HS2.1.11 | Advances in forest hydrology
Advances in forest hydrology
Convener: Daniele Penna | Co-conveners: Luisa Hopp, Rodolfo NóbregaECSECS, Alicia CorreaECSECS
| Thu, 18 Apr, 16:15–18:00 (CEST)
Room 2.31
Posters on site
| Attendance Thu, 18 Apr, 10:45–12:30 (CEST) | Display Thu, 18 Apr, 08:30–12:30
Hall A
Posters virtual
| Attendance Thu, 18 Apr, 14:00–15:45 (CEST) | Display Thu, 18 Apr, 08:30–18:00
vHall A
Orals |
Thu, 16:15
Thu, 10:45
Thu, 14:00
Forests are primary regulators of water, energy, and carbon cycles. Maintaining forest functional integrity is fundamental to the sustainability of ecosystems, societies, and human development as described in the UN Sustainable Development Goals.
Global change and anthropogenic intervention are putting enormous pressure on forests, affecting the ecosystem services they provide through water quantity and quality, and biogeochemical cycles. The conventional wisdom that forest hydrology emphasizes the role of forests and forest management practices on runoff generation and water quality has expanded in light of rapid global change. Some of the largest pristine forest areas are in the tropics and have undergone drastic changes in land use in recent decades. Although novel modeling and observational techniques have been applied as alternatives to develop cutting-edge research, these tropical systems remain notably underrepresented in hydrological studies compared to temperate regions, especially concerning long-term experimental setups and monitoring networks.
Improving our understanding of how hydrological processes in forest ecosystems are determined by time-invariant factors and time-varying controls, as well as how forested catchments respond to dynamic environmental conditions and disturbances, will depend critically on understanding forest-water interactions. Building this knowledge requires interdisciplinary approaches in combination with new monitoring methods and modeling efforts.
This session brings together studies that will improve our understanding of water-forest interactions and stimulate debate on the impact of global change on hydrological processes in forest ecosystems at different scales.
We invite field experimentalists and modelers working in forests from boreal to tropical regions to submit contributions that:
1) Improve our understanding of forest (eco)hydrological processes using an experimental or modeling approach or a combination of both;
2) Assess the hydrology-related impacts of land use/cover change and environmental disturbances on forested ecosystems;
3) Feature innovative methods and observational techniques, such as optical sensors, tracer-based experiments, monitoring networks, citizen science, and drones, that reveal new insights or data sources in forest hydrology;
4) Include interdisciplinary research that supports consideration of overlooked soil-plant-atmosphere components in hydrological studies.

Session assets

Orals: Thu, 18 Apr | Room 2.31

Chairpersons: Daniele Penna, Alicia Correa, Rodolfo Nóbrega
On-site presentation
Laurent Pfister, Bonanno Enrico, Fabiani Ginevra, Gourdol Laurent, Hissler Christophe, Huck Viola, Iffly Jean François, Keim Richard, Martínez-Carreras Núria, Mestdagh Xavier, Montemagno Alessandro, Penna Daniele, Schymanski Stan, and Zehe Erwin

For decades, field data collection has been largely in decline in favour of environmental modelling – the latter being considered less labour and cost-intensive. However, this trend goes against the grain with new observational field data having repeatedly been the source of breakthroughs in science (e.g., high-frequency measurements in stream water, in-situ monitoring of the isotopic composition of tree xylem water, imaging of infiltration pathways with electrical resistivity tomography, or time-lapse mapping of surface-saturation dynamics with thermal infrared imagery). Hypotheses generated from this type of novel, integrative, observations offer the potential to free hydrological concepts from the restrictions of typical datasets.

However, recent technological developments in field instrumentation have also revealed an increasingly complex landscape heterogeneity. General organizing principles have been proposed to explain river basin complexity. Deciphering this heterogeneity remains very challenging – essentially because eco-hydrologic processes occur over a wide range of spatial and temporal scales and vary by multiple orders of magnitude. The dilemma here is that we could continue instrumenting our catchments to the point of littering, and still miss out on processes or features that we were simply not looking for.

Here, we demonstrate the potential for time-lapse photography to unravel the complex organisation of eco-hydrologic processes at various temporal and spatial scales. This technique (also called undercranking) consists of taking regular frames with a camera and subsequently speeding up the action during playback. We installed a wildlife monitoring camera (RECONYX Hyperfire 2 Professional White Flash Camera) in the forested Weierbach experimental catchment (WEC) – an interdisciplinary Critical Zone observatory dedicated to the long-term study of hydrological, hydro-geochemical, and eco-hydrological processes. The rainfall-runoff response of the WEC is characterized by a strong seasonality, with pronounced summer low flows and winter high flows (resulting from a complex interplay of multiple eco-hydrological processes). The full time-lapse video of a hillslope-riparian zone-stream transect in the Weierbach catchment spans from December 2020 to July 2022 and is available online via https://youtu.be/74S7DfT7Uhs.

