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Groundwater represents the largest accessible terrestrial water storage. As such, it is of crucial importance for society, as it can uphold provision of hydrological services to society when there is a lack of water during drought. However, when droughts extend over longer periods of time and groundwater abstraction is increased, longer-lasting groundwater droughts can develop, aggravating existing water shortages. This complex interaction results in an ambivalence of groundwater as a drought mitigation agent, and groundwater as a drought hazard impacting society. In theory, it is understood that this interaction exists. However, to date no study exists looking at empirical data regarding how and when groundwater is factually reported as a mitigation agent or as an impact agent. In this study, we build on previous efforts to synthesize text-based reports on drought impacts in the European Drought Impact Inventory (EDII). We extended the EDII with additional, groundwater-related reports. We then analysed the EDII data to find out: a) how is society impacted groundwater drought?, b) how is groundwater used during drought and what is its role?, c) is it used differently in different climatic or legislative settings?, and d) did the role of groundwater change over time? To answer these questions, we isolated the Europe-wide reports in the EDII that directly relate to groundwater, and merged them to new groundwater-related impact categories. We qualitatively summarized the way groundwater is reported as an impact, or as a mitigation agent, and provide statistics on the frequency of each occurrence. We further present the temporal development of the reporting, and compare the type and number of occurrence of groundwater-related reports between the UK with a humid climate and the Iberian Peninsula (Spain and Portugal) with a Mediterranean climate. The study will provide the first systematic documentation of groundwater drought impacts. It will thus provide guidance for researchers and water resource professionals considering whether groundwater can be used sustainably as an alternative resource during drought under different circumstances. [ABSTRACT FROM AUTHOR]

Groundwater level fluctuations are caused by spatial and temporal superposition of processes within and outside the aquifer system. Most of the subsurface processes are usually observed on a small scale. Upscaling to the regional scale, as required for future climate change scenarios, is difficult due to data scarcity and increasing complexity. In contrast to the limited availability of system characteristics, high-resolution data records of groundwater hydrographs are more generally available. Exploiting the information contained in these records should thus be a priority for analysis of the chronical lack of data describing groundwater system characteristics. This study analyses the applicability of 63 indices derived from daily hydrographs to quantify different dynamics of groundwater levels in unconfined gravel aquifers from three groundwater regions (Bavaria, Germany). Based on the results of two different skill tests, the study aids index selection for different dynamic components of groundwater hydrographs. [ABSTRACT FROM AUTHOR]

A new concept is proposed for describing, analysing and predicting the dynamic behaviour of groundwater resources based on classification and similarity. The concept makes use of the ideas put forward by the "PUB" (predictions in ungauged basins) initiative in surface-water hydrology. One of the approaches developed in PUB uses the principle that similar catchments, exposed to similar weather conditions, will generate a similar discharge response at the catchment outlet. This way, models developed for well-observed catchments can be used to make predictions for ungauged catchments with similar properties (topography, land use, etc.). The concept proposed here applies the same idea to groundwater systems, with the goal to make predictions of the dynamic behaviour of groundwater in poorly observed systems using similarities to well-observed and understood systems. This paper gives an overview of the main ideas, the methodological background, the progress so far, and the challenges that the authors regard as most crucial for further development. One of the main goals of this article is thus to raise interest for this new concept within the groundwater community. There are a multitude of highly interesting aspects to investigate, and a community effort, as with PUB, is required. A second goal is to foster and exchange ideas between the groundwater and surface water research communities who, while often working on similar problems, have often missed the opportunity to learn from each other. [ABSTRACT FROM AUTHOR]

High frequency as well as long-term fluctuations of groundwater levels are consequences of many different processes within the aquifer system. Groundwater levels are influenced both by natural processes (e.g. groundwater recharge, interaction with river systems) and anthropogenic influences (e.g. water abstraction, artificial recharge and piling). Spatial and temporal superposition of these processes cause fluctuations of groundwater levels, referred to as groundwater dynamics, at the position of the well's screen. Due to superposition, the differentiation between multiple driving forces (input signals) is difficult and requires knowledge of hydro(geo)logical properties of the system, such as characteristics of the surface, vadose and phreatic zone on different scales. The exploration of those characteristics is complex and time-consuming and, therefore, normally not available on a regional scale. In contrast to the low availability of data on system characteristics, high-resolution records of groundwater hydrographs are available. Analysis of high-resolution hydrographs is a frequently applied tool for the prediction of ungauged (surface water) basins (PUB, e.g. Blöschl et al., 2013; Hrachowitz et al., 2013). The PUB community introduced a number of different quantitative indices to characterize different parts of river flow dynamics. Based on similarities of these indices, river catchments are clustered in groups of similar dynamics and subsequently linked to system characteristics. Due to various differences between surface and sub-surface catchments, unrestricted transferability of river flow indices on groundwater time series is disputable.This study is focused on the question of how transferable these river flow indices are to groundwater time series measured in alluvial aquifers in Bavaria (Southern Germany). More than 50 indices are calculated from sub-monthly groundwater level time series from different regions, with the purpose of covering various hydro(geo)logical settings. The result of hierarchical clustering (Ward Linkage algorithm) of these 50 groundwater hydrographs serves as a baseline. The ability of each index to express hydrograph similarity is quantified by comparing indices' values from observation wells with high spatial proximity and congruent hydrographs. The alikeness of index values is quantified by measuring the Euclidean distance between the values. Additionally, the study also analyses the ability of different indices to express similarities of groundwater hydrographs from different regions but with similar hydro(geo)logical settings.References:Blöschl G, Sivapalan M, Wagener T, Viglione A, Savenije H. (2013): Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales, Cambridge University Press.Hrachowitz, M. et al. (2013): A decade of Predictions in Ungauged Basins (PUB) - a review, Hydrol. Sci. J. 58 (6), 1198–1255, doi:10.1080/02626667.2013.803183. [ABSTRACT FROM AUTHOR]