ALBERT

Description: Well defined productivity-precipitation relationships of ecosystems are needed as benchmarks for the validation of land-models used for future projections. The productivity-precipitation relationship may be studied in two ways: the spatial approach relates differences in productivity to those in precipitation among sites along a precipitation gradient (the spatial fit, with a steeper slope); the temporal approach relates inter-annual productivity changes to variation in precipitation within sites (the temporal fits, with flatter slopes). Precipitation-reduction experiments in natural ecosystems represent a complement to the fits, because they can reduce precipitation below the natural range and are thus well suited to study potential effects of climate drying. Here, we analyze the effects of dry treatments in eleven multi-year precipitation-manipulation experiments, focusing on changes in the temporal fit. We expected that structural changes in the dry treatments would occur in some experiments, thereby reducing the intercept of the temporal fit and displacing the productivity-precipitation relationship downward the spatial fit. The majority of experiments (72%) showed that dry treatments did not alter the temporal fit. This implies that current temporal fits are to be preferred over the spatial fit to benchmark land-model projections of productivity under future climate within the precipitation ranges covered by the experiments. Moreover, in two experiments, the intercept of the temporal fit unexpectedly increased due to mechanisms that reduced either water- or nutrient losses. The expected decrease of the intercept was observed in only one experiment, and only when distinguishing between the late and the early phases of the experiment. This implies that we currently do not know at which precipitation-reduction level or at which experimental duration structural changes will start to alter ecosystem productivity. Our study highlights the need for experiments with multiple, including more extreme, dry treatments, to identify the precipitation boundaries within which the current temporal fits remain valid. This article is protected by copyright. All rights reserved.

Description: We have developed a probabilistic model to simulate the fate and transport of non-conservative constituents in urban watersheds. The approach implemented here extends previous studies that rely on the Geomorphological Instantaneous Unit Hydrograph concept to include non-conservative constituents. This is implemented with a factor χ that affects the transfer functions and therefore accounts for the loss (gain) of mass associated with the constituent as it travels through the watershed. Using this framework we developed an analytical solution for the dynamics of dissolved oxygen (DO) and biochemical oxygen demand (BOD) in urban networks based on the Streeter and Phelps model. This model breaks-down the catchment into a discreet number of possible flow paths through the system, requiring less data and implementation effort than well-established deterministic models. Application of the model to one sewer catchment in the Chicago area with available BOD information proved its ability to predict the BOD concentration observed in the measurements. In addition, comparison of the model with a calibrated Storm Water Management Model (SWMM) of another sewer catchment from the Chicago area showed that the model predicted the BOD concentration as well as the widely accepted SWMM. The developed model proved to be a suitable alternative to simulate the fate and transport of constituents in urban catchments with limited and uncertain input data. This article is protected by copyright. All rights reserved.

Description: Continuous monitoring of the atmospheric boundary layer (ABL) depth ( z i ) is important for investigations of trace gases with near-surface sources. This study investigates the temporal variability of z i on both diurnal and seasonal time scales over a full year (2011) and relates these changes to the atmospheric 222 Rn concentrations ( C Rn ) measured near the top of a 200 m tower at a rural site (Trainou) in France. Continuous z i estimates were made using a combination of lidar and hourly 4-height carbon dioxide (CO 2 ) profile measurements. Over the diurnal cycle, the 180 m C Rn reached a maximum in the late morning as the growing ABL passed through the inlet height (180 m) transporting upward high C Rn air from the nocturnal boundary layer. During late afternoon, a minimum in the C Rn occurred mainly due to ABL-mixing. We argue that ABL-dilution occurs in two stages: firstly, during the rapid morning growth into the residual layer, and secondly, during afternoon with the free atmosphere when z i has reached its quasi-stationary height (around 750 m in winter, or 1700 m in summer). An anti-correlation (R 2 of −0.49) was found while performing a regression analysis between the daily z i growth rates and the corresponding changes in the C Rn illustrating ABL-dilution effect. We also investigated the numerical proportions of the time within a season when z i remained lower than the inlet height and found a clear seasonal variability for the nighttime measurements with higher number of cases with shallow z i (〈200 m) in winter (67.3 %) than in summer (33.9 %) and spring (54.5 %). Thus, this pilot study helps delineate the impact of z i on C Rn at the site mainly for different regimes of ABL, in particular, during the times when the z i is above the measurement height. It is suggested that when the z i is well below the inlet height, measurements are most possibly indicative of the residual layer 222 Rn; an important issue that should be considered in the mass budget approach.