Potential groundwater contribution to Amazon evapotranspiration

Climate and land ecosystem models simulate a dry-season vegetation stress in the Amazon forest, but observations do not support these results, indicating adequate water supply. Proposed mechanisms include larger soil water store and deeper roots in nature and the ability of roots to move water up and down (hydraulic redistribution), both absent in the models. Here we provide a first-order assessment of the potential importance of the upward soil water flux from the groundwater driven by capillarity. We present a map of equilibrium water table depth from available observations and a groundwater model simulation constrained by these observations. We then present a map of maximum capillary flux these water table depths, combined with the fine-textured soils in the Amazon, can potentially support. The maps show that the water table beneath the Amazon can be shallow in lowlands and river valleys (<5 m in 36% and <10 m in 60% of Amazonia). These water table depths can potentially accommodate a maximum capillary flux of 2.1 mm day−1 to the land surface averaged over Amazonia, but varies from 0.6 to 3.7 mm day−1 across nine study sites. We note that the results presented here are based on limited observations and simple equilibrium model calculations, and as such, have important limitations and must be interpreted accordingly. The potential capillary fluxes are not indicative of their contribution to the actual evapotranspiration, and they are only an assessment of the possible rate at which this flux can occur, to illustrate the power of soil capillary force acting on a shallow water table in fine textured soils. They may over-estimate the actual flux where the surface soils remain moist. Their contribution to the actual evapotranspiration can only be assessed through fully coupled model simulation of the dynamic feedbacks between soil water and groundwater with sub-daily climate forcing. The equilibrium water table obtained here serves as the initial state for the dynamic simulation, and together with the equilibrium potential capillary flux, will serve as a baseline to evaluate the diurnal, event, seasonal and inter-annual dynamicsSupport comes from the Ramon y Cajal program of the Spanish Ministry of Education and Science and Centro de Supercomputacion de Galicia, CESGA at Santiago de ´ Compostela, Galicia, Spain, the US National Science Foundation, and the Rutgers University Academic Excellence FundS

Application of an improved global-scale groundwater model for water table estimation across New Zealand

Many studies underline the importance of groundwater assessment at the larger, i.e. global, scale. The groundwater models used for these assessments are dedicated to the global scale and therefore not often applied for studies in smaller areas, e.g. catchments, because of their simplifying assumptions. In New Zealand, advanced numerical groundwater flow models have been applied in several catchments. However, that application is piecemeal: only for a limited amount of aquifers and through a variety of groundwater model suites, formats, and developers. Additionally, there are large areas where groundwater models and data are sparse. Hence, an inter-catchment, inter-regional, or nationwide overview of important groundwater information, such as the water table, does not exist. The investment needed to adequately cover New Zealand with high-resolution groundwater models in a consistent approach would be significant and is therefore not considered possible at this stage. This study proposes a solution that obtains a nationwide overview of groundwater that bridges the gap between the (too-)expensive advanced local models and the (too-)simple global-scale models. We apply an existing, global-scale, groundwater flow model and improve it by feeding in national input data of New Zealand terrain, geology, and recharge, and by slight adjustment of model parametrisation and model testing. The resulting nationwide maps of hydraulic head and water table depths show that the model points out the main alluvial aquifers with fine spatial detail (200 m grid resolution). The national input data and finer spatial detail result in better and more realistic variations of water table depth than the original, global-scale, model outputs. In two regional case studies in New Zealand, the hydraulic head shows excellent correlation with the available groundwater level data. Sensitivity and other analyses of our nationwide water tables show that the model is mostly driven by recharge, model resolution, and elevation (gravity), and impeded by the geology (permeability). The use of this first dedicated New Zealand-wide model can aid in provision of water table estimates in data-sparse regions. The national model can also be used to solve inconsistency of models in areas of trans-boundary aquifers, i.e. aquifers that cover more than one region in New Zealand. Comparison of the models, i.e. the national application (National Water Table model: NWT) with the global model (Equilibrium Water Table model: EWT), shows that most improvement is achieved by feeding in better and higher-resolution input data. The NWT model still has a bias towards shallow water tables (but less than the EWT model because of the finer model resolution), which could only be solved by feeding in a very high resolution terrain model that incorporates drainage features. Although this is a model shortcoming, it can also be viewed as a valuable indicator of the pre-human water table, i.e. before 90 % of wetlands were drained for agriculture since European settlement in New Zealand. Calibration to ground-observed water level improves model results but can of course only work where there are such data available. Future research should therefore focus on both model improvements and more data-driven, improved estimation of hydraulic conductivity, recharge, and the digital elevation model. We further surmise that the findings of this study, i.e. successful application of a global-scale model at smaller scales, will lead to subsequent improvement of the global-scale model equationsThis research has been part of a PhD study of the lead author at the University of Waikato, New Zealand, supervised by Moira Steyn-Ross. It has been performed as part of the Smart Aquifer Characterisation (SAC) Programme, funded by the Ministry of Business, Innovation and Employment, New Zealand. This project has received co-funding from the European Union's Seventh Framework Programme for Research and Technological Development under grant agreement no. 603608, eartH2Observe. We furthermore would like to thank the reviewers for their valuable comments, Waikato Regional Council and Environment Canterbury for their ground-observed data used in the results section, and Jeremy White (GNS Science) for his advice on the sensitivity analysesS

