No transforms were required to achieve normality. Infiltration rates by site and treatment were separated using Tukey's HSD test. The Spearman correlation was used to test for the correlation between rainfall simulator and MDI infiltration results. MANOVA repeated measures analysis was used to detect significant changes in some soil conditions following the rainfall simulations.

Analytical findings

We considered the results in terms of soil, runoff, infiltration and particle-size distribution. The soil section included measurements of several soil properties, which were repeated to test for changes before and after rainfall simulations. Runoff timing, sediment yield and organic matter content were contrasted between the two treatments (surfactant and untreated water). Infiltration rates were compared between and within (by treatment) each site for both methods. The MDI and rainfall-simulator data were also compared. Finally, particle-size distribution analysis revealed differences by soil type and any correlations between particle-size fractions and infiltration.

A rainfall simulator, above, at Bliss State Park was used at different rates to generate runoff and measure its sediment content.

A rainfall simulator, above, at Bliss State Park was used at different rates to generate runoff and measure its sediment content.

Pre- and post-rainfall simulation site conditions

TABLE 2. Pre- and post-rainfall simulation site conditions

Soils. Soil physical properties were measured before and after each treatment (table 2). Mulch depth did not differ significantly by site or treatment. Cone penetrometer depths, used as an index of soil strength, usually slightly increased following treatment, presumably as a result of increased soil moisture. The within-subjects MANOVA test comparing the results of depth-to-refusal measurements before and after treatments resulted in nonsignificant differences between means — thereby removing treatment as a variable affecting soil strength.

Averaged initial soil-moisture levels ranged between 1.6% and 3.0%. At all sites except Blackwood Canyon (volcanic soil), surfactant treatments resulted in higher final soil-moisture levels than untreated water. This difference was most pronounced on granitic soils, where postsimulation soil moisture was more than four times higher with surfactant than with water at Bliss, and three times higher at Meyers. The depth to continuous wetting differed significantly by soil type (P = 0.0355), treatment (P < 0.0001) and soil type/ treatment interaction (P = 0.0078).

Rainfall-simulation runoff results

TABLE 3. Rainfall-simulation runoff results

Mini-disk infiltrometer (MDI) and rainfall-simulator infiltration rates

TABLE 4. Mini-disk infiltrometer (MDI) and rainfall-simulator infiltration rates

At every site, the surfactant caused deeper wetting than untreated water. The untreated water was more effective in wetting volcanic soils than granitic soils, which were nearly completely resistant to wetting (these soils had significantly less wetting with untreated water than the other soil type/ treatment combinations). When surfactant was used, the virtually unwettable granitic soils were wetted to a depth of approximately 6 inches (15 centimeters). While dry “pockets” or layers above wetted soil were observed at all sites, preferential flow was most obvious at Meyers with the surfactant treatment.

Runoff. The effectiveness of surfactant treatment on runoff rates was obvious in the field and samples collected during runoff simulation (table 3); runoff was produced by all 12 untreated-water plots, but only two of 12 plots that received surfactant. The lack of runoff data from many surfactant plots made the statistical analysis of some variables difficult or impossible, but several comparisons are worth noting.

While the granitic soils produced no runoff when surfactant was used, each of the volcanic soils produced runoff from one surfactant treatment plot. Though runoff occurred from these two plots with surfactant treatment, the time to runoff was different. At Truckee, the single runoff-producing surfactant plot took 16 times longer to run off than the average time for the untreated-water plots. At Blackwood Canyon, the surfactant required about four times longer to produce runoff than the average from the untreated-water plots. Sediment yield and concentration was highest at Bliss State Park, followed by the Blackwood Canyon, Truckee and Meyers road cut sites. Comparison of similar soil types showed that Blackwood Canyon produced about four times as much sediment as Truckee, while runoff from Bliss contained more than seven times as much sediment as Meyers. Runoff sediment organic-matter fractions were highest at Meyers (65%), followed by Blackwood Canyon (44%) then Bliss and Truckee (35%).

Infiltration. Rainfall-simulator-determined infiltration rates differed significantly by treatment at the Bliss site only (table 4). Factorial analysis revealed a significant interaction between site and treatment for the MDI results; treatment had different effects depending on the site. Additional analyses indicated that there was also an interaction between treatment and soil type, suggesting that whatever controlled the treatment effect at different sites was associated with soil type.

