SUMMARY OF CUMULATIVE WATERSHED EFFECTS PROCESS

Transcription

1 SUMMARY OF CUMULATIVE WATERSHED EFFECTS PROCESS KLAMATH NATIONAL FOREST PREPARED BY: ANGIE BELL, FOREST GEOLOGIST 12 OCTOBER 2012 INTRODUCTION The Klamath National Forest currently utilizes three separate models as tools to estimate the cumulative watershed effects of management activities. The three models are the Universal Soil Loss Equation (USLE), the Equivalent Roaded Area (ERA) and the Mass Wasting Model (GEO), known collectively as the CWE models. The models are mathematical equations that represent physical processes. The USLE model estimates soil erosion potential, the GEO model estimates landsliding potential and the ERA model is a proxy for impacts to peak flow. The CWE models are applied spatially across the watersheds using ArcMap (ESRI). ASSUMPTIONS AND LIMITATIONS Spatial scale: Project (site)-specific actions are modeled within the context of 7 th -field watersheds (drainages, from 3,000 to 10,000 acres in size). If required, 7 th -field watersheds can be aggregated to characterize 6 th or 5 th -field watersheds. Temporal scale: Surface erosion model predicts quantities of delivered sediment for the first winter season following the action and is expected to diminish thereafter. Non road-related sediment yields return to near background in three to seven years, depending on local site conditions. Mass-wasting (landslide) model predicts delivered sediment over a decade following the project. High rates of landsliding are typically associated with episodic flood events, which recur every years. Non road-related sediment yields return to near background in ten to twenty years, depending on local site conditions. Non road-related ERA modeled disturbances recover gradually over forty years. Since roads are forever and do not recover like vegetative disturbance, road-related sedimentation is constant year after year. Positive changes can occur by stormproofing, decommissioning, and administrative decisions that affect use levels. Stochastic element: Altering the condition of a land surface is a game played in a stochastic environment random, but probabilistic. For example, several dry winters might follow the clearing of a hillslope; in which case, high rates of landsliding would not occur. On the other hand, a flood event might occur the winter after; in which case, copious landsliding would occur. These triggering flood events remain essentially unpredictable, but we can define the probability and estimate the risk of such erosional events. CWE modeling seeks to predict the increased risk of slope failure and sedimentation from our proposed action not absolute sedimentation

2 volumes. This concept of risk combines a statement of probability of an event with an estimation of the resultant magnitude. METHODS Past, present and reasonably foreseeable actions are modeled using a combination of the following parameters: 1. Silvicultural prescription - Since silvicultural prescriptions very widely between projects, even between units and alternatives in one project, the CWE impact of proposed silvicultural activities are categorized and modeled as high, moderate, low or no disturbance. An example of a silviculture practices with high impacts would be regeneration harvest, a moderate impacts would include group select, and a low impact would include a thin-from-below prescription. 2. Logging system The model differentiates between disturbance from tractor, skyline, helicopter, and feller buncher yarding. Landings and temporary roads can also be included in the model if proposed. 3. Site preparation The model differentiates between tractor piling and burning, hand piling and burning, broadcast burning and underburning if they are proposed. 4. Wildfire Wildfire events are included in the models with the disturbance being determined by the soil burn severity with focus being on the high and moderate soil burn severity. 5. Existing Road System the model takes into account the road surface, road width, road template (in-sloped or outsloped), maintenance level and it current status (decommissioned, stormproofed, or other). USLE The USLE model estimates soil loss and delivered to a stream channel. The USLE model is represented by the following equation: SSSSSSSSSSSSSSSS dddddddddddddddddd (yyyy 3 aaaa yyyyyyyy) = [(0.7)xx RR xx LLLL xx DD xx KK xx CC] x A Where: A = Area (acres) of polygon 0.7 = conversion factor (tons to yd 3 ) R = Rainfall/runoff factor (what is a 2 year rain event) D = Delivery factor (what % of sediment will be delivered) C = Cover factor (related to disturbance class) Cumulative Watershed Effects Process Page 2

3 LS = Slope-length/slope steepness factor K = Soil erodibility factor (related to Soil Map Unit) The runoff, slope-length/steepness, and soil erodibility factors are constant for any given point on the Forest. The delivery factor assumes a 10% delivery (0.10) of the sediment to the stream from the hillslopes. The delivery factor for roads is based on the template of the road (in-slope, out-slope, etc.) and ranges from 15% -40% assumed delivery. GEO The geological cumulative watershed effects model (GEO) compares the landslide sediment production as a result of existing road conditions, harvest and fire disturbances to the production if the watershed was undisturbed. It assumes a winter storm event with a 10 year return interval. The coefficients (c in equation) were developed in the Salmon River basin and the model assumes the geomorphic terranes react identically regardless of elevation. The risk ratio is based on background over the current landslide sediment production. Mass Wasting (yds 3 /decade) = C x Geoterrain disturbance (acres) ERA The ERA model provides a simplified accounting system for tracking disturbances that affect watershed processes, in particular, estimates in changes in peak runoff flows influenced by ground-disturbing activities. Unlike the surface erosion (USLE) and mass wasting (GEO) models, ERA is not intended to be a process-based sediment model. It does, however, provide an indicator of watershed conditions. This model compares the current [& proposed] level of disturbance within a given watershed (expressed as % ERA) with the theoretical maximum disturbance level acceptable (expressed as % threshold of concern). The threshold of concern (TOC) is a measure of watershed sensitivity. TOC is calculated based on channel sensitivity, beneficial uses, soil erodibility, hydrologic response, and slope stability of each watershed. For example, a watershed with sensitive channels, highly productive anadromous streams (high beneficial use), highly erodible soils, high landslide densities &/or high percentage of granitic lands (slope stability), and high percentage of watershed in the "rainon-snow" zone (~3,500' to 5,000' elevation; hydrologic response) would have a high "watershed sensitivity level" and therefore a low TOC. Some use ERA as a run off risk model which estimates the level of hydrological disturbance or relative risk of increased peak flows and consequent potential for channel alteration and general adverse watershed impacts. Coefficients are additive. Coefficients for harvest prescription are added to logging system coefficients, which are added site prep coefficients. For example, a Cumulative Watershed Effects Process Page 3

