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.

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.

Reference

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.

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.

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.

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.

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.

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.

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.

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.

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.