Effectiveness of Prescribed Fire to Re-Establish Sagebrush Steppe Vegetation and Ecohydrologic Function on Woodland-Encroached Sagebrush Rangelands, Great Basin, USA: Part I: Vegetation, Hydrology, and Erosion Responses

Pinyon (Pinus spp.) and juniper (Juniperus spp.) woodland encroachment has imperiled a broad ecological domain of the sagebrush steppe (Artemisia spp.) ecosystem in the Great Basin Region, USA. As these conifers increase in dominance on sagebrush rangelands, understory vegetation declines and ecohydrologic function can shift from biotic (vegetation) controlled retention of soil resources to abiotic (runoff) driven loss of soil resources and long-term site degradation. Scientists, public land management agencies, and private land owners are challenged with selecting and predicting outcomes to treatment alternatives to improve ecological structure and function on these rangelands. This study is the first of a two-part study to evaluate effectiveness of prescribed fire to re-establish sagebrush steppe vegetation and improve ecohydrologic function on mid- to late-succession pinyon-and juniper-encroached sagebrush sites in the Great Basin. We used a suite of vegetation and soil measures, small-plot (0.5 m2) rainfall simulations, and overland flow experiments (9 m2) to quantify the effects of tree removal by prescribed fire on vegetation, soils, and rainsplash, sheetflow, and concentrated flow hydrologic and erosion processes at two woodlands 9-yr after burning. For untreated conditions, extensive bare interspace (87% bare ground) throughout the degraded intercanopy (69–88% bare ground) between trees at both sites promoted high runoff and sediment yield from combined rainsplash and sheetflow (~45 mm, 59–381 g m−2) and concentrated flow (371–501 L, 2343–3015 g) processes during high intensity rainfall simulation (102 mm h−1, 45 min) and overland flow experiments (15, 30, and 45 L min−1, 8 min each). Burning increased canopy cover of native perennial herbaceous vegetation by \u3e5-fold, on average, across both sites over nine growing seasons. Burning reduced low pre-fire sagebrush canopy cover (30 yr. Enhanced herbaceous cover in interspaces post-fire reduced runoff and sediment yield from high intensity rainfall simulations by \u3e2-fold at both sites. Fire-induced increases in herbaceous canopy cover (from 34% to 62%) and litter ground cover (from 15% to 36%) reduced total runoff (from 501 L to 180 L) and sediment yield (from 2343 g to 115 g) from concentrated flow experiments in the intercanopy at one site. Sparser herbaceous vegetation (49% cover) and litter cover (8%) in the intercanopy at the other, more degraded site post-fire resulted in no significant reduction of total runoff (371 L to 266 L) and sediment yield (3015 g to 1982 g) for concentrated flow experiments. Areas underneath unburned shrub and tree canopies were well covered by vegetation and ground cover and generated limited runoff and sediment. Fire impacts on vegetation, ground cover, and runoff and sediment delivery from tree and shrub plots were highly variable. Burning litter covered areas underneath trees reduced perennial herbaceous vegetation and increased invasibility to the fire-prone annual cheatgrass (Bromus tectorum L.). Cheatgrass cover increased fro

Effectiveness of Prescribed Fire to Re-Establish Sagebrush Steppe Vegetation and Ecohydrologic Function on Woodland-Encroached Sagebrush Reangelands, Great Basin, USA: Part II: Runoff and Sediment Transport at the Patch Scale

Woody species encroachment into herbaceous and shrub-dominated vegetations is a concern in many rangeland ecosystems of the world. Arrival of woody species into affected rangelands leads to changes in the spatial structure of vegetation and alterations of biophysical processes. In the western USA, encroachment of pinyon (Pinus spp.) and juniper (Juniperus spp.) tree species into sagebrush steppes poses a threat to the proper ecohydrological functioning of these ecosystems. Prescribed fire has been proposed and used as one rangeland improvement practice to restore sagebrush steppe from pinyon-juniper encroachment. Short-term effects of burning on the ecohydrologic response of these systems have been well documented and often include a period of increased hydrologic and erosion vulnerability immediately after burning. Long-term ecohydrologic response of sagebrush steppe ecosystems to fire is poorly understood due to lack of cross-scale studies on treated sites. The aim of this study is to evaluate long-term vegetation, hydrologic, and erosion responses at two pinyon-juniper-encroached sagebrush sites 9 years after prescribed fire was applied as a restoration treatment. Thirty-six rainfall simulation experiments on 6 m × 2 m plots were conducted for 45 min under two conditions: a dry run (70 mm h−1; dry antecedent soils) and a wet run (111 mm h−1; wet antecedent soils). Runoff and erosion responses were compared between burned and unburned plots. Overall, increases in herbaceous cover in the shrub-interspace areas (intercanopy area between trees) at both sites 9 years post-burn resulted in runoff- and erosion-reduction benefits, especially under the wet runs. While the initially more degraded site characterized by 80% bare ground pre-burn, registered a higher overall increase (40% increase) in canopy cover, greater post-fire reductions in runoff and erosion were observed at the less degraded site (57% bare ground pre-burn). Runoff and erosion for the wet runs decreased respectively by 6.5-fold and 76-fold at the latter site on the burned plots relative to control plots, whereas these decreases were more muted at the more degraded site (2.5 and 3-fold respectively). Significant fragmentation of flow paths observed at the more-degraded site 9 years post-fire, suggests a decreased hydrologic connectivity as a mechanism of runoff and erosion reduction during post-fire recovery

