ALBERT

Description: Impacts of climate and land cover changes on streamflow were assessed using a hydrological modeling. The precipitation runoff modeling system of the US Geological Survey was modified in order to consider wetlands as a separate hydrological response unit. Initial model parameters were obtained from a previously modeled adjacent catchment and subsequent calibration and validation were carried out. The model calibration and validation periods were divided into three. The calibration period was a five years period (1981–1986). The validation period was divided into two: validation 1 (1986–1991) and validation 2 (1996–2002). Model performance was evaluated by using joint plots of daily and monthly observed and simulated runoff hydrographs and different coefficients of efficiency. The model coefficients of efficiency were 0.71 for the calibration period and 0.69 and 0.66 for validation periods 1 and 2, respectively. A "delta-change" method was used to formulate climatic scenarios. One land cover change scenario was also used to assess the likely impacts of these changes on the runoff. The results of the scenario analysis showed that the basin is more sensitive to increase in rainfall (+80% for +20%) than to a decrease (−62% for −20%). The rainfall elasticity is 4:1 for a 20% increase in rainfall while it is 3:1 for a 20% reduction. A 1.5°c increase in temperature resulted in a 6% increase in potential evapotranspiration and 13% decrease in streamflow. This indicates that the watershed is more elastic to rainfall increase than temperature. The proposed land cover scenario of converting areas between 2000 to 3000 m a.s.l. to woodland also resulted in a significant decrease in streamflow (11.8%). The study showed that properly calibrated and validated models could help understand likely impacts of climate and land cover changes on catchment water balance.

Description: Impacts of climate variability and land use change on catchment runoff of the Meki River basin were assessed using hydrological modeling. The Modular Modeling System (MMS) was used to build a suitable Precipitation Runoff Modeling System (PRMS) for the study area, perform sensitivity analysis, model calibration and validation, and scenario analysis. The model calibration and validation periods in this study were divided into three. The calibration period was a five years period (1981–1986). The validation period was divided into two: validation 1 (1986–1991) and validation 2 (1996–2002). Model performance was evaluated by using joint plots of daily and monthly observed and simulated runoff hydrographs and the Nash-Sutcliffe coefficient of efficiency to statistically analyze model performance in simulating daily and monthly runoff. Daily observed and simulated hydrographs showed a reasonable agreement for both calibration and validation periods. The model coefficients of efficiency were 0.71 for the calibration period and 0.69 and 0.66 for validation period 1 and 2, respectively. Simulated runoff was generally greater than the observed runoff values for the calibration and validation periods. The model was also limited in its capability of simulating complex hydrograph shapes and peak discharge values. However, the model performed well in simulating dry season flows for both validation and calibration periods. A 20% of change in rainfall and 1.5 °C increase in temperature was considered for climatic scenarios and one land use change scenario were used to assess the likely impacts of these changes on the runoff of the Meki River. The results of the scenario analysis showed that the basin is more sensitive to increase in rainfall (+80% for +20%) than to a decrease (−62% for −20%). Increase in temperature has also a significant impact both on the potential evapotranspiration and stream flow of the basin. Increase by 1.5 °C in temperature resulted in increase in potential evapotranspiration (6.02%) and decrease in stream flow (13%). The proposed land use scenario of converting areas between 2000 to 3000 m a.s.l. to woodland also resulted in a significant decrease in stream flow (11.8%) and increase in evapotranspiration (2.2%) of the study area.

Description: We explore the reliability of large-scale climate variables, namely precipitation and temperature, as inputs for a basin-lake hydrological model in central Argentina. We used data from two regions in NCEP/NCAR reanalyses and three regions from LMDZ model simulations forced with observed sea surface temperature (HadISST) for the last 50 years. Reanalyses data cover part of the geographical area of the Sali-Dulce Basin (region A) and a zone at lower latitudes (region B). The LMDZ selected regions represent the geographical area of the Sali-Dulce Basin (box A), and two areas outside of the basin at lower latitudes (boxes B and C). A statistical downscaling method is used to connect the large-scale climate variables inferred from LMDZ and the reanalyses, with the hydrological Soil Water Assessment Tool (SWAT) model in order to simulate the Rio Sali-Dulce discharge during 1950–2005. The SWAT simulations are then used to force the water balance of Laguna Mar Chiquita, which experienced an abrupt level rise in the 1970's attributed to the increase in Rio Sali-Dulce discharge. Despite that the lowstand in the 1970's is not well reproduced in either simulation, the key hydrological cycles in the lake level are accurately captured. Even though satisfying results are obtained with the SWAT simulations using downscaled reanalyses, the lake level are more realistically simulated with the SWAT simulations using downscaled LMDZ data in boxes B and C, showing a strong climate influence from the tropics on lake level fluctuations. Our results highlight the ability of downscaled climatic data to reproduce regional climate features. Laguna Mar Chiquita can therefore be considered as an integrator of large-scale climate changes since the forcing scenarios giving best results are those relying on global climate simulations forced with observed sea surface temperature. This climate-basin-lake model is a promising approach for understanding and simulating long-term lake level variations.

