@article{9905, abstract = {Vaccines are thought to be the best available solution for controlling the ongoing SARS-CoV-2 pandemic. However, the emergence of vaccine-resistant strains may come too rapidly for current vaccine developments to alleviate the health, economic and social consequences of the pandemic. To quantify and characterize the risk of such a scenario, we created a SIR-derived model with initial stochastic dynamics of the vaccine-resistant strain to study the probability of its emergence and establishment. Using parameters realistically resembling SARS-CoV-2 transmission, we model a wave-like pattern of the pandemic and consider the impact of the rate of vaccination and the strength of non-pharmaceutical intervention measures on the probability of emergence of a resistant strain. As expected, we found that a fast rate of vaccination decreases the probability of emergence of a resistant strain. Counterintuitively, when a relaxation of non-pharmaceutical interventions happened at a time when most individuals of the population have already been vaccinated the probability of emergence of a resistant strain was greatly increased. Consequently, we show that a period of transmission reduction close to the end of the vaccination campaign can substantially reduce the probability of resistant strain establishment. Our results suggest that policymakers and individuals should consider maintaining non-pharmaceutical interventions and transmission-reducing behaviours throughout the entire vaccination period.}, author = {Rella, Simon and Kulikova, Yuliya A. and Dermitzakis, Emmanouil T. and Kondrashov, Fyodor}, issn = {2045-2322}, journal = {Scientific Reports}, number = {1}, publisher = {Springer Nature}, title = {{Rates of SARS-CoV-2 transmission and vaccination impact the fate of vaccine-resistant strains}}, doi = {10.1038/s41598-021-95025-3}, volume = {11}, year = {2021}, } @phdthesis{9418, abstract = {Deep learning is best known for its empirical success across a wide range of applications spanning computer vision, natural language processing and speech. Of equal significance, though perhaps less known, are its ramifications for learning theory: deep networks have been observed to perform surprisingly well in the high-capacity regime, aka the overfitting or underspecified regime. Classically, this regime on the far right of the bias-variance curve is associated with poor generalisation; however, recent experiments with deep networks challenge this view. This thesis is devoted to investigating various aspects of underspecification in deep learning. First, we argue that deep learning models are underspecified on two levels: a) any given training dataset can be fit by many different functions, and b) any given function can be expressed by many different parameter configurations. We refer to the second kind of underspecification as parameterisation redundancy and we precisely characterise its extent. Second, we characterise the implicit criteria (the inductive bias) that guide learning in the underspecified regime. Specifically, we consider a nonlinear but tractable classification setting, and show that given the choice, neural networks learn classifiers with a large margin. Third, we consider learning scenarios where the inductive bias is not by itself sufficient to deal with underspecification. We then study different ways of ‘tightening the specification’: i) In the setting of representation learning with variational autoencoders, we propose a hand- crafted regulariser based on mutual information. ii) In the setting of binary classification, we consider soft-label (real-valued) supervision. We derive a generalisation bound for linear networks supervised in this way and verify that soft labels facilitate fast learning. Finally, we explore an application of soft-label supervision to the training of multi-exit models.}, author = {Bui Thi Mai, Phuong}, issn = {2663-337X}, pages = {125}, publisher = {Institute of Science and Technology Austria}, title = {{Underspecification in deep learning}}, doi = {10.15479/AT:ISTA:9418}, year = {2021}, } @inproceedings{9416, abstract = {We study the inductive bias of two-layer ReLU networks trained by gradient flow. We identify a class of easy-to-learn (`orthogonally separable') datasets, and characterise the solution that ReLU networks trained on such datasets converge to. Irrespective of network width, the solution turns out to be a combination of two max-margin classifiers: one corresponding to the positive data subset and one corresponding to the negative data subset. The proof is based on the recently introduced concept of extremal sectors, for which we prove a number of properties in the context of orthogonal separability. In particular, we prove stationarity of activation patterns from some time onwards, which enables a reduction of the ReLU network to an ensemble of linear subnetworks.