Sources of Uncertainty in Canadian Low Flow Hydrometric Data

Abstract

The uncertainty of estimated daily mean discharge is 5% at the 95% confidence interval (Herschy, 1999a). The use of this unique value requires acceptance of the assumption of uniformity of uncertainty within the hydrometric dataset. It is the implicit responsibility of each researcher to challenge this assumption with respect to any given hypothesis test. This paper evaluates this assumption of uniformity for a subset of the hydrometric dataset-low flow in Canada. Studies of low flow phenomena are becoming more prevalent with increasing recognition of the importance of low flows for viable ecosystems and sustainable economies and as a sentinel of change. Environmental and operational circumstances are identified that elevate the opportunity for error with respect to measurement of low flows. These factors are examined qualitatively and are found to exist throughout all steps of the data production process. The uncertainty of low flows in Canada is very likely different from the uncertainty of the global hydrometric dataset. The magnitude of low flow uncertainty remains undefined because no field experiments were conducted as part of this study. It is hoped that these findings will inspire the design of future research needed to overcome this deficiency. Résumé : L'incertitude liée au débit moyen quotidien estimatif est de 5 %, avec un intervalle de confiance à 95 % (Herschy, 1999a). Le recours à cette valeur unique implique l'acceptation du postulat d'uniformité de l'incertitude dans l' ensemble de données hydrométriques. Chaque chercheur a la responsabilité implicite de remettre en question ce postulat dans le contexte d'une vérification d'hypothèse. La présente communication évalue ce postulat d'uniformité pour un sous-ensemble de l' ensemble de données hydrométriques-basses eaux au Canada. Les études portant sur les phénomènes d' étiage (basses eaux) sont de plus en plus répandues étant donné la reconnaissance croissante de l'importance des basses eaux pour la viabilité des écosystèmes et la durabilité des économies, et aussi comme sentinelle du changement. Sont cernées les circonstances environnementales et opérationnelles qui élèvent les possibilités d' erreur relativement à la mesure des basses eaux. Ces facteurs sont examinés qualitativement et, selon les constatations, ils existent à tous les stades du processus de production de données. L'incertitude liée à l' étiage au Canada est fort probablement différente de l'incertitude liée à l' ensemble des données hydrométriques globales. L'ampleur de l'incertitude liée à l' étiage n'a toujours pas été définie car aucune expérience sur le terrain n'a été menée en tant que volet de l' étude. Ces résultats, espère-ton , serviront d'inspiration pour l' élaboration d'une méthodologie de recherche future permettant de surmonter cette lacune.

Evaluating the ability of a hydrologic model to replicate hydro‐ecologically relevant indicators

“…streamflow gauging and stage–discharge relationship uncertainties) will lead to a certain degree of deviations between the observed and simulated results. In particular, the ability to obtain reliable low‐flow measurements is complicated by factors such as hyporheic flow exchange and the presence of river ice and aquatic vegetation (Hamilton, ), leading to considerable uncertainties in the measured discharge. The fact that over 35% of the observed data used are obtained under ice conditions (Water Survey of Canada, hydrometric data: http://www.wsc.ec.gc.ca/applications/H2O/index-eng.cfm) implies considerable uncertainty in the measured low‐flow values.…”

“…errors in precipitation measurement, and interpolation uncertainties due to elevation and orographic effects; (ii) output uncertainty, e.g. errors in observed discharge data due to hydrometric data uncertainties (Pelletier, ; Hamilton, ; Hamilton and Moore, ) and stage–discharge relationship uncertainties (Shrestha et al ., ); (iii) structural uncertainty, arising from simplified and/or incomplete representation of hydrologic processes in the model; and (iv) parameter uncertainty, arising from the need to calibrate parameter values based on observed streamflow. Difficulties in reproducing some of the hydrologic indicators using the goodness‐of‐fit (GOF) measures [coefficient of determination, R 2 , and Nash–Sutcliffe coefficient of efficiency (NSE)] (Nash and Sutcliffe, ) have been documented by previous studies, such as simultaneous representation of low flow and high flow with a single set of parameter values (Wagener, ; Fenicia et al ., ).…”

Modelling of climate-induced hydrologic changes in the Lake Winnipeg watershed

“…The results of the SWAT model calibration may also be affected by a number of other factors, such as river-ice, which can affect the quality of discharge records, especially during break-up (e.g., Pelletier, 1990;Hamilton, 2008). These discharge uncertainties influence the model calibration and validation results.…”

A national‐scale assessment of long‐term water budget closures for Canada's watersheds

“…In fact, once an ice‐cover forms or during dynamic ice events, the usefulness of water level monitoring for estimating Q remains questionable. Detailed analyses of Canadian Q data suggested that the uncertainty in Q values under various situations particularly in the north could well exceed the theoretical uncertainty that is generally assumed for the typical global hydrometric data set, but remains to be quantified [ Hamilton , ; Hamilton and Moore , ].…”