Abstract. Effects of climate change on the ecosystem productivity and water fluxes have been studied in various types of experiments. However, it is still largely unknown whether and how the experimental approach itself affects the results of such studies. We employed two contrasting experimental approaches, using high-precision weighable monolithic lysimeters, over a period of 4 years to identify and compare the responses of water fluxes and aboveground biomass to climate change in permanent grassland. The first, manipulative, approach is based on controlled increases of atmospheric CO2 concentration and surface temperature. The second, observational, approach uses data from a space-for-time substitution along a gradient of climatic conditions. The Budyko framework was used to identify if the soil ecosystem is energy limited or water limited. Elevated temperature reduced the amount of non-rainfall water, particularly during the growing season in both approaches. In energy-limited grassland ecosystems, elevated temperature increased the actual evapotranspiration and decreased aboveground biomass. As a consequence, elevated temperature led to decreasing seepage rates in energy-limited systems. Under water-limited conditions in dry periods, elevated temperature aggravated water stress and, thus, resulted in reduced actual evapotranspiration. The already small seepage rates of the drier soils remained almost unaffected under these conditions compared to soils under wetter conditions. Elevated atmospheric CO2 reduced both actual evapotranspiration and aboveground biomass in the manipulative experiment and, therefore, led to a clear increase and change in seasonality of seepage. As expected, the aboveground biomass productivity and ecosystem efficiency indicators of the water-limited ecosystems were negatively correlated with an increase in aridity, while the trend was unclear for the energy-limited ecosystems. In both experimental approaches, the responses of soil water fluxes and biomass production mainly depend on the ecosystems' status with respect to energy or water limitation. To thoroughly understand the ecosystem response to climate change and be able to identify tipping points, experiments need to embrace sufficiently extreme boundary conditions and explore responses to individual and multiple drivers, such as temperature, CO2 concentration, and precipitation, including non-rainfall water. In this regard, manipulative and observational climate change experiments complement one another and, thus, should be combined in the investigation of climate change effects on grassland.

Abstract. Hydrological processes are affected by changing climatic conditions. In grassland areas, changes in the ecosystem water balance components will alter aboveground biomass production (AGB), which in turn is of great importance for ecological and economic benefits of grassland. However, the effects of climate change on the ecosystem productivity and water fluxes are often derived from climate change experiments. It is still largely unknown whether and how the experimental approach itself affects the results of such studies. The aim of this investigation was to identify the effects of climate change on the water balance and the productivity of grassland ecosystems by comparing results of two contrasting approaches of climate change experiments. The first (manipulative) climate change approach uses increased atmospheric CO2 concentrations and surface temperatures. The second (observational) approach uses data from a space-for-time substitution approach along a gradient in climatic conditions. The climate change effects on the ecosystem’s water balance was determined by using high-precision weighable monolithically lysimeters at each site over a period of four years, including the exceptionally dry year 2018. The aridity index, defined as the grass-reference evapotranspiration (ET0) to precipitation (P), was used to characterize the hydrological status of the regime (i.e. energy- or water limited system). The observational approach (grassland ecosystem moved to a drier and warmer site), resulted in a large decrease of precipitation (P) and non-rainfall water (NRW), an increase in actual evapotranspiration (ETa) and upward directed water fluxes from deeper soil and hence a decline of seepage water as well a decrease in AGB and water use efficiency (WUE). The manipulative approach (grassland ecosystem treated in place) resulted in decreasing P and NRW under conditions of elevated temperature but responded with increasing NRW for elevated CO2 as compared to the reference. Similarly, an elevated CO2 and heating increased the ecosystem’s water loss by ETa. However, the effect of increasing CO2 on ETa was largely compensated by the opposite effect of an elevated temperature in the combined treatment. The seepage water rate also increased with elevated CO2, whereas it clearly decreased for the heating treatment as compared to the reference. All treatments led to a reduction of the grassland productivity in terms of the AGB and to reduced WUE as compared to the grassland ecosystem under reference conditions. The consideration of changes in NRW and P by the treatments needs to be considered in climate change experiments to avoid an over- (elevated temperature) or underestimation (elevated CO2) of the effects of climate change on ecosystems response, especially for sites where water limitation plays a role. The impact of drought periods on seepage rates (potentially leading to groundwater recharge) was more pronounced for the relatively humid site with a longer ETa period without water stress than for a relatively dry site. The hydroclimatological and ecohydrological indicators were similarly affected by changes in temperature, atmospheric CO2 concentrations, and precipitation in both manipulative and observational climate change experiments except for the responses of ETa and AGB in the dry and warm year 2018. The resulting response differences between the two climate change approaches were explained by the actual soil moisture status. The results suggest that energy limited ecosystems tend to increase their ETa and AGB production (excluding effects from elevated CO2 and temperature), but water limited ecosystems respond with a decrease in ETa as a result of water stress, which leads to a clear decline of AGB. The results also suggest that climate change experiments should account for the possible change of the hydrological status of the ecosystem and impose sufficiently extreme levels of climatic conditions within their set-up to allow such changes to occur for capturing the full response of the ecosystem. The results may help to better understanding the impact of climate change on future ecosystem functioning.