}, author = {Bui Thi Mai, Phuong and Lampert, Christoph}, booktitle = {9th International Conference on Learning Representations}, location = {Virtual}, title = {{The inductive bias of ReLU networks on orthogonally separable data}}, year = {2021}, } @article{22472, abstract = {Increasing urban tree cover is an often proposed mitigation strategy against urban heat as trees are expected to cool cities through evapotranspiration and shade provision. However, trees also modify wind flow and urban aerodynamic roughness, which can potentially limit heat dissipation. Existing studies show a varying cooling potential of urban trees in different climates and times of the day. These differences are so far not systematically explained as partitioning the individual tree effects is challenging and impossible through observations alone. Here, we conduct numerical experiments removing and adding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model. Simulations are presented for four cities in different climates (Phoenix, Singapore, Melbourne, Zurich) considering the seasonal and diurnal cycles of air and surface temperatures. Results show that evapotranspiration of well-watered trees alone can decrease local 2 m air temperature at maximum by 3.1– 5.8 °C in the four climates during summer. Further cooling is prevented by stomatal closure at peak temperatures as high vapour pressure deficits limit transpiration. While shading reduces surface temperatures, the interaction of a non-transpiring tree with radiation can increase 2 m air temperature by up to 1.6 – 2.1 °C in certain hours of the day at local scale, thus partially counteracting the evapotranspirative cooling effect. Furthermore, in the analysed scenarios, which do not account for tree wind blockage effects, trees lead to a decrease in urban roughness, which inhibits turbulent energy exchange and increases air temperature during daytime. At night, single tree effects are variable likely due to differences in atmospheric stability within the urban canyon. These results explain reported diurnal, seasonal and climatic differences in the cooling effects of urban trees, and can guide future field campaigns, planning strategies, and species selection aimed at improving local microclimate using urban greenery.}, author = {Meili, Naika and Manoli, Gabriele and Burlando, Paolo and Carmeliet, Jan and Chow, Winston T.L. and Coutts, Andrew M. and Roth, Matthias and Velasco, Erik and Vivoni, Enrique R. and Fatichi, Simone}, issn = {1610-8167}, journal = {Urban Forestry & Urban Greening}, number = {3}, publisher = {Elsevier}, title = {{Tree effects on urban microclimate: Diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects}}, doi = {10.1016/j.ufug.2020.126970}, volume = {58}, year = {2021}, } @article{22474, abstract = {In light of globally increasing temperatures, accentuated in cities by the urban heat island effect, urban planners and designers are looking for new, quantitative methods to assess the performance of their designs in terms of ecosystem services provided by vegetation. Among these ecosystem services, improved microclimate conditions are particularly important for human thermal comfort and health. In this study, an urban scene in the tropical city of Singapore is numerically investigated with a fully-integrated, three-dimensional urban microclimate model implemented in OpenFOAM. Mass and heat transport in air and storage effect in the urban environment are coupled so that the daily turbulent transport in air using steady Reynolds-averaged Navier-Stokes (RANS) can be solved iteratively with the unsteady heat and moisture transfer from urban surfaces. Vegetation is modeled as a porous medium for the flow of moist air and a leaf energy balance model is used to determine the heat fluxes and transpiration at leaf surfaces. The analysis shows the influence of an urban park upon air temperatures and thermal comfort. Cooling intensity of 1 °C is observed downwind of the park within a region of 27 m for an incoming wind speed of 2.3 m s−1, which reduces to 0.6 °C at a distance of 117 m from the park. The Universal Thermal Comfort Index (UTCI) shows a reduction in thermal stress in and around the park. The approach presented here can provide specific guidelines for urban planners and frame expectations on magnitude and spatial extent of local microclimate modifications generated by an urban park in a tropical city.