Summer Evapotranspiration Research Articles

Towards understanding the influence of seasons on low-groundwater periods based on explainable machine learning

Abstract. Seasons are known to have a major influence on groundwater recharge and therefore groundwater levels; however, underlying relationships are complex and partly unknown. The goal of this study is to investigate the influence of the seasons on groundwater levels (GWLs), especially during low-water periods. For this purpose, we train artificial neural networks on data from 24 locations spread throughout Germany. We exclusively focus on precipitation and temperature as input data and apply layer-wise relevance propagation to understand the relationships learned by the models to simulate GWLs. We find that the learned relationships are plausible and thus consistent with our understanding of the major physical processes. Our results show that for the investigated locations, the models learn that summer is the key season for periods of low GWLs in fall, with a connection to the preceding winter usually only being subordinate. Specifically, dry summers exhibit a strong influence on low-water periods and generate a water deficit that (preceding) wet winters cannot compensate for. Temperature is thus an important proxy for evapotranspiration in summer and is generally identified as more important than precipitation, albeit only on average. Single precipitation events show by far the largest influences on GWLs, and summer precipitation seems to mainly control the severeness of low-GWL periods in fall, while higher summer temperatures do not systematically cause more severe low-water periods.

Divergent Responses of Summer Terrestrial Evapotranspiration to Cloud Increase in East Asia

AbstractCloud impact on terrestrial evapotranspiration (ET) can be significant, while a fundamental understanding of ET response to cloud change has not been addressed in regional aspects. In this study, seven ET datasets from satellite and land surface models are used to analyze the summer ET in response to low cloud cover (LCC) over East Asia, in combination with satellite microwave and optical vegetation data, as well as ERA5 climatic and surface parameters. In general, all ET datasets present a strong negative relationship with LCC over dense vegetation (e.g., forests), which leads to the ET reduction of more than −0.5 mmday−1 under the large LCC increase, compared with the least cloudy sky conditions. Such reduction is stronger under more humid and denser forests (aridity index AI > 0.5 and enhanced vegetation index EVI > 0.6). In contrast, a positive relationship between ET and LCC presents over arid and sparse lands (AI < 0.2 and EVI < 0.2), accompanied by enhanced ET (>0.2 mmday−1). Analysis shows that under increased LCC conditions, substantial reduction in radiation availability results in the decline in both plant transpiration (ETveg) and soil evaporation (ETsoil) over humid dense vegetation, dominating the overall negative response. The increase of near‐surface relative humidity over arid sparse vegetation may indicate a stronger moisture availability, which promotes ETsoil and partly offsets the negative effect of radiation reduction. The change of vegetation water content, indicated by a satellite microwave vegetation index, is important for ETveg over arid mountainous lands with cloud cover. These findings contribute new insights to the understanding of vegetation‐atmosphere interactions.

Hydrological changes in Serra da Mantiqueira Range watersheds (Southeast Brazil)

The Serra da Mantiqueira Environmental Protection Area is one of the most important Atlantic Forest biodiversity hotspots in the World, and one of the main water sources for the Brazilian Southeast Region. In this study, we analyze the existence and characteristics of streamflow changes in two watersheds (Sapucaí-Mirim and Sapucaí-Guaçu River watersheds) of the Serra da Mantiqueira Range, and if these changes are due to climate change, alterations of land cover and/or external factors. Mann-Kendall and Pettit tests were used to detect changes in streamflow, rainfall and evapotranspiration data series, at different time scales. The characteristics and magnitude of land cover changes were evaluated from multi-temporal maps, produced from satellite images and the supervised classification technique. The results show an increase in winter rainfall at the two watersheds studied. No changes were detected in the streamflows of the Sapucaí-Mirim River Basin. The summer streamflow of Sapucaí-Guaçu River watershed reduced at a rate of −0.035 m³.s−1. year−1. In addition, there were sudden streamflow changes in the summer (from the year 1967) and autumn (from the year 1988) in Sapucaí-Guaçu watershed. Furthermore, the reference summer evapotranspiration of the Sapucaí-Guaçu watershed showed an increasing trend and an abrupt increase (in 1988), besides a sudden reduction of actual evapotranspiration in 2006. The integrated analysis of these results, together with the changes in land cover and population, allow us to conclude that the reductions in summer and autumn streamflows at the Sapucaí-Guaçu watershed were probably due to the increase in the water withdrawal for public supply, added to the effects of urbanization on the hydrology of the watershed.

Seasonal and diurnal urban energy balance and carbon exchanges over a residential neighborhood in a humid subtropicalcity

Urban energy and carbon exchanges based on the eddy covariance observation over a neighborhood of a humid subtropical city (Guangzhou) were measured from November 2020 to September 2021. The observed urban albedo (average:0.1), median peaks of net all-wave radiation (Q*: 419–563 W m−2) and sensible heat fluxes (QH: 128–156 W m−2) have small seasonal variabilities, differing from mid-latitude cities (e.g., Shanghai and London) and similar to tropical city (e.g., Singapore). However, latent heat fluxes (QE) demonstrate a distinct difference between summer (median peak QE: 131 W m−2, mean daytime QE/Q* = 27%) and other seasons (median peak QE < 75 W m−2, mean daytime QE/Q* = 16%), owing to the synthetic effect of precipitation and radiation. This suggests summer evapotranspiration is significant in cooling such humid cities. For all seasons, the mean daytime QH/Q* and QE/Q* are 31% and 22%, causing a mean Bowen ratio of 1.42. The anthropogenic heat flux was substantial (average: 51.6 W m−2), resulting in a larger storage heat flux (median peak ΔQS: 317 to 357 W m−2, mean daytime ΔQS/Q* = 67%) than other suburbs or low-rise neighborhoods. Carbon dioxide concentrations and fluxes have similar two-peaks dominated by rush-hour traffic emissions, whereas their seasonality is different. This study is expected to reinforce the global dataset of urban energy exchanges in subtropical cities and evaluate surface-atmosphere exchange processes and urban climate models.

