Brunner, Philip

2022-1-1, Dahrabou, Asmae, Valley, Benoît, Meier, Peter, Brunner, Philip, Alcolea, Andres

Deep geothermal boreholes, often drilled to the crystalline basement, suffer from borehole breakouts that compromise borehole stability and/or lead to low drilling performance. These issues increase the cost of deep geothermal projects and lead to irregular cross-sectional geometries that may entangle well completion (e.g., packer isolation for zonal stimulation, cementing, etc.). Thus, the proper knowledge of rock strength, state of stress and their interactions at the closest vicinity of the borehole is key to the success of deep geothermal drilling. Typically, the magnitudes of the vertical and minimum horizontal principal stresses, 𝑆𝑣 and 𝑆ℎ𝑚𝑖𝑛, respectively, can be estimated while 𝑆𝐻𝑚𝑎𝑥 is difficult to constrain. This paper presents a systematic methodology to jointly evaluate the heterogeneous distributions of the stress tensor principal components and orientations, and the rock strength properties (e.g. cohesion, friction). Model parameters are estimated from measurements available during or shortly after drilling, i.e., breakout width, breakout extent/depth of penetration, breakout orientation and drilling induced tensile fractures. Additionally, measurements of estimated parameters or transformations of them can be considered in the calibration in a generic manner (e.g., 𝑆ℎ𝑚𝑖𝑛 interpreted from XLOT). For illustration purposes, the methodology is applied to the extensive borehole data set along the crystalline section of the borehole BS-1, in Basel (Switzerland). The methodology allows us (1) to derive plausible sets of stress and strength parameters reproducing the complex distribution of breakouts along BS-1, and (2) to unveil the paradox of having no borehole breakouts at sections with high density of natural fractures.

2021-10, Glaser, Barbara, Hopp, Luisa, Partington, Daniel, Brunner, Philip, Therrien, René, Klaus, Julian

Knowledge of the sources of surface water in riparian zones and floodplains is critical to understanding its role in runoff generation and impact on biogeochemical and ecological processes. In this study, we demonstrate the potential of integrated surface-subsurface hydrologic modeling (HydroGeoSphere) in combination with a hydraulic mixing-cell approach to decipher different sources of surface water and their mixing in space and time. We present a novel approach to processing the model data that allowed us to compare which mechanisms ultimately transferred water to the surface (delivery processes) and from where the surface water originated (geographical sources) for varying wetness states and phases of wetting or drying across 36 test locations within the riparian-stream continuum of an intensively-studied, humid-temperate, forested headwater catchment (45 ha). Consistent with current process understanding for the study site, water exfiltrating from the subsurface was simulated as the dominant source for riparian surface water and intermittent streamflow. The model further helped to specify the relevance of different subsurface stores, revealing a wetness-dependent activation of upslope source areas. Contributions of riparian overland flow and precipitation were minor during all investigated phases of wetting and drying. Moreover, the spatial variability of surface water sources proved to be smaller than expected for the heterogeneous patterns and frequencies of the surface saturation observed and simulated. Based on these findings, we discuss the value of hydraulic mixing-cell modeling to complement the planning and interpretation of field investigations and to enhance process understanding regarding the spatio-temporal sources of surface water.

2021-8, Yan, Renhua, Brunner, Philip, Gao, Junfeng

A new modelling framework, the Polder Hydrology and Nitrogen modelling System (PHNS), was developed to simulate the nitrogen dynamics and processes in polder systems. PHNS is a mass-balance model that simulates water and nitrogen dynamics in soil and surface water systems through integrating the WALRUS-paddy, MUSLE, and INCA models. The model explicitly considers the interactions among surface water, groundwater, and vadose water, as well as irrigation, pumping, and fertilizer application, which are the key processes controlling the nitrogen cycle in polders. The sensitivity analysis, calibration, and validation of the developed model were conducted in a Chinese agricultural polder by using three years of measured hydro-meteorological data. The calibrated and validated results proved that the model has a good performance with an R2 of 0.748 and a Nash- Sutcliffe (NS) efficiency coefficient of 0.619 for total nitrogen (TN) concentration during the validation period. The nitrogen budget results (net export of 40.4 kg/ha/yr) revealed that the polder is a nitrogen source for downstream freshwater systems. Reducing the amount of fertilizers, retaining crop residues, and restoring aquatic plants in surface water are effective countermeasures for reducing nitrogen export from polders. This study provides an efficient modelling tool and useful insights into polder management.

