Lee H. MacDonald

Measurement and prediction of post-fire erosion at the hillslope scale, Colorado Front Range

Post-fire soil erosion is of considerable concern because of the potential decline in site productivity and adverse effects on downstream resources. For the Colorado Front Range there is a paucity of post-fire erosion data and a corresponding lack of predictive models. This study measured hillslope-scale sediment production rates and site characteristics for three wild and three prescribed fires over two summers and one winter using 48 sediment fences. Over 90% of the sediment was generated by summer convective storms. Sediment production rates from recent, high-severity wildfires were 0.2-1.0 kg m −2 year −1 . Mean sediment production rates from areas recently burned at moderate and low severity were only 0.02 and 0.005 kg m −2 year −1 , respectively. For a given severity, sediment production rates from prescribed fires were generally lower than from wildfires, but there was considerable variability between plots and within fire severity classes. Fire severity, percent bare soil, rainfall erosivity, soil water repellency and soil texture explained 77% of the variability in sediment production rates, while a two-parameter model using percentage bare soil and rainfall erosivity explained 62% of the variability. Model validation confirmed the usefulness of these empirical models. The improved understanding of post-fire erosion rates can help guide forest management and post-fire rehabilitation efforts.

Fundamental Elements of Ecologically Healthy Watersheds in the Pacific Northwest Coastal Ecoregion

Characteristics of streams and rivers reflect variations in local geomorphology, climatic gradients, spatial and temporal scales of natural disturbances, and the dynamic features of the riparian forest. This results in a variety of stream types which, when coupled with the many human uses of the Pacific Northwest coastal ecoregion, presents a difficult challenge in identifying and evaluating fundamental, system-level components of ecologically healthy watersheds. Over 20 types of streams are found in western Oregon, Washington, and British Columbia and in southeastern Alaska, a region where extractive forest, agricultural, fishing, and mining industries and a rapidly increasing urban population are severely altering the landscape. Yet stream characteristics remain the best indicators of watershed vitality, provided the fundamental characteristics of healthy streams are accurately known. The premise of this article is that the delivery and routing of water, sediment, and woody debris to streams are the key processes regulating the vitality of watersheds and their drainage networks in the Pacific Northwest coastal ecoregion. Five fundamental components of stream corridors are examined: basin geomorphology, hydrologic patterns, water quality, riparian forest characteristics, and habitat characteristics. Ecologically healthy watersheds require the preservation of lateral, longitudinal, and vertical connections between system components as well as the natural spatial and temporal variability of those components. The timing and mode of interdependencies between fundamental components are as important as the magnitude of individual components themselves.

Characteristics of log and clast bed-steps in step-pool streams of northwestern Montana, USA

The role of log and clast steps was studied in 53 step-pool reaches in 32 streams in northwestern Montana. In each reach, 20 consecutive steps were sampled. Drainage areas ranged from 0.5 to 11.5 km2. Mean step spacing was found to be proportional to channel gradient, width of the bankfull channel, and drainage area; mean step height was proportional to the width of the channel. No significant difference exists between clast and log steps with regard to step spacing and step height. This suggested either that (1) woody debris is mobile in even the smallest channels in the study area, or (2) channel flows arrange clast steps around immobile woody debris. Calculations of theoretical entrainment and transport of sediment suggest that the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels. The presence of immobile log steps (composed of living roots) in the upstream reaches, an increase in the percentage of log steps with gradient, and a decrease in this percentage with the width of the bankfull channel and drainage area, imply that woody debris is mobile primarily in the downstream reaches. Calculations of flow resistance suggest that most steps serve to maximize flow resistance. These results support the second hypothesis, that the height and spacing of clast steps are adjusted around immobile log steps to maintain consistent step characteristics.