This was also confirmed by a similar treatment effect at the granitic sites, although Truckee and Blackwood differed from one another. Water treated with surfactant infiltrated much more efficiently than untreated water at all sites except Blackwood (P = 0.2747), where the infiltration rate with surfactant exceeded that for water by only 20%. The surfactant rate was greater than the untreated-water rate by a factor of about three at Truckee (P = 0.0029), four at Bliss (P = 0.0003) and eight at Meyers (P < 0.0001). The greatest infiltration rate was found at Blackwood Canyon using untreated water, but that site had the lowest rate with surfactant. The untreated-water infiltration rate at Blackwood was about twice that of Truckee or Bliss, and five times higher than at Meyers. Surfactant infiltration rates at Bliss and Truckee, which were nearly equal, were 25% higher than those at Meyers and 60% higher than Blackwood Canyon. MDI infiltration rates were much greater than those from the rainfall simulator, though they were significantly correlated (Spearman R = 0.83).

Runoff sample sediment particle-size distributions for Tahoe Basin soils

TABLE 5. Runoff sample sediment particle-size distributions for Tahoe Basin soils

At the Meyers road cut site, a soil excavation shows water infiltrating the soil in “fingers” of flow.

At the Meyers road cut site, a soil excavation shows water infiltrating the soil in “fingers” of flow.

At the Truckee site, dry patches or layers of organic material remained above the wetted mineral soil.

At the Truckee site, dry patches or layers of organic material remained above the wetted mineral soil.

Particle-size distributions. As found by Grismer et al. (2008), volcanic soils were much finer than granitic soils at each particle-size percentile (D10, D25, D50, D75, D90), and particle sizes differed significantly by soil type (table 5). Volcanic particles were typically about one-fourth the size of granitic particles. Ten percent of particles occurring in runoff from volcanic soils were less than 8 ???, a size fraction considered detrimental to lake clarity (Swift et al. 2006). The relationship between particle-size distribution and infiltration rate appeared to be nonlinear, making the Spearman correlation an appropriate test. All particle sizes were strongly, negatively correlated with infiltration rates. For the rainfall-simulator-based infiltration rates, Spearman correlations for the D10, D25, D50, D75 and D90 particle sizes were R = −0.86, −0.91, −0.83, −0.89 and −0.69, respectively; similarly, for the MDI-based infiltration rates, the Spearman correlations were R = −0.80, −0.70, −0.74, −0.68 and −0.76, respectively.

Surfactants and repellency

Differences in infiltration rates due to the surfactant treatment were unmistakable, as rates always increased — by a statistically significant margin at one site for the rainfall simulators, and at three of four sites for the MDIs. Increased infiltration rates with surfactants demonstrated the importance of soil hydrophobicity to possible runoff and erosion, and, if surfactant is a good model of wettable conditions, that repellency has a substantial effect on infiltration rates into mineral soil. However, the infiltration rates found with the MDI remained very high with untreated water, suggesting that the persistence of repellency in mineral soil upon contact with water is minimal. Much lower infiltration rates resulted from the rainfall simulators when native covers were maintained. Therefore, surfactant efficacy and the actual magnitude of the infiltration rate depended to a large degree on the soil cover conditions.

Surface litter. Surface litter layers were an important factor affecting wetting patterns following rainfall simulation. Native litter cover was most substantial at Truckee and Meyers. Beneath a layer of identifiable pine needle mulch was a layer of decomposed organic material (hemic/sapric < 16% plant material still discernible) with a high degree of fungal activity mats (fungal mycelia with organic matter). Excavations following rainfall simulation indicated that this layer was different from the pine needle mulch because it was neither a storage zone delaying runoff nor a structural barrier encouraging lateral rather than downward movement. The highly decomposed organic layer was strongly and persistently hydrophobic, restricting infiltration into the mineral soil below. Discontinuities in this layer were responsible for preferential flow and wetting.

These wetting patterns were most obvious at the sites with the most litter, Truckee and Meyers. “Fingered” flow was most evident in the Meyers surfactant plots, while at Truckee there was considerable wetting below a large, dry, mineral layer. In the finer-textured Truckee soil, it appeared that runoff had preferentially infiltrated several inches into the mineral profile and then began to wet upward via capillary action. Sites with less litter, Blackwood and Bliss, had correspondingly less developed or nonexistent decomposed organic layers. Infiltration was not as concentrated, and preferential zones were not as obvious, although upon excavation it was apparent that dry patches or layers of organic material remained above the wetted mineral soil.