}, author = {Mughal, Muhammad Omer and Kubilay, Aytac and Fatichi, Simone and Meili, Naika and Carmeliet, Jan and Edwards, Peter and Burlando, Paolo}, issn = {2212-0955}, journal = {Urban Climate}, publisher = {Elsevier}, title = {{Detailed investigation of vegetation effects on microclimate by means of computational fluid dynamics (CFD) in a tropical urban environment}}, doi = {10.1016/j.uclim.2021.100939}, volume = {39}, year = {2021}, } @article{22468, abstract = {Vegetation establishment, growth and succession in riparian ecosystems are linked to river flow dynamics and groundwater table fluctuations. This is especially true in Alpine gravel-bed rivers with wide floodplains, geomorphically active floods and a strong river-aquifer exchange. The role of short-term groundwater fluctuations is not always clear in these ecosystems, as it is assumed that phreatophytic vegetation close to rivers is adapted to such conditions. Here, we provide data evidence of riparian plant response to short-term groundwater table fluctuations in a braided gravel-bed river (Maggia). We used indirect physiological variables for photosynthesis and transpiration—stomatal conductance gs and daily variation in stem diameter ΔDd—which we measured at six mature riparian trees of the Salicaceae family at two sites with different mean depths to groundwater during two growing seasons. The data demonstrate that (a) short-term variation of the groundwater table affects riparian vegetation—at the site with deeper groundwater, the water table depth was the best predictor of gs variability, while at the site with shallower groundwater, temperature and vapour pressure deficit (VPD) were the best predictors of ΔDd variability; (b) instantaneous stomatal conductance is related to VPD, but conditioned by groundwater levels, with higher stomatal conductance for the same radiative input and VPD when the water table was higher for all trees; and (c) local microclimate measured at tree locations had a stronger predictive power for gs than valley scale climate, suggesting local climate controls on vegetated stands on gravel bars. Our results provide evidence of riparian trees undertaking physiological adjustments to transpiration in response to groundwater stage, depending on their riparian floodplain setting.}, author = {Martinetti, Stefano and Fatichi, Simone and Floriancic, Marius and Burlando, Paolo and Molnar, Peter}, issn = {1936-0592}, journal = {Ecohydrology}, number = {2}, publisher = {Wiley}, title = {{Field evidence of riparian vegetation response to groundwater levels in a gravel‐bed river}}, doi = {10.1002/eco.2264}, volume = {14}, year = {2021}, } @article{22430, abstract = {Research to detect changes in precipitation variables has become a topic of particular interest to understand modifications in water resources availability. The review is focused on the Italian territory, outlining the “state of the art” of changes in precipitation regime through a review of 54 published studies on observed rainfall trend analyses, in the period 1999–2018. The aim is to combine a large body of knowledge in a single review and to explain the main patterns of rainfall changes occurred in Italy over the last decades. The analysis focused on the Total Precipitation (TP) and the number of Wet Days (WDs) indices at the annual and seasonal scale. A weight factor is introduced to take into account the differences among studies in geographical area, time series length, and number of stations. The review is accompanied by the discussion of other rainfall related variables, that is, precipitation intensity, extreme rainfall events and meteorological droughts, which are useful to provide a broader picture of rainfall changes. Overall, there is an agreement about the tendency of a decrease in wet days on the entire Italy, with limited discrepancies in the various regions. A decrease in wet days is accompanied by a negative trend (although less evident) in total precipitation, especially in winter. Nevertheless, a univocal direction of trends (or lack of thereof) in annual total precipitation and mostly hydrological extreme events is difficult to achieve.