Description: In the Sahelian belt, Lake Chad is a key water body for 13 million people who live on its resources. It experiences, however, substantial and frequent surface changes. Located at the center of one of the largest endorheic basins in the world, its waters remain surprisingly fresh. Its low salinity has been attributed to a low infiltration flow whose value remains poorly constrained. Understanding the lake's hydrological behavior in response to climate variability requires a better constraint of the factors that control its water and chemical balance. Based on the three-pool conceptualization of Lake Chad proposed by J. C. Bader, J. Lemoalle, and M. Leblanc (Bader et al., 2011), this study aims to quantify the total water outflow from the lake, the respective proportions of evaporation (E), transpiration (T) and infiltration (I), and the associated uncertainties. A Bayesian inversion method based on lake-level data was used, leading to total water loss estimates in each pool (ETI). Sodium and stable isotope mass balances were then used to separate total water losses into E, T and I components. Despite the scarcity of representative data available on the lake, the combination of these two geochemical tracers is relevant to assess the relative contribution of these three outflows involved in the control of the hydrological budget. Mean evapotranspiration rates were estimated at 2070 ± 100 and 2270 ± 100 mm yr−1 for the southern and northern pools respectively. Infiltration represents between 100 and 300 mm yr−1 but most of the water is evapotranspirated in the first few kilometers from the shorelines and does not efficiently recharge the Quaternary aquifer. Transpiration is shown to be significant, around 300 mm yr−1 and reaches 500 mm yr−1 in the vegetated zone of the archipelagos. Hydrological and chemical simulations reproduce the marked hydrological change between the normal lake state that occurred before 1972 and the small lake state after 1972 when the lake surface shrunk to a tenth of its size. According to our model, shrinking phases are efficient periods for salt evacuation from the lake towards the phreatic aquifer.

Description: Siliceous sponge spicules constitute an important siliceous component of lacustrine sediments, together with widespread diatom frustules. In contrast to diatom frustules, siliceous spicules are formed in sponges in an enzymatic way. Previous attempts to use their oxygen isotopic signature (δ18Osilica) as a paleoenvironmental proxy have led to contradictory conclusions. These attempts demonstrated the need to further assess whether sponges form their silica in oxygen isotopic equilibrium with water. For this reason, we measured the δ18O signature of sponge spicules from a single freshwater species (Metania spinata) grown on natural and artificial supports over nine months in a small Brazilian pond (Lagoa Verde, northwestern Minas Gerais). The δ18Osilica values were obtained using the infrared (IR) laser-heating fluorination technique following a controlled isotopic exchange (CIE). The δ18O values (δ18Owater) and temperature of the pond water were periodically measured and reconstructed over the course of the sponge growth. Assuming that silica may form continuously in the spicules, temperature and δ18Owater values over the months of growth were weighted using a sponge growth coefficient previously established for Metania spinata. The δ18Osilica values of sponges grown simultaneously and on similar substrates were scattered. No relationships were observed between the Δ18Osilica-water and water temperature when the reconstructed values were considered. Conversely, a positive correlation was obtained, with a coefficient of 0.3‰ °C–1 (R2 = 0.63), when δ18Owater values and water temperature at the time of sample collection were considered. Such a positive temperature coefficient clearly indicates that the freshwater sponge Metania spinata does not form its siliceous spicules in oxygen isotopic equilibrium with the pond water. Instead, one or several biologically controlled kinetic fractionation mechanisms may be in play during the various steps of silica formation. Our results suggest that the latest precipitation gives its δ18O imprint to the entire spicules assemblage. The amplitude of the apparent fractionations increases with temperature, but other controlling parameters, such as dissolved Si concentration and nutrient availability, co-varying with temperature may intervene. These results prevent the use of δ18Osilica values from the spongillites of northwestern Minas Gerais as a direct proxy for past δ18Owater and/or temperature changes.