Runoff and road erosion at the plot and road segment scales, St John, US Virgin Islands

Previous studies have identified unpaved roads as the primary source of erosion on St John in the US Virgin Islands, but these studies estimated road erosion rates only as annual averages based primarily on road rill measurements. The goal of this project was to quantify the effect of unpaved roads on runoff and sediment production on St John, and to better understand the key controlling factors. To this end runoff and sediment yields were measured from July 1996 to March 1997 from three plots on naturally vegetated hillslopes, four plots on unpaved road surfaces and two cutslope plots. Sediment yields were also measured from seven road segments with contributing areas ranging from 90 to 700 m2. With respect to the vegetated plots, only the two largest storm events generated runoff and there was no measurable sediment yield. Runoff from the road surface plots generally occurred when storm precipitation exceeded 6 mm. Sediment yields from the four road surface plots ranged from 0·9 to 15 kg m−2 a−1, and sediment concentrations were typically 20–80 kg m−3. Differences in runoff between the two cutslope plots were consistent with the difference in upslope contributing area. A sprinkler experiment confirmed that cross-slope roads intercept shallow subsurface stormflow and convert this into surface runoff. At the road segment scale the estimated sediment yields were 0·1 to 7·4 kg m−2 a−1. Road surface runoff was best predicted by storm precipitation, while sediment yields for at least three of the four road surface plots were significantly correlated with storm rainfall, storm intensity and storm runoff. Sediment yields at the road segment scale were best predicted by road surface area, and sediment yields per unit area were most strongly correlated with road segment slope. The one road segment subjected to heavy traffic and more frequent regrading produced more than twice as much sediment per unit area than comparable segments with no truck traffic. Particle-size analyses indicate a preferential erosion of fine particles from the road surface and a rapid surface coarsening of new roads. Published in 2001 by John Wiley & Sons, Ltd.

Detecting cumulative watershed effects: the statistical power of pairing

Abstract The statistical power for detecting change in water quality should be a primary consideration when designing monitoring studies. However, some of the standard approaches for estimating sample size result in a power of less than 50%, and doubling the pre- and post-treatment sample size are necessary to increase the power to 80%. The ability to detect change can be improved by including an additional explanatory variable such as paired watershed measurements. However, published guidelines have not explicitly quantified the benefits of including an explanatory variable or the specific conditions that favor a paired watershed design. This paper (1) presents a power analysis for the statistical model (analysis of covariance) commonly used in paired watershed studies; (2) discusses the conditions under which it is beneficial to include an explanatory variable; and (3) quantifies the benefits of the paired watershed design. The results show that it is beneficial to include an explanatory variable when its correlation to the water quality variable of concern is as low as about 0.3. The ability to detect change increases non-linearly as the correlation increases. Power curves quantify sample size requirements as a function of the correlation and intrinsic variability. In general, the temporal and spatial variability of many watershed-scale characteristics, such as annual sediment loads, makes it very difficult to detect changes within time spans that are useful for land managers or conducive to adaptive management.

Modelling road surface sediment production using a vector geographic information system

Field investigations indicate that unpaved roads are the largest sediment source on St John, US Virgin Islands. Crosssectional measurements of eroded road surfaces were used to establish an empirical relationship to predict annual road surface erosion as a function of road gradient and contributing drainage area. A model (ROADMOD) for estimating and mapping average annual sediment production from a road network was developed by combining this empirical relationship with a series of network algorithms to analyse road data stored in a vector geographic information system. ROADMOD was used to estimate road surface erosion in two St John catchments with very different road densities but similar land cover, topography and soils. Unpaved roads were found to increase sediment production in the more densely roaded catchment by a factor of three to eight, and in the less-roaded catchment by a factor of 1.3-2.0. Turbidity measurements in the receiving bays of these two catchments are consistent with model predictions and observed sediment delivery processes. Although this model was developed specifically for St John, it can easily be adapted to other locations by substituting a locally derived predictive equation for road erosion. Model assumptions, limitations and potential improvements are discussed. @ 1998 John Wiley & Sons, Ltd.