}, author = {Caporali, Enrica and Lompi, Marco and Pacetti, Tommaso and Chiarello, Valentina and Fatichi, Simone}, issn = {1097-0088}, journal = {International Journal of Climatology}, number = {S1}, pages = {E1--E25}, publisher = {Wiley}, title = {{A review of studies on observed precipitation trends in Italy}}, doi = {10.1002/joc.6741}, volume = {41}, year = {2021}, } @article{22457, abstract = {An increase in urban vegetation is an often proposed mitigation strategy to reduce urban heat and improve outdoor thermal comfort (OTC). Vegetation can alter urban microclimate through changes in air temperature, mean radiant temperature, humidity, and wind speed. In this study, we model how street tree and ground vegetation cover and their structural, optical, interception, and physiological traits control the diurnal cycle of OTC in different urban densities in a tropical city (Singapore). For this purpose, we perform a variance based sensitivity analysis of the urban ecohydrological model UT&C. Model performance is evaluated through a comparison with local microclimate measurements and OTC is assessed with the Universal Thermal Climate Index (UTCI). We find a pronounced daily cycle of vegetation effects on UTCI. Tree cover fraction is more efficient in decreasing UTCI during daytime, while a higher vegetated ground fraction provides more cooling during night. Generally, increasing vegetation cover fractions do not deter OTC, except in certain urban densities during some periods of the day. An increase in tree and ground vegetation fractions provides a higher average UTCI reduction compared to a change in vegetation traits (0.9 – 2.9 °C vs. 0.7 – 1.1 °C during midday, 10 month average). The increase in humidity related to plant transpiration prevents further reduction of UTCI. However, the choice of vegetation traits enhancing tree transpiration can decrease UTCI during hot periods. These results can inform urban planners on the selection of vegetation amount and traits to achieve feasible OTC improvements in tropical cities.}, author = {Meili, Naika and Acero, Juan Angel and Peleg, Nadav and Manoli, Gabriele and Burlando, Paolo and Fatichi, Simone}, issn = {0360-1323}, journal = {Building and Environment}, publisher = {Elsevier}, title = {{Vegetation cover and plant-trait effects on outdoor thermal comfort in a tropical city}}, doi = {10.1016/j.buildenv.2021.107733}, volume = {195}, year = {2021}, } @article{22448, abstract = {Groundwater is a key water resource in semiarid and seasonally dry regions around the world, which is replenished by intermittent precipitation events and mediated by vegetation, soil, and regolith properties. Here, a climate reconstruction of 4500 years for the Jerusalem region was used to determine the relation between climate, vegetation, and groundwater recharge. Despite changes in air temperature and vegetation characteristics, simulated recharge remained linearly related to precipitation over the entire analyzed period, with drier decades having lower rates of recharge for a given annual precipitation due to soil memory effects. We show that in recent decades, the lack of changes in the precipitation–groundwater recharge relation results from the compensating responses of vegetation to increasing CO2, i.e., increased leaf area and reduced stomatal conductance. This multicentury relation is expected to be modified by climate change, with changes up to −20% in recharge for unchanged precipitation, potentially jeopardizing water resource availability.}, author = {Fatichi, Simone and Peleg, Nadav and Mastrotheodoros, Theodoros and Pappas, Christoforos and Manoli, Gabriele}, issn = {2375-2548}, journal = {Science Advances}, number = {37}, publisher = {American Association for the Advancement of Science}, title = {{An ecohydrological journey of 4500 years reveals a stable but threatened precipitation–groundwater recharge relation around Jerusalem}}, doi = {10.1126/sciadv.abe6303}, volume = {7}, year = {2021}, } @article{22452, abstract = {Model fidelity and accuracy in process representations have been the crux of scientific hydrological modeling, creating a pressing need for a better linkage between the development of hydrological models and the growing number of data sources and measurement techniques. Improved representation of process dynamics in hydrological models can provide new insights into complex hydrological systems and point out less understood natural phenomena that need further investigation. This special issue includes contributions that offer potential solutions and strategies to improve and test the representation of hydrological processes. We have organized the special issue contributions into four topical categories: (a) Beyond streamflow, which looks into the power of complementary data sources in addition to traditionally used streamflow for process inference. (b) Challenge of subsurface hydrology, that reflects on lesser understood processes under the surface and their impact on the model structure. (c) Evaporation in hydrological modeling, linking ecological aspects to the hydrological functioning of the natural system. Finally, (d) top down vs. bottom up modeling approaches, relied upon for process representation analysis. The special issue and our reflection on the contributions present a snapshot of ongoing efforts for integrating new concepts, knowledge, and data in process representation in hydrological models.