The high-speed playback of pictures recorded between December 2020 and July 2022, combined with in-situ eco-hydrological measurements reveals a comprehensive view of contrasted seasons with gradually changing processes. In winter, snowfall events trigger a slow but gradual snow-fed groundwater recharge (recorded by soil moisture probes and groundwater wells). Balmy weather in spring announces the onset of leaf-out and a recession in groundwater levels and hydrographs. In summer, vegetation is highly dynamic and growing, while groundwater levels and discharge evolve between high and low levels along successive dry and wet sequences. With cooler autumn temperatures and wet weather, leaf senescence starts, and the groundwater system switches back to a rain-fed recharge state.

The combination of a four-season-long time-lapse sequence of the pulse of the Weierbach with a high-frequency, multi-parameter dataset is offering an innovative opportunity for combining ‘soft’ and ‘hard’ data across multiple scales and eventually improving the dialogue between experimentalists and modelers. Such an alternative source of information may eventually become the starting point for a new cycle of hypothesis framing and testing.

Further information on this study is available at https://doi.org/10.1002/hyp.15026.

How to cite: Pfister, L., Enrico, B., Ginevra, F., Laurent, G., Christophe, H., Viola, H., Jean François, I., Richard, K., Núria, M.-C., Xavier, M., Alessandro, M., Daniele, P., Stan, S., and Erwin, Z.: Time-lapse photography – a tool for unravelling the intricate complexity of eco-hydrologic processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11844, https://doi.org/10.5194/egusphere-egu24-11844, 2024.

On-site presentation
Anke Hildebrandt, Fischer-Bedtke Christine, Metzger Johanna Clara, Demir Gökben, and Wutzler Thomas

Heterogeneity in throughfall, caused by the redistribution of precipitation in the vegetation canopy, has repeatedly been hypothesized to influence the variation in soil water content and runoff behavior, especially in forests. However, observational studies directly relating the spatial variation in the soil water content or dynamics to net precipitation are rare. Here, we investigate how throughfall patterns affect the spatial heterogeneity in the soil water response in the main rooting zone. We assessed rainfall, throughfall, and soil water content (at two depths, 7.5 and 27.5 cm) in a 1 ha temperate mixed beech forest plot in Germany during the 2015 and 2016 growing seasons using independent, high-resolution, stratified, random designs. Because the throughfall and soil water content cannot be measured at the same location, we used kriging to derive the throughfall values at the locations where the soil water content was measured.

Spatial patterns of throughfall were related to canopy density. Although spatial autocorrelation decreased with increasing event size, temporally stable throughfall patterns emerged, resulting in the reoccurrence of higher- and lower throughfall locations across precipitation events. Linear mixed-effects model analysis showed that while soil water content patterns were poorly related to spatial patterns of throughfall, the increase in soil water content after rainfall was strongly related. More water was stored in the soil in areas where throughfall was elevated. At the same time, however, the local soil water response was modified by the soil wetness itself in a way that suggests processes of rapid drainage and runoff. Locations with a lower than average topsoil water content tended to store less of the input water, indicating locally enhanced preferential flow. In contrast, in the subsoil, locations with above average water content stored less water than their drier counterparts. In addition, macroporosity also modified how much water was retained in soil storage.

Overall, throughfall patterns influenced soil water content much less than soil water dynamics shortly after rainfall events. Furthermore, drainage reduced the soil moisture variation within hours to days, when returning from the wetted to the dry state. Therefore, we conclude that percolation rather than the soil water content is affected by small-scale spatial heterogeneity in canopy input patterns.


Fischer-Bedtke, C., Metzger, J. C., Demir, G., Wutzler, T., and Hildebrandt, A.: Throughfall spatial patterns translate into spatial patterns of soil moisture dynamics – empirical evidence, Hydrol. Earth Syst. Sci., 27, 2899–2918, https://doi.org/10.5194/hess-27-2899-2023, 2023.

How to cite: Hildebrandt, A., Christine, F.-B., Johanna Clara, M., Gökben, D., and Thomas, W.: Are spatial patterns of soil moisture or percolation affected by throughfall heterogeneity? Empirical evidence from a beech-dominated forest., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11093, https://doi.org/10.5194/egusphere-egu24-11093, 2024.

On-site presentation
Milad Asgarimehr, Jens Wickert, Adriano Camps, and Dara Entekhabi

The dynamics of Vegetation Water Content (VWC) throughout the day reflect how plants cope with water stress, trying to replenish lost water during daylight hours. Traditional radar sensors have shown sensitivity to diurnal vegetation moisture fluctuations but struggle due to their limited sampling rates, making it difficult to monitor daily patterns effectively. Innovations like Global Navigation Satellite Systems Reflectometry (GNSS-R) present a promising solution to overcome these limitations.

In this study, we leverage GNSS-R measurements from the NASA Cyclone (CYGNSS) mission, launched in late 2016, to study diurnal VWC cycles in Amazon's evergreen forests. CYGNSS offers high sampling rates and increased sensitivity to VWC, penetrating vegetation layers effectively with longer L-band wavelengths. The eight satellites of CYGNSS provide frequent measurements in tropical regions across different times of the day.

Our results uncover distinct differences between morning and evening VWCs over Amazon. We have observed a strong correlation (R = 0.75) between VWC and Vapor Pressure Deficit (VPD) throughout 2019, indicating VPD as a crucial factor influencing water stress. The diurnal VWC cycles in the Amazonian peatland demonstrate disruptions during arid periods and emphasize the significant role of VPD in governing vegetation diurnal moisture dynamics.

