Coupling statistically downscaled GCM outputs with a basin-lake hydrological model in subtropical South America: evaluation of the influence of large-scale precipitation changes on regional hydroclimate variability

International audienceWe 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

Impact of soil and water conservation measuren on catchment hydrological response: a case in north Ethiopia

Impact studies of catchment management in the developing world rarely include detailed hydrological components. Here, changes in the hydrological response of a 200-ha catchment in north Ethiopia are investigated. The management included various soil and water conservation measures such as the construction of dry masonry stone bunds and check dams, the abandonment of post-harvest grazing, and the establishment of woody vegetation. Measurements at the catchment outlet indicated a runoff depth of 5 mm or a runoff coefficient (RC) of 1·6% in the rainy season of 2006. Combined with runoff measurements at plot scale, this allowed calculating the runoff curve number (CN) for various land uses and land management techniques. The pre-implementation runoff depth was then predicted using the CN values and a ponding adjustment factor, representing the abstraction of runoff induced by the 242 check dams in gullies. Using the 2006 rainfall depths, the runoff depth for the 2000 land management situation was predicted to be 26·5mm(RCD 8%), in line with current RCs of nearby catchments. Monitoring of the ground water level indicated a rise after catchment management. The yearly rise in water table after the onset of the rains (ΔT) relative to the water surplus (WS) over the same period increased between 2002-2003 (ΔT/WS D 3·4) and 2006 (ΔT/WS >11·1). Emerging wells and irrigation are other indicators for improved water supply in the managed catchment. Cropped fields in the gullies indicate that farmers are less frightened for the destructive effects of flash floods. Due to increased soil water content, the crop growing period is prolonged. It can be concluded that this catchment management has resulted in a higher infiltration rate and a reduction of direct runoff volume by 81% which has had a positive influence on the catchment water balance. © 2010 John Wiley & Sons, Ltd

Unraveling the hydrological budget of isolated and seasonally contrasted subtropical lakes

Complete understanding of the hydrological functioning of large-scale intertropical watersheds such as the Lake Chad basin is becoming a high priority in the context of climate change in the near future and increasing demographic pressure. This requires integrated studies of all surface water and groundwater bodies and of their quite-complex interconnections. We present here a simple method for estimating the annual mean water balance of sub-Sahelian lakes subject to high seasonal contrast and located in isolated regions with no road access during the rainy season, a situation which precludes continuous monitoring of in situ hydrological data. Our study focuses for the first time on two lakes, Iro and Fitri, located in the eastern basin of Lake Chad. We also test the approach on Lake Ihotry in Madagascar, used as a benchmark site that has previously been extensively studied by our group. We combine the δ18O and δ2H data that we measured during the dry season with altimetry data from the SARAL satellite mission in order to model the seasonal variation of lake volume and isotopic composition. The annual water budget is then estimated from mass balance equations using the Craig–Gordon model for evaporation. We first show that the closed-system behavior of Lake Ihotry (i.e., precipitation equal to evaporation) is well simulated by the model. For lakes Iro and Fitri, we calculate evaporation to influx ratios (E∕I) of 0.6±0.3 and 0.4±0.2, respectively. In the case of the endorheic Lake Fitri, the estimated output flux corresponds to the infiltration of surface water toward the surface aquifer that regulates the chemistry of the lake. These results constitute a first-order assessment of the water budget of these lakes, in regions where direct hydrological and meteorological observations are very scarce or altogether lacking. Finally, we discuss the implications of our data on the hydro-climatic budget at the scale of the catchment basins. We observe that the local evaporation lines (LELs) obtained on both lake and aquifer systems are slightly offset from the average rainfall isotopic composition monitored by IAEA at N'Djamena (Chad), and we show that this difference may reflect the impact of vegetation transpiration on the basin water budget. Based on the discussion of the mass balance budget we conclude that, while being broadly consistent with the idea that transpiration is on the same order of magnitude as evaporation in those basins, we cannot derive a more precise estimate of the partition between these two fluxes, owing to the large uncertainties of the different end-members in the budget equations.</p

Unraveling the hydrological budget of isolated and seasonally contrasted subtropical lakes