Meeuwig (1971) originally underscored the litter layer's importance as an infiltration-limiting factor, linking eight distinct wetting patterns and corresponding infiltration curves on bare and covered sites northeast of Lake Tahoe. At forested sites, the hydrologic effects of mineral-soil repellency are at some level subsumed by those of the partially decomposed organic layer. The differences in wetting of mineral soil between treatments were most pronounced on coarse, granitic soils, indicating that hydrophobicity plays a more important role with these soils compared to volcanic soils. Granitic soils exhibited almost no wetting by untreated water, but about 6 inches (15 centimeters) of continuous and more than 10 inches (25 centimeters) of intermittent wetting with surfactant.

The texture and moisture content of soils at each site did not necessarily correlate with the degree of repellency found in the study. For example, at Blackwood Canyon infi ltration rates with untreated water in the finer-textured volcanic soils were twice as high as any other site, and the response to surfactant was relatively subdued.

The texture and moisture content of soils at each site did not necessarily correlate with the degree of repellency found in the study. For example, at Blackwood Canyon infi ltration rates with untreated water in the finer-textured volcanic soils were twice as high as any other site, and the response to surfactant was relatively subdued.

Soil moisture and texture. The changes in soil moisture content also revealed the impact of the surfactant treatment on granitic soils. Following water treatment moisture contents doubled, but with the surfactant treatments they increased sixfold. The effects of soil texture on the establishment and degree of repellent conditions are complex. Coarse-textured soils have been associated with repellency because coarse particles have less surface area per unit volume than finer particles, making them more susceptible to coating by a limited supply of hydrophobic substances (Crockford et al. 1991). However, repellency is not exclusive to coarse soils; if fine-textured (25% to more than 40% clay) soils form aggregates (presumably with greater organic-matter content) they, too, are susceptible to the development of repellency conditions (Wallis et al. 1991). In some cases, very fine fractions have the highest degree of repellency (de Jonge et al. 1999).

Texture alone does not imply a degree of repellency because aggregation and the supply of hydrophobic material are controlled by many other factors; contradictions in the relationship between soil texture and repellency may also be due to confusion between the effects of texture and structure (Fox et al. 2007). Further confusion results because fine fractions are not necessarily associated with fine textures, nor are coarse fractions necessarily associated with coarse textures; the effects of soil aggregate formation must be considered. Fine-textured soils have exhibited the highest degree of repellency, while coarse soils appear to be more susceptible to developing fire-induced or other repellency (de Jonge et al. 1999; Doerr et al. 2000).

Though the Blackwood site comprised finer-textured volcanic soils with scant litter cover, its untreated-water infiltration rates (MDI) were nearly twice as high as those of any other site, and the response to surfactant was relatively subdued. The limited response to surfactant suggests that the litter at Blackwood was neither physically inhibiting infiltration nor providing hydrophobic substances adequate to coat the relatively fine mineral particles.

Untreated plots. All of our untreated-water plots produced runoff. At the volcanic soil sites, infiltration rates were similar to previous studies and ranged from 3.4 to 4.7 inches (86 to 119 millimeters) per hour at Truckee and 4.0 to 4.7 inches (103 to 119 millimeters) per hour at Blackwood. The average sediment concentrations were 0.22 gram per liter at Truckee and 0.85 gram per liter at Blackwood. In previous studies conducted on plots with pine needle cover at Bliss State Park, and at Rubicon on granitic Meeks series soils (about 60% slopes), infiltration rates were about 2 inches (51 millimete

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References

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Grismer ME, Ellis AL, Fristensky A. Runoff sediment particle sizes associated with soil erosion in the Lake Tahoe Basin, USA. Land Degrad Dev. 2008. 19(3):331-50.

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Grismer ME, Schnurrenberger C, Arst R, Hogan MP. Integrated monitoring and assessment of soil restoration treatments in the Lake Tahoe Basin. Env Monit Assess. 2009. 150:365-83.

Hatch LK. The Generation, Transport and Fate of Phosphorus in the Lake Tahoe Ecosystem. 1997. UC Davis: Doctoral dissertation. 212p.