2021, Le Bayon, Renée-Claire, Bullinger-Weber, G, Schomburg, Andreas Cédric, Turberg, Pascal, Brunner, Philip, Schlaepfer, Rodolphe, Guenat, Claire

The importance of engineers is increasingly recognized in soil science because of their implication in most important pedological processes. Furthermore, they contribute to ecological functions provided by soils in both natural and human‐modified environments. In this review, we focus on the role of two ecosystem engineers: (1) plants, their root system, and associated microorganisms and (2) earthworms. First, we explain why they are considered as major soil engineers, and which variables (texture, porosity, nutrient, and moisture dynamics) control their activities in space and time (hotspots and hot moments). Then, their roles in three processes of soil formation are reviewed, namely, rock and mineral weathering, soil structure (formation, stabilization, and disintegration), and bioturbation. For each of them, the involved mechanisms that occur at different spatial scales (from local to landscape) are presented. On one hand, tree uprooting plays a key role in rock weathering and soil profile bioturbation. In addition, living and dead roots also contribute to rock alteration and aggregation. On the other hand, earthworms are mainly involved in the formation of aggregates and burrows through their bioturbation activities and to a less extent in weathering processes. The long‐term effects of such mechanisms on soil heterogeneity, soil development, and pathways of pedogenesis are discussed. Finally, we show how these two main ecosystem engineers contribute to provisioning and regulating services. Through their physical activities of burrowing and soil aggregation, earthworms and plants increase plant productivity, water infiltration, and climate warming mitigation. They act as catalysts and provide, transform, and translocate organic matter and nutrients throughout the soil profile. Finally, due to inter‐ and intraspecific interactions and/or symbiosis with microorganisms (arbuscular fungi, bacteria), they enhance soil fertility, decrease parasitic action, and bioremediate some pollutants. Future research is, however, still needed for a better understanding of the relationships between adequate soil management, agricultural practices, and soil biota in a perspective of relevant maintenance and durability of ecological services.

2021-11-10, Koutantou, Kalliopi, Mazzotti, Giulia, Brunner, Philip

Snow depth mapping in Alpine forests is of high importance for hydrogeology, ecology, tourism, and natural hazards prevention. Different remote sensing approaches have been employed for the precise mapping of snow depth within forests. However, optical sensors cannot provide below-canopy information. While Airborne Laser Scanning (ALS) systems have been used successfully in this context and allow obtaining data below canopies, the costs of acquisitions are very high, not allowing frequent data acquisitions. UAV-based Lidar technology potentially can provide the critical below-canopy information at lower cost and allows for frequent acquisitions. First attempts to employ a UAV-based Lidar system in forests have proven promising, but they are limited to flat forests and to grid-level snow depth calculations. In this study, we present UAV-based Lidar data of both flat and steep forests. Different Lidar processing workflows are analyzed and compared, and snow depth algorithms are used both at the point and the grid level. Whereas the UAV-Lidar system proved capable of mapping snow in both environments, the steep forests' data processing comes with greater challenges, especially for the 3D registration, ground classification, and point-to-point snow depth calculations.