Potential hydrologic changes in the Amazon by the end of the 21st century and the groundwater buffer

This study contributes to the discussions on the future of the Amazon rainforest under a projected warmer-drier climate from the perspectives of land hydrology. Using IPCC HadGEM2-ES simulations of the present and future Amazon climate to drive a land hydrology model that accounts for groundwater constraint on land drainage, we assess potential hydrologic changes in soil water, evapotranspiration (ET), water table depth, and river discharge, assuming unchanged vegetation. We ask: how will ET regimes shift at the end of the 21st century, and will the groundwater help buffer the anticipated water stress in some places-times? We conducted four 10 yr model simulations, at the end of 20th and 21st century, with and without the groundwater. Our model results suggest that, first, over the western and central Amazon, ET will increase due to increased potential evapotranspiration (PET) with warmer temperatures, despite a decrease in soil water; that is, ET will remain PET or atmospheric demand-limited. Second, in the eastern Amazon dry season, ET will decrease in response to decreasing soil water, despite increasing PET demand; that is, ET in these regions-seasons will remain or become more soil water or supply-limited. Third, the area of water-limited regions will likely expand in the eastern Amazonia, with the dry season, as indicated by soil water store, even drier and longer. Fourth, river discharge will be significantly reduced over the entire Amazon but particularly so in the southeastern Amazon. By contrasting model results with and without the groundwater, we found that the slow soil drainage constrained by shallow groundwater can buffer soil water stress, particularly in southeastern Amazon dry season. Our model suggests that, if groundwater buffering effect is accounted for, the future Amazon water stress may be less than that projected by most climate modelsFunding comes from NSF (NSF-AGS-1045110), US EPA (EPA-STAR-RD834190) and the European Commission FP7 (EartH2Observe). Computation used the NCAR's Computational and Information Systems Laboratory, sponsored by NSF and other agenciesS

Groundwater influence on soil moisture memory and land–atmosphere fluxes in the Iberian Peninsula

