Wood in neotropical headwater streams, Costa Rica

Department Head: Sally J. Sutton.2010 Summer.Includes bibliographical references.Wood has been shown to be an integral component of forest streams throughout the temperate climate zone, both in terms of the physical structure of the channel and in terms of aquatic ecosystem function, but the function of wood in undisturbed tropical streams has not been studied. This dissertation represents the first systematic analysis of instream wood in a tropical setting to be published. This study was limited to the headwater streams (drainage area <8.5 km2) of La Selva Biological Station, on the Atlantic margin of Costa Rica, a wet tropical site with limited landslide activity. Although the results are instructive and enable comparisons with the vast temperate instream wood literature, they should not be construed as representative of debris flow-dominated wet tropical forest streams or of dry or seasonal tropical forest streams. Wood loads in the thirty 50-m-long study reaches examined ranged from 3.0 to 34.7 m3 of wood per 100 m of channel length and 41 to 612 m3 of wood per ha of channel area. Average values are 12.3 m3/100 m and 189 m3/ha. These values fall generally in the lower range of wood load reported for temperate streams, with values typically lower than those reported from the Pacific Northwest region and the Great Lakes region and within the range of those reported from the Rocky Mountain region and from Southern Hemisphere study sites. Comparisons to study sites in eastern North America, Europe, and Japan are problematic because La Selva is a generally undisturbed forest, whereas studies from those regions are conducted in streams with significant human impact and tend to have very small wood loads. Flow hydraulics appear to be the dominant control on the lateral distribution of wood in the channels of La Selva, but they are only a partial control on the longitudinal distribution of wood, explaining about half of the variation in wood load among the study sites. The remainder of the variation is likely caused by the stochastic nature of large tree fall. In spite of the high temporal variability of lateral input of wood to the channels, spatial variability is small, partially because of the paucity of landslides at La Selva. Therefore, I propose that instream transport has a greater influence on the longitudinal distribution of wood than lateral input variability. Wood in a representative subset of 10 of the 50-m-long study reaches was monitored for 2.3 years. The wood in the streams of La Selva is more transient than wood in most sites studied in the temperate zone, with piecewise mean residence times ranging from 2 to 12 years and volume-wise mean residence times ranging from 2 to 83 years among the 10 sites monitored. Average values were 5 and 7 years, respectively. These are roughly an order of magnitude shorter than mean residence times reported from the Pacific Northwest, but similar to times reported from the Colorado Rocky Mountains. The short residence times may be a result of more frequent large floods caused by the wet tropical climate, higher decay rates caused by the warm tropical climate, or both. Perhaps because of this transience, wood was found to have minimal influence on flow resistance in a subset of 6 of the 50-m-long study reaches. In contrast, wood has been shown to be a major control on flow resistance in temperate mountain streams. It is possible that the channel geometry and bed material size are adjusted to the frequent high discharges, which also mobilize and rework the wood, causing grain and form resistance to overwhelm any resistance contribution from wood. Instream wood at La Selva also appears to have a minimal influence on sediment transport. Jams in sand-bed channels and jams in boulder-bed channels had no associated residual elevation drop. Jams in gravel-bed channels did alter bed elevation by trapping sediment wedges behind them, but analysis of tracer clast movement at one gravel-bed jam resulted in no observable difference in transport distances or mobility between clasts placed upstream of the jam and those placed downstream. An additional forest-stream interaction that was documented is diel cycles in stream discharge associated with groundwater withdrawal by the forest for evapotranspiration. Analysis of the cycles indicates a strong correlation with vapor pressure differential, which previous researchers have found to correlate with sap flow. Further analysis of the cycles suggests that at low-stage conditions transmissivity dominates groundwater flow into the channel, while at high-stage conditions hydraulic gradient is dominant

The influence of large woody debris on post-wildfire debris flow sediment storage

Debris flows transport large quantities of water and granular material, such as sediment and wood, and this mixture can have devastating effects on life and infrastructure. The proportion of large woody debris (LWD) incorporated into debris flows can be enhanced in forested areas recently burned by wildfire because wood recruitment into channels accelerates in burned forests. In this study, using four small watersheds in the Gila National Forest, New Mexico, which burned in the 2020 Tadpole Fire, we explored new approaches to estimate debris flow velocity based on LWD characteristics and the role of LWD in debris flow volume retention. To understand debris flow volume model predictions, we examined two models for debris flow volume estimation: (1) the current volume prediction model used in US Geological Survey debris flow hazard assessments and (2) a regional model developed to predict the sediment yield associated with debris-laden flows. We found that the regional model better matched the magnitude of the observed sediment at the terminal fan, indicating the utility of regionally calibrated parameters for debris flow volume prediction. However, large wood created sediment storage upstream from the terminal fan, and this volume was of the same magnitude as the total debris flow volume stored at the terminal fans. Using field and lidar data we found that sediment retention by LWD is largely controlled by channel reach slope and a ratio of LWD length to channel width between 0.25 and 1. Finally, we demonstrated a method for estimating debris flow velocity based on estimates of the critical velocity required to break wood, which can be used in future field studies to estimate minimum debris flow velocity values.</p

HYDROLOGIC EFFECTS OF LARGE SOUTHWESTERN USA WILDFIRES SIGNIFICANTLY INCREASE REGIONAL WATER SUPPLY: FACT OR FICTION?