Our findings bridge the information gap on water stress in vegetation, showing the potential of VWC derived from advanced remote sensing technologies. It complements in-situ data on water potential gradients, offering valuable insights into vegetation water status in these critical ecosystems.

How to cite: Asgarimehr, M., Wickert, J., Camps, A., and Entekhabi, D.: Diurnal Vegetation Moisture Dynamics and Water Stress: Insights from GNSS Reflectometry-Derived Vegetation Water Content, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15823, https://doi.org/10.5194/egusphere-egu24-15823, 2024.

On-site presentation
Pascal Benard, Julian Schoch, Andrea Mazza, Peter Lehmann, and Andrea Carminati

Soil water repellency has been observed across a range of ecosystems, including forests. Among other parameters such as organic matter content and quality, climate, and soil texture, the magnitude and persistence of water repellency is controlled by the initial soil moisture content. With the increasing risk of prolonged and recurrent drought events across Europe causing significant increases in tree mortality, the feedback between soil moisture and soil rewetting is of increasing importance, as delayed soil rewetting may prolong water stress beyond drought events and reduce the plant available water. In this study, we quantified the local contact angle (sessile drop method) and in-situ rewetting dynamics (electrical resistivity tomography), including their relationship with initial soil moisture (sorptivity), of forest soils with contrasting vegetation.

The results showed a fundamental difference in the persistence of water repellency and rewetting dynamics between an oak and a spruce stand. Despite prolonged precipitation (> 100 mm) following a dry summer, the sandy loam topsoil under spruce did not rewet after rain events, indicating persistent water repellency and fast water percolation to greater depth via preferential flow paths. In contrast, the sandy loam topsoil under oak rewetted after rainfall and was unaffected by water repellency, despite the similarly high initial contact angle of about 85° of dry soil at both sites.

Our results highlight the importance of moisture dependence and persistence of soil water repellency for plant-available soil water in forests. The striking persistence of low soil moisture in topsoil, in combination with shallow-rooted spruce may explain in parts the severe dieback of spruce across Europe. Furthermore, the hydrological response of repellency-affected forest sites is likely to be influenced by the feedback between initial soil moisture, soil wettability and its persistence, and soil rewetting dynamics.

How to cite: Benard, P., Schoch, J., Mazza, A., Lehmann, P., and Carminati, A.: Soil moisture dynamics of forest soils – magnitude, persistence and implications of soil water repellency , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7687, https://doi.org/10.5194/egusphere-egu24-7687, 2024.

On-site presentation
Shaochun Huang, Stephanie Eisner, Wai Kwok Wong, and Nicolas Cattaneo

Potential climate change impacts on water resources have been extensively assessed in Norway due to substantial changes in climate in the recent decades. However, the combined and isolated effects of forest and forest management have been rarely considered in the climate impact studies in Norway although about 38% of the land area is covered by forest. This study aims to improve hydrological impact projections in forest dominant catchments by considering the effects of forest growth and management and to attribute hydrological changes to climate and forest changes. The eco-hydrological model SWIM (Soil and Water Integrated Model) was applied to simulate hydrological processes and extremes for two micro-scale, two meso-scale and two macro-scale catchments, accounting for the effects of spatial scale. The climate projections were generated by three EURO-CORDEX (Coordinated Downscaling Experiment for the European domain) regional climate models (RCMs) for two RCPs (Representative Concentration Pathways, RCP2.6 and RCP4.5) and were bias corrected using the quantile-mapping method. Forest development over time was simulated as a function of climate determining growth and SSP-dependent harvest levels determining wood outtake. The simulations were initialized with the forest status of the year 2020 and different forest types are distinguished according to structural characteristics represented by three key parameters: leaf area index, mean tree height and surface albedo. Preliminary simulation results show that there are minor changes (within ±5%) in hydrological processes under the combinations of the climate and forest scenarios for these catchments. Climate change is the major driver of hydrological change at the catchment scale whereas forest development mainly influences the spatial distribution of the hydrological fluxes. The results further indicate that forest growth under a warming climate helps to reduce the risk of the floods and drought slightly by reducing surface runoff in wet periods and increasing base flow in dry periods, respectively.

How to cite: Huang, S., Eisner, S., Wong, W. K., and Cattaneo, N.: The potential impacts of climate change and forest management on water resources for micro-, meso- and macro-scale catchments in cold regions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18681, https://doi.org/10.5194/egusphere-egu24-18681, 2024.

On-site presentation
Roberto Corona, Serena Sirigu, Nicola Montaldo, and Gabriel G. Katul

Sardinia island is a reference for ecohydrological studies on past and future climate change effects, representing typical conditions of the western Mediterranean Sea basin. Ecosystems are heterogenous, and trees optimize the use of water through the root systems, uptaking water from the deep layers.

