Younes Alila


Relevant Thesis-Based Degree Programs


Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Understanding and predicting forest harvesting effects on peak flows in snow environments with nonstationary frequency modelling (2020)

A century of paired watershed studies evaluated the effects of forest harvesting on peak flows by pairing events by equal chronology. This method has recently come under repeated criticism and calls have been made to abandon the practice and pair by equal frequency instead. However, the stationarity assumption imposed by conventional frequency analyses complicates the use of frequency pairing because peak flows contain change-point and trend-shifting nonstationarities caused by continuous harvesting and forest regrowth. Here, a new nonstationary frequency pairing method was introduced to evaluate harvesting effects by allowing the parameters of peak flow frequency distributions to change in time using physically-based covariates. This research falls within the emerging field of “attribution science”, which uses observations and models to identify separately the factors contributing to extremes.The outcomes of applying this new method at five treatment-control, snow-dominated watersheds (37 km2 to 3550 km2) revealed how harvesting increased the magnitude and frequency of not only small (10-yr return period) peak flows. This was a consequence of changes to both the means (+28% to +113%) and standard deviations (no effects to +110%) of the peak flow frequency distributions. Large peak flows became 3-4 (10-20) times more frequent in the least (most) sensitive watershed. The different treatment effects reveal contrasting watershed sensitivities to harvesting, owing to different physiographic characteristics and logging histories. Based on the collective outcomes, a physical model encapsulating harvesting effects at the stand- and watershed-levels was developed to advance the probabilistic understanding of the forests and floods relation.Advantages of the new method include: (i) relaxing the watershed size and proximity constraints between control and treatment watersheds; (ii) detecting small levels of change in the peak flow time series with moderate harvesting levels; (iii) bypassing the need for a calibration equation, hence eliminating associated sources of uncertainty; (iv) making use of larger sample sizes to conduct frequency analysis, which make better inferences about the effects on extreme events; and (v) allowing for the estimation of harvesting effects at different historic snapshots of a watershed, thus providing an evaluation of hydrologic recovery.

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An investigation into the Flow Duration Curve in eastern United States: environmental controls and prediction at ungauged basins (2018)

The Flow Duration Curve (FDC) is a probabilistic flow representation relevant to streamflow investigation and physical understanding given its use in wide range of hydrological and ecological applications. A regional study that investigates FDCs and their prediction at ungauged catchments is important to develop causal models and provide insights to solve issues of water resources planning and management of aquatic ecosystems and habitats. Hydrological modelling and model parametrization in gauged and ungauged catchments are fundamental steps preceding the regional investigation of flow response using FDC. By means of Sacramento rainfall-runoff model (SAC-SMA), in 73 catchments from the eastern United States, I investigated the effect of SAC-SMA a priori parameters in the constrained calibration of the model. This analysis revealed and discussed limitations of a priori parameters that are intensively used to facilitate model calibration and make predictions at ungauged basins (PUB). The PUB using parameter transfer within homogeneous regions of similar climate and flow characteristics outweighed in performance the a priori parameters. The FDC was advantageous in revealing the effect of lack of efficiency and bias. A parameter regionalization approach is more efficient for PUB than a priori parameters. The ultimate limitations of the within-region parameter transfer are recognized and discussed. The interaction between climate and landscape properties was central to develop the physical understanding of the FDC. The high precipitation variability does not necessarily lead to FDC of a steep slope. Characteristics of the catchment— equivalent to a precipitation filter— interplayed with the precipitation and affected FDC shapes. The analysis highlighted the role of soil infiltration rates in specific conditions of soil moisture storage capacity and predominant runoff generation mechanism. The study revealed that processes underlying FDC and the FDC shapes are by far more complex than being characterized in the wider literature using characteristics of landscape or climate. In conditions of humid climate and perennial flow, a meta-analysis utilizing a process-based investigation showed that mean monthly runoff FDC —readily available at ungauged catchments—predicts only FDC middle third (exceedance probabilities between 33% and 66%). The method is constrained by the value of flow variability (slope of the FDC).