In recent years climate change and historic fire suppression have increased the frequency of large wildfires in the southwestern USA, motivating study of the hydrological consequences of these wildfires at point and watershed scales, typically over short periods of time. These studies have revealed that reduced soil infiltration capacity and reduced transpiration due to tree canopy combustion increase streamflow at the watershed scale. However, the degree to which these local increases in runoff propagate to larger scales—relevant to urban and agricultural water supply—remains largely unknown, particularly in semi-arid mountainous watersheds co-dominated by winter snowmelt and the North American monsoon. To address this question, we selected three New Mexico watersheds—the Jemez (1223 km ^2 ), Mogollon (191 km ^2 ), and Gila (4807 km ^2 )—that together have been affected by over 100 wildfires since 1982. We then applied climate-driven linear models to test for effects of fire on streamflow metrics after controlling for climatic variability. Here we show that, after controlling for climatic and snowpack variability, significantly more streamflow discharged from the Gila watershed for three to five years following wildfires, consistent with increased regional water yield due to enhanced infiltration-excess overland flow and groundwater recharge at the large watershed scale. In contrast, we observed no such increase in discharge from the Jemez watershed following wildfires. Fire regimes represent a key difference between the contrasting responses of the Jemez and Gila watersheds with the latter experiencing more frequent wildfires, many caused by lightning strikes. While hydrologic dynamics at the scale of large watersheds were previously thought to be climatically dominated, these results suggest that if one fifth or more of a large watershed has been burned in the previous three to five years, significant increases in water yield can be expected

Tectonic and climatic controls on endorheic-exorheic transitions in narrow continental rifts

Narrow continental rifts such as the East African Rift zone and the Rio Grande rift consist of a series of half-grabens separated by accommodation or transfer zones, bounded by border faults and uplifted rift shoulders with alternating polarity. Within this structural framework, drainages develop that transport water and sediments into and out of the rift basins. Dependent on sediment supply volume and climate, and the rate of accommodation space created (related to rift-opening rate), rift basins may be closed basins (internally drained), be in a state of balance-fill, or become open, overfilled basins with a throughgoing river system. Starting approximately 8 million years ago, the Rio Grande rift, a narrow continental rift zone in the southwestern U.S., transitioned from a series of internally drained basins (endorheic) to an open, externally drained Rio Grande river (exorheic). This endorheic-exorheic transition occurred several tens of millions of years after rift-opening started. We use landscape-evolution models to study the effect of tectonics (extension rates) on endorheic-exorheic transitions in narrow continental rifts in both ¿semi-arid¿ and ¿wet¿ climates. Sediment- and water discharge responses are analyzed before, during, and after rifting. We show how extension rate, basin area, and climate affect endorheic-exorheic transitions. The landscape that surrounds the rift basins (including rift-shoulders) and the rift basin area are also identified as important parameters in endorheic-exorheic transitions. We further find that the extension rate of a rift basin has a significant control on downstream discharge. These results suggest that accelerated integration of axial basins in the Rio Grande rift could be a function of upstream basin size.Peer Reviewe

Landscape evolution modeling of endorheic-exorheic transitions in active continental rifts

AGU Fall Meeting 2019, in San Francisco, 9-13 december 2019We use tectonic-landscape evolution models to understand endorheic-exorheic transitions in active continental rift zones such as the Rio Grande rift and the East African rift. Tectonic extension of continental lithosphere creates accommodation space in which sediments are deposited, and the balance between the creation of this accommodation space and the sediment volume that is deposited determines whether an active rift is endorheic or not. Our numerical experiments show that slow rift-opening rates, a slowing-down of rift opening, or increase of headwater topography (e.g., upstream epeirogenic uplift), are tectonic situations that can cause a transition from an endorheic to an exorheic drainage state in a rift basin. A crucial model parameter in these landscape evolution models is the erodibility. Analyses of model results reveal that landscape drainage formation and topographic roughness are closely linked to the erodibility coefficient that was used to model the landscape. We show that with a training dataset consisting of hundreds of thousands synthetic landscapes, machine learning algorithms can be used to identify the topographic patterns of a landscape. This method may provide a path toward better understanding landscape memory and erodibility values for natural landscapes

Validation and Comparison of a Model of the Effect of Sea-Level Rise on Coastal Wetlands

Models are used to project coastal wetland distribution under future sea-level rise scenarios to assist decision-making. Model validation and comparison was used to investigate error and uncertainty in the Sea Level Affecting Marshes Model, a readily available model with minimal validation, particularly for wetlands beyond North America. Accurate parameterisation is required to improve the performance of the model, and indeed any spatial model. Consideration of tidal attenuation further enhances model performance, particularly for coastal wetlands located within estuaries along wave-dominated coastlines. The model does not simulate vegetation changes that are known to occur, particularly when sedimentation exceeds rates of sea-level rise resulting in shoreline progradation. Model performance was reasonable over decadal timescales, decreasing as the time-scale of retrospection increased due to compounding of errors. Comparison with other deterministic models showed reasonable agreement by 2100. However, given the uncertainty of the future and the unpredictable nature of coastal wetlands, it is difficult to ascertain which model could be realistic enough to meet its intended purpose. Model validation and comparison are useful for assessing model efficacy and parameterisation, and should be applied before application of any spatially explicit model of coastal wetland response to sea-level rise