Papers by François Birgand
ABSTRACT Excessive nutrient export in watersheds has led to the development of large monitoring p... more ABSTRACT Excessive nutrient export in watersheds has led to the development of large monitoring programs and the implementation of Best Management Practices throughout the western countries. Documenting water quality improvement or degradation is not an easy endeavor. On a particular site, perceived improvement or degradation may result from climate variability from year to year. High uncertainties may actually result from the actual sampling and monitoring design, and particularly on the frequencies at which water is sampled and analyzed. We conducted an analysis of uncertainties on nitrate fluxes and concentration indicators (average, median, 90th and 95th percentiles, maximum) associated with infrequent sampling in Brittany, France. We used a database of 50 watershed-year datasets for which high temporal resolution data (hourly, daily at worst) was available for flow and nitrate concentrations. For each dataset, we calculated yearly reference fluxes and indicators. We then numerically simulated sampling for a set frequency and calculation strategy and compared the result to the reference ones. The choice of the algorithm used to compute the nitrate fluxes largely determines the expected accuracy and imprecision of the strategy. All methods that do not use the continuous record of flow performed very poorly and we recommend not to use them. The flow weighted average concentration ratio method was showed to best perform across the 50 datasets in Brittany. Ranges of errors were found to be correlated to a flow duration indicator, which lead to the drawing of reference guideline curves. These may serve as a template to determine, for a given watershed and its hydrological reactivity, expected errors for a given sampling frequency or to better design sampling frequency for a given uncertainty level. Annual values computed using monthly sampling showed that for a moderately ‘flashy’ watershed nitrate flux estimates would be between -12% and +11% of the ‘true’ load, average and 95th percentile concentrations would be within ±6% and between -7% and +2% of the reference value, respectively.
Trans. …, 2010
In water quality monitoring programs, standard sampling frequency schemes tend to be applied thro... more In water quality monitoring programs, standard sampling frequency schemes tend to be applied throughout entire regions or states. Ideally, the common standard among monitoring stations ought not to be the sampling frequency but instead the level of uncertainty of the estimated water quality indicators. Until now, there was no obvious way of doing this. This article proposes, for the first time, guidelines to select appropriate sampling frequencies to harmonize the level of uncertainty in the case of yearly nitrate indicators for the regional river water quality monitoring network in Brittany, France. A database of 50 watershed-year datasets (nine watersheds of 4 to 252 km 2 in size) was used for which high temporal resolution data (hourly and daily) were available for flow and nitrate concentrations. For each dataset, the uncertainty levels were calculated by numerically simulating sampling intervals varying from 2 to 60 days. The precision limits of the uncertainties were successfully correlated to a hydrological reactivity index. The correlations were used to derive sampling frequency charts. These charts can be used by watershed managers to optimize the sampling frequency scheme for any watershed for a desired uncertainty level, provided that the dimensionless local hydrological reactivity can be calculated from previous records of continuous flow rates. The sampling frequency charts also suggest that, depending on the hydrological reactivity, expected uncertainties generated by monthly sampling range between ±6% and ±14% for the annual load and between-5% and +2.5% to +7.2% for the annual concentration average.
Proc. ASABE Annual …, 2010
... Kirsti Granlund Finnish Environment Institute, PO Box 140, FI-00251 Helsinki, Finland. Katri ... more ... Kirsti Granlund Finnish Environment Institute, PO Box 140, FI-00251 Helsinki, Finland. Katri Rankinen, PhD ... The BFI determines the portion of effective rainfall that will contribute to groundwater instead of only passing through the soil zone. ...