Complete understanding of the hydrological functioning of large-scale intertropical watersheds such as the Lake Chad basin is becoming a high priority in the context of climate change in the near future and increasing demographic pressure. This requires integrated studies of all surface water and groundwater bodies and of their quite-complex interconnections. We present here a simple method for estimating the annual mean water balance of sub-Sahelian lakes subject to high seasonal contrast and located in isolated regions with no road access during the rainy season, a situation which precludes continuous monitoring of in situ hydrological data. Our study focuses for the first time on two lakes, Iro and Fitri, located in the eastern basin of Lake Chad. We also test the approach on Lake Ihotry in Madagascar, used as a benchmark site that has previously been extensively studied by our group. We combine the δ18O and δ2H data that we measured during the dry season with altimetry data from the SARAL satellite mission in order to model the seasonal variation of lake volume and isotopic composition. The annual water budget is then estimated from mass balance equations using the Craig–Gordon model for evaporation. We first show that the closed-system behavior of Lake Ihotry (i.e., precipitation equal to evaporation) is well simulated by the model. For lakes Iro and Fitri, we calculate evaporation to influx ratios (E∕I) of 0.6±0.3 and 0.4±0.2, respectively. In the case of the endorheic Lake Fitri, the estimated output flux corresponds to the infiltration of surface water toward the surface aquifer that regulates the chemistry of the lake. These results constitute a first-order assessment of the water budget of these lakes, in regions where direct hydrological and meteorological observations are very scarce or altogether lacking. Finally, we discuss the implications of our data on the hydro-climatic budget at the scale of the catchment basins. We observe that the local evaporation lines (LELs) obtained on both lake and aquifer systems are slightly offset from the average rainfall isotopic composition monitored by IAEA at N'Djamena (Chad), and we show that this difference may reflect the impact of vegetation transpiration on the basin water budget. Based on the discussion of the mass balance budget we conclude that, while being broadly consistent with the idea that transpiration is on the same order of magnitude as evaporation in those basins, we cannot derive a more precise estimate of the partition between these two fluxes, owing to the large uncertainties of the different end-members in the budget equations.</p

A global database of water vapor isotopes measured with high temporal resolution infrared laser spectroscopy

The isotopic composition of water vapour provides integrated perspectives on the hydrological histories of air masses and has been widely used for tracing physical processes in hydrological and climatic studies. Over the last two decades, the infrared laser spectroscopy technique has been used to measure the isotopic composition of water vapour near the Earth's surface. Here, we have assembled a global database of high temporal resolution stable water vapour isotope ratios (delta O-18 and delta D) observed using this measurement technique. As of March 2018, the database includes data collected at 35 sites in 15 Koppen climate zones from the years 2004 to 2017. The key variables in each dataset are hourly values of delta O-18 and delta D in atmospheric water vapour. To support interpretation of the isotopologue data, synchronized time series of standard meteorological variables from in situ observations and ERA5 reanalyses are also provided. This database is intended to serve as a centralized platform allowing researchers to share their vapour isotope datasets, thus facilitating investigations that transcend disciplinary and geographic boundaries

Quelles relations entre la mer, les nappes souterraines et les eaux de surface dans le delta du Rhône ?