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Modelling sediment dynamics at the basin scale: implications of changes in climate and hydrological regimes (2018)

Basin-wide sediment dynamics are closely linked to hydrological processes and landscape and therefore expected to be susceptible to climate change. Simulating sediment transport through large basins presents a challenging problem to modellers; the relationship between water flux and sediment load is complex and non-linear, and significant sediment generation can occur over small spatial and time scales. To date, most studies have employed lumped empirical models that predict annual load at the outlet of a study basin, but do not consider variability across the basin or sub-annually. In this study, we adapt a physically-based, distributed suspended sediment transport model for large-scale use. The sediment model is integrated into the Terrestrial Hydrology Model with Biochemistry (THMB) as a routine to make use of THMB’s dynamic water routing. The coupled model is applied to the 230,000 km² Fraser River Basin (FRB) in British Columbia, Canada using 1) historical hydrological input to test the model and 2) synthetic input derived from Intergovernmental Panel on Climate Change (IPCC) scenarios A1B, A2, and B1 to study potential impacts of climate change. In both cases the input data is provided by the Pacific Climate Impacts Consortium (PCIC) and comes from simulations using the Variable Infiltration Capacity (VIC) model. Simulation results using historical inputs are compared with observations at five stations using the coefficient of determination (R²), Nash-Sutcliffe coefficient of efficiency (NSE), and percent bias (PBIAS) metrics. Overall, simulated load values match well with observed values, with the monthly simulations at the station nearest the outlet scoring R² = 0.78, NSE = 0.77, and PBIAS = -20%. Simulation results using climate scenario-driven inputs are studied for potential future changes in sediment dynamics. Results re- veal a general shift in hillslope erosion and sediment yield towards larger values from autumn to spring, reduced summer values, and an overall annual increase, with hillslope generation growing 35-45% from baseline levels and yield at the basin’s outlet increasing 10-15%. These physically-based results offer unique insights into the impacts of climate change on sediment processes within a large basin and their potential implications.

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Forests, floods and channel processes: Illuminating links between forest harvesting, the flood regime and channel response in snowmelt headwater streams (2014)

A meta-analysis of four snowmelt catchments with moderate harvest levels (30% to 40%) utilizing a frequency-based approach demonstrates, despite a century’s-worth of studies to the contrary, how harvesting increases the magnitude and frequency of all floods on record including the largest floods (R.I. = 1:50 yrs.) and how such effects increase unchecked with increasing return period as a consequence of changes to both the mean and standard deviation of the flood frequency distribution. Additionally, meta-analysis outcomes reveal up to three-fold increases in the number and duration of peak flows with return periods ranging from 0.6*Q₁.₅ to Q₁₀, which includes floods capable of mobilizing bedload and altering the form of gravel-bed streams. A frequency-based meta-analysis provides new insights concerning the physical processes governing the relation between forests and floods in snowmelt environments that were previously unrecognized using traditional chronological-pairing methods. The dominant process responsible for flood regime changes following harvesting is the increase in basin-average snowmelt rates that are amplified or mitigated by physical characteristics such as aspect distribution, elevation range, slope gradient and amount of alpine area. The outcomes of a high-resolution, nested-monitoring investigation of hydrologic and geomorphic controls on bedload mobility indicate that flow regime changes from harvesting in snowmelt streams can alter rates of bedload transport, a first-order determinant of channel form in fluvial systems. Regression analysis shows that annual sediment yield is controlled by the number of peaks-over-threshold discharge. During peak events, repeated destabilization of channel armor and re-mobilization of sediment temporarily stored behind LWD generates bedload transport across the entire snowmelt season. However, the potential for channel response to changes in the flow regime depends on characteristics of channel morphology. Study results indicate that patterns of bedload entrainment and mobility are influenced by flow resistance associated with channel form, grain size and LWD while the value of the critical dimensionless shear stress varies with channel gradient, relative and absolute grain effects and flow resistance. Differences in bed texture, gradient and channel form contributes to variations in rates and characteristics of bed mobility, hence differences in potential for alteration due to changes in the flow regime.