Two micrometeorological towers have been installed in two different sites under different precipitation conditions. The first is installed in Orroli (annual precipitation of about 600 mm), a case study of the ALTOS European project, which is a patchy mixture of wild olive trees and C3 herbaceous that grow in a shallow under a rocky layer of basalt, partially fractured (soil depth 15 40 cm), with a tree cover percentage of 33% in the footprint. Instead, the second is in a mountainous forest site of Quercus ilex characterized by steeper slopes and rocky outcrops (mean annual precipitation of about 800 mm), and tree cover percentage of 68% in the footprint. In both sites land surface fluxes and CO2 fluxes are estimated using the eddy correlation technique, soil moisture was estimated with water content reflectometers, and periodically leaf area index (LAI) were estimated, while tree transpiration component is estimated using the sap flow sensors.

The following objectives are addressed:1) pointing out the dynamics of land surface fluxes, soil moisture and CO2 for two contrasting sites; 2) assess the impact of vegetation dynamics and type on the CO2 and water balance dynamics; 3) evaluate the soil effect on water and energy budgets.

The Orroli site is more controlled by rainfall seasonality, and vegetation species use the source of water stored in the deep rocky layer to sustain their physiological activity. In the Orroli site we found seasonal dynamics in the CO2 flux and in the evapotranspiration (ET) terms, which are higher when grass and woody vegetation species are present and lower when the grass component dies. Instead, we found a constant flux of ET in the Marganai highlighting the high efficiency of tree species in extract the deep sources of water. ET is higher in the Orroli site as long as the grass species are present in live form, and then LE is higher in the Marganai forest. The ET of Quercus ilex in the Marganai forest seems being not controlled by surface soil moisture, because the annual precipitation is enough for sustain the transpiration needs of that fraction of tree cover. The results confirm a threshold of 700 mm/year of rain, below which rain can restrict tree cover growth.

How to cite: Corona, R., Sirigu, S., Montaldo, N., and Katul, G. G.: On the Evapotranspiration estimates of two contrasting and Heterogenous Ecosystems in a Mediterranean region, Sardinia, under water limited conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19390, https://doi.org/10.5194/egusphere-egu24-19390, 2024.

On-site presentation
Camyla Innocente dos Santos, Julian Klaus, and Pedro Luiz Borges Chaffe

Predictions from Global Climate Models point to increasing water scarcity across the Global South. However we currently lack understanding of how these predicted changes propagate through the hydrological cycle to smaller-scale systems. This lack of understanding is due to limited knowledge of hydrological processes, which persists in tropical, subtropical, and arid environments of the Global South. Clear blueprints exist for experimental catchment setups that allow us to understand spatiotemporal catchment processes (e.g., the Long Term Ecological Research LTER, the Critical Zone Observatory network CZO, and the Terrestrial Environmental Observatories TERENO); yet, it is not feasible to maintain such dense experimental networks in the understudied Global South. Reconciling snapshot baseflow campaigns as sources of data can be an alternative to expanding hydrological observation, particularly in developing countries where long-term records are scarce and pressure on water resources is growing. Here, we present results from baseflow campaigns in small nested catchments across different landscape elements to improve a rainfall-runoff model (geomorphologic instantaneous unit hydrograph) and provide insights into spatial patterns of flow, catchment water storage, and estimates of streamflow sources. The Peri Lake Experimental Catchment (19 km²) is characterized by granite and diabase dike and covered by the Atlantic rainforest. We measured baseflow discharge and sampled isotopes (δ18O and δ2H) at 25 catchments (areas ranging from 0.02 to 5.33 km²). Through combining flow velocity and discharge, we incorporated spatial variations of velocity in the channels during runoff, using a constant relationship between velocity and celerity. The Nash values were above 0.80, and we eliminated the need for concentration time formulas, where uncertainty reaches 500%. Combining isotopes and discharge enhanced our knowledge of the role of geology, with the Spearman coefficient between the percentage of granite and specific discharge being -0.68 (p-value < 0.05). We conceptualize that the diabase dikes are shallower with greater permeability, functioning as a conductor and supplier of water during baseflow. Simulations with a 3D surface-subsurface hydrological model  verify the capacity of the observed baseflow patterns in this catchment with heterogeneous geology. The results suggest that measurements in nested catchment during baseflow conditions reflect the heterogeneity of the different sources that contribute to streamflow. Snapshot measurement and sampling  campaigns are a powerful tool to understand runoff generation patterns in subtropical and tropical catchments of the Global South where continuous monitoring is hard to implement.

How to cite: Innocente dos Santos, C., Klaus, J., and Chaffe, P. L. B.: Improving runoff generation knowledge through baseflow campaigns, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-336, https://doi.org/10.5194/egusphere-egu24-336, 2024.

On-site presentation
Aline Meyer Oliveira, Fernanda Gianasi, Patrícia Vieira Pompeu, Rubens Manoel dos Santos, and Ilja van Meerveld

Floodplain forests are unique ecosystems at the interface between rivers and terrestrial environments. They provide important ecosystem services, such as biodiversity conservation and flood control. However, they are also one of the most threatened ecosystems. Vegetation composition in floodplain forests depends on the flood regime, but there is a lack of knowledge on the relation between the water level regime and forest composition and functioning for the seasonally dry tropics. As a result, it is unclear how these forests will be impacted by climate change.