Heyvaert AC. Biogeochemistry and Paleolimnology of Sediments from Lake Tahoe, California-Nevada. 1998. UC Davis: Doctoral dissertation, Ecology Graduate Group. p.127.

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Dry-season soil water repellency affects Tahoe Basin infiltration rates

Erin C. Rice, Mark E. Grismer
Webmaster Email: wsuckow@ucanr.edu

Dry-season soil water repellency affects Tahoe Basin infiltration rates

Share using any of the popular social networks Share by sending an email Print article
Share using any of the popular social networks Share by sending an email Print article

Authors

Erin C. Rice, UC Davis
Mark E. Grismer, UC Davis

Publication Information

California Agriculture 64(3):141-148. https://doi.org/10.3733/ca.v064n03p141

Published July 01, 2010

PDF  |  Citation  |  Permissions

Author Affiliations show

Abstract

Lake Tahoe's declining clarity makes the identification of runoff and erosion sources and evaluation of control measures vitally important. We treated relatively undisturbed, native, forested sites of 10% to 15% slope with surfactant and used a rainfall simulator to investigate the effects of repellency. We compared infiltration measurements made by the simulator and a mini-disk infiltrometer (MDI). Runoff was produced by all plots with untreated water, but only two of 12 plots with surfactant. At volcanic soil sites, infiltration rates using surfactant exceeded those with water by only 20% when there was little litter cover, but with substantial litter the infiltration rates increased threefold. Similarly, at the granitic soil sites surfactant-enhanced infiltration rates were four times greater with scant litter, and eight times greater with substantial litter cover. Postsimulation soil moisture content and wetting depths were greater with the surfactant treatment. Excavations under surfactant treatments revealed that discontinuities in the soil's hydrophobic organic layer resulted in preferential infiltration zones in the mineral soils below.

Full text

Lake Tahoe is a spectacular, deep mountain lake of exceptional clarity, historically maintained due to low nutrient (ultraoligotrophic) conditions. Since continuous water-quality monitoring began in the early 1960s, algal growth has increased by more than 5% per year, with a corresponding 1-foot-per-year decline in water clarity.

Lake Tahoe's famed clarity is declining due to increases in sediments and nutrients, which coincide with nearly a half-century of urban growth in the region. Soil repellency may be a factor contributing to erosion and runoff into the lake.

Lake Tahoe's famed clarity is declining due to increases in sediments and nutrients, which coincide with nearly a half-century of urban growth in the region. Soil repellency may be a factor contributing to erosion and runoff into the lake.

The consensus among researchers is that: (1) the documented decline in clarity coincides with more than 40 years of growth in urban areas (now 10% of total land area), which contribute 72% of fine particles to the lake (TERC 2008); (2) lake-floor sediment accumulations correspond with periods of human activity (Heyvaert 1998); (3) annual phosphorus loading to the lake depends directly on sediment concentrations (Hatch 1997); and (4) fine (1 to 8 microns [???]) particles diminish the lake's clarity by transporting adsorbed nutrients and scattering light while in suspension (Swift et al. 2006). Understanding the sources, transport and means of controlling fine-particle delivery is essential to stem the water quality decline.

Infiltration, runoff and erosion near Tahoe have been studied extensively, yet knowledge of repellent (hydrophobic) soil conditions often remains anecdotal or oversimplified. Soil water repellency can be induced by fire (Doerr et al. 2010) and also occurs during late-season dry conditions. The commonly acknowledged paradigm that hydrophobicity is responsible for greater runoff and erosion after fires (Robichaud 2000), while accurate in some locales, has not always been verified (Doerr and Moody 2004).

Larsen et al. (2009) noted that high-severity fires alter the vegetative cover and characteristics of mineral soil, making it difficult to separate the effects of fire-induced soil water repellency from other changes in soil characteristics and surface cover. In the Western states, Pierson et al. (2008) found that repellency was greatest on unburned slopes and that dry-season variability had a more substantial impact than fire. Postfire hydrologic responses were not attributed to intensified repellency, but rather to the increased connectivity of runoff sources following the removal of vegetation and soil cover. Seasonal, non-fire-induced repellency has been considered a function of soil moisture, but its recurrence following wet periods appears to depend not only on soil drying, but also on input or the redistribution of hydrophobic substances (Doerr and Thomas 2000).