Vegetation, Hydrologic, and Erosion Responses of Sagebrush Steppe 9 Yr Following Mechanical Tree Removal

Land managers across the western United States are faced with selecting and applying tree-removal treatments on pinyon (Pinus spp.) and juniper (Juniperus spp.) woodland-encroached sagebrush (Artemisia spp.) rangelands, but current understanding of long-term vegetation and hydrological responses of sagebrush sites to tree removal is inadequate for guiding management. This study applied a suite of vegetation and soil measures (0.5 − 990 m2), small-plot rainfall simulations (0.5 m2), and overland flow experiments (9 m2) to quantify the effects of mechanical tree removal (tree cutting and mastication) on vegetation, runoff, and erosion at two mid- to late-succession woodland-encroached sagebrush sites in the Great Basin, United States, 9 yr after treatment. Low amounts of hillslope-scale shrub (3 − 15%) and grass (7 − 12%) canopy cover and extensive intercanopy (area between tree canopies) bare ground (69 − 88% bare, 75% of area) in untreated areas at both sites facilitated high levels of runoff and sediment from high-intensity (102 mm • h− 1, 45 min) rainfall simulations in interspaces (~ 45 mm runoff, 59 − 381 g • m− 2 sediment) between trees and shrubs and from concentrated overland flow experiments (15, 30, and 45 L • min− 1, 8 min each) in the intercanopy (371 − 501 L runoff, 2 342 − 3 015 g sediment). Tree cutting increased hillslope-scale density of sagebrush by 5% and perennial grass cover by twofold at one site while tree cutting and mastication increased hillslope-scale sagebrush density by 36% and 16%, respectively, and perennial grass cover by threefold at a second more-degraded (initially more sparsely vegetated) site over nine growing seasons. Cover of cheatgrass (Bromus tectorum L.) was \u3c 1% at the sites pretreatment and 1 − 7% 9 yr after treatment. Bare ground remained high across both sites 9 yr after tree removal and was reduced by treatments solely at the more degraded site. Increases in hillslope-scale vegetation following tree removal had limited impact on runoff and erosion for rainfall simulations and concentrated flow experiments at both sites due to persistent high bare ground. The one exception was reduced runoff and erosion within the cut treatments for intercanopy plots with cut-downed-trees. The cut-downed-trees provided ample litter cover and tree debris at the ground surface to reduce the amount and erosive energy of concentrated overland flow. Trends in hillslope-scale vegetation responses to tree removal in this study demonstrate the effectiveness of mechanical treatments to reestablish sagebrush steppe vegetation without increasing cheatgrass for mid- to late-succession woodland-encroached sites along the warm-dry to cool-moist soil temperature − moisture threshold in the Great Basin. Our results indicate improved hydrologic function through sagebrush steppe vegetation recruitment after mechanical tree removal on mid- to late-succession woodlands can require more than 9 yr. We anticipate intercanopy runoff and erosion rates will decrease over time at both sites as shrub and grass cover continue to increase, but follow-up tree removal will be needed to prevent pinyon and juniper recolonization. The low intercanopy runoff and erosion measured underneath isolated cut-downed-trees in this study clearly demonstrate that tree debris following mechanical treatments can effectively limit microsite-scale runoff and erosion over time where tree debris settles in good contact with the soil surface

Developing a Parameterization Approach for Soil Erodibility for the Rangeland Hydrology and Erosion Model (RHEM)

Soil erodibility is a key factor for estimating soil erosion using physically based models. In this study, a new parameterization approach for estimating erodibility was developed for the Rangeland Hydrology and Erosion Model (RHEM). The approach uses empirical equations that were developed by applying piecewise regression analysis to predict the differences of erodibility before and after disturbance (i.e., wildfire, prescribed fire, and tree encroachment) and across a wide range of soil textures as a function of vegetation cover and surface slope angle. The approach combines rain splash, sheet flow, and concentrated flow erodibilities into a single parameter for modeling erodibility in most cases. We evaluated the new approach for sites representing different degrees of disturbance associated with burning and tree encroachment. Our results show that the new erodibility approach in RHEM predicts erosion at the plot scale with a satisfactory range of error in all cases. The new approach extends the applications of RHEM to a greater scope of landscapes and soil texture