In the WatForFun (“Water level regime and forest functioning in floodplain forests”) project, we brought together an interdisciplinary team of hydrologists and ecologists to better understand the relation between flood dynamics (e.g., flood frequency and duration) and tree species composition, phylogenetic diversity, functional diversity and taxonomic diversity. Fieldwork was conducted in six seasonally flooded forests in the state of Minas Gerais in southeastern Brazil. Three floodplains are located in the Rio Grande basin (Capivari, Jacaré, and Aiuruoca), and another three in the São Francisco basin (Jequitaí, Verde Grande, and Carinhanha). The floodplains encompass a gradient in climate, from humid subtropical to seasonal dry tropical. For each floodplain, we identified five geomorphological eco-units based on the vegetation composition: marginal levee, lower terrace, upper terrace, lower plain, and higher plain. We surveyed the vegetation at each site and installed groundwater wells and surface water level loggers to monitor the water level regime. We sampled xylem water, soil water, groundwater, surface water and precipitation to identify the sources for tree water uptake based on the stable isotopes of hydrogen and oxygen.

The eco-units differ from each other with regards to vegetation composition, phylogenetic and taxonomic diversity, and in terms of flood duration and flood frequency. The terraces remained flooded for longer periods of time than the other eco-units. The flood duration for the levees differed for the two basins. The xylem water was more depleted during the wet season than during the dry season, suggesting that trees change water uptake strategies depending on water availability. These findings help us to better understand the relation between floods and vegetation composition and to predict the impacts of climate change on vegetation composition and diversity.  

How to cite: Meyer Oliveira, A., Gianasi, F., Vieira Pompeu, P., Manoel dos Santos, R., and van Meerveld, I.: Water level regimes, forest composition, and forest functioning in floodplain forests in southeastern Brazil, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-896, https://doi.org/10.5194/egusphere-egu24-896, 2024.

On-site presentation
Zhen Cui and Fuqiang Tian

Bimodal runoff behavior, characterized by two distinct peaks in flow response, often leads to significant stormflow and associated flooding. Understanding and characterizing this phenomenon is crucial for effective flood forecasting. However, this runoff behavior has been understudied and poorly understood in semi-humid regions. This study delves into the mechanisms behind delayed stormflow generation in a mountainous forested watershed within the semi-humid regions of North China. We assess the influence of soil water content and groundwater levels on the threshold behavior of delayed stormflow. Results indicate that the threshold behavior of the bimodal hydrograph is jointly controlled by soil water storage and groundwater levels, with soil water storage serving as the initiating factor for delayed stormflow. The groundwater replenishment and subsequent rise in groundwater level, crucial for the delayed stormflow, occur specifically when the soil water storage reaches 200 mm amid rainfall. At this point, shallow groundwater flow is promptly mobilized, swiftly moving into the channel and leading to the initiation of delayed stormflow. Notably, upon reaching a specific threshold groundwater level, each hillslope responds almost simultaneously, establishing a more extensive hydrological connectivity between the hillslopes and the stream channel. A substantial volume of shallow groundwater is released within a day, resulting in a hybrid bimodal hydrograph. These findings can enhance our understanding of the groundwater stormflow generation mechanism in semi-humid forest watersheds and contribute to the refinement of related runoff generation theories. 

How to cite: Cui, Z. and Tian, F.: Bimodal Hydrographs in Semi-humid Forested Watershed: Controlling Factors and Generation Mechanism , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7747, https://doi.org/10.5194/egusphere-egu24-7747, 2024.

On-site presentation
Yafeng Zhang, Chuan Yuan, Ning Chen, and Delphis Levia

Rainfall partitioning into stemflow, throughfall, and interception loss by vegetation alters hydrological and biogeochemical fluxes between vegetation and soil, and further affects water and nutrient balances at local, catchment, and regional scales. Here, we compiled a comprehensive dataset of rainfall partitioning by vegetation (forests, shrublands, croplands, and grasslands) in China. Based on this dataset, we delineate the general characteristics of rainfall partitioning in China from field observations. We summarize the best-fit functions reported for rainfall partitioning fluxes as a function of rainfall amount, as well as the rainfall thresholds for throughfall and stemflow initiation. We explore the patterns of the proportions of stemflow, throughfall, and interception loss to the gross rainfall across vegetation types in China. We determine whether and to what extent the chemical composition of rainwater is altered during rainfall partitioning processes. We use a machine learning method (boosted regression trees) to model the relative effects of cross-site biotic and abiotic predictors on each of the rainfall partitioning fluxes (%) and on the magnitude of chemical alteration in throughfall and stemflow. Our study avails a global readership to the findings of a large cache of Chinese studies that have been inaccessible hitherto, would aid in an accurate estimation of water and nutrients budget in vegetated ecosystems worldwide, and are helpful for making viable strategies to enhance forestry water resources management.

How to cite: Zhang, Y., Yuan, C., Chen, N., and Levia, D.: Rainfall partitioning by vegetation in China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8425, https://doi.org/10.5194/egusphere-egu24-8425, 2024.

