John W. Pomeroy

IAHS decade on predictions in ungauged basins (PUB), 2003-2012: Shaping an exciting future for the hydrological sciences

Abstract Drainage basins in many parts of the world are ungauged or poorly gauged, and in some cases existing measurement networks are declining. The problem is compounded by the impacts of human-induced changes to the land surface and climate, occurring at the local, regional and global scales. Predictions of ungauged or poorly gauged basins under these conditions are highly uncertain. The IAHS Decade on Predictions in Ungauged Basins, or PUB, is a new initiative launched by the International Association of Hydrological Sciences (IAHS), aimed at formulating and implementing appropriate science programmes to engage and energize the scientific community, in a coordinated manner, towards achieving major advances in the capacity to make predictions in ungauged basins. The PUB scientific programme focuses on the estimation of predictive uncertainty, and its subsequent reduction, as its central theme. A general hydrological prediction system contains three components: (a) a model that describes the key processes of interest, (b) a set of parameters that represent those landscape properties that govern critical processes, and (c) appropriate meteorological inputs (where needed) that drive the basin response. Each of these three components of the prediction system, is either not known at all, or at best known imperfectly, due to the inherent multi-scale space—time heterogeneity of the hydrological system, especially in ungauged basins. PUB will therefore include a set of targeted scientific programmes that attempt to make inferences about climatic inputs, parameters and model structures from available but inadequate data and process knowledge, at the basin of interest and/or from other similar basins, with robust measures of the uncertainties involved, and their impacts on predictive uncertainty. Through generation of improved understanding, and methods for the efficient quantification of the underlying multi-scale heterogeneity of the basin and its response, PUB will inexorably lead to new, innovative methods for hydrological predictions in ungauged basins in different parts of the world, combined with significant reductions of predictive uncertainty. In this way, PUB will demonstrate the value of data, as well as provide the information needed to make predictions in ungauged basins, and assist in capacity building in the use of new technologies. This paper presents a summary of the science and implementation plan of PUB, with a call to the hydrological community to participate actively in the realization of these goals.

A decade of Predictions in Ungauged Basins (PUB)—a review

Abstract The Prediction in Ungauged Basins (PUB) initiative of the International Association of Hydrological Sciences (IAHS), launched in 2003 and concluded by the PUB Symposium 2012 held in Delft (23–25 October 2012), set out to shift the scientific culture of hydrology towards improved scientific understanding of hydrological processes, as well as associated uncertainties and the development of models with increasing realism and predictive power. This paper reviews the work that has been done under the six science themes of the PUB Decade and outlines the challenges ahead for the hydrological sciences community. Editor D. Koutsoyiannis Citation Hrachowitz, M., Savenije, H.H.G., Blöschl, G., McDonnell, J.J., Sivapalan, M., Pomeroy, J.W., Arheimer, B., Blume, T., Clark, M.P., Ehret, U., Fenicia, F., Freer, J.E., Gelfan, A., Gupta, H.V., Hughes, D.A., Hut, R.W., Montanari, A., Pande, S., Tetzlaff, D., Troch, P.A., Uhlenbrook, S., Wagener, T., Winsemius, H.C., Woods, R.A., Zehe, E., and Cudennec, C., 2013. A decade of Predictions in Ungauged Basins (PUB)—a review. Hydrological Sciences Journal, 58 (6), 1198–1255.