4 tractor logged regeneration harvest unit using tractor harvest and masticating slash would have a total ERA coefficient of :.19 (total ERA per acre) =.12 (GTR Rx) +.04 (tractor,modified) +.03 (mastication) RECOVERY Recovery of the watershed to disturbance is assumed to be dependent on the disturbance and is model specific. Areas disturbed by harvest or fire are assumed to recover to background levels over time. No recovery on roads is assumed unless they receive some type of treatment (e.g. stormproofing or decommissioning). The USLE assumes that recovery is relative to the post-disturbance cover factor. The recovery is exponential meaning that the hillslope recovers quickly and then the recovery levels off. The equation for recovery is C=0.244t (-1.54). Where C is the cover factor post disturbance and t is the time since disturbance. The GEO model assumes no recovery for the first 10 years then a linear recovery to background over the next 40 years. So the GEO model assumes full recovery to background in 50 years. The ERA model recovery rates are relative to the initial ERA coefficient for the disturbance. The ERA remains constant for a fixed period of time and then recovers linearly to zero through time. The ERA recovers more quickly for a small disturbance (e.g. non-commercial thin) and assumes a longer recovery rate for areas with more disturbance (e.g. regeneration harvest). RISK RATIOS Klamath National Forest CWE assessments model the disturbances and land sensitivity. Results fall on a continuum. As disturbances increase (and recover) over time and space, at some point, the risk of initiating or contributing to existing adverse cumulative watershed impacts becomes a cause for concern. These model-specific levels are called inference points or TOC and are used to inform land management decisions. Ecologically, a transition exists from lower to higher risk of adverse effects to beneficial uses from insignificant to potentially significant. From a management perspective, inference points are intended to represent the center of that transition zone. Inference points do not represent the exact point at which cumulative watershed effects will occur. Rather, they serve as yellow flag indicators of increasing susceptibility for significant adverse effects occurring within a watershed. Modeled CWE levels, relative to defined inference point values, are expressed as risk ratios. These ratios are calculated by dividing accelerated sedimentation and ERA values by an inference point value. For the GEO and USLE models existing levels are shown as percent over background, which is a measure of accelerated sedimentation. For the ERA/TOC model Cumulative Watershed Effects Process Page 4

5 existing disturbance levels are expressed as equivalent roaded acres (ERA). Inference point values for each model have been identified at the following levels: (1) USLE (surface erosion) model = 400% over background, (2) GEO (mass-wasting) model = 200% over background, and (3) ERA/TOC model = watersheds TOC value. For example, a watershed with GEO model-estimated sediment delivery from mass wasting of 100% over background would yield a risk ratio of.50 [100% divided by 200%]. Risk ratio values represent a continuum of and serve as indicators of relative watershed condition. Inference point values will need to be reviewed, and either validated or changed. Circumstances warranting change include acquisition of new or more refined information and/or better understanding of watershed processes and interactions. Watersheds with elevated CWE model numbers near or over these inference point values typically share common characteristics. These include high road densities and/or large areas of wildfire (high/moderate burn intensities) or harvest (high/moderate impact silvicultural prescriptions). These disturbances commonly lie on sensitive landscapes, with respect to geology, landforms and soils. Examples include inner gorges, active landslides and weathered granitic soils. During episodic flood events, these watersheds typically exhibit high rates of landsliding and channel alteration. WHAT IS TYPICALLY MODELED? [1] Vegetative disturbances/removal, except those listed below; typically harvest units, but can include other vegetation manipulation activities, such as conversions, etc. [2] New road construction temporary or otherwise [3] New or enlarged landing construction [4] Other fuel treatments associated with timber sales [5] Fuels treatment projects not associated with timber harvest [6] Actions on private lands as shown in Timber Harvest Plans [7] Wildfire high and moderate burn intensities [8] Prescribed underburns [9] Road improvements beyond routine maintenance; chiefly treatments associated with stormproofing (e.g., outsloping, rocking, slide stabilization, crossing fixes critical dips to vented fords) [10] Road decommissioning meeting KNF decommissioning guidelines WHAT IS NOT TYPICALLY MODELED? Cumulative Watershed Effects Process Page 5

6 These actions are not modeled because (1) effects are minor or not significant and/or (2) data is unavailable not complete, not consistent, not credible. Because we cannot numerically model certain actions does NOT mean they should be ignored in CWE assessments. Effects of these other activities can be discussed qualitatively or semi-quantitatively. We can attempt quantitative modeling upon special request. [1] Very light silvicultural treatments, including roadside salvage & pre-commercial thin [2] Skid trails [3] Fire lines unless large tractor lines or requested [4] Road maintenance (routine) [5] Re-opening existing landings [6] Re-opening existing roads [7] Trails for hiking and off highway vehicle (OHV) use [8] Power transmission line corridors [9] Old railroad grades [10] Quarries or rock pits [11] Wildfire low burn intensity [12] Historic mining activities [13] Grazing activities [14] Lop & scatter, gill-poke, and other non-heavy equipment types of site preparation [15] Minor or mitigating roadwork, such as spot rocking REFERENCES Elder, D and Reichert, M Cumulative Watershed Effects Analysis Process Paper. USDA, Klamath National Forest. Cumulative Watershed Effects Process Page 6