Description: Stable isotopes of the water vapor represent a powerful tool for tracing atmospheric vapor origin and mixing processes. Laser spectrometry recently allowed high time resolution measurements, but despite an increasing number of experimental studies, there is still a need for a better understanding of the main drivers of isotopic signal variability at different time scales. We present results of in situ measurements of δ18O and δD during 36 consecutive days in summer 2011 in atmospheric vapor of a Mediterranean coastal wetland exposed to high evapotranspiration (Camargue, Rhône River delta, France). A calibration protocol was tested and instrument stability was analysed over the period. The mean composition of atmospheric vapor during the campaign is δ18O = −14.66‰ and δD = −95.4‰, with δv data plotting clearly above the local meteoric water line, and an average deuterium excess (dv) of 21.9‰. At daily time step, we show a clear separation of isotopic characteristics with respect to the air mass back trajectories, with the Northern air masses providing depleted compositions (δ18O = −15.83‰, δD = −103.5‰) compared to Mediterranean air masses (δ18O = −13.13‰, δD = −86.5‰). There is also a clear separation between dv corresponding to these different air mass origins, but not in the same direction as was previously evidenced from regional rainfall data, with higher dv found for Northern air masses (23.2‰) than for Mediterranean air masses (18.6‰). Based on twenty-four average hourly data, we propose a depiction of typical daily evolution of water vapor isotopic composition. High diurnal variations in dv is attributed to a dominant control of evapotranspiration, over entrainment of free atmosphere. Daily cycles in dv are more pronounced for Mediterranean than for North Atlantic air mass origin and are discussed in terms of local evapotranspiration versus regional signatures. We calculate the composition of the vapor source that produces the day-time increase in dv for the different air mass origins, and propose an atmospheric water and isotopic mass balance.

Description: Stable isotopes of water vapour represent a powerful tool for tracing atmospheric vapour origin and mixing processes. Laser spectrometry recently allowed high time-resolution measurements, but despite an increasing number of experimental studies, there is still a need for a better understanding of the isotopic signal variability at different time scales. We present results of in situ measurements of δ18O and δD during 36 consecutive days in summer 2011 in atmospheric vapour of a Mediterranean coastal wetland exposed to high evaporation (Camargue, Rhône River delta, France). The mean composition of atmospheric vapour (δv) is δ18O = −14.66 ‰ and δD = − 95.4 ‰, with data plotting clearly above the local meteoric water line on a δ18O-δD plot, and an average deuterium excess (d) of 21.9 ‰. Important diurnal d variations are observed, and an hourly time scale analysis is necessary to interpret the main processes involved in its variability. After having classified the data according to air mass back trajectories, we analyse the average daily cycles relating to the two main meteorological situations, i.e. air masses originating from North Atlantic Ocean and Mediterranean Sea. In both situations, we show that diurnal fluctuations are driven by (1) the influence of local evaporation, culminating during daytime, and leading to an increase in absolute water vapour concentration associated to a δv enrichment and d increase; (2) vertical air mass redistribution when the Planetary Boundary Layer collapses in the evening, leading to a d decrease, and (3) dew formation during the night, producing a δv depletion with d remaining stable. Using a two-component mixing model, we calculate the average composition of the locally evaporated vapour (δE). We find higher d(E) under North Atlantic air mass conditions, which is consistent with lower humidity conditions. We also suggest that δv measured when the PBL collapses is the most representative of a regional signal. Strong, cold and dry winds coming from the north bring an isotopically depleted vapour, while light, warm and wet winds coming from the south bring an isotopically enriched vapour. Under northern conditions, a strong advection rate dilutes the contribution of the locally evaporated vapour (δE) to the ambient moisture (δv). The higher d values measured under northern conditions, compared to the Mediterranean situation, thus results from the combination of a higher d in both local and regional vapour. This depiction of typical daily cycles of water vapour isotopic composition can be used as a framework for further quantitative analyses of vapour sources during specific days.