<p>HYDRUS-1D is a popular software suite for one-dimensional modeling of flow and transport through the vadose zone [1]. Models can be handled through the Graphical User Interface (GUI), made freely available by the original authors (https://www.pc-progress.com/). As the program is file-based, the HYDRUS-1D GUI already ensures a certain degree of reproducibility, as these files contain all information about a model. The original FORTRAN code of the HYDRUS-1D model is also made available and is used in many publications to perform more complicated analysis of flow and transport through the unsaturated zone. For each of these publications new code was programmed to change the input files and perform a specific analysis. Being a popular hydrological model, it seems only logical to start reusing such code and structurally develop its capabilities. In the presentation, we introduce Phydrus, an open source Python package to create, optimize and visualize HYDRUS-1D models. Python scripts or Jupyter Notebooks are used for all steps of the modeling process, documenting the entire workflow and ensuring reproducibility of the analysis. Connecting HYRDUS-1D to Python makes it easier to perform repetitive tasks on models, and potentially opens up a whole new set of possibilities and applications. While introducing Phydrus, this presentation will also focus on the process of creating the Python Package and why we think it is worthwhile for the hydrologic community to interface existing (older) code with newer programming languages popular in the hydrological scientific community.</p><p><strong>References<br></strong>[1] Šimůnek, J. and M. Th. van Genuchten (2008) Modeling nonequilibrium flow and transport with HYDRUS, Vadose Zone Journal.</p>

<p>Evapotranspiration (ET) is a major component of the hydrological cycle and accurate estimates of the flux are important to the water and agricultural sector, among others. Due to difficulties in the direct observation of ET in the field, the flux is often estimated from other meteorological data using empirical formulas. There is a wide variety of such formulas, with different levels of input data and parameter requirements. While some Python packages are available in the Python ecosystem for these tasks, they typically focus on one specific formula or data type. The goal of PyEt is to provide a Python package for the estimation of ET that works with many different data types, is well documented and tested, and simple to use. The source code is hosted at GitHub (https://github.com/phydrus/PyEt) and Pypi can be used to install the package. PyEt currently contains nine different methods to estimate ET and various methods to estimate surface and aerodynamic resistance. The methods are tested against other open source data to ensure proper functioning of the methods. While the methods currently are only implemented for 1D data (e.g. time series data), future work will focus on enabling the methods on 2D and 3D data as well (such as Numpy Arrays, XArray, and NetCDF files). The package allows hydrologists to compute and compare evapotranspiration estimates using different approaches with minimum effort. The presentation will focus on the problems associated with reproducibility in ET estimation and linkage with existing Python libraries to perform complex sensitivity and uncertainty analyses.</p>

Keywords: Vrbanski Plato aquifer, FREEWAT HORIZON 2020 project, water resource management, artificial groundwater recharge. riassunto: Il caso studio situato in Slovenia, all'interno del progetto EU HORIZON 2020 FREEWAT ha avuto come oggetto l'acquifero dell'altopiano di Vrbanski. In termini di bacini fluviali, la Slovenia è divisa in due distretti: il Danubio e l'Adriatico settentrionale. L'acquifero dell'altopiano di Vrbanski, che presenta una ricarica dal subalveo del fiume Drava sia di tipo naturale che artificiale, fa parte del distretto del bacino del Danubio ed è la fonte di acqua più importante per 14 comuni della parte nordest della Slovenia. Gli Autori hanno studiato l'interazione fra le acque superficiali del fiume Drava e quelle sotterranee dell'acquifero poroso che giace al di sotto dell'antico letto del fiume, assieme alla valutazione di una possibile riduzione dell'impatto ambientale dovuto alla presenza del vicino agglomerato urbano. Questo sito rappresenta in Slovenia la più antica opera di ricarica gestita delle acque sotterranee, utilizzando la tecnica dell'induzione di ricarica da subalveo, e registra ad oggi più di trent'anni di funzionamento efficiente. Rappresenta un impianto piuttosto particolare, caratterizzato da molta abbondanza di risorsa confinata in un piccolo spazio, indipendente dalla siccità e dai cambiamenti climatici, ma vulnerabile a causa dell'impatto della vicina città. Proprio sotto la città è presente uno spartiacque,,che periodicamente si sposta di posizione a causa delle diverse condizioni operative di gestione dell'acqua. Per definire una gestione ottimale della risorsa, gli Autori hanno deciso di utilizzare il plug- in FREEWAT incluso nel software QGIS. Con il nuovo plug-in FREEWAT, sviluppato durante l'omonimo progetto, gli Autori hanno sviluppato un modello sia in stato stazionario che in transitorio, per definire lo spostamento dello spartiacque sotterraneo in dipendenza delle diverse condizioni di gestione. Il modello è stato progettato in modo da identificare e descrivere tutti gli aspetti principali del sistema idrogeologico fisico e della gestione idrica. Durante le attività di progetto, è avvenuto un incidente di fuoriuscita di olio di riscaldamento nell'area della città, proprio in corrispondenza dello spartiacque. Pertanto, il modello transitorio (accoppiato ad un modello di trasporto) è stato utilizzato anche per proporre metodologie di riduzione dell'impatto della fuoriuscita di olio, utilizzando una barriera idraulica per implementare una tecnica di pump and treat sul luogo dell'incidente. L'esperienza di applicazione della piattaforma FREEWAT durante il caso studio di Vrbanski è stata molto positiva. La possibilità di gestire i dati direttamente in ambiente GIS, la licenza libera e gratuita, l'organizzazione dei dati in un database relazionale, e gli approcci modellistici inclusi, hanno garantito la funzionalità di uno strumento modellistico adattato ad essere applicato con approccio professionale e a comunicare i risultati del modello agli stakehold...