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Modeling deep groundwater flow through fractured bedrock in a mountainous headwater catchment using a coupled surface water-groundwater model, Okanagan Basin, British Columbia (2013)

Quantifying recharge to the mountain block from headwater catchments in snowmelt dominated upland mountainous regions is an important aspect of hydrologic studies. This study contributes to understanding of the interaction between surface water, soil water and deep groundwater flow in headwater catchments. A novel approach was developed for estimating the bedrock hydraulic conductivity of a regional-scale fractured bedrock aquifer using discrete fracture network (DFN) modeling. The methodology was tested in the mountainous Okanagan Basin, British Columbia, Canada. Discrete fractures were mapped in outcrops, and larger-scale fracture zones (corresponding to lineaments) were mapped from orthophotos and LANDSAT imagery. Outcrop fracture data were used to generate DFN models for estimating hydraulic conductivity for the fractured matrix (Km). The mountain block hydraulic conductivity (Kmb) was estimated using larger-scale DFN models. Simulated Km and Kmb values range from 10⁻⁸ to 10⁻⁷ m/s, are consistent with estimates from regional modeling studies, and are greatest in a N-S direction, coinciding with the main strike direction of Okanagan Valley Fault Zone. Kmb values also decrease away from the fault, consistent with the decrease in lineament density. Simulated hydraulic conductivity values also compare well with those estimated from pumping tests. The estimates of Kmb were then used to represent the deep bedrock in a coupled surface water - groundwater model using MIKE SHE for the Upper Penticton Creek 241 headwater catchment in the Okanagan Basin. Although highly uncertain due to parameter uncertainty and calibration error, recharge to deep groundwater was ~4% of the annual water budget. An specified outward flux from the catchment boundary, representing ~6% of annual water budget, did not significantly impact streamflow calibration, indicating that such deep groundwater losses from the catchment can be accommodated in a model. This outflow may contribute to cross-catchment flow and, ultimately, to groundwater inflow to lower elevation catchments in the mountain block. The modeling exercise is one of the first in catchment hydrology modeling within steep mountainous terrain in which the lower boundary of the model is not treated as impermeable, and in which recharge to the deep bedrock and discharge to the surrounding mountain block were estimated.

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Bankfull and effective discharge in small mountain streams of British Columbia (2012)

In this thesis, the concept of channel-forming discharge developed for large lowland rivers is critically re-evaluated for small mountain streams. In large lowland rivers under equilibrium conditions, the effective discharge (the discharge interval that transports the greatest proportion of sediment) approximates the bankfull discharge. The effective and bankfull discharges therefore provide measurable analogues for the theoretical channel-forming discharge, responsible for the form and dimensions of the stream channel. In small mountain streams this concordance of bankfull and effective discharges has been suspected to break down, in part because the channel form and dimensions are determined by the non-fluvial sediment supply as well as by fluvial processes. This interaction of non-fluvial sediment supply with fluvial processes makes the application of models and concepts developed for lowland rivers to mountain streams difficult. The history of past glaciation in mountain environments adds complexity in the form of increased sediment supply and changes in streamflow regime over time.Bankfull and effective discharges are measured and estimated for small mountain streams in three distinct hydroclimatic zones in southern British Columbia. These discharges are determined using surveyed stream cross-sections, photogrammetric analysis of in-channel sediment, long-term Water Survey of Canada gauging records, and a two-fraction sediment transport model.Results indicate multiple orders of magnitude of variability in the frequency and magnitude of bankfull and effective discharge for the studied streams, and little correlation between the bankfull and effective discharges. Bankfull frequency can be related to hydraulic and hydroclimatic variables while effective discharge frequency cannot. Most of the studied streams are incised, with bankfull discharges one or more orders of magnitude greater than their effective discharge, a condition attributable to the legacy of Pleistocene glaciation. In these streams the effective discharge is not a channel-forming discharge; at best, it is a channel-maintaining flow, and at worst it is geomorphically meaningless. In a few streams, the bankfull and effective discharges are both large, rare events; these are the only streams in the study in which the bankfull and effective discharges approximate a truly channel-forming discharge.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Hypersensitivity of peak flows to clearcut logging in British Columbia’s snow environment (2023)

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Frequency pairing reveals how large peak flows can be highly sensitive to forest treatment in rain environment: impacts increase nonlinearly with event size (2022)

