Younes Alila

Forests and floods: A new paradigm sheds light on age‐old controversies

[1] The science of forests and floods is embroiled in conflict and is in urgent need of reevaluation in light of changing climates, insect epidemics, logging, and deforestation worldwide. Here we show how an inappropriate pairing of floods by meteorological input in analysis of covariance (ANCOVA) and analysis of variance (ANOVA), statistical tests used extensively for evaluating the effects of forest harvesting on floods smaller and larger than an average event, leads to incorrect estimates of changes in flood magnitude because neither the tests nor the pairing account for changes in flood frequency. We also illustrate how ANCOVA and ANOVA, originally designed for detecting changes in means, do not account for any forest harvesting induced change in variance and its critical effects on the frequency and magnitude of larger floods. The outcomes of numerous studies, which applied ANCOVA and ANOVA inappropriately, are based on logical fallacies and have contributed to an ever widening disparity between science, public perception, and often land-management policies for decades. We demonstrate how only an approach that pairs floods by similar frequency, well established in other disciplines, can evaluate the effects of forest harvesting on the inextricably linked magnitude and frequency of floods. We call for a reevaluation of past studies and the century-old, preconceived, and indefensible paradigm that shaped our scientific perception of the relation between forests, floods, and the biophysical environment.

A hierarchical approach for the regionalization of precipitation annual maxima in Canada

The L moments are used in the three stages of regional frequency analysis: the delineation of homogeneous regions, the identification of a regional parent distribution, and the estimation of distribution parameters. Numerical analysis is conducted on 5 min to 24 hours annual rainfall extremes from 375 precipitation gaging stations in Canada. The numerical analysis concluded that Canada could be considered as a single homogeneous region in which the L skewness and L kurtosis display no significant spatial variability. Also, on the basis of mean annual precipitation (MAP), Canada can be divided into climatologically homogeneous subregions, in which the L coefficient of variation is virtually constant. The parent distribution was identified as the general extreme value (GEV), the parameters of which depend on the MAP and storm duration. A hierarchical regional approach is proposed for fitting the identified GEV distribution, where the L skewness, L coefficient of variation, and mean are estimated on a regional, subregional, and single-site basis, respectively. Monte Carlo simulations indicate that design storms estimated by the proposed hierarchical approach are substantially more accurate than those estimated by the single-site method. The simulations also demonstrate that the proposed hierarchical approach makes the estimation of design storms at ungaged sites less dependent on the availability of precipitation data.

Regional rainfall depth‐duration‐frequency equations for Canada

The geographical variation of short-duration rainfall extremes in Canada is first evaluated in terms of depth-duration and depth-frequency ratios. Depth-duration ratios, defined as the ratios of the t-min to the 60-min rainfall depths of the same return period, are found to be independent of return period and geographical location for any storm duration of less than 60 min. However, for storms of longer durations, depth-duration ratios are found to depend on both the return period and geographical location indexed by the at-site mean annual precipitation. Depth-frequency ratios, defined as the ratios of the T-year to the 10-year rainfall depths of the same storm duration, are also found to depend on the return period and geographical location. Generalized expressions of depth-duration and depth-frequency ratios are then combined to develop regional depth-duration-frequency equations. A split sampling experiment has verified that the proposed equations reproduce satisfactorily the design storms at long-term record stations in different hydrologic zones. The proposed equations represent a viable alternative to current interpolation procedures as they eliminate the need for isoline maps of both mean and standard deviation of annual rainfall maxima for various durations.

Regional rainfall distribution for Canada

Abstract Current point rainfall frequency analysis techniques used in engineering design are outdated. In this study, data distribution and analytical techniques are reviewed, and a new method for regional analysis of rainfall is presented. The regional analysis is performed using data across Canada, and takes advantage of recently developed linear order statistics of L-moments. Numerical analysis of 320 stations showed that Canada may be considered as one homogeneous region with respect to L-skewness and L-kurtosis. However, the L-coefficient of variation shows regional variability that is related to the mean annual precipitation (MAP). A regional parent distribution is identified as the general extreme value (GEV), with parameters depending on MAP and storm duration. These findings differ from present methodology, whereby the Gumbel I distribution is used irrespective of storm duration.

Regional analysis of annual maxima precipitation using L-moments

Abstract Current precipitation frequency analysis used as a standard for urban runoff calculations are out of date. In this paper, the form of the distribution governing rainfall intensity of various durations in a regional context is investigated using the method of L -moments. The results indicate that the currently used Gumbel type I distribution is not generally valid for all the durations considered in this analysis. The form of the distribution is dependent on the duration of the event being considered.

