A limited number of gauging stations, especially for nested catchments, hampers a process understanding of the interaction between streamflow, groundwater and water usage during drought. Non-commercial measurement devices can help overcome this lack of monitoring, but they need to be thoroughly tested. The Dreisam River in the South-West of Germany was affected by several hydrological drought events from 2015 to 2020 during which parts of the main stream and tributaries fell dry. Therefore it provided a useful case study area for a flexible longitudinal water quality and quantity monitoring network. Among other measurements the setup employs an image-based method with QR codes as fiducial marker. In order to assess under which conditions the QR-code based water level loggers (WLL) deliver data according to scientific standards, we compared its performance to conventional capacitive based WLL. The results from 20 monitoring stations reveal that the riverbed was dry for >50% at several locations and even for >70% at most severely affected locations during July and August 2020, with the north western parts of the catchment being especially concerned. Highly variable longitudinal drying patterns of the stream reaches emerged from the monitoring. The image-based method was found valuable for identification and validation of zero level occurrences. Nevertheless, a simple image processing approach (based on an automatic thresholding algorithm) did not compensate for errors due to natural conditions and technical setup.Our findings highlight that the complexity of measurement environments is a major challenge when working with image-based methods.

Hard rock aquifers (HRAs) in West Africa (WA) are located within a thick regolith layer. The representation of thick tropical regolith in integrated hydrological models lacks consensus on aquifer geometries and parameter ranges. Our main objective was to determine the knowledge requirements on saturated hydraulic conductivity (Ks) to model the critical zone (CZ) of HRAs in WA. A parametric sensitivity analysis with a focus on the representation of the Ks heterogeneity of the regolith was conducted with a critical zone model (Parflow‐CLM [Community Land Model]) of the Upper Ouémé catchment in Benin (14,000 km2) at a 1‐ × 1‐km2 resolution. The impact of parameter changes in the near subsurface (0.3‐to‐5‐m depth) and in the deeper regolith aquifer (24‐ and 48‐m maximum depth) was assessed in five modeling experiments. Streamflow was largely dependent on Ks and on clay distribution in the near subsurface and less on the properties of the deeper subsurface. Groundwater table depths and amplitudes were controlled by vegetation and topography as observed on instrumented hillslopes and for Ks within the literature range. Experiments with higher Ks suggested a Ks threshold where dynamics become less determined by one‐dimensional vertical and more determined by lateral processes. Such heterogeneity impacts from smaller scales need to be accounted for when hydrological models are upscaled to larger domains (1‐ × 1‐km2 resolution or coarser). Our findings highlight the need for a new conceptual approach to represent clay distribution in order to develop catchment‐scale CZ models of HRAs in WA that capture the observed processes.

Process understanding of the interaction between streamflow, groundwater and water usages under drought are hampered by a limited number of gauging stations, especially in tributaries. Recent technological advances facilitate the application of non-commercial measurement devices for monitoring environmental systems. The Dreisam River in the South-West of Germany was affected by several hydrological drought events from 2015 to 2020, when parts of the main stream and tributaries fell dry. A flexible longitudinal water quality and quantity monitoring network was set up in 2018. Among other measurements it employs an image based method with QR codes as fiducial marker. In order to assess under which conditions the QR-code based water level loggers (WLL) deliver data according to scientific standards, we present a comparison to conventional capacitive based WLL. The results from 20 monitoring stations reveal that the riverbed was dry for > 50 \% at several locations and even for > 70 \% at most severely affected locations during July and August 2020, with the north western parts of the catchment being especially concerned. Thus, the highly variable longitudinal drying patterns of the stream reaches could be monitored. The image-based method was found to be a valuable asset for identification of confounding factors and validation of zero level occurrences. Nevertheless, a simple image processing approach (based on an automatic thresholding algorithm) did not compensate for errors due to natural conditions and technical setup. Our findings highlight that the complexity of measurement environments is a major challenge when working with image-based methods.

<p>Currently 40 % of Africa's population still lacks access to clean water. Twice as many rural people live in hard rock areas as compared to sedimentary areas. In these hard rock areas a thick weathered regolith layer covers the crystalline basement, where groundwater (GW) circulates. In the Sudanian area of West Africa (WA) ,groundwater levels are shallow enough to interact directly with the surface water. Therefore, constructing coupled surface-groundwater models helps to estimate quantities of both, GW and surface flows, and their evolution over time to facilitate integrated water management. However, the sensitivity of such models to aquifer properties (saturated hydraulic conductivity (Ks), porosity, geometry), which are difficult to obtain in heterogeneous crystalline contexts, is still poorly constrained. The heterogeneity of aquifer properties at the scale at which most water management decisions are taken, is twofold: 1) bimodal vertical heterogeneity with an unconsolidated weathered zone (high porosity, low Ks) overlying a fissured zone (low porosity, high Ks) and 2) lateral heterogeneity controlled by substratum features and weathering history. We assessed the sensitivity of a coupled surface-groundwater model (PARFLOW-CLM) to vertical and lateral heterogeneity of Ks. The sensitivity to the lateral heterogeneity was explored either using simulations with homogeneous or distributed Ks following random field approaches with a range of spatial correlation lengths. The representation of a vertically uniform aquifer layer was compared to a two-layer scenario for each of the lateral heterogeneity cases. Here, we focused our analysis on the Northern Oueme catchment in Benin (14 000 km²) and we constructed a model with a spatial resolution of 1 km², preventing the analysis of smaller-scale features, such as macropores or clay accumulations. Hydraulic conductivity and aquifer geometry data to constrain the sensitivity experiments were derived from the literature specific to the target area, but also from regional hard rock aquifers in West Africa. As an output of the model, we obtained streamflow, water table head and evapotranspiration time series (in a monthly and daily resolution). The results that we gained with our model configuration (and resolution) point towards a low sensitivity of the model to lateral and vertical heterogeneity. However, we observed a significant impact of the magnitude of Ks on water table head and particularly on the streamflow amplitude. Regarding the water balance our results show that further exploration of the subsurface is crucial to improve critical zone modeling in the context of WA.</p>