Posters on site: Thu, 18 Apr, 10:45–12:30 | Hall A

Display time: Thu, 18 Apr 08:30–Thu, 18 Apr 12:30
Chairpersons: Rodolfo Nóbrega, Alicia Correa, Daniele Penna
Clara Rohde, Alberto Iraheta, Matthias Beyer, Gökben Demir, and Maren Dubbert

In 2003, 2015, and 2018 extreme droughts caused severe depletion in soil water storage, decreased groundwater tables, and severe damage to forest ecosystems in Europe, such as increased mortality rate. In the future, such extreme droughts will be more likely to occur due to climate change. Therefore, it is crucial to understand and develop mitigation strategies and responses to reduced water availability of European trees for drought resilience.

Tree species significantly differ in their response to drought. Isohydric trees, for example, which are often deep-rooted, close their stomata when sensing a change in soil water potential while anisohydric trees - often shallow-rooted - continue to transpire even when soil moisture declines. As a result, anisohydric trees have an increased risk of hydraulic failure under drought stress because of their high stomatal conductance. Moreover, it is assumed that deep-rooted trees are more resilient to droughts than shallow-rooted trees because these trees possess an enhanced capacity to better withstand periods of drought. However, when naturally deep-rooted and isohydric trees lose their stable water source connection they might be strongly susceptible to drought. In this study, we examine the different below and above-ground mitigation strategies of common central European tree species in a temperate climate.

We chose a mixed forest stand composed of Fagus sylvatica L., Carpinus betulus L., Fraxinus excelsior L., and Quercus spp. trees on a hillslope in NW Germany where a natural gradient of groundwater distance (> 4 m top site, ~ 1.50 m valley) exists and variable rooting depths are found. Observations of soil and plant water status, as well as groundwater level at three hillslope positions (top, slope, valley), started in May 2023. Two point-dendrometers per tree species and hillslope position providing annual tree growth were used to determine tree water potential. Sap flow sensors (three per tree species and position) were installed to estimate plant water use and stem water content as well as for upscaling to tree and stand-level photosynthesis. All sensors will run another growing season for comparative analysis.

Although the year 2023 was not particularly dry and no severe soil water depletion was observed, the first growing season measurements indicate that e.g. C. betulus and F. excelsior are performing better - i.e. showing higher sap flux velocities and higher growth rates - in the valley than at the top position. Potential reasons could include the proximity of the groundwater table in the valley, trees being less limited in their transpiration efforts. However, other factors such as differing shading and less competition to C. betulus trees, which are more abundant uphill, need to be explored further.

How to cite: Rohde, C., Iraheta, A., Beyer, M., Demir, G., and Dubbert, M.: Understanding water use strategies of Central European tree species in dependency on groundwater depth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1529, https://doi.org/10.5194/egusphere-egu24-1529, 2024.

Lutz Klein, Bruce Dudley, Julian Klaus, and Dean Meason

Pinus radiata, which grows under a wide range of climate conditions, makes up the majority of the planted forests in New Zealand leading to concerns over the impact of this non-native species on water availability and quality. Transit times of water through the vadose zone reflect water fluxes and affect runoff chemistry. However, little is known on how vadose zone transit times differ for forests of the same species under different precipitation regimes. Our goal here is to evaluate how root water uptake (RWU), water transit times, and groundwater recharge in Pinus radiata plantations differ under variant precipitation. We investigated soil water fluxes and RWU in nine soil profiles in two Pinus radiata forests with greatly differing annual precipitation amounts (2934 mm vs. 725 mm) in New Zealand’s South Island. We estimated water age of vadose zone and xylem water using an isotope-enabled version of the one dimensional hydrological model Hydrus 1-D. We inversely derived the model parameters using a Monte-Carlo simulation with Latin hypercube sampling with times series of soil moisture and soil and xylem water stable isotopes. 
At the dry forest site, we found that transpiration and recharge accounted for over 80%, and around 10% of annual precipitation, respectively. At the wet forest site transpiration accounted for 24% and recharge 70% of annual precipitation. RWU at the dry forest site was nearly constantly soil moisture-limited while vapour pressure deficit was the limiting factor at the wet forest site. At the dry forest site, a large range of water ages contributed to RWU. During dry periods water age of RWU was high, but dropped sharply in surface soil layers and RWU following intense precipitation events. This was not observed at the wet forest site where soil water age and xylem water age were less variable. While at the wet forest site Pinus radiata almost exclusively relied on water from the current season for RWU, at the dry forest site significant amounts of RWU in the summer stemmed from winter precipitation.   Our results demonstrate not only greater impacts of these plantation forests on soil water balances in more arid climates, but also suggest greater susceptibility of dryland forests to variation in precipitation regimes.  

How to cite: Klein, L., Dudley, B., Klaus, J., and Meason, D.: Soil and plant water ages in two Pinus radiata forests with markedly different annual precipitation amounts , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3398, https://doi.org/10.5194/egusphere-egu24-3398, 2024.