Recycling and transport of evapotranspiration over the Tibetan Plateau: Detected by a water vapour tracer method embedded in regional climate model

AbstractWith an average elevation of 4000 m, the Tibetan Plateau (TP) exerts a global influence on weather and climate. To investigate and quantify the impact of terrestrial evapotranspiration (ET) over the TP on both local and downstream regions, we employed the water vapour tracer (WVT) method integrated into the Weather Research and Forecasting (WRF) model to track the propagation of evaporative moisture across the TP. Based on precipitation landing location, termination of evaporative moisture over the TP was categorized into recycled ET (ETin, which precipitates inside the TP) and moving‐out ET (ETout, which precipitates outside the TP). Our findings reveal that ETin dominates 70% of ET moisture termination over the TP, exhibiting a decrease gradient from eastern to western regions. The majority of ETout spreads eastward with some reaching southward. Seasonal variations indicate that more than 75% of ET recycles during summer, while more ET moves out from TP in spring and autumn. The interplay between convection and advection leads to different reach‐outs for moving‐out ET in summer compared to other seasons: strong convection and diabatic heating plus a relative weak large‐scale advection result in shorter reach‐outs during summer, whereas weaker convection combined with strong advection leads to longer ‐reach‐outs in other seasons. This study enhances our understanding of terrestrial ET moisture transport over the TP as well as its mechanism influencing both local water cycle dynamics and downstream processes.

Exploring the contribution of environmental factors to evapotranspiration dynamics in the Three-River-Source region, China

Evapotranspiration (ET) exerts substantial effects on terrestrial ecosystem carbon and water cycles. A deep understanding of the spatial and temporal variability in ET and its dominant controls has crucial implications for future ecological management, especially for the ecologically-fragile Three-River-Source region (TRSR) in China. The study examined the performance of three ET products including Global Land Evaporation Amsterdam Model (GLEAM), Famine Early Warning Systems Network Land Data Assimilation System (FLDAS) and Global Land Surface Satellite (GLASS) based on eddy covariance-based flux observations. On this basis, ET dynamics in the TRSR from 1982 to 2018 were analyzed at two time scales using the GLASS ET product. Spatially, the pattern in ET showed a gradual upward trend from northwest to southeast. Meanwhile, ET in the majority of regions significantly enhanced, with multiannual mean ET and summer ET (June to August) increasing by 0.609 mm/yr and 0.765 mm/yr, respectively. Especially, ET in summer dominated the interannual dynamics of ET in the TRSR. The individual contributions of five environmental factors including Leaf Area Index (LAI), solar radiation (Rs), air temperature (Ta), precipitation (PRE) and vapour pressure deficit (VPD), to the variability in ET were isolated using ridge regression. Significant differences appeared when explaining ET over different time scales. Rs (33.05 %) and LAI (25.29 %) were the dominant control variables in shaping the interannual ET, with VPD contributing the least by 7.19 %. However, the long-term dynamics in summer ET were mainly controlled by LAI (35.73 %) and PRE (31.19 %), and Ta exerted the least effect by 5.73 %. All analyses will strengthen our understanding of the relationships between climate, vegetation and ET in the TRSR area, China.

Productivity and water use efficiency of summer soybean-winter wheat rotation system under limited water supply in the North China Plain

Development of novel crop rotations with superior water- and N-use efficiency will be fundamental in the quest for securing sustainable food supplies under environments that are increasingly resource limited. Here, our aim was to examine how replacing maize with soybean as part of winter wheat rotations impacted the nutrient-equivalent yield (EY), evapotranspiration (ET) and water-use efficiency (WUE) of rotation systems. We contrasted summer soybean-winter wheat (SW) and summer maize-winter wheat (MW) cropping systems under three nitrogen (N) fertilization regimes: no, medium, and high nitrogen application rates (0 N, MN, and HN, respectively). Results showed that SW not only resulted in lower annual yields, but also caused more variable inter-annual wheat productivity compared with MW. SW had similar annual productivities to MW in wet (2019–2020) and dry (2020–2021) years, but lower productivity than MW in an average precipitation year (45% reduction, 2018–2019). As well, SW had higher average annual ETs (633–854 mm) than MW, mainly was attributed to higher ETs (268–440 mm) in summer. SW reduced WUEs by 17% and 28% in average (2018–2019) and dry (2020–2021) years, respectively. Regardless of cropping system, N fertilization increased annual EYs and ETs by 10–18% and 5–18% compared with no fertilization, respectively. Together these comparable increments in EY and ET, we found that N fertilization had rare effects on annual WUE of both systems. Overall, our study demonstrated that replacing maize with soybean in winter wheat rotations increased annual ET and neither maintained productivity nor improved water productivity. Thus, we suggest that practitioners explore other drought tolerant grain legumes to generate more reliable yields and less water consumption under legume-based crop rotations.

A Hybrid Framework for Simulating Actual Evapotranspiration in Data-Deficient Areas: A Case Study of the Inner Mongolia Section of the Yellow River Basin

Evapotranspiration (ET) plays an important role in transferring water and converting energy in the land–atmosphere system. Accurately estimating ET is crucial for understanding global climate change, ecological environmental problems, the water cycle, and hydrological processes. Machine learning (ML) algorithms have been considered as a promising method for estimating ET in recent years. However, due to the limitations associated with the spatial–temporal resolution of the flux tower data commonly used as the target