Researchers studied soil repellency with and without surfactant treatments at four sites in the Lake Tahoe Basin.

Researchers studied soil repellency with and without surfactant treatments at four sites in the Lake Tahoe Basin.

The realization that few studies actually isolated the hydrologic effects of repellency prompted new research directions. Leighton-Boyce et al. (2007) modified earlier methods developed in Southern California, in which surfactants were applied during rainstorms on burned slopes. Surfactants may be used as wetting agents to induce infiltration and mimic normal infiltration conditions. The 2007 study in Portugal used surfactant-treated water in a rainfall simulator to isolate hydrophobic effects.

We investigated plot-scale hydrologic responses — including infiltration, runoff and sediment yield — due to seasonal hydrophobicity at four relatively undisturbed, native, forested sites in the Tahoe Basin (fig. 1). (The entire basin was logged in the 1850s, and partially again in the 1920s.) We present baseline hydrologic responses to repellency that may be used for comparison to similar data gathered at disturbed sites targeted for erosion-control measures. Data from two infiltration measurement devices, a rainfall simulator and the more readily deployed mini-disk infiltrometer (MDI), was also compared and evaluated.

Tahoe Basin site map. Source: relief map, mytopo.com.

Fig. 1. Tahoe Basin site map. Source: relief map, mytopo.com.

Lake Tahoe study areas

The Truckee and Blackwood Canyon sites had volcanic soil, and Bliss State Park and Meyers road cut were granitic. The sites were similar in slope (10% to 15%), and rainfall simulations had been conducted previously at all four under similar conditions to those considered here (Grismer et al. 2009) (table 1).

Surfactants reduce water surface tension, are commonly used (e.g., detergents) and generally nontoxic. We modified methods presented by Leighton-Boyce et al. (2007), and used Pro-Spreader Activator surfactant (Target Specialty Products, Fresno, Calif.) mixed with available groundwater to a concentration of 0.25%, the upper limit of the manufacturer's recommendation. Initial field tests showed that this concentration was suitable to induce infiltration through repellent soils in the Tahoe Basin.

After plots were established at each site, we measured initial soil moisture (Campbell Scientific TDR moisture meter) and soil strength (cone penetrometer depth to refusal, 350 pounds per square inch).

Following the artificial rainfall test to determine infiltration and runoff rates, moisture and density (using cone penetrometer depths as an index) were again measured along with litter depth and composition, which was visually estimated. Measurements were taken with an MDI in areas adjacent to the rainfall-simulator plot frames at each site. Soils were hand-excavated to 10 inches, to observe wetting patterns and depths.

The rainfall-simulator tests were also slightly modified from the description by Grismer and Hogan (2004). Without foreknowledge of the treatment to be applied, 6.9-square-foot (0.64-square-meter) plot frames were installed, and simulated rainfall was applied at 4.7 inches per hour (120 millimeters per hour) for the duration necessary to produce steady runoff and fill sequential 6-ounce (175-milliliter) sample bottles. This sometimes took more than 70 minutes.

Rainfall simulations. Infiltration rates were calculated as the difference between the applied rainfall and runoff rates, and were assumed to be greater than the application rate when no runoff occurred. Three replicates of each treatment were conducted at each site for a total of 24 rainfall simulations. Following a series of surfactant treatment simulations, all equipment was cleaned with a mild bleach solution prior to the untreated-water simulations. Collected runoff samples were filtered in the lab (Whatman #541 and 0.45-micron filters). The sediment samples were oven-dried at 221°F (105°C) and then combusted at 806°F (430°C) to determine organic matter content (Grismer et al. 2008).

MDI. An MDI (Decagon Devices, Pullman, Wash.) was also used at each site to determine infiltration rates. These devices have been deployed by the U.S. Forest Service to evaluate hydrophobicity. Water held in a chamber resembling a graduated cylinder infiltrates when suction is sufficient to break surface tension across a porous disk at the base (Robichaud et al. 2008). The constant-head (water level) adjustment was set at 0.79 inch (20 millimeters), and measurements were taken for 1 minute with the difference in volume used to calculate the infiltration rate. At each site surfactant and untreated water were each replicated 10 times.