Integrative approaches to assess holistic ecosystem reactions to environmental threats and perturbations - an ecohydrological perspective
Maren Dubbert, Adrian Dahlmann, David Dubbert, Angelika Kübert, Gökben Demir, Shrijana Vaidya, Jürgen Augustin, and Mathias Hoffmann
Brune Raynaud--Schell, Jérôme Demarty, Jordi Etchanchu, Chloé Ollivier, Léna Collet, Jean Kempf, Jean-Marc Limousin, Olivier Marloie, Albert Olioso, Jean-Marc Ourcival, Guillaume Simioni, and Véronique Leonardi

Droughts are a major factor in the vulnerability of Mediterranean ecosystems, particularly forest ecosystems, which are mainly located in karstic environments. Under the effects of global change, these environments are exposed to increasingly frequent and intense droughts. Recent ecophysiological and isotopic studies have shown that tree roots are able to feed deep enough in the epikarst to support transpiration during periods of water stress. However, the quantification of stocks and temporal dynamics are not yet fully established. This calls for the development of models adapted to the complexity of the environment, with the aim of improving our knowledge of both aquifer recharge and the hydric functioning of forests. The work carried out in this study goes in this direction. It aims to suggest, implement and test a SVAT-type model of energy and water exchanges at the soil-vegetation-atmosphere interface, adapted to Mediterranean forest environments in karstic zones. The modelling objective is dual: i) to jointly simulate the processes of diffuse infiltration into the soil (i.e. the superficial part of the root zone) and rapid infiltration into the network of karstic fractures (i.e. the deep part of the root zone); ii) to simulate the transpiratory and water extraction processes throughout the root zone. To do this, an adaptation of the SiSPAT model was developed and then deployed for the first time on two sites in the ICOS network, namely the forest sites of Font-Blanche (Bouches-du-Rhône, P.I. URFM) and Puéchabon (Hérault, P.I. CEFE). The results highlight the importance to represent both diffuse and preferential flows in SVAT modelling for karstic areas. It particularly shows that preferential infiltration builds up deep water reserves throughout the year. It helps to reproduce better observed transpiration by the plant canopy during periods of water stress. It also significantly affects the different hydrological components of the surface, e.g. runoff and drainage to the aquifers.

How to cite: Raynaud--Schell, B., Demarty, J., Etchanchu, J., Ollivier, C., Collet, L., Kempf, J., Limousin, J.-M., Marloie, O., Olioso, A., Ourcival, J.-M., Simioni, G., and Leonardi, V.: Modelling surface water and energy transfer in karstic mediterranean forests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10997, https://doi.org/10.5194/egusphere-egu24-10997, 2024.

Daniela Sauer, Simon Drollinger, Michael Dietze, Dominik Seidel, Daniel Schwindt, and Jago Birk

Climate change models suggest increasing rain variability for Europe in the next decades, with hypothesised cascading effects on ecosystems. We evaluate decadal-scale data of a measuring plot in a beech forest in central Germany to test these model-based suggestions and potential implications by empirical evidence.

Based on 15 min resolution metrics of precipitation and subsequent water pathways towards and within the soil, we show medium-term trends in rainfall characteristics and their modulation by biota.

Rain event durations and rain amounts per event tended to decrease over the observation period, while rain intensity increased, accommodating the effect of the two former parameters on annual precipitation. This change in precipitation patterns, together with canopy structure caused a systematic decrease in throughfall ratios and an exponentially enhanced throughfall variability.

Our results suggest that changing rainfall and throughfall patterns will progressively decouple hydrological links in one of Europe’s most extensive ecosystem types. Based on the observed trends, we discuss effects of changing vegetation-modulated rain-input on ecosystem functioning and soil-hydrological trajectories to be anticipated in the near to mid-term future.

How to cite: Sauer, D., Drollinger, S., Dietze, M., Seidel, D., Schwindt, D., and Birk, J.: Climate change drives hydrological decoupling of a central European beech forest, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11540, https://doi.org/10.5194/egusphere-egu24-11540, 2024.

Raunak Kirti, Alejandra Valdés-Uribe, and Dirk Hölscher

Evapotranspiration (ET) is a critical process within the hydrological cycle, susceptible to shifts due to changes in land use. In tropical forest regions, widespread transformations often result in mosaic patterns of land-use types. Our goal was to explore the importance of vegetation structure, topography, meteorology and soil for the spatial variability of ET in a tropical mosaic landscape. We used a random forest machine learning technique for spatial data, employing forward feature selection and cross-validation to prevent overfitting. Our study region is situated in north-eastern Madagascar and is mainly composed of forest fragments, vanilla agroforests, rice fields and fallow land of shifting cultivation. We used a combination of open-source data products derived from various satellite experiments. Daily ET data were retrieved from the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS). Forest structure predictors from GEDI and PROBA-V, meteorological data from ERA5, topography from JAXA and soil data from ISRIC were obtained. The variables included in the L3 algorithm to calculate ECOSTRESS ET daily data were not included in the study to prevent bias in the models. The models achieved high accuracy for the spatial prediction of ET (R2) of 0.76 and 0.82 for different days. Besides other biophysical variables, leaf area index, tree cover and tree height were important variables in predicting ET. Our findings thereby underscore the crucial role of forest structure on ET even in complex structured tropical mosaic landscapes.

How to cite: Kirti, R., Valdés-Uribe, A., and Hölscher, D.: Importance of vegetation structure for predicting evapotranspiration in a tropical mosaic landscape, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12425, https://doi.org/10.5194/egusphere-egu24-12425, 2024.

