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Abstract. Understanding groundwater recharge processes is important for sustainable water resource management. Experimental approaches to study recharge in karst areas often focus on analysing the aquifer response using a disintegration of its outlet signals, but only a few approaches directly investigate the recharge processes that occur at the surface of the system. Soil moisture measurements have a high potential to investigate water infiltration to deeper soil depth or epikarst with an easy and not too intrusive installation. They can yield long-term measurements with high temporal resolution. Using these advantages, we developed and tested a method to estimate recharge based on soil moisture measurements. The method consists of the extraction of linked events in rainfall, soil moisture, and discharge time series, as well as a subsequent fitting of the parameters of a simple drainage model to calculate karst recharge from soil moisture metrics of individual events. The fitted parameters could be interpreted in physically meaningful terms and were related to the properties of the karstic system. The model was tested and validated in a karst catchment located in southwest Germany with hourly precipitation, soil moisture, and discharge data of 8 years duration. The soil moisture measurements were distributed among grassland (n = 8) and woodland areas (n = 7) at 20 cm depth. A threshold of about 35 % (± 8 %) of volumetric water content was necessary to initiate effective infiltration. Soil moisture averaged during the wetting period of each event was the best metric for the prediction of recharge. The model performed reasonably well, estimating recharge during single rainfall events. It was also capable of simulating 88 % of the average annual recharge volume despite considerable differences in the performance between years. The event-based approach is potentially applicable to other karstic systems where soil moisture and precipitation measurements are available to predict karst groundwater recharge.

This paper shares an Early-Career Scientist (ECS) perspective on potential themes for the upcoming International Association of Hydrological Sciences (IAHS) scientific decade (SD). Six discussion sessions were organised in four countries in western Europe in spring 2022. Early-career hydrologists were invited to join the sessions to formulate potential SD themes, to provide feedback on themes proposed in earlier sessions, and to further develop the proposed themes. This community paper summarizes the outcome of these discussion sessions, where three potential themes were identified that could serve to direct the broader hydrological community: “Tipping points and thresholds in hydrology”, “Intensification of the water cycle”, and “Water services under pressure”. Additionally, four trends were distinguished concerning the way in which hydrological research is conducted: big data, bridging science and practice, open science, and inter- and multidisciplinarity. These themes and trends can provide valuable input for future discussions on the theme for the next IAHS SD and they can also serve as a guidance for future research pathways.

<p>Citizen science observations are potentially useful to complement existing monitoring networks. This is also the case in hydrology, where we often lack spatially distributed observations. Engaging the public might help to overcome the lack of data in hydrology. So far, most hydrological citizen science projects have been based on the use of different instruments or installations. For stream level observations, a staff gauge is installed in the river but it is difficult to scale this type of citizen science approach to a large number of sites because these gauges cannot be installed everywhere (or by everyone). Here, we present an evaluation of the CrowdWater smartphone app that allows the collection of hydrological data without any physical installation or specialized instruments. With the help of a free app, citizens can report the stream level, soil moisture conditions, the presence of water in temporary streams, plastic pollution in streams and on streambanks, as well as general information on streams. The approach is similar to geocaching, with the difference that instead of finding treasures, hydrological measurement sites are set up. These sites can be found by the initiator or other citizen scientists to take additional measurements at a later time. For the water level measurements, a virtual staff gauge approach is used instead of a physical staff gauge. A picture of a staff gauge is digitally inserted into a photo of a stream bank or a bridge pillar and serves as a reference of the water level. During a subsequent field visit, the stream level is compared to the virtual staff gauge on the first picture. In this presentation, we discuss how well the water level class observations agreed with measured stream levels, and in which months and during which flow conditions citizens submitted their stream level observations. We also highlight methods to ensure data quality, and illustrate how these water level data can be used in hydrological model calibration. We also give an update on new activities in the CrowdWater project.</p>