}, author = {Guse, Björn and Fatichi, Simone and Gharari, Shervan and Melsen, Lieke A.}, issn = {1944-7973}, journal = {Water Resources Research}, number = {11}, publisher = {American Geophysical Union}, title = {{Advancing process representation in hydrological models: Integrating new concepts, knowledge, and data}}, doi = {10.1029/2021wr030661}, volume = {57}, year = {2021}, } @unpublished{10579, abstract = {We consider a totally asymmetric simple exclusion process (TASEP) consisting of particles on a lattice that require binding by a "token" to move. Using a combination of theory and simulations, we address the following questions: (i) How token binding kinetics affects the current-density relation; (ii) How the current-density relation depends on the scarcity of tokens; (iii) How tokens propagate the effects of the locally-imposed disorder (such a slow site) over the entire lattice; (iv) How a shared pool of tokens couples concurrent TASEPs running on multiple lattices; (v) How our results translate to TASEPs with open boundaries that exchange particles with the reservoir. Since real particle motion (including in systems that inspired the standard TASEP model, e.g., protein synthesis or movement of molecular motors) is often catalyzed, regulated, actuated, or otherwise mediated, the token-driven TASEP dynamics analyzed in this paper should allow for a better understanding of real systems and enable a closer match between TASEP theory and experimental observations.}, author = {Kavcic, Bor and Tkačik, Gašper}, booktitle = {arXiv}, title = {{Token-driven totally asymmetric simple exclusion process}}, doi = {10.48550/arXiv.2112.13558}, year = {2021}, } @article{22546, abstract = {Increasing urban green spaces and canopy cover requires careful planning of irrigation strategies, especially in arid and semiarid areas. This study investigates how vegetation cover and irrigation affect the water balance and vegetation productivity of a small urban reserve in the Melbourne metropolitan area, Australia. Using a mechanistic ecohydrological model, a series of numerical experiments were carried out for the period 1999–2018, which included a prolonged drought. Results indicated that irrigation played an essential role in helping both trees and grass productivity by increasing soil moisture and vegetation water access during the drought. With 10% tree cover, grass benefitted more than trees by increasing irrigation, and trees coped well with drought even without additional water. However, trees strongly relied on irrigation to maintain productivity when tree cover increased, highlighting the need for a sustainable balance between increasing urban greening and water conservation. Differences in soil properties and rooting strategies were also found to strongly modify the need for irrigation and the competition for water. These results provide quantitative insights on how increasing tree cover and vegetation diversity may impact irrigation requirements, highlighting the key role of mechanistic numerical models to support urban planners in the evaluation and design of urban green spaces.}, author = {Marchionni, V. and Fatichi, Simone and Tapper, N. and Walker, J.P. and Manoli, G. and Daly, E.}, issn = {1872-6062}, journal = {Landscape and Urban Planning}, keywords = {Urban green spaces, Remnant vegetation, Irrigation, Stormwater harvesting, Ecohydrological modeling}, publisher = {Elsevier}, title = {{Assessing vegetation response to irrigation strategies and soil properties in an urban reserve in southeast Australia}}, doi = {10.1016/j.landurbplan.2021.104198}, volume = {215}, year = {2021}, } @article{22570, abstract = {Atmospheric carbon dioxide concentration ([CO 2 ]) is increasing, which increases leaf-scalephotosynthesis and intrinsic water-use efficiency. These direct responses have the potential toincrease plant growth, vegetation biomass, and soil organic matter; transferring carbon from theatmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sinkwould slow the rate of [CO 2] increase and thus climate change. However, ecosystem CO2responses are complex or confounded by concurrent changes in multiple agents of global changeand evidence for a [CO 2]-driven terrestrial carbon sink can appear contradictory. Here wesynthesize theory