Site information and soil classifi cations

TABLE 1. Site information and soil classifi cations

Smaller sequential samples were also collected for texture analysis. The Coulter LS-230 particle-size analyzer uses laser-light scattering to produce particle-size distributions by volume. A revised version of the protocol developed by Eshel et al. (2004) was used to process the runoff samples. In the field, we collected 48 runoff samples from both volcanic and granitic sites and made composites for each site as needed for the analyses.

Statistics. Factorial analyses were conducted to test for significant interactions between site and treatment effects for rainfall simulation, MDI and particle-size distribution. For the rainfall simulation results this interaction was nonsignificant, providing the rationale to use a randomized design. No transforms were required to achieve normality. Infiltration rates by site and treatment were separated using Tukey's HSD test. The Spearman correlation was used to test for the correlation between rainfall simulator and MDI infiltration results. MANOVA repeated measures analysis was used to detect significant changes in some soil conditions following the rainfall simulations.

Analytical findings

We considered the results in terms of soil, runoff, infiltration and particle-size distribution. The soil section included measurements of several soil properties, which were repeated to test for changes before and after rainfall simulations. Runoff timing, sediment yield and organic matter content were contrasted between the two treatments (surfactant and untreated water). Infiltration rates were compared between and within (by treatment) each site for both methods. The MDI and rainfall-simulator data were also compared. Finally, particle-size distribution analysis revealed differences by soil type and any correlations between particle-size fractions and infiltration.

A rainfall simulator, above, at Bliss State Park was used at different rates to generate runoff and measure its sediment content.

A rainfall simulator, above, at Bliss State Park was used at different rates to generate runoff and measure its sediment content.

Pre- and post-rainfall simulation site conditions

TABLE 2. Pre- and post-rainfall simulation site conditions

Soils. Soil physical properties were measured before and after each treatment (table 2). Mulch depth did not differ significantly by site or treatment. Cone penetrometer depths, used as an index of soil strength, usually slightly increased following treatment, presumably as a result of increased soil moisture. The within-subjects MANOVA test comparing the results of depth-to-refusal measurements before and after treatments resulted in nonsignificant differences between means — thereby removing treatment as a variable affecting soil strength.

Averaged initial soil-moisture levels ranged between 1.6% and 3.0%. At all sites except Blackwood Canyon (volcanic soil), surfactant treatments resulted in higher final soil-moisture levels than untreated water. This difference was most pronounced on granitic soils, where postsimulation soil moisture was more than four times higher with surfactant than with water at Bliss, and three times higher at Meyers. The depth to continuous wetting differed significantly by soil type (P = 0.0355), treatment (P < 0.0001) and soil type/ treatment interaction (P = 0.0078).

Rainfall-simulation runoff results

TABLE 3. Rainfall-simulation runoff results

Mini-disk infiltrometer (MDI) and rainfall-simulator infiltration rates

TABLE 4. Mini-disk infiltrometer (MDI) and rainfall-simulator infiltration rates

At every site, the surfactant caused deeper wetting than untreated water. The untreated water was more effective in wetting volcanic soils than granitic soils, which were nearly completely resistant to wetting (these soils had significantly less wetting with untreated water than the other soil type/ treatment combinations). When surfactant was used, the virtually unwettable granitic soils were wetted to a depth of approximately 6 inches (15 centimeters). While dry “pockets” or layers above wetted soil were observed at all sites, preferential flow was most obvious at Meyers with the surfactant treatment.

Runoff. The effectiveness of surfactant treatment on runoff rates was obvious in the field and samples collected during runoff simulation (table 3); runoff was produced by all 12 untreated-water plots, but only two of 12 plots that received surfactant. The lack of runoff data from many surfactant plots made the statistical analysis of some variables difficult or impossible, but several comparisons are worth noting.

While the granitic soils produced no runoff when surfactant was used, each of the volcanic soils produced runoff from one surfactant treatment plot. Though runoff occurred from these two plots with surfactant treatment, the time to runoff was different. At Truckee, the single runoff-producing surfactant plot took 16 times longer to run off than the average time for the untreated-water plots. At Blackwood Canyon, the surfactant required about four times longer to produce runoff than the average from the untreated-water plots. Sediment yield and concentration was highest at Bliss State Park, followed by the Blackwood Canyon, Truckee and Meyers road cut sites. Comparison of similar soil types showed that Blackwood Canyon produced about four times as much sediment as Truckee, while runoff from Bliss contained more than seven times as much sediment as Meyers. Runoff sediment organic-matter fractions were highest at Meyers (65%), followed by Blackwood Canyon (44%) then Bliss and Truckee (35%).