Groundwater plays an important role in the terrestrial water cycle, interacting with the land surface via vertical fluxes through the water table and distributing water resources spatially via gravity-driven lateral transport. It is therefore essential to have a correct representation of groundwater processes in land surface models, as land–atmosphere coupling is a key factor in climate research. Here we use the LEAFHYDRO land surface and groundwater model to study the groundwater influence on soil moisture distribution and memory, and evapotranspiration (ET) fluxes in the Iberian Peninsula over a 10-year period. We validate our results with time series of observed water table depth from 623 stations covering different regions of the Iberian Peninsula, showing that the model produces a realistic water table, shallower in valleys and deeper under hilltops. We find patterns of shallow water table and strong groundwater–land surface coupling over extended interior semi-arid regions and river valleys. We show a strong seasonal and interannual persistence of the water table, which induces bimodal memory in the soil moisture fields; soil moisture “remembers” past wet conditions, buffering drought effects, and also past dry conditions, causing a delay in drought recovery. The effects on land–atmosphere fluxes are found to be significant: on average over the region, ET is 17.4 % higher when compared with a baseline simulation with LEAFHYDRO's groundwater scheme deactivated. The maximum ET increase occurs in summer (34.9 %; 0.54 mm d−1). The ET enhancement is larger over the drier southern basins, where ET is water limited (e.g. the Guadalquivir basin and the Mediterranean Segura basin), than in the northern Miño/Minho basin, where ET is more energy limited than water limited. In terms of river flow, we show how dry season baseflow is sustained by groundwater originating from accumulated recharge during the wet season, improving significantly on a free-drain approach, where baseflow comes from water draining through the top soil, resulting in rivers drying out in summer. Convective precipitation enhancement through local moisture recycling over the semi-arid interior regions and summer cooling are potential implications of these groundwater effects on climate over the Iberian Peninsula. Fully coupled land surface and climate model simulations are needed to elucidate this question.This research has been supported by the Spanish Department of Education and Science and by the European Commission (grant no. 603608: eartH2Observe). This study has also received funding from the UK National Environment Research Council (NERC), under the Hydro-JULES programme (grant no. NE/S017380/1)S

Multi-decadal hydrologic change and variability in the Amazon River basin: understanding terrestrial water storage variations and drought characteristics

We investigate the interannual and interdecadal hydrological changes in the Amazon River basin and its sub-basins during the 1980–2015 period using GRACE satellite data and a physically based, 2 km grid continental-scale hydrological model (LEAF-Hydro-Flood) that includes a prognostic groundwater scheme and accounts for the effects of land use–land cover (LULC) change. The analyses focus on the dominant mechanisms that modulate terrestrial water storage (TWS) variations and droughts. We find that (1) the model simulates the basin-averaged TWS variations remarkably well; however, disagreements are observed in spatial patterns of temporal trends, especially for the post-2008 period. (2) The 2010s is the driest period since 1980, characterized by a major shift in the decadal mean compared to the 2000s caused by increased drought frequency. (3) Long-term trends in TWS suggest that the Amazon overall is getting wetter (1.13 mm yr−1), but its southern and southeastern sub-basins are undergoing significant negative TWS changes, caused primarily by intensified LULC changes. (4) Increasing divergence between dry-season total water deficit and TWS release suggests a strengthening dry season, especially in the southern and southeastern sub-basins. (5) The sub-surface storage regulates the propagation of meteorological droughts into hydrological droughts by strongly modulating TWS release with respect to its storage preceding the drought condition. Our simulations provide crucial insight into the importance of sub-surface storage in alleviating surface water deficit across Amazon and open pathways for improving prediction and mitigation of extreme droughts under changing climate and increasing hydrologic alterations due to human activities (e.g., LULC change).This research has been supported by the National Science Foundation (grant no. 1639115)S

Understanding precipitation recycling over the Tibetan Plateau using tracer analysis with WRF

The precipitation recycling (PR) ratio is an important indicator that quantifies the land-atmosphere interaction strength in the Earth system’s water cycle. To better understand how the heterogeneous land surface in the Tibetan Plateau (TP) contributes to precipitation, we used the water-vapor tracer (WVT) method coupled with the Weather Research and Forecasting (WRF) regional climate model. The goals were to quantify the PR ratio, in terms of annual mean, seasonal variability and diurnal cycle, and to address the relationships of the PR ratio with lake treatments and precipitation amount. Simulations showed that the PR ratio increases from 0.1 in winter to 0.4 in summer when averaged over the TP with the maxima centered at the headwaters of three major rivers (Yangtze, Yellow and Mekong). For the central TP, the highest PR ratio rose to over 0.8 in August, indicating that most of the precipitation was recycled via local evapotranspiration in summer. The larger daily mean and standard deviation of the PR ratio in summer suggested a stronger effect of land-atmosphere interactions on precipitation in summer than in winter. Despite the relatively small spatial extent of inland lakes, the treatment of lakes in WRF significantly impacted the calculation of the PR ratio over the TP, and correcting lake temperature substantially improved both precipitation and PR ratio simulations. There was no clear relationship between PR ratio and precipitation amount; however, a significant positive correlation between PR and convective precipitation was revealed. This study is beneficial for the understanding of land-atmosphere interaction over high mountain regionsThis work is jointly supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDA2006010202), by the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (2019QZKK010314), by the National Natural Science Foundation of China (91537105 and 91537211), and by the NCAR Water System Program. Support of the Fudan University-Tibet University Joint Laboratory for Biodiversity and Global Change is appreciated. We thank the ECMWF for the free access of the ERA-Interim Reanalysis dataset. The work was carried out at the National Supercomputer Center in TianjinS