and broad, multidisciplinary evidence for the effects of increasing [CO 2](iCO 2) on the global terrestrial carbon sink. Evidence suggests a substantial increase in globalphotosynthesis since pre-industrial times. Established theory, supported by experiments,indicates that iCO 2 is likely responsible for about half of the increase. Global carbon budgeting,atmospheric data, and forest inventories indicate a historical carbon sink, and these apparentiCO 2 responses are high in comparison to experiments and predictions from theory. Plantmortality and soil carbon iCO 2 responses are highly uncertain. In conclusion, a range of evidencesupports a positive terrestrial carbon sink in response to iCO2 , albeit with uncertain magnitudeand strong suggestion of a role for additional agents of global change.}, author = {Walker, Anthony P. and De Kauwe, Martin G. and Bastos, Ana and Belmecheri, Soumaya and Georgiou, Katerina and Keeling, Ralph F. and McMahon, Sean M. and Medlyn, Belinda E. and Moore, David J. P. and Norby, Richard J. and Zaehle, Sönke and Anderson‐Teixeira, Kristina J. and Battipaglia, Giovanna and Brienen, Roel J. W. and Cabugao, Kristine G. and Cailleret, Maxime and Campbell, Elliott and Canadell, Josep G. and Ciais, Philippe and Craig, Matthew E. and Ellsworth, David S. and Farquhar, Graham D. and Fatichi, Simone and Fisher, Joshua B. and Frank, David C. and Graven, Heather and Gu, Lianhong and Haverd, Vanessa and Heilman, Kelly and Heimann, Martin and Hungate, Bruce A. and Iversen, Colleen M. and Joos, Fortunat and Jiang, Mingkai and Keenan, Trevor F. and Knauer, Jürgen and Körner, Christian and Leshyk, Victor O. and Leuzinger, Sebastian and Liu, Yao and MacBean, Natasha and Malhi, Yadvinder and McVicar, Tim R. and Penuelas, Josep and Pongratz, Julia and Powell, A. Shafer and Riutta, Terhi and Sabot, Manon E. B. and Schleucher, Juergen and Sitch, Stephen and Smith, William K. and Sulman, Benjamin and Taylor, Benton and Terrer, César and Torn, Margaret S. and Treseder, Kathleen K. and Trugman, Anna T. and Trumbore, Susan E. and van Mantgem, Phillip J. and Voelker, Steve L. and Whelan, Mary E. and Zuidema, Pieter A.}, issn = {1469-8137}, journal = {New Phytologist}, keywords = {Beta factor, Carbon dioxide, CO2 fertilization, CO2-fertilization hypothesis, Free-air CO2 enrichment (FACE), Global carbon cycle, Land–atmosphere feedback, Terrestrial ecosystems}, number = {5}, pages = {2413--2445}, publisher = {Wiley}, title = {{Integrating the evidence for a terrestrial carbon sink caused by increasing atmospheric CO2}}, doi = {10.1111/nph.16866}, volume = {229}, year = {2021}, } @article{15279, abstract = {We formulate and prove an analog of Poonen’s finite-field Bertini theorem with Taylor conditions that holds in the Grothendieck ring of varieties. This gives a broad generalization of the work of Vakil and Wood, who treated the case of smooth hypersurface sections, and is made possible by the use of motivic Euler products to write down candidate motivic probabilities. As applications, we give motivic analogs of many results in arithmetic statistics that have been proven using Poonen’s sieve, including work of Bucur and Kedlaya on complete intersections and Erman and Wood on semiample Bertini theorems.}, author = {Bilu, Margaret and Howe, Sean}, issn = {1944-7833}, journal = {Algebra & Number Theory}, keywords = {Algebra and Number Theory}, number = {9}, pages = {2195--2259}, publisher = {Mathematical Sciences Publishers}, title = {{Motivic Euler products in motivic statistics}}, doi = {10.2140/ant.2021.15.2195}, volume = {15}, year = {2021}, } @unpublished{12076, abstract = {We find an asymptotic formula for the number of primitive vectors $(z_1,\ldots,z_4)\in (\mathbb{Z}_{\neq 0})^4$ such that $z_1,\ldots, z_4$ are all squareful and bounded by $B$, and $z_1+\cdots + z_4 = 0$. Our result agrees in the power of $B$ and $\log B$ with the Campana-Manin conjecture of Pieropan, Smeets, Tanimoto and V\'{a}rilly-Alvarado.}, author = {Shute, Alec L}, booktitle = {arXiv}, title = {{Sums of four squareful numbers}}, doi = {10.48550/arXiv.2104.06966}, year = {2021}, } @article{9260, abstract = {We study the density of rational points on a higher-dimensional orbifold (Pn−1,Δ) when Δ is a Q-divisor involving hyperplanes. This allows us to address a question of Tanimoto about whether the set of rational points on such an orbifold constitutes a thin set. Our approach relies on the Hardy–Littlewood circle method to first study an asymptotic version of Waring’s problem for mixed powers. In doing so we make crucial use of the recent resolution of the main conjecture in Vinogradov’s mean value theorem, due to Bourgain–Demeter–Guth and Wooley.