Potential impacts of climate change on water availability for crops in the Okanagan Basin, British Columbia

Crop water demand in the Okanagan Basin was determined for 1961 to 1990, 2010 to 2039, 2040 to 2069, and 2070 to 2099. Daily station temperature data were spatially interpolated to a 1 × 1 km grid and adjusted for elevation. Daily precipitation data were estimated across four climatic regions. Output from three global climate models (GCM), CGCM2, CSIROMk2 and HadCM3 was used to create future daily climate. Daily potential evapo-transpiration (grass reference) was estimated from an empirical relationship between Bellani- plate atmometer readings, temperature and extra-terrestrial solar radiation, and then modified by crop coefficients for all crops except pasture. Depending on GCM, projected water demand increased by 12–20% (2010 to 2039), 24–38% (2040 to 2069) and 40–61% (2070 to 2099). Possible elevated CO2 effects on stomatal conductance, which may reduce water demand, were not accounted for. Comparisons with modeled Okanagan Lake inflows indicated that, on average, high water demand and low supply scen...

A New Low-Cost, Stand-Alone Sensor System for Snow Monitoring

Abstract Monitoring continuous changes in snowpack dynamics and its meteorological drivers is critical for understanding key aspects of water resources, climate variability, and ecology. While manual snow surveys have traditionally been used to evaluate snow processes, their high costs and discrete measurements can lead to biased estimations of accumulation and ablation rates. Ultrasonic range sensors offer an alternative to continuously monitor snow depth but their widespread employment has been limited because of high prices. This paper describes the development of an inexpensive prototype ultrasonic sensor suite characterized by a ready-to-use stand-alone design and flexibility to incorporate additional meteorological instruments. The performance of 48 units was tested during a winter season in central British Columbia, recording snow depth and air temperature data consistent with those from nearby weather stations and manual measurements. Despite a relatively small underestimation of snow depth due to...

Generation of an Hourly Meteorological Time Series for an Alpine Basin in British Columbia for Use in Numerical Hydrologic Modeling

Abstract Spatially distributed numerical hydrologic models are useful tools for examining the long-term impact of forest harvesting in mountainous basins on streamflow regime properties. Such models require the input of long-duration subdaily meteorological time series data that are not routinely available in mountainous headwater basins. A relatively simple method is presented for extending short-duration records by using a combined stochastic–empirical technique, and the approach is demonstrated using the Redfish Creek in British Columbia, Canada. Synthetic hourly precipitation, precipitation gradient, air temperature, temperature lapse rate, wind speed, relative humidity, solar beam and diffuse irradiance, and downward longwave irradiance for two station locations are generated in a three-step process: 1) hourly precipitation is generated using a clustered rectangular pulse point process, 2) daily meteorology is generated using a multivariate first-order autoregressive process, and 3) final hourly nonp...

Reply to comment by Jack Lewis et al. on “Forests and floods: A new paradigm sheds light on age-old controversies”

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Effective discharge in small formerly glaciated mountain streams of British Columbia: Limitations and implications

Episodic sediment supply, past glaciation, and slow responses to disturbance make small mountain streams transitional alluvial regimes in which nonequilibrium conditions are common. Bed load effective discharge in these streams is on average a low-magnitude, high-frequency event, but is highly variable. Using a two-phase sediment transport model and long-term discharge records, we distinguish between three types of streams; streams in which gravel (sediment > 8 mm diameter) moves frequently and effective discharge occurs during gravel transport (Frequently Mobile Gravel (FMG)), streams in which gravel moves infrequently but effective discharge nonetheless occurs during gravel transport (Infrequently Mobile Gravel (IMG)), and streams in which sand (sediment < 8 mm diameter) moves over largely immobile gravel and effective discharge occurs frequently during sand-phase transport (Sand over Immobile Gravel (SG)). Using only effective discharge frequency or magnitude to characterize a stream, without information on mobile sediment type, is insufficient to distinguish between FMG and SG streams. Only the IMG streams have large, rare effective discharges that approximate the bankfull discharge; in FMG and SG streams the effective discharge is much more frequent and smaller than the bankfull. Only in the IMG streams does the effective discharge approximate a channel-forming discharge. In FMG and SG streams, the effective discharge bears little relation to the size or dimensions of the channel and is at best a channel-maintaining flow; at worst it is geomorphically meaningless. Effective discharge should not therefore be used in isolation as a proxy for channel-forming discharge for mountain stream channel design or management.