A physical concept in the press: the case of the jet stream

[Abstract] In recent years, science has hardened the discourse on the emergency of global warming, pointing out that the next decades will be decisive to maintaining the stability of the climate system and, thus, avoiding a cascade effect of events that increase the average temperature above safe limits. The scientific community warns that there are different tipping points that could produce a chain reaction in the global climate. One of them is related to the jet stream. However, despite the importance of this air current in atmospheric dynamics in the Northern Hemisphere and the changes it is experiencing in the context of global warming, the public is still not familiar with this kind of physical concept nor with other much simpler concepts. As concerns about the climate crisis rise, climate literacy remains stagnant. To advance the learning of the science of climate change, in general, and of concepts such as the jet stream, in particular, specific scientific communication formats are required that can successfully tackle the difficult task of explaining such complex problems to the general public. These formats should be included in the media, as the characteristics of the formats (daily section, scientific dissemination, historic perspective, teleconnections and specialization) make them well suited to taking on the challenge of explaining the complexity of climate science. In this article, we present a communication proposal existent in a newspaper published in Spain. We argue that this communication format represents a good model to disseminate climate science, educate readers and even to make physical concepts such as the jet stream accessible. We believe that this format conforms to and complies with the enunciation of Article 12 of the Paris Agreement, which calls on the signatory countries to promote education and training on climate change

Spatial and temporal expansion of global wildland fire activity in response to climate change

Global warming is expected to alter wildfire potential and fire season severity, but the magnitude and location of change is still unclear. Here, we show that climate largely determines present fire-prone regions and their fire season. We categorize these regions according to the climatic characteristics of their fire season into four classes, within general Boreal, Temperate, Tropical and Arid climate zones. Based on climate model projections, we assess the modification of the fire-prone regions in extent and fire season length at the end of the 21st century. We find that due to global warming, the global area with frequent fire-prone conditions would increase by 29%, mostly in Boreal (+111%) and Temperate (+25%) zones, where there may also be a significant lengthening of the potential fire season. Our estimates of the global expansion of fire-prone areas highlight the large but uneven impact of a warming climate on Earth’s environmentFunding comes from the Spanish Ministerio de Economia y Competitividad OPERMO (CGL2017-89859-R) and the CRETUS strategic partnership (AGRUP2015/02). All these programs are co-funded by the European Union ERDF. M.S.R. acknowledges Xunta de Galicia for a predoctoral grant (Programa de axudas á etapa predoutoral 2019, ED481A-2019/112). D.I.C. was awarded a pre-doctoral FPI (PRE2018-084425) grant from the Spanish Ministry of Science, Innovation and UniversitiesS

Local and remote moisture sources for extreme precipitation: a study of the two catastrophic 1982 western Mediterranean episodes