}, author = {Browning, Timothy D and Yamagishi, Shuntaro}, issn = {1432-1823}, journal = {Mathematische Zeitschrift}, pages = {1071–1101}, publisher = {Springer Nature}, title = {{Arithmetic of higher-dimensional orbifolds and a mixed Waring problem}}, doi = {10.1007/s00209-021-02695-w}, volume = {299}, year = {2021}, } @article{10711, abstract = {In this paper, we investigate the distribution of the maximum of partial sums of families of m -periodic complex-valued functions satisfying certain conditions. We obtain precise uniform estimates for the distribution function of this maximum in a near-optimal range. Our results apply to partial sums of Kloosterman sums and other families of ℓ -adic trace functions, and are as strong as those obtained by Bober, Goldmakher, Granville and Koukoulopoulos for character sums. In particular, we improve on the recent work of the third author for Birch sums. However, unlike character sums, we are able to construct families of m -periodic complex-valued functions which satisfy our conditions, but for which the Pólya–Vinogradov inequality is sharp.}, author = {Autissier, Pascal and Bonolis, Dante and Lamzouri, Youness}, issn = {1570-5846}, journal = {Compositio Mathematica}, keywords = {Algebra and Number Theory}, number = {7}, pages = {1610--1651}, publisher = {Cambridge University Press}, title = {{The distribution of the maximum of partial sums of Kloosterman sums and other trace functions}}, doi = {10.1112/s0010437x21007351}, volume = {157}, year = {2021}, } @article{22506, abstract = {Alpine grasslands sustain local economy by providing fodder for livestock. Intensive fertilization is common to enhance their yields, thus creating negative externalities on water quality that are difficult to evaluate without reliable estimates of nutrient fluxes. We apply a mechanistic ecosystem model, seamlessly integrating land-surface energy balance, soil hydrology, vegetation dynamics, and soil biogeochemistry, aiming at assessing the grassland response to fertilization. We simulate the major water, carbon, nutrient, and energy fluxes of nine grassland plots across the broad European Alpine region. We provide an interdisciplinary model evaluation by confirming its performance against observed variables from different datasets. Subsequently, we apply the model to test the influence of fertilization practices on grassland yields and nitrate (NO3) losses through leaching under both current and modified climate scenarios. Despite the generally low NO3 concentration in groundwater recharge, the variability across sites is remarkable, which is mostly (but not exclusively) dictated by elevation. In high-Alpine sites, short growing seasons lead to less efficient nitrogen (N) uptake for biomass production. This combined with lower evapotranspiration rates results in higher amounts of drainage and NO3 leaching to groundwater. Scenarios with increased temperature lead to a longer growing season characterized by higher biomass production and, consequently, to a reduction of water leakage and N leaching. While the intersite variability is maintained, climate change impacts are stronger on sites at higher elevations. The local soil hydrology has a crucial role in driving the NO3 use efficiency. The commonly applied fixed threshold limit on fertilizer N input is suboptimal. We suggest that major hydrological and soil property differences across sites should be considered in the delineation of best practices or regulations for management. Using distributed maps informed with key soil and climatic attributes or systematically implementing integrated ecosystem models as shown here can contribute to achieving more sustainable practices.}, author = {Botter, Martina and Zeeman, Matthias and Burlando, Paolo and Fatichi, Simone}, issn = {1726-4189}, journal = {Biogeosciences}, number = {6}, pages = {1917--1939}, publisher = {Copernicus Publications}, title = {{Impacts of fertilization on grassland productivity and water quality across the European Alps under current and warming climate: insights from a mechanistic model}}, doi = {10.5194/bg-18-1917-2021}, volume = {18}, year = {2021}, } @article{22529, abstract = {Mountainous catchments cover a broad range of elevations and their response to a warming climate is expected to vary significantly in space. Nevertheless, studies on climate change impacts typically examine the changes in flow statistics only at the catchment outlet. In this study, we instead demonstrate the high variability of the hydrological response to climate change at the sub-catchment scale, investigating