Younes Alila

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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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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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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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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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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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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