Measurements and modelling of snow interception in the boreal forest

Snow accumulation and ablation processes are particularly important to the hydrology of cold climate forests. In order to calculate the distribution of snow cover and the loss of snow to sublimation, the amount of snowfall intercepted by forest canopies must be determined. This paper introduces a physically-based snowfall interception model that scales snowfall interception processes from branch to canopy. Previous models of snow interception have neglected the persistent presence and subsequent unloading of intercepted snow in cold climates and hence have only been applicable to regions where snow is quickly lost from the canopy. To investigate how snow is intercepted at the forest stand scale, measurements of wind speed, air temperature, above- and below-canopy snowfall, accumulation of snow on the ground and the load of snow intercepted by a suspended, weighed, full-size conifer were collected from spruce and pine stands in the southern boreal forest. These data show that interception increases at a declining rate with increasing snowfall, to a point where the intercepted load overcomes the strength of branches to support it. Leaf area, tree species and initial canopy snow load determine the snow storage capacity of the canopy. These factors, canopy coverage and snowfall are used to calculate snow interception, presuming an exponential decay in incremental interception as cumulative snowfall increases. The subsequent unloading of intercepted snow is additionally modelled as an exponential function of time. The sensitivity of the combined model to temperature, wind speed, snowfall, snow load and canopy structure is examined for weekly time-steps. The examination shows that interception efficiency is particularly sensitive to snowfall amount, canopy density and time since snowfall. A comparison of the model with weekly measurements of snow interception suggests that the method can be used to calculate snow interception successfully in a physically-based manner.

An evaluation of snow accumulation and ablation processes for land surface modelling

This paper discusses the development and testing of snow algorithms with specific reference to their use and application in land surface models. New algorithms, developed by the authors, for estimating snow interception in forest canopies, blowing snow transport and sublimation, snow cover depletion and open environment snowmelt are compared with field measurements. Existing algorithms are discussed and compared with field observations. Recommendations are made with respect to: (a) density of new and aged snow in open and forest environments; (b) interception of snow by evergreen canopies; (c) redistribution and sublimation of snow water equivalent by blowing snow; (d) depletion in snow-covered area during snowmelt; (e) albedo decay during snowmelt; (f) turbulent transfer during snowmelt; and (g) soil heat flux during meltwater infiltration into frozen soils. Preliminary evidence is presented, suggesting that one relatively advanced land surface model, CLASS, significantly underestimates the timing of snowmelt and snowmelt rates in open environments despite overestimating radiation and turbulent contributions to melt. The cause(s) may be due to overestimation of ground heat loss and other factors. It is recommended that further studies of snow energetics and soil heat transfer in frozen soils be undertaken to provide improvements for land surface models such as CLASS, with particular attention paid to establishing the reliability of the models in invoking closure of the energy equation. #1998 John Wiley & Sons, Ltd.

Evaluation of forest snow processes models (SnowMIP2)

Thirty-three snowpack models of varying complexity and purpose were evaluated across a wide range of hydrometeorological and forest canopy conditions at five Northern Hemisphere locations, for up t ...

The Prairie Blowing Snow Model: characteristics, validation, operation

Abstract Physically based algorithms that estimate saltation, suspension and sublimation rates of blowing snow using readily available meteorological and land use data are presented. These algorithms are assembled into a model, the Prairie Blowing Snow Model (PBSM), and used to describe snow transport on fields in a Canadian Prairie environment. Validation tests of PBSM using hourly meteorological data indicate differences between modelled and measured seasonal snow accumulations between 4 and 13%. Application of the blowing snow model using meteorological records from the Canadian prairies shows that the annual proportion of snow transported above any specific height increases notably with mean seasonal wind speed. An observed decrease in annual blowing snow transport and sublimation quantities with increasing surface roughness height becomes more apparent with higher seasonal wind speeds and temperatures. The annual quantity of snow transported off a fetch increases with fetch length up to lengths of between 300 and 1000 m, then remains relatively constant or slowly declines. Within the first 300 m of fetch 38–85% of annual snowfall is removed by snow transport, the amount increasing with wind speed. Beyond 1000 m of fetch, blowing snow sublimation losses dominate over transport losses. In Saskatchewan, sublimation losses range from 44 to 74% of annual snowfall over a 4000 m fetch, depending on winter climate. Notably, as a result of steady-state transport, the sum of snowcover loss due to blowing snow transport and sublimation does not change appreciably from its 1000 m fetch value for fetches 500 to 4000 m. The transition from primarily transport to primarily sublimation losses at the 1000 m fetch distance may be useful in assessing the effect of scale in snow hydrology.