&... more <p>The effects of intermittent drying of normally saturated organic systems such as peatlands, swamps, or wetlands has not been reported quite as often as those of wetting and drying cycles of normally dry soils. We report here the effects of weekly drying and rewetting events on saturated woodchips used as denitrification bed. We used denitrification rates and gas effluxes as indicators of the response of normally saturated organic substrate to intermittent aerobic conditions. We used replicated eight upflow columns in the lab fed with nitrated water, and undergoing variable duration of intermittent aerobic conditions (none, 2, 8, and 24 hours) over a 400d experiment.  We used high-frequency sensors to measure in- and outflow nitrate and DOC concentrations on a 2-hour basis, from which we calculated denitrification rates. We also measured the CO2 and N2O effluxes in the headspace on an hourly basis. The results show a burst of respiration activity during drying events and for several days after rewetting. Isotopic data suggest that respiration was bacterial denitrification. Intermittent aerobic conditions seem to provide the conditions conducive to the generation of more and better quality DOC, which microbes use during subsequent saturated conditions. Our results suggest that intermittent aerobic conditions may have lasting impacts on microbial respiration in wetlands.</p>
Journal of Irrigation and Drainage Engineering
Hydrology and Earth System Sciences
Land Degradation & Development
Journal of Environment Quality
Water Resources Research, 2016
We compared three stream-based sampling methods to study the fate of nitrate in groundwater in a ... more We compared three stream-based sampling methods to study the fate of nitrate in groundwater in a coastal plain watershed: point measurements beneath the streambed, seepage blankets (novel seepage-meter design), and reach mass-balance. The methods gave similar mean groundwater seepage rates into the stream (0.3-0.6 m/d) during two 3-4 day field campaigns despite an order of magnitude difference in stream discharge between the campaigns. At low flow, estimates of flowweighted mean nitrate concentrations in groundwater discharge ([NO 2 3 ] FWM) and nitrate flux from groundwater to the stream decreased with increasing degree of channel influence and measurement scale, i.e., [NO 2 3 ] FWM was 654, 561, and 451 mM for point, blanket, and reach mass-balance sampling, respectively. At high flow the trend was reversed, likely because reach mass-balance captured inputs from shallow transient high-nitrate flow paths while point and blanket measurements did not. Point sampling may be better suited to estimating aquifer discharge of nitrate, while reach mass-balance reflects full nitrate inputs into the channel (which at high flow may be more than aquifer discharge due to transient flow paths, and at low flow may be less than aquifer discharge due to channel-based nitrate removal). Modeling dissolved N 2 from streambed samples suggested (1) about half of groundwater nitrate was denitrified prior to discharge from the aquifer, and (2) both extent of denitrification and initial nitrate concentration in groundwater (700-1300 mM) were related to land use, suggesting these forms of streambed sampling for groundwater can reveal watershed spatial relations relevant to nitrate contamination and fate in the aquifer.
La mesure des débits d`eau à l`exutoire des « petits » bassins versants (< 3 500 Ha) est un pr... more La mesure des débits d`eau à l`exutoire des « petits » bassins versants (< 3 500 Ha) est un préalable essentiel à l`estimation des flux de polluants générés par l`activité agricole. Il existe beaucoup de méthodes normées pour mesurer les flux d`eau (28 recensées dans la norme ISO/TR8363: 1997). Dans cet article, nous proposons une méthode relativement simple de la mesure des débits dans les canaux ouverts qui utilise les débitmètres ultrasoniques Doppler, désormais couramment disponibles sur le marché. Cette méthode en général ne fait pas encore l`objet de norme mais des lignes directrices ont récemment été établies (ISO/TS 15769:2000(E)). Nous proposons en premier lieu un rappel du principe et des limitations de la méthode. Plusieurs exemples de stations sont proposés pour plusieurs gammes de débits, et pour lesquels la performance de mesure a été évaluée. Nous pensons que cette méthode peut être très appropriée pour la mesure des débits à l`exutoire des bassins versants agricol...
Deux méthodes complémentaires de la mesure des flux de MES dans les petits bassins versants ont é... more Deux méthodes complémentaires de la mesure des flux de MES dans les petits bassins versants ont été testées. La première méthode consiste à corréler des valeurs de turbidité mesurées par un turbidimètre à des concentrations en MES. Cette corrélation établie, les flux sont calculés à partir des données de débit et de turbidité obtenues à pas de temps court, de l`ordre de quelques minutes, ce qui permet de prendre en compte les variations rapides des flux instantanés. La deuxième méthode consiste à piéger une partie des matières en suspension par filtration dans des collecteurs placés horizontalement dans la colonne d`eau des ruisseaux et relevés tous les quinze jours. Ces deux méthodes ont été mises en oeuvre durant une année, à l`exutoire de deux petits bassins versants agricoles de Basse-Normandie, de 2,2 et 4,5 km². Les turbidimètres nécessitent des traitements de données assez longs, du fait de variations diurnes et de divers phénomènes de dérive. La valeur de la méthode dépend d...
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Papers by François Birgand