Decades of literature on forest treatment – peak flow relations have generated considerable disagreements and turned the topic into what have been regarded as enigmatic. Using the new science of causation to investigate forest treatment peak flow relations is a necessary step needed to advance the understanding and prediction of forest treatment effects on peak flows. The causes of peak flows are multiple and chancy and, hence, can only be investigated via a probabilistic approach. We analyze peak flow data using the peak flow frequency distribution framework using two pairs of control-treatment watersheds in the rain environment of Coweeta - United States Department of Agriculture Forest Service experimental forest situated in North Carolina, south eastern United States of America. We demonstrate how a wide range of forest treatments change the magnitude and frequency of all peak flows on record and how such effects can increase with increasing event size as a consequence of changes to the peak flow frequency distribution. The dominant process responsible for the changes to the parameters characterizing the frequency distribution such as mean, variance, and skewness of peak flows is the treatment-induced suppression of evapotranspiration and changes to non-vegetative factors; which alter the soil storage capacity, moisture available for runoff, and the efficiency with which such runoff is delivered to the outlet of the watershed. The dominant topographical aspect of the treatment watershed, the seasonal differences in storm types, the extent to which the storm events are in-phase or out-of-phase with high antecedent soil moisture, and lagged runoff responses and watershed memory emerged as key indicators of the sensitivity of peak flow regime to forest treatment. We call for more research on forest treatment – peakflow regime relations using the stochastic approach to physics and prediction, which is standard practice in the wider hydrology but not as commonly used in forest hydrology.

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Nonstationary stochastic paired watershed approach: investigating forest harvesting effects on floods in two large, nested, and snow-dominated watersheds in British Columbia, Canada (2022)

Drawing on advances in nonstationary frequency analysis and the science of causation and attribution, this study employs a newly developed nonstationary stochastic paired watershed approach to determine the effect of forest harvesting on floods. Moreover, this study furthers the application of stochastic physics to evaluate the environmental controls and drivers of flood response. Physically-based climate and time-varying harvesting data are used as covariates to drive the nonstationary flood frequency distribution parameters to detect, attribute, and quantify the effect of harvesting on floods in the snow-dominated Deadman River (878 km²) and nested Joe Ross Creek (99 km²) watersheds. Harvesting only 21% of the watershed caused a 38% and 84% increase in the mean but no increase in variability around the mean of the frequency distribution in the Deadman River and Joe Ross Creek, respectively. Consequently, the 7-year, 20-year, 50-year, and 100-year flood events became approximately two, four, six, and ten times more frequent in both watersheds. An increase in the mean is posited to occur from an increase in moisture availability following harvest from suppressed snow interception and increased net radiation reaching the snowpack. Variability was not increased because snowmelt synchronization was inhibited by the buffering capacity of abundant lakes, evenly distributed aspects, and widespread spatial distribution of cutblocks in the watersheds, preventing any potential for harvesting to increase the efficiency of runoff delivery to the outlet. Consistent with similar recent studies, the effect of logging on floods is controlled not only by the harvest rate but most importantly the physiographic characteristics of the watershed and the spatial distribution of the cut blocks. Imposed by the probabilistic framework to understanding and predicting the relation between extremes and their environmental controls, commonly used in the general sciences but not forest hydrology, it is the inherent nature of snowmelt-driven flood regimes which cause even modest increases in magnitude, especially in the upper tail of the distribution, to translate into surprisingly large changes in frequency. Contrary to conventional wisdom, harvesting influenced small, medium, and very large flood events, and the sensitivity to harvest increased with increasing flood event size and watershed area.

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Revealing forest harvesting effects on large peakflows in rain-on-snow environment with new stochastic physics (2017)