Estimates of Threshold Wind Speeds for Snow Transport Using Meteorological Data

Abstract The threshold wind speed for snow transport is related to properties of the surface snowpack: snow particle bonding, cohesion, and kinetic friction. These properties are controlled by meteorological factors. A method is proposed that relates the threshold wind speed for the initiation of snow transport to standard surface meteorological observations. A complete dataset on the hourly threshold condition for snow transport as determined from visual observation was developed for 16 stations on the prairies of western Canada over six winters. The threshold wind speeds for wet snow transport are significantly different from those for dry snow transport. The majority of recorded threshold 10-m wind speeds ranged from 7 to 14 m s−1 with an average of 9.9 m s−1 for wet snow transport, and from 4 to 11 m s−1 with an average of 7.7 m s−1 for dry snow transport. The observations display a nonlinear but generally positive correlation between threshold wind speed and air temperature. An empirical model betwee...

A distributed model of blowing snow over complex terrain

Physically-based models of blowing snow and windflow are used to develop a distributed model of blowing snow transport and sublimation over complex terrain. The model is applied to an arctic tundra basin. A reasonable agreement with results from snow surveys is obtained, provided sublimation processes are included; a simulation without sublimation produces much greater snow accumulations than were measured. The model is able to reproduce some observed features of redistributed snowcovers: distributions of snow mass, classified by vegetation type and landform, can be approximated by lognormal distributions, and standard deviations of snow mass along transects follow a power law with transect length up to a cut-off. The representation used for the downwind development of blowing snow with changes in windspeed and surface characteristics is found to have a large moderating influence on snow redistribution.

APPLICATION OF A DISTRIBUTED BLOWING SNOW MODEL TO THE ARCTIC

Transportation, sublimation and accumulation of snow dominate snow cover development in the Arctic and produce episodic high evaporative fluxes. Unfortunately, blowing snow processes are not presently incorporated in any hydrological or meteorological models. To demonstrate the application of simple algorithms that represent blowing snow processes, monthly snow accumulation, relocation and sublimation fluxes were calculated and applied in a spatially distributed manner to a 68-km 2 catchment in the low Arctic of north-western Canada. The model uses a Landsat-derived vegetation classification and a digital elevation model to segregate the basin into snow ‘sources’ and ‘sinks’. The model then relocates snow from sources to sinks and calculates in-transit sublimation loss. The resulting annual snow accumulation in specific landscape types was compared with the result of intensive surveys of snow depth and density. On an annual basis, 28% of annual snowfall sublimated from tundra surfaces whilst 18% was transported to sink areas. Annual blowing snow transport to sink areas amounted to an additional 16% of annual snowfall to shrub‐tundra and an additional 182% to drifts. For the catchment, 19.5% of annual snowfall sublimated from blowing snow, 5.8% was transported into the catchment and 86.5% accumulated on the ground. The model overestimated snow accumulation in the catchment by 6%. The application demonstrates that winter precipitation alone is insuAcient to calculate snow accumulation and that blowing snow processes and landscape patterns govern the spatial distribution and total accumulation of snow water equivalent over the winter. These processes can be modelled by relatively simple algorithms, and, when distributed by landscape type over the catchment, produce reasonable estimates of snow accumulation and loss in wind-swept regions. #1997 John Wiley & Sons, Ltd.

The Energy Balance of the Winter Boreal Landscape

Abstract During the winter of 1993/94 a study to quantify the winter energy balance of the main cover types of the boreal landscape took place. The study was based on the southern edge of boreal forest in Canada. Measurements were made over a mature jack pine stand and a frozen lake. Shortwave albedos of 12% to 14% over the jack pine and 20% to 88% on the frozen lake (both depending on snow cover) were measured. There were correspondingly large contrasts in the total radiation inputs and the turbulent heat fluxes. The mean net all-wave radiation input was large and positive into the forest and negative over the lake. The sensible heat fluxes were of the same sign as the radiative inputs with positive values over the forest peaking at +200 W m−2 and failing to − 100 W m−2 over the lake. The evaporation from the forest depended on whether the there was snow on the canopy. When the canopy was snow-free, the evaporation was low, about 50% of net radiation but, when there was snow on the canopy, the evaporatio...