Floods and flash floods are frequent in the south of Europe resulting from heavy rainfall events that often produce more than 200 mm in less than 24 h. Even though the meteorological conditions favourable for these situations have been widely studied, there is a lingering question that still arises: what humidity sources could explain so much precipitation? To answer this question, the regional atmospheric Weather Research and Forecasting (WRF) model with a recently implemented moisture tagging capability has been used to analyse the main moisture sources for two catastrophic flood events that occurred during the autumn of 1982 (October and November) in the western Mediterranean area, which is regularly affected by these types of adverse weather episodes. The procedure consists in selecting a priori potential moisture source regions for the extreme event under consideration, and then performing simulations using the tagging technique to quantify the relative contribution of each selected source to total precipitation. For these events we study the influence of four possible potential sources: (1) evaporation in the western Mediterranean; (2) evaporation in the central Mediterranean; (3) evaporation in the North Atlantic; and (4) advection from the tropical and subtropical Atlantic and Africa. Results show that these four moisture sources explain most of the accumulated precipitation, with the tropical and subtropical input being the most relevant in both cases. In the October event, evaporation in the western and central Mediterranean and in the North Atlantic also had an important contribution. However, in the November episode tropical and subtropical moisture accounted for more than half of the total accumulated rainfall, while evaporation in the western Mediterranean and North Atlantic played a secondary role and the contribution of the central Mediterranean was almost negligible. Therefore, remote sources were crucial: in the October event they played a similar role to local sources, whereas in the November case they were clearly dominant. In both episodes, long-distance moisture transport from the tropics and subtropics mostly occurred in mid-tropospheric layers, via well-defined moisture plumes with maximum mixing ratios at medium levels

Validation of a new SAFRAN-based gridded precipitation product for Spain and comparisons to Spain02 and ERA-Interim

Offline Land-Surface Model (LSM) simulations are useful for studying the continental hydrological cycle. Because of the nonlinearities in the models, the results are very sensitive to the quality of the meteorological forcing; thus, high-quality gridded datasets of screen-level meteorological variables are needed. Precipitation datasets are particularly difficult to produce due to the inherent spatial and temporal heterogeneity of that variable. They do, however, have a large impact on the simulations, and it is thus necessary to carefully evaluate their quality in great detail. This paper reports the quality of two high-resolution precipitation datasets for Spain at the daily time scale: the new SAFRAN-based dataset and Spain02. SAFRAN is a meteorological analysis system that was designed to force LSMs and has recently been extended to the entirety of Spain for a long period of time (1979/80-2013/14). Spain02 is a daily precipitation dataset for Spain and was created mainly to validate Regional Climate Models. In addition, ERA-Interim is included in the comparison to show the differences between local high-resolution and global low-resolution products. The study compares the different precipitation analyses with rain gauge data and assesses their temporal and spatial similarities to the observations. The validation of SAFRAN with independent data shows that this is a robust product. SAFRAN and Spain02 have very similar scores, although the later slightly surpasses the former. The scores are robust with altitude and throughout the year, save perhaps in summer, when a diminished skill is observed. As expected, SAFRAN and Spain02 perform better than ERA-Interim, which has difficulty capturing the effects of the relief on precipitation due to its low resolution. However, ERA-Interim reproduces spells remarkably well, in contrast to the low skill shown by the high-resolution products. The high-resolution gridded products overestimate the number of precipitation days, which is a problem that affects SAFRAN more than Spain02 and is likely caused by the interpolation method. Both SAFRAN and Spain02 underestimate high precipitation events, but SAFRAN does so more than Spain02. The overestimation of low precipitation events and the underestimation of intense episodes will probably have hydrological consequences once the data are used to force a land surface or hydrological model.We are grateful to the French National Centre for Meteorological Research (CNRM UMR3539, Météo-France CNRS) for allowing us to use the code of the SAFRAN analysis system for our studies, the Spanish State Meteorological Agency (AEMET) for sharing their very valuable observational data with us and the European Centre for Medium-Range Weather Forecasts (ECMWF) for making their ERA-Interim product openly available. This is a contribution to the FP7 eartH2Observe project (http://www.earth2observer.eu), which received funding from the European Union’s Seventh Programme for research, technological development and demonstration under grant agreement no. 603608. This work has been funded by the Spanish Economy and Competitiveness Ministry and the European Regional Development Fund through grant CGL2013-47261-R. This work has been supported by the Metropolitan Area of Barcelona Project (no. 308321; flood evolution in the metropolitan area of Barcelona from a holistic perspective: past, present and future) and the Spanish Project HOPE (CGL2014-52571-R) supported by the Ministry of Economy and Competitiveness. This work is a contribution to the HyMeX program (Hydrological cycle in the Mediterranean EXperiment; http://www.hymex.org)