Franciele de Bastos and Hubert Hasenauer

Mountain forests are essential for reducing runoff, sediment transport, and risk of natural hazards. In this analysis we address the protection function of mountain forests by assessing the interactions among the forest structure and the water dynamics. We are specifically interested in the forest's ability to reduce the outflow during a 10-day rainfall period according to the leaf area index (LAI) of the forested areas. The study was conducted using 31 Norway spruce (Picea abies) forest stands covering a wide range of LAI located in the Rindbach watershed in Austria. The elevation ranges from 446 m to 1379 m, and the predominant soil type is the Orthic Rendzina. From 1960 to 2022, the mean average annual precipitation was 1498 mm, and the mean average annual temperature was 6.6 °C. We use the biogeochemical ecosystem model Biome-BGC with its parameter settings for European tree species to simulate the daily carbon, nitrogen, water, and energy flux dynamics and assess the relative proportion of the daily water balance parameters during the 10-day rain period grouped according to the leaf area index (LAI). Our results for the 10-day rain period with a total accumulation of 135.3 mm (about 9.7 % of the annual rainfall in the area) suggest: (i) Norway spruce forest areas with an LAI < than 1 m²/m², outflow was evident on the first day of rainfall while Norway spruce forests with an LAI ≥ 7 m2/m-2 exhibited the first outflow on the ninth day of rainfall. (ii) This resulted in a 4 to 5 times lower outflow compared to forest stands with an LAI < 1m²/m² (e.g. 51.3 versus 11.1 mm, within 10 days). This emphasizes the importance of forest vegetation coverage in reducing runoff, avoiding flooding, mudslides, and sediment transport, and improving the protection function of mountain forests.

How to cite: de Bastos, F. and Hasenauer, H.: Modeling the effects of forest stand characteristics on the water dynamics of mountain forests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16271, https://doi.org/10.5194/egusphere-egu24-16271, 2024.

Vincent Haagmans, Giulia Mazzotti, Clare Webster, and Tobias Jonas

Canopy surface temperature is a critical state variable of land surface models. During winter, it plays a key role in modulating energy fluxes between atmosphere, canopy air space, and sub-canopy snowpack. Understanding these surface temperature dynamics spatially and temporally is becoming increasingly important as recent hyper resolution models are now capable of resolving snow-forest interactions at the scale of individual trees and within concrete canopy structure.

Here, we present a novel dataset and analyze spatio-temporal wintertime canopy surface temperaturedynamics derived from ground-based thermal infrared (ThIR) images. Panoramic ThIR images were captured in forest gaps and dense stands at up to hourly intervals throughout diurnal cycles in boreal and sub-alpine forests. Postprocessing enabled documentation of absolute vertical and azimuthal tree surface temperature distributions within the forest under varying meteorological conditions. Our observations revealed the spatiotemporal dynamics of canopy temperatures offsets relative to ambient air temperatures. Positive offsets mainly followed direct insolation patterns within the 3-dimensional canopy structure in case of clear sky conditions. Insolated stems in forest gaps were observed to be up to 20 degrees above the surrounding canopy, while at the same time shaded stems could be up to 3 degrees colder than the canopy. Moreover, combining ThIR observations with RGB imagery further demonstrated evidence of insolation driven unloading of snow intercepted by the canopy, providing valuable data for further development of hyper resolution forest snow models.

How to cite: Haagmans, V., Mazzotti, G., Webster, C., and Jonas, T.: Wintertime Tree Surface Temperature Dynamics in Boreal and Sub-Alpine Forests Revealed by Thermal Infrared Imaging, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18190, https://doi.org/10.5194/egusphere-egu24-18190, 2024.

Posters virtual: Thu, 18 Apr, 14:00–15:45 | vHall A

Display time: Thu, 18 Apr 08:30–Thu, 18 Apr 18:00
John Van Stan and Juan Pinos

Plant canopies divert a portion of precipitation to the base of their stems through “stemflow”, a phenomenon that influences the canopy water balance, soil microbial ecology, and intrasystem nutrient cycling. However, a comprehensive integration of stemflow into theoretical and numerical models in natural science remains limited. This perspective examines three unresolved, fundamental questions hindering this integration, spanning the canopy to the soil. First, the precise source area within the canopy that generates stemflow is undefined. Thus, we asked, “whence stemflow?” Current common assumptions equate it to the entire tree canopy, a potentially misleading simplification that could affect our interpretation of stemflow variability. Second, we asked what are the various conditions contributing to stemflow generation—beyond rain, to dew and intercepted ice melt—and could the exclusion of these volumes consequently obscure an understanding of the broader implications of stemflow? Third, we explored ”whither stemflow?” This question extends beyond how much stemflow infiltrates where, into what uptakes it and from where. Addressing these questions is constrained by current observational and analytical methods. Nevertheless, by confronting these challenges, the stemflow research community stands to make significant strides in comprehending this unique hydrological component and situating it within the broader context of natural science.

How to cite: Van Stan, J. and Pinos, J.: Three Fundamental Challenges to the Advancement of Stemflow Research and Its Integration into Natural Science, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12393, https://doi.org/10.5194/egusphere-egu24-12393, 2024.