Hydrologic restoration of a fen in Rocky Mountain National Park, Colorado, USA

Big Meadows, a 63-ha fen in Rocky Mountain National Park (RMNP), was ditched for agricultural purposes in the early part of this century. Although use of the ditch ceased after the establishment of RMNP in 1915, it continued to intercept sheet flows in the central and southern portions of the fen, causing the ground-water level to decrease and aerobic soil conditions to develop in the mid- to late-summer of most years. In 1990, the ditch was blocked in an attempt to restore the hydrologic regime in the central and southern portions of the fen. Water-level data from three years prior to restoration and four years after restoration show that blocking the ditch successfully restored surface sheet flow, high late-summer watertable levels, and anaerobic soil conditions in much of the central and southern portions of the fen. Conditions in these areas are now similar to those in the northern portion of the fen. The long-term data from this site also indicate that summer rainfall has a greater influence on the magnitude of late-summer drying than the size of the winter snowpack. In a post-restoration year with extremely low rainfall in July and August, water levels throughout the fen decreased to levels similar to those observed throughout most of the pre-restoration period. The study suggests that this and other fens in the southern Rocky Mountains are extremely sensitive to summer precipitation and the hydrologic changes created by even small ditches or water diversions.

Predicting post-fire hillslope erosion in forest lands of the western United States

Many forests and their associated water resources are at increasing risk from large and severe wildfires due to high fuel accumulations and climate change. Extensive fuel treatments are being proposed, but it is not clear where such treatments should be focussed. The goals of this project were to: (1) predict potential post-fire erosion rates for forests and shrublands in the western United States to help prioritise fuel treatments; and (2) assess model sensitivity and accuracy. Post-fire ground cover was predicted using historical fire weather data and the First Order Fire Effects Model. Parameter files from the Disturbed Water Erosion Prediction Project (WEPP) were combined with GeoWEPP to predict post-fire erosion at the hillslope scale. Predicted median annual erosion rates were 0.1–2 Mg ha–1 year–1 for most of the intermountain west, ~10–40 Mg ha–1 year–1 for wetter areas along the Pacific Coast and up to 100 Mg ha–1 year–1 for north-western California. Sensitivity analyses showed the predicted erosion rates were predominantly controlled by the amount of precipitation rather than surface cover. The limited validation dataset showed a reasonable correlation between predicted and measured erosion rates (R2 = 0.61), although predictions were much less than measured values. Our results demonstrate the feasibility of predicting post-fire erosion rates on a large scale. The validation and sensitivity analysis indicated that the predictions are most useful for prioritising fuel reduction treatments on a local rather than interregional scale, and they also helped identify model improvements and research needs.

Spatial and Temporal Active Layer Dynamics along Three Glacial Meltwater Streams in the McMurdo Dry Valleys, Antarctica

Abstract Active layer thickness was monitored along three ephemeral streams in the Taylor Dry Valley of Antarctica during the 1997–1998 summer season. Five to seven cross sections were established on each stream, and the thickness of the active layer was measured every 1.5 m over intervals ranging from 2 to 30 days. Active layer depths ranged from a minimum of 3 cm in early November to a maximum of 60 cm in late January, and the depth of the active layer increased rapidly as summer temperatures climbed above freezing. While there were significant differences in the thickness of the active layer among the three streams, the timing of rapid thaw was similar for all cross sections. Changes in active layer thickness were responsive to both daily and seasonal changes in air temperature. There was more rapid thaw under the areas with flowing water, suggesting a transfer of heat from meltwater into the underlying sediments, and some evidence of an insulating effect during cold periods. Active layer thickness was not strongly related to modeled differences in incoming solar radiation using 30 m grid cells.

BEYOND THE GUIDEliNES: PRACTICAL LESSONS FOR MONITORING

A series of workshops have provided extensive feedback on a recently published manual, Monitoring Guidelines to Evaluate Effects of Forestry Activities on Streams in the Pacific Northwest and Alaska (Guidelines) (MacDonald et al., 1991). These workshops and other discussions have led to the identification of fourteen additional ‘lessons’ for monitoring. These lessons are concepts which either were not incorporated into the Guidelines, were not sufficiently emphasized, or which are needed to put the Guidelines in context. The topics include: monitoring as a continuum; defining objectives and hypotheses; peer review; uncertainty and risk; upslope vs. instream monitoring; photo sequences; scale considerations; data storage, data interpretation, and data base management; ‘activities monitoring’; and personal commitment as a critical component in monitoring projects. Many of these lessons might appear self-evident, but our experience indicates that they are often ignored. Like the Guidelines, these lessons are widely applicable and should be explicitly recognized when formulating and conducting monitoring projects.