2021-8, Cochand, Fabien, Brunner, Philip, Hunkeler, Daniel, Rössler, Ale, Holzkämper, Annelie

Climate change affects both water resources and agricultural production.With rising temperatures and decreasing summer precipitation, it is expected that agricultural production will be increasingly limited by drought. Where surface- or groundwater resources are available for irrigation, an increase inwaterwithdrawals for irrigation is to be expected. Therefore, quantitative approaches are required to anticipate and manage the expected conflicts related to increased water abstraction for irrigation. This project aims to investigate how agricultural production,water demand for irrigation, runoff and groundwater dynamics are affected by future climate change and howclimate change impacts combinedwith changes in agriculturalwater use affect groundwater dynamics. To answer these research questions, a comprehensive, loosely coupled model approach was developed, combining models from three disciplines: an agricultural plant growth model, a hydrological model and a hydrogeological model. The model coupling was implemented and tested for an agricultural area located in Switzerland inwhich groundwater plays a significant role in providing irrigationwater. Our suggested modelling approach can be easily adapted to other areas. The model results show that yield changes are driven by drought limitations and rising temperatures. However, an increase in yieldmay be realized with an increase in irrigation. Simulation results showthat thewater requirement for irrigation without climate protection (RCP8.5) could increase by 40% by the end of the century with an unchanged growing season and by up to 80%with varietal adaptations. With climate changemitigation (RCP2.6) the increase inwater demand for irrigationwould be limited to 7%. The increase in irrigation (+12mm) and the summer decrease in recharge rates (~20mm/month)with decreasing summer precipitation causes a lowering of groundwater levels (40 mm) in the area in the late summer and autumn. This impact may be accentuated by an intensification of irrigation and reduced by extensification.

2021-7, Thornton, James Matthew, Brauchli, Tristan, Mariéthoz, Grégoire, Brunner, Philip

In mountainous terrain, reliable snow simulations are crucial for many applications. However, except in highly instrumented research catchments, meteorological data are usually limited, and so the interpolated spatial fields used to force snow models are uncertain. Moreover, certain potentially important processes cannot presently be simulated at catchment scales using entirely physical algorithms. It is therefore often appropriate to introduce empirical parameters into otherwise physically-based snow models. Many opportunities to incorporate snow observations into the parameter estimation process now exist, but they remain to be fully exploited. In this context, a novel approach to the calibration of an energy balance-based snow model that additionally accounts for gravitational redistribution is presented. Several important parameters were estimated using an efficient, gradient-based method with respect to two complementary observation types – Landsat 8-derived snow extent maps, and reconstructed snow water equivalent (SWE) time-series. When assessed on a per-pixel basis, observed patterns were ultimately reproduced with a mean accuracy of 85%. Spatial performance metrics compared favourably with those previously reported, whilst the temporal evolution of SWE at the stations was also satisfactorily captured. Subsequent uncertainty and data worth analyses revealed that: i) the propensity for model predictions to be erroneous was substantially reduced by calibration, ii) pre-calibration uncertainty was largely associated with two parameters which modify the longwave component of the energy balance, but this uncertainty was greatly diminished by calibration, and iii) a lower elevation SWE series was particularly valuable, despite containing comparatively few observations. Overall, our work demonstrates that contemporary snow models, observation technologies, and inverse approaches can be combined to both constrain and quantify the uncertainty associated with simulations of alpine snow dynamics.

2021-11, Nogueira, G.E.H., Schmidt, C., Brunner, Philip, Graeber, D., Fleckenstein, Jan H.