Using nine pairs of control-treatment watersheds with varying climate, physiography, and harvesting practices in the Rain-On-Snow (ROS) environment of the Pacific Northwest region, this thesis demonstrates the linkage between environmental control and the sensitivity of peakflow response to harvesting effects. Compared to previous paired watershed studies in ROS environment, this study, for the first time, employed an experimental design of Frequency Pairing to isolate the effects of disturbances on systems’ response. The use of frequency distributions for evaluating the relation between forest harvesting and peakflows is a well-established framework outside forest hydrology literature. The results show how harvesting can dramatically increase the magnitude of all peakflows on record and how such effects can increase with increasing return periods, as a consequence of substantial increases to the mean and variance of the peakflow frequency distribution. Most critically, peakflows with return period larger than 10 years can increase in frequency, where the larger the peakflow event the more frequent it may become. The sensitivity of the upper tail of the frequency distribution of peakflows was found to be linked to the physiographic and climatic characteristics via a unifying synchronization / desynchronization spatial scaling mechanism that controls the generation of rain-on-snow runoff. This new physically-based stochastic hydrology understanding on the response of watersheds in ROS environments runs counter the deterministic prevailing wisdom of forest hydrology, which presumes a limited and diminishing role of forest cover as the magnitude of the peakflow event increases. By demonstrating the need for invoking the dimension of frequency in the understanding and prediction of the effects of harvesting on peakflows, this study added another brick to the pile of evidence in calling for the abandonment of the outdated pure deterministic hypotheses and experimental designs that have misguided forest hydrology research for over a century on this topic.

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Hydrogeomorphic distrubance, landscape development and riparian vegetation dynamics of an alluvial, temperate rainforest in the Carmanah River Valley, British Columbia, Canada (2012)

The alluvial forest of the Carmanah River valley on the west coast of Vancouver Island, British Columbia, was studied to examine the role of hydrogeomorphic disturbance in perpetuating the shifting-mosaic of habitats within this diverse ecosystem. Field-based research was complemented by a landscape-scale analysis that examined changes in the extent of specific forest types using a 70-year aerial photographic record. Thirty-eight plots containing 4509 trees were sampled for forest structure, composition, age, understory composition, and elevation above the contemporary channel. These field data, including a vegetation chronosequence spanning over 500 years, were used to examine vegetation dynamics. Over the past century, Carmanah River has eroded nearly 30% of the alluvial forest in this study area – 65% over the past 500 years. High magnitude floods result in diminished floodplain forest area by converting forests to channel. This results in a subsequent course of vegetation succession and geomorphic development. Fluvial deposits are colonized by a high density of Alnus rubra accompanied by a subcanopy of Picea sitchensis individuals. As Alnus die off after 60-100 years, Picea increasingly dominates the canopy while Tsuga heterophylla regenerate within the understory. The original cohort of Picea dies off after 300-500 years, which allows Tsuga to dominate old growth terrace forests. Picea or Alnus do not tend to regenerate under these dense canopies and without disturbance Tsuga may remain dominant indefinitely. Understory composition was related to landform age, however species distributions at low elevation floodplain sites were also driven by elevation above thalweg and flood frequency. Light availability was also a significant factor in driving community composition. It appears that understory dynamics were linked to overstory succession and geomorphic development processes, which alter environmental conditions at the understory level. That is, species distributions are driven by dynamic environmental filters, which change as a result of biogeomorphic succession. Mature forest patches tended to persist longer than young forests. The landscape composition reflects a balance between episodes of hydrogeomorphic disturbance and periods of successional development. Increased hydrogeomorphic disturbance rates due to climate change have the capacity to alter the landscape composition resulting in diminished mature forest area.

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Stage discharge estimation using a 1d river hydraulic model and spatially-variable roughness (2010)

Stage-discharge relations (rating curves) are integral to stream gauging, yet the existing empirical calibration methods are expensive, particularly in remote areas, and are limited to low flows. Numerical modelling can provide stage-discharge relations from a single site survey, reducing the overall cost, and can be fit to changing surface conditions. This study explores a one-dimensional model to calculate theoretical stage-discharge relations for four field sites in British Columbia that range in bed stability, bed structure, hydrology and sediment supply. However, due to the non-linear relation between flow and roughness we do not assume the conventional reach-averaged roughness and instead employ a spatially-distributed roughness model. Furthermore, based on local grain size distribution and refined field survey technique, new formulae for wetted perimeter, flow area, and flow depth were developed that eliminate commonly held modelling assumptions and reduce topographic error. The results show (1) good agreement with Water Survey of Canada measurements, (2) distributed roughness provided an improvement over spatially-averaged roughness, (3) spatial variability of the geomorphology within the channel reach leads to shifts in the stage-discharge relations and high sediment amplifies those shifts, and (4) the relations must be re-evaluated following events that mobilize the bed. The method can be used to estimate high flows and flows in remote locations and it does not require calibration.

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