National audienceLes échanges d'eau sont contrôlés par un système de vannes et pertuis qui relient les étangs à la mer à travers la « Digue à la mer ». Sur la période 1994-2010, les volumes d'eau moyens échangés au niveau du pertuis de la Fourcade sont d'environ 11 millions de m 3 / an d'entrée marine et 20 millions de m 3 /an de sortie vers la mer. Malgré ce système, de grandes variations interannuelles se produisent. En effet, les flux restent dépendants des différences de niveau d'eau de part et d'autre, qui eux dépendent des facteurs environnementaux « internes » (apports d'eau par l'irrigation des rizières) et « externes » : les précipitations et le vent qui influencent le niveau d'eau des étangs, ainsi que les variations du niveau de la mer. Ces variations se produisent à court terme en fonction de la marée et des conditions météorologiques, avec notamment les phénomènes de surcotes marines, et à long terme en relation avec la remontée du niveau de la mer (1 à 7,5 mm/an-voir question 17 : « Changement climatique et surcotes marines, quel avenir pour les plages camarguaises ? »). Les relations entre les eaux souterraines et la mer Il existe une continuité des formations géologiques de part et d'autre du trait de côte permettant des échanges d'eau entre les eaux souterraines et la mer (fig. 1). Les différences de niveau entre la mer et les nappes régissent la direction et l'intensité des flux d'eau. Le bilan de ces flux se produit majoritairement en direction de la mer. Mais il existe néanmoins une intrusion saline, qui explique la salinité plus élevée des eaux de la nappe captive le long de la côte. L'augmentation progressive du niveau de la mer favorise l'intrusion de l'eau de mer dans les terres, ce qui entraînera à plus ou moins long terme une accentuation de la salinisation des eaux. L'eau partout présente en Camargue provient soit de la mer, soit des précipitations, soit de son fleuve. Depuis l'endiguement du Rhône, ce sont essentiellement les canaux d'irrigation et de drainage qui déterminent la distribution de l'eau du fleuve à travers son delta (photo 1). Les eaux continentales souterraines et en surface L'île de Camargue reçoit à la fois des eaux de pluie (470 millions de m 3 /an en moyenne) et des eaux d'irrigation issues du Rhône (296 et 362 millions de m 3 /an pour 1996 et 1993, Fondation Sansouire). En surface, le système lagunaire du Vaccarès est le réceptacle principal des flux hydriques de la zone (photo 2). Dans le sous-sol, on distingue deux types de nappes (fig. 1). Les sédiments récents du Rhône se trouvent sur une épaisseur variant de quelques mètres vers Arles jusqu'à environ 50 m vers le littoral et abritent des nappes plus ou moins connectées et de salinité très variable : douce, salée et même sursalée (jusqu'à 110 g/l, la mer étant à 38 g/l). Au-dessous, une formation de cailloutis grossiers abrite une nappe captive (séparée de la surface par une couche imperméable), dont la salinité augmente progressivement en direction de la mer depuis des eaux presque douces (≈2 g/l au nord) à des eaux de composition marine au sud. Les relations entre les eaux de surface et les eaux souterraines Les échanges d'eau entre la surface et le sous-sol sont complexes et se produisent soit par infiltration d'eau, soit par remontées artésiennes (sous pression). En fonction de l'occupation du sol, les eaux qui s'infiltrent proviennent de l'irrigation des rizières ou de la pluie dans les zones naturelles. Les remontées d'eau vers la surface se produisent quand la pression de l'eau souterraine le permet. On observe ce phénomène là où la topographie est légèrement plus basse, par exemple en bordure nord du Vaccarès où se trouve un forage artésien, mais il est le plus souvent invisible et difficile à quantifier. Ainsi, les étangs de Camargue, et de Vaccarès en particulier, reçoivent une quantité importante d'eau souterraine, estimée à environ un quart des apports de surface. Quelles relations entre la mer, les nappes souterraines et les eaux de surface dans le delta du Rhône

Analysis of the hydrological response of a tropical terminal lake, Lake Abiyata (Main Ethiopian Rift Valley) to changes in climate and human activities

Integrated dynamic water and chloride balance models with a catchment-scale hydrological model (PRMS) are used to investigate the response of a terminal tropical lake, Lake Abiyata, to climate variability and water use practices in its catchment. The hydrological model is used to investigate the response of the catchment to different climate and land-use change scenarios that are incorporated into the lake model. Lake depth-area-volume relationships were established from lake bathymetries. Missing data in the time series were filled using statistical regression techniques. Based on mean monthly data, the lake water balance model produced a good agreement between the simulated and observed levels of Lake Abiyata for the period 1968-83. From 1984 onwards the simulated lake level is overestimated with respect to the observed one, while the chloride concentration is largely underestimated. This discrepancy is attributed to human use of water from the influent rivers or directly from the lake. The simulated lake level and chloride concentration are in better agreement with observed values (r2 = 0*96) when human water use for irrigation and salt exploitation are included in the model. A comparison of the simulation with and without human consumption indicates that climate variability controls the interannual fluctuations and that the human water use affects the equilibrium of the system by strongly reducing the lake level. Sensitivity analysis based on a mean climatic year showed that, after prolonged mean climatic conditions, Lake Abiyata reacts more rapidly to an abrupt shift to wetter conditions than to dry conditions. This study shows the significant sensitivity of the level and salinity of the terminal Lake Abiyata to small changes in climate or land use, making it a very good 'recorder' of environmental changes that may occur in the catchment at different time scales