Browsing by Author "MacDonald, Lee H., advisor"
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Item Open Access Predicting cumulative watershed effects in small forested watersheds(Colorado State University. Libraries, 2009) Litschert, Sandra E., author; MacDonald, Lee H., advisorCumulative watershed effects (CWEs), such as the hydrologic and sedimentary changes due to management activities, are a critical concern for many land managers. The goal was to develop a suite of GIS-based models for assessing CWEs in forested watersheds. The models need to be easy-to-use, spatially and temporally explicit, and scientifically based. Delta-Q and FOREST (FORest Erosion Simulation Tools) are a series of empirical and conceptual models that calculate the annual changes in discharge (Delta-Q) and annual sediment production, delivery and yield (FOREST) from roads, fires, and forest management. Given the paucity of data on hillslope sediment delivery, a field study also was conducted to assess the connectivity of sediment pathways from timber harvest units. Delta-Q and FOREST were verified using data from the Eldorado National Forest in California. The models were evaluated using measured data from three experimental watersheds. The predicted and measured 50th percentile flows were closer than the 1st and 99th percentiles, as extreme flows are more sensitive to climatic fluctuations. While predicted bedload sediment yields usually fell within the range of measured values, suspended sediment yields were generally overpredicted. FOREST results were most sensitive to changes in mean annual precipitation and GIS data scale. FOREST generates GIS layers to show the hillslopes, roads, and stream reaches with the greatest risk for altering runoff and erosion, and inducing sedimentation. Both models can be used to compare management scenarios within a watershed or among watersheds. By design the models take a middle approach between commonly used, unvalidated empirical models, and complex physically based models that are rarely used by land managers. During the field study in the Sierra Nevada mountains of California, the downslope edges of nearly 200 timber harvest units were traversed. Only 19 rills or sediment plumes were found that originated from harvest units. Five of the six features that extended through the stream-side management zone to a stream channel were generated by runoff from skid trails. The results indicate that harvest units rarely deliver sediment to streams, but post-harvest skid trail treatments may be needed to reduce surface runoff and sediment delivery to streams.Item Open Access Road sediment production and delivery: effects of fires, traffic, and road decommissioning(Colorado State University. Libraries, 2016) Sosa Peréz, Gabriel, author; MacDonald, Lee H., advisor; Kampf, Stephanie, committee member; Lefsky, Michael, committee member; Butters, Gregory, committee memberUnpaved roads often are a major source of sediment to streams in forested watersheds, and an increase in sediment production and delivery can adversely degrade water quality and aquatic habitat. The first part of this study quantifies the effects of wildfires on road erosion and road-stream connectivity as a function of fire severity and road segment characteristics. The data were collected along 6.8 km of an unpaved road after the High Park wildfire in Colorado. The second and third parts of this dissertation investigate how traffic and two road decommissioning treatments affect road sediment production and road-stream connectivity through the use of rainfall simulations, sediment production measurements at the road segment scale, and repeated surveys of 12.3 km of decommissioned roads. The segment-scale and road survey data were collected over a three-year period that included one summer prior to decommissioning and the first two years after decommissioning. The road-wildfire study indicated that road surface rill erosion increased with hillslope burn severity due to the increasing amounts of runoff, but the length and area of rilling also increased with road segment slope. Segments with a slope ≤5% tended to capture sediment from the hillslope. Road segment area was only important for roads in areas burned at low severity, indicating that hillslopes become a progressively less important source of runoff as burn severity decreases. All of the road segments in areas burned at moderate and high severity and 78% of the segments in areas burned at low severity were connected to the stream due to the increased runoff from upslope, the concentration of hillslope and road surface runoff to a single drainage point, and the reduced infiltration and trapping capacity of the hillslopes below the road. After wildfires land managers need to increase the frequency of drainage structures, and a more integrated modeling approach is needed to further our understanding of the complex interactions between burned hillslope and roads. The rainfall simulations showed that the infiltration capacity for the decommissioning treatment of only ripping had little effect on infiltration and significantly increased sediment yields compared to closed roads. Mulching after ripping doubled the final infiltration rate and decreased sediment yields by nearly a factor of five compared to only ripping. Eighty passes of an all-terrain vehicle on two closed roads had no effect on infiltration capacity, but increased sediment yields by a factor of three. The results at the road segment-scale showed that traffic was the dominant control on sediment production, and both decommissioning treatments greatly reduced road sediment production as nearly all of the eroded sediment was trapped in the furrows. Decommissioning reduced road-stream connectivity from 12% of the total length to only 2%, with most of the connected segments being immediately adjacent to a stream. These results can help calibrate and validate road erosion models, and guide the design of future road decommissioning treatments.Item Open Access Sediment production and delivery from hillslopes and forest roads in the southern Sierra Nevada, California(Colorado State University. Libraries, 2011) Stafford, Allison K., author; MacDonald, Lee H., advisor; Stednick, John D., committee member; Butters, Gregory, committee memberUnpaved roads often are a major source of sediment to streams in forested watersheds, and an increase in sediment production and delivery can adversely affect the overall health of a stream. The goals of this study were to first quantify the effects of climate and soil type on hillslope and road sediment production and delivery, and then evaluate the effects of graveling, grading, and waterbar construction on road sediment production and delivery. Sediment fences were used to collect 109 fence-years of data from water years 2008 and 2009 in the more rain dominated José Basin (800-1200 m) and 193 fence-years of data in the snow dominated Kings River Experimental Watersheds (KREW) (1485-2420 m), both located in Sierra National Forest (SNF) in California. Detailed road surveys assessed road segment characteristics and road-stream connectivity. Mean hillslope sediment production in José Basin was 3.7 x 10-3 kg m-2 yr-1, which was similar to the value of 4.1 x 10-3 kg m-2 yr-1 in KREW. Native surface road segments in José Basin had a mean sediment production rate of 1.8 kg m-2 yr-1, and the estimated total sediment production from the 67 km of native surface roads is 680 metric tons per year. An estimated 30% of the native surface road length is connected to the stream network, indicating that up to 210 metric tons of sediment may be delivered to streams each year. There was no significant difference in sediment production and delivery between road segments in the highly erodible Holland soil and road segments in other soil types. Mean sediment production for the native surface road segments in the KREW watersheds was 0.13 kg m-2 yr-1, which was more than an order of magnitude lower than the mean value in José Basin, and road-stream connectivity was only 3%. There was no significant difference in sediment production from native and gravel surface road segments in José Basin due to the high variability and the gravel segments still averaged 51% bare soil. The gravel surface segments had shorter drainage features than native surface segments, but 40% of the gravel roads were connected as they tended to be closer to streams. Graveled roads in the Providence Creek watersheds produced 0.16 kg m-2 yr-1, which was only 22% as much sediment as the native surface roads, and had 11% connectivity. In José Basin grading initially decreased the mean segment length from 65 m to 41 m, but one year after grading 22% of the waterbars had failed, leading to a 15% increase in mean segment length. Graded road segments in José Basin produced eight and three times more sediment per unit area than ungraded segments in WY2008 and WY2009, respectively, and this can be attributed to extensive rilling. Sediment production rates decreased by 40-60% from the first to the second year after grading. Sediment production and delivery from forest roads can be reduced by: 1) using more than 30% gravel cover on native surface roads, 2) minimizing grading, and 3) improving the construction of waterbars to better withstand and direct overland flow.