Infiltration. Rainfall-simulator-determined infiltration rates differed significantly by treatment at the Bliss site only (table 4). Factorial analysis revealed a significant interaction between site and treatment for the MDI results; treatment had different effects depending on the site. Additional analyses indicated that there was also an interaction between treatment and soil type, suggesting that whatever controlled the treatment effect at different sites was associated with soil type.

This was also confirmed by a similar treatment effect at the granitic sites, although Truckee and Blackwood differed from one another. Water treated with surfactant infiltrated much more efficiently than untreated water at all sites except Blackwood (P = 0.2747), where the infiltration rate with surfactant exceeded that for water by only 20%. The surfactant rate was greater than the untreated-water rate by a factor of about three at Truckee (P = 0.0029), four at Bliss (P = 0.0003) and eight at Meyers (P < 0.0001). The greatest infiltration rate was found at Blackwood Canyon using untreated water, but that site had the lowest rate with surfactant. The untreated-water infiltration rate at Blackwood was about twice that of Truckee or Bliss, and five times higher than at Meyers. Surfactant infiltration rates at Bliss and Truckee, which were nearly equal, were 25% higher than those at Meyers and 60% higher than Blackwood Canyon. MDI infiltration rates were much greater than those from the rainfall simulator, though they were significantly correlated (Spearman R = 0.83).

Runoff sample sediment particle-size distributions for Tahoe Basin soils

TABLE 5. Runoff sample sediment particle-size distributions for Tahoe Basin soils

At the Meyers road cut site, a soil excavation shows water infiltrating the soil in “fingers” of flow.

At the Meyers road cut site, a soil excavation shows water infiltrating the soil in “fingers” of flow.

At the Truckee site, dry patches or layers of organic material remained above the wetted mineral soil.

At the Truckee site, dry patches or layers of organic material remained above the wetted mineral soil.

Particle-size distributions. As found by Grismer et al. (2008), volcanic soils were much finer than granitic soils at each particle-size percentile (D10, D25, D50, D75, D90), and particle sizes differed significantly by soil type (table 5). Volcanic particles were typically about one-fourth the size of granitic particles. Ten percent of particles occurring in runoff from volcanic soils were less than 8 ???, a size fraction considered detrimental to lake clarity (Swift et al. 2006). The relationship between particle-size distribution and infiltration rate appeared to be nonlinear, making the Spearman correlation an appropriate test. All particle sizes were strongly, negatively correlated with infiltration rates. For the rainfall-simulator-based infiltration rates, Spearman correlations for the D10, D25, D50, D75 and D90 particle sizes were R = −0.86, −0.91, −0.83, −0.89 and −0.69, respectively; similarly, for the MDI-based infiltration rates, the Spearman correlations were R = −0.80, −0.70, −0.74, −0.68 and −0.76, respectively.

Surfactants and repellency

Differences in infiltration rates due to the surfactant treatment were unmistakable, as rates always increased — by a statistically significant margin at one site for the rainfall simulators, and at three of four sites for the MDIs. Increased infiltration rates with surfactants demonstrated the importance of soil hydrophobicity to possible runoff and erosion, and, if surfactant is a good model of wettable conditions, that repellency has a substantial effect on infiltration rates into mineral soil. However, the infiltration rates found with the MDI remained very high with untreated water, suggesting that the persistence of repellency in mineral soil upon contact with water is minimal. Much lower infiltration rates resulted from the rainfall simulators when native covers were maintained. Therefore, surfactant efficacy and the actual magnitude of the infiltration rate depended to a large degree on the soil cover conditions.

Surface litter. Surface litter layers were an important factor affecting wetting patterns following rainfall simulation. Native litter cover was most substantial at Truckee and Meyers. Beneath a layer of identifiable pine needle mulch was a layer of decomposed organic material (hemic/sapric < 16% plant material still discernible) with a high degree of fungal activity mats (fungal mycelia with organic matter). Excavations following rainfall simulation indicated that this layer was different from the pine needle mulch because it was neither a storage zone delaying runoff nor a structural barrier encouraging lateral rather than downward movement. The highly decomposed organic layer was strongly and persistently hydrophobic, restricting infiltration into the mineral soil below. Discontinuities in this layer were responsible for preferential flow and wetting.