During the flow of stream water from losing reaches through aquifer sediments, aerobic and anaerobic respiration (denitrification) can deplete dissolved oxygen and nitrate (NO3 - ), impacting water quality in the floodplain and downstream gaining reaches. Such processes, which vary in time with short and longterm changes in stream flow and temperature, need to be assessed at the stream corridor scale to fully capture their effects on net turnover, but this has rarely been done. To address this gap, we combine a fully-integrated 3D transient numerical flow model with temperature-dependent reactive transport along advective subsurface flow paths to assess aerobic and anaerobic respiration dynamics at the stream corridor scale in a predominantly losing stream. Our results suggest that given carbon availability (as an electron donor), complete NO3 - removal occurred further away from the stream after complete oxygen depletion and was relatively insensitive to variations in temperature and transit-times. Conversely, transittimes and oxygen concentrations constrained nitrate removal along short hyporheic flow paths. Even under limited carbon availability and low-temperatures, NO3 - removal fractions (RNO3) will be greater at locations further from the stream than along shorter hyporheic flow paths (RNO3=0.4 and RNO3=0.1, respectively). With increasing temperature, the relative effects of stream flow and solute concentrations on biogeochemical turnover and the redox zonation around the stream decreased. The study highlights the importance of seasonal variations of stream flow and temperature for water quality at the streamcorridor scale. It also provides an adaptive framework to assess and quantify reach-scale turnover around dynamic streams.

2021-8, Zhang, Zaiyong, Wang, Wenke, Gong, Chengcheng, Zhao, Ming, Hendricks Franssen, Harrie-Jan, Brunner, Philip

Afforestation can reduce desertification and soil erosion. However, the hydrologic implications of afforestation are not well investigated, especially in arid and semi-arid regions. China has the largest area of afforestation in the world, with one-third of the world's total plantation forests. How the shrubs affect evapotranspiration, soil moisture dynamics, and groundwater recharge remains unclear. We designed two pairs of lysimeters, one being 1.2 m deep and the other one 4.2 m deep. Each pair consists of one lysimeter with bare soil, while on the other one a shrub is planted. The different water table depths were implemented to understand how depth to groundwater affects soil moisture and water table dynamics under different hydrological conditions. Soil moisture, water table depth, sap flow, and rainfall were measured concurrently. Our study confirms that for the current meteorological conditions in the Ordos plateau recharge is reduced or even prohibited through the large-scale plantation Salix psammophila. Shrubs also raise the threshold of precipitation required to increase soil moisture of the surface ground. For the conditions we analyzed, a minimum of 6 mm of precipitation was required for infiltration processes to commence. In addition to the hydrological analysis, the density of root distribution is assessed outside of the lysimeters for different water table depths. The results suggest that the root-density distribution is strongly affected by water table depth. Our results have important implications for the determination of the optimal shrub-density in future plantations, as well as for the conceptualization of plant roots in upcoming numerical models.

2021-2, Popp, Andrea L., Pardo-Alvarez, Alvaro, Schilling, Oliver, Scheidegger, Andreas, Musy, Stephanie, Peel, Morgan, Brunner, Philip, Purtschert, Roland, Hunkeler, Daniel, Kipfer, Rolf

Understanding the mixing between surface water and groundwater as well as groundwater travel times in vulnerable aquifers is crucial to sustaining a safe water supply. Age dating tracers used to infer apparent travel times typically refer to the entire groundwater sample. A groundwater sample, however, consists of a mixture of waters with a distribution of travel times. Age dating tracers only reflect the proportion of the water that is under the dating range of the used tracer, thus their interpretation is typically biased. Additionally, end-member mixing models are subject to various sources of uncertainties, which are typically neglected. In this study, we introduce a new framework that untangles groundwater mixing ratios and travel times using a novel combination of in-situ noble gas analyses. We applied this approach during a groundwater pumping test carried out in a pre-alpine Swiss valley. First, we calculated transient mixing ratios between recently infiltrated river water and regional groundwater present in a wellfield, using helium-4 concentrations combined with a Bayesian end-member mixing model. Having identified the groundwater fraction of recently infiltrated river water (Frw) consequently allowed us to infer the travel times from the river to the wellfield, estimated based on radon-222 activities of Frw. Furthermore, we compared tracer-based estimates of Frw with results from a calibrated numerical model. We demonstrate (i) that partitioning of major water sources enables a meaningful interpretation of an age dating tracer of the water fraction of interest and (ii) that the streambed has a major control on the estimated travel times.