in detail the contribution of all components of the hydrological cycle in two mountainous catchments (Thur and Kleine Emme) in the Swiss Alps. The analysis was conducted with a two-dimensional weather generator model that simulated gridded climate variables at an hourly and 2-km resolution until the end of the 21st century for the RCP8.5 emission scenario. The climate ensemble was used as input into a distributed hydrological model to estimate the changes in hydrological processes at 100-m and hourly resolutions. Climate models show that precipitation intensifies during winter but weakens during summer in the order of ± 5–10% toward the end of the century. Temperature will rise by up to 4°C, leading to a 50% reduction in snowmelt, 10% increase in evapotranspiration, and shift in precipitation type from snowfall to rainfall. As a result, streamflow is projected to increase by 40% in winter but decrease by 20% to 40% during summer, with winter floods becoming more frequent. The changes to streamflow (mean and extreme low and high flows) at the sub-catchments show a strong dependency with elevation. In contrast to the small changes projected at the outlet of the catchments, streamflow shows a reduction at higher elevations (up to −20% change in mean streamflow for sub-catchments at elevations exceeding 1400 m) and an increase at lower elevations (up to +5% for Kleine Emme and +20% for the Thur at elevations below 600 m). These impacts are tied to the changes in precipitation, as well as changes in snowmelt (at high elevation) and evapotranspiration (at low elevation). The results reveal the causes and diversity of hydrological response to climate change, emphasizing the importance of investigating the distributed impacts of climate change in mountainous environments.}, author = {Moraga, Jorge Sebastián and Peleg, Nadav and Fatichi, Simone and Molnar, Peter and Burlando, Paolo}, issn = {1879-2707}, journal = {Journal of Hydrology}, keywords = {Catchment modelling, Climate change impacts, Weather generator, Distributed hydrological model, Streamflow extremes, Hydrological response}, publisher = {Elsevier}, title = {{Revealing the impacts of climate change on mountainous catchments through high-resolution modelling}}, doi = {10.1016/j.jhydrol.2021.126806}, volume = {603}, year = {2021}, } @article{22497, abstract = {This study evaluates the simulation of water balance components at half-hourly time steps from the Canadian Land Surface Scheme (CLASS) when driven by a 500-year stochastic meteorological data set produced by the Advanced WEather GENerator (AWE-GEN) at two boreal sites with contrasting water availability. The CLASS was driven by ERA5 reanalysis data (CLASS-CTL) over 39 years and its output was used as a surrogate for land surface observations. At both sites, the mean monthly and annual values of all meteorological variables used to drive CLASS, including precipitation, are well captured by AWE-GEN, but their variability is, sometimes, biased. In general, CLASS driven by stochastic data (CLASS-WG) tends to produce higher evapotranspiration compared to values simulated by CLASS-CTL, especially during spring and summer at the wet site. The interannual evapotranspiration-precipitation and runoff-precipitation relationships derived from CLASS-WG and those derived from CLASS-CTL were very similar to each other at the dry site; they both indicate that evapotranspiration and runoff are limited by water availability. At the wet site, however, CLASS-WG only captured well the interannual runoff-precipitation relationship. The sensitivity analysis shows that CLASS water fluxes are particularly affected by the replacement of physically consistent input time series of incoming short-wave radiation, precipitation, temperature, and specific humidity. In conclusion, the results show that even though a weather generator can produce coherent climate time series, the use of this synthetic data as meteorological forcing in a physically based land surface model does not necessarily reproduce the complex surface processes, such as the surface water fluxes. More studies are encouraged to further analyze the constraints of this framework.}, author = {Alves, Marco and Nadeau, Daniel F. and Music, Biljana and Anctil, François and Fatichi, Simone}, issn = {1434-4483}, journal = {Theoretical and Applied Climatology}, pages = {215--244}, publisher = {Springer Nature}, title = {{Can we replace observed forcing with weather generator in land surface modeling? Insights from long-term simulations at two contrasting boreal sites}}, doi = {10.1007/s00704-021-03615-y}, volume = {145}, year = {2021}, }