These wetting patterns were most obvious at the sites with the most litter, Truckee and Meyers. “Fingered” flow was most evident in the Meyers surfactant plots, while at Truckee there was considerable wetting below a large, dry, mineral layer. In the finer-textured Truckee soil, it appeared that runoff had preferentially infiltrated several inches into the mineral profile and then began to wet upward via capillary action. Sites with less litter, Blackwood and Bliss, had correspondingly less developed or nonexistent decomposed organic layers. Infiltration was not as concentrated, and preferential zones were not as obvious, although upon excavation it was apparent that dry patches or layers of organic material remained above the wetted mineral soil.

Meeuwig (1971) originally underscored the litter layer's importance as an infiltration-limiting factor, linking eight distinct wetting patterns and corresponding infiltration curves on bare and covered sites northeast of Lake Tahoe. At forested sites, the hydrologic effects of mineral-soil repellency are at some level subsumed by those of the partially decomposed organic layer. The differences in wetting of mineral soil between treatments were most pronounced on coarse, granitic soils, indicating that hydrophobicity plays a more important role with these soils compared to volcanic soils. Granitic soils exhibited almost no wetting by untreated water, but about 6 inches (15 centimeters) of continuous and more than 10 inches (25 centimeters) of intermittent wetting with surfactant.

The texture and moisture content of soils at each site did not necessarily correlate with the degree of repellency found in the study. For example, at Blackwood Canyon infi ltration rates with untreated water in the finer-textured volcanic soils were twice as high as any other site, and the response to surfactant was relatively subdued.

The texture and moisture content of soils at each site did not necessarily correlate with the degree of repellency found in the study. For example, at Blackwood Canyon infi ltration rates with untreated water in the finer-textured volcanic soils were twice as high as any other site, and the response to surfactant was relatively subdued.

Soil moisture and texture. The changes in soil moisture content also revealed the impact of the surfactant treatment on granitic soils. Following water treatment moisture contents doubled, but with the surfactant treatments they increased sixfold. The effects of soil texture on the establishment and degree of repellent conditions are complex. Coarse-textured soils have been associated with repellency because coarse particles have less surface area per unit volume than finer particles, making them more susceptible to coating by a limited supply of hydrophobic substances (Crockford et al. 1991). However, repellency is not exclusive to coarse soils; if fine-textured (25% to more than 40% clay) soils form aggregates (presumably with greater organic-matter content) they, too, are susceptible to the development of repellency conditions (Wallis et al. 1991). In some cases, very fine fractions have the highest degree of repellency (de Jonge et al. 1999).

Texture alone does not imply a degree of repellency because aggregation and the supply of hydrophobic material are controlled by many other factors; contradictions in the relationship between soil texture and repellency may also be due to confusion between the effects of texture and structure (Fox et al. 2007). Further confusion results because fine fractions are not necessarily associated with fine textures, nor are coarse fractions necessarily associated with coarse textures; the effects of soil aggregate formation must be considered. Fine-textured soils have exhibited the highest degree of repellency, while coarse soils appear to be more susceptible to developing fire-induced or other repellency (de Jonge et al. 1999; Doerr et al. 2000).

Though the Blackwood site comprised finer-textured volcanic soils with scant litter cover, its untreated-water infiltration rates (MDI) were nearly twice as high as those of any other site, and the response to surfactant was relatively subdued. The limited response to surfactant suggests that the litter at Blackwood was neither physically inhibiting infiltration nor providing hydrophobic substances adequate to coat the relatively fine mineral particles.

Untreated plots. All of our untreated-water plots produced runoff. At the volcanic soil sites, infiltration rates were similar to previous studies and ranged from 3.4 to 4.7 inches (86 to 119 millimeters) per hour at Truckee and 4.0 to 4.7 inches (103 to 119 millimeters) per hour at Blackwood. The average sediment concentrations were 0.22 gram per liter at Truckee and 0.85 gram per liter at Blackwood. In previous studies conducted on plots with pine needle cover at Bliss State Park, and at Rubicon on granitic Meeks series soils (about 60% slopes), infiltration rates were about 2 inches (51 millimete

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