Understanding ephemeral gully erosion: Relation to the apparent soil erodibility concept

Ephemeral gully erosion is considered one of the dominant sources of soil loss from the agricultural landscape but has been historically overlooked in traditional soil erosion assessment. Efforts to develop an adequate modeling framework addressing ephemeral gully erosion are hampered by the lack of data and research tools tailored to this erosion pathway. Ephemeral gullies are assumed to form where surface flow concentration exerts hydraulic forces exceeding a given threshold for channel initiation. Consequently, models attempting to predict ephemeral gully location and erosion only consider surface flow as the main controlling factor. Subsurface hydrology is often given a secondary role in ephemeral gully erosion since only its effect on surface flow quantity is considered. Recent developments in soil erosion research support the hypothesis of a first order control of subsurface hydrology on soil erosion. This evidential control of subsurface hydrology on soil erosion is a special case of a more general concept describing soil erodibility as an apparent property composed of a constant intrinsic part and an extrinsic component depending on field conditions. Difficulties of current ephemeral gully erosion prediction models to address gully erosion on fields with limited runoff such as those managed under No-Till farming suggest that the apparent soil erodibility concept might play a significant role in gully initiation and development. This thesis uses the specific case where soil erodibility was modified by subsurface hydrology to show the relationship between the apparent soil erodibility concept and ephemeral gully erosion. To complete this study, research tools specifically designed to address ephemeral gully processes were developed. A 9.75 m × 3.66 m hillslope section with controllable surface and subsurface hydrologic conditions was designed and built to adequately represent natural hillslope scale processes. To monitor soil erosion and deposition, channel network development and hillslope form evolution, a digital photogrammetric technique was developed and tested for soil erosion applications. A series of rainfall-runoff experiments was conducted on the hillslope set under drainage or oversaturation (seepage) condition. Soil loss was monitored by collecting runoff samples and by digitizing the soil surface at regular time intervals using digital photogrammetry. Results suggest that subsurface hydrology had a significant impact on erosion rate and channel network development. On average, erosion rates were 1.85 times higher under seepage than they were under drainage. From the elevation change data, we found that channels developed at a rate 1.5 times higher under seepage condition than they did under drainage condition. The mechanism of channel development was also affected by subsurface hydrology. While channel development occurred at a steady rate under drainage condition, two phases of development with contrasting rates were observed under seepage condition. Finally, soil loss and elevation change data were also used to validate key theories in soil erosion modeling. Measured effects of rainfall-runoff intensity on interrill sediment load, support a previously proposed model for interrill erosion relating sediment load to rainfall intensity and to the square root of runoff rate while elevation change patterns observed during each rainfall-runoff event were found to be more consistent with the simultaneous erosion deposition theory as opposed to the sediment transport capacity concept for sediment transport mechanism. This study demonstrated that subsurface hydrology might be a controlling factor in the location, initiation and rate of development of ephemeral gullies

Exploring the Jordanian rangeland status transition merging the restoration experiment with modeling - 71st Annual Meeting, Technical Training and Trade Show January 28 - February 2, 2018

Due to recurring droughts and severe overgrazing, Jordan’s dry rangelands are exceptionally prone to degradation. Establishing both restoration and sustainable rangeland management practices are crucial to reverse the negative impacts on the ecosystem. However, a primary estimate of the native baseline’s water and sediment fluxes is essential to properly target a sustainable transition from degraded to a potentially revegetated landscape status. In Jordan, a widely applied restoration technique is the mechanized micro-Water Harvesting basin (WH) approach on sloping, degraded, and hard-crusted rangelands. Small basins (6m long by 0.6 cm deep) are scooped out of the soil along the contour with the spoils piled on the down slope side of the basin. Within the micro-pit basins native shrubs are planted supporting the development of shrub islands. This technique captures overland flow and reduces soil erosion. The Rangeland Hydrology and Erosion Model (RHEM) was used to explore three different rangeland ecological states and their implications on water and soil fluxes: i) the supposed historical vegetation condition (baseline), using literature review and scientist and community questionnaire data, ii) the actual degraded status, and iii) micro-WH based restored equilibrium scenario, based on field monitoring and modeling. Rangeland experimental site near Amman, Jordan, provided diverse monitoring and validation data for RHEM modeling. RHEM was applied to evaluate 1) long-term stability of the 3 different ecological states of the hillslopes (i-iii), and 2) event based spatially distributed watershed modeling to estimate how the different scenarios of landscape vegetation patterns would alter hydrologic processes. The spatial-temporal assessment of water and sediment transport in baseline, degraded, and restored Jordanian rangelands provide information on the sustainability of the WH restoration approach to different rainfall events (i.e., 10 year runoff event) and allow us to approximate resilient equilibrium water and soil dynamics that is available to support the restored vegetation

Inferring Sediment Transport Capacity from Soil Microtopography Changes on a Laboratory Hillslope

In hillslope erosion modeling, the Transport Capacity (Tc) concept describes an upper limit to the flux of sediment transportable by a flow of given hydraulic characteristics. This widely used concept in process-based erosion modeling faces challenges due to scarcity of experimental data to strengthen its validity. In this paper, we test a methodology that infers the exceedance of transport capacity by concentrated flow from changes to soil surface microtopography sustained during rainfall-runoff events. Digital Elevation Models (DEMs) corresponding to pre- and post-rainfall events were used to compute elevation change maps and estimate spatially-varying flow hydraulics ω taken as the product of flow accumulation and local slope. These spatial data were used to calculate a probability of erosion PE at regular flow hydraulics intervals. The exceedance of Tc was inferred from the crossing of the PE = 0.5 line. The proposed methodology was applied to experimental data collected to study the impact of soil subsurface hydrology on soil erosion and sediment transport processes. Sustained net deposition occurred under drainage condition while PE for seepage conditions mostly stayed in the net erosion regime. Results from this study suggest pulsating erosion patterns along concentrated flow networks with intermittent increases in PE to local maxima followed by declines to local minima. These short-range erosion patterns could not be explained by current Tc-based erosion models. Nevertheless, Tc-based erosion models adequately capture observed decline in local PE maxima as ω increased. Applying the proposed approach suggests a dependence of Tc on subsurface hydrology with net deposition more likely under drainage conditions compared to seepage conditions

Restoring Degraded Rangelands in Jordan: Optimizing Mechanized Micro Water Harvesting using Rangeland Hydrology and Erosion Model (RHEM).

Jordan's rangelands, the so called Badia, home of the Bedouins, are threatened through a combination of over-exploitation of the ecosystem services and a changing climate towards drier seasons and highly erratic rainfalls. In the recent decades, the once productive grazing lands transformed into sparsely vegetated and crusted desert grounds not capable of retaining the sporadic rainwater within the landscape - and consequential surface runoff inevitably accelerates soil erosion and gullying. To counter-measure the imminent rangeland degradation the International Center for Agricultural Research in the Dry Areas (ICARDA) is investigating mechanized micro Water Harvesting (WH) based restoration technique using the Vallerani plow. This technique intermediately breaks up the crusted surface soil layers and hard pans to approximately 50cm depth. Thus, creating dispersed micro-catchments, well-protected and suitable for the plantation of shrub seedlings, supporting the initial vegetation growth and eventually leading to shrub-island evolvement over the landscape. However, optimum design, particularly the spacing between the WH plow lines, depend on various environmental conditions. In this research, Rangeland Hydrology and Erosion Model (RHEM) was used to assess degraded rangeland hydrological response to transparently suggest on WH layout optimized for the magnitudes and the occurrence probabilities of runoff, soil erosion and sediment accumulation affecting the storage capacity of the micro-catchments. The study combined physical based modeling and ground truthing through different runoff and sediment related experiments. Preliminary results demonstrate good potential of the RHEM-based WH design approach; case study results will be presented for the first time at the conference. Eventually, a fully developed rangeland assessment system will support transparent target area selection and sound WH design interlinked with a risk analysis approach that accounts for the variable environmental patterns of the Badia

Evaluation of structure from motion for soil microtopography measurement

Recent developments in low-cost structure-from-motion (SfM) technologies offer new opportunities for geoscientists to acquire high-resolution soil microtopography data at a fraction of the cost of conventional techniques. However, these new methodologies often lack easily accessible error metrics and hence are difficult to evaluate. In this research, a framework was developed to evaluate a SfM approach for soil microtopography measurement through assessment of uncertainty sources and quantification of their potential impact on overall 3D reconstruction. Standard deviations of camera interior orientation parameters estimated from SfM self-calibration within five different soil surface models were several orders of magnitude larger than precisions expected from pattern-based camera calibration. Sensitivity analysis identified the principal point position as the dominant source of calibration-induced uncertainty. Overall, surface elevation values estimated from both technologies were similar in magnitude with a root mean square (RMS) of elevation difference of 0·2 mm. Nevertheless, the presence of deformation in either SfM or traditional photogrammetric point clouds highlights the importance of quality assurance safeguards (such as a judicious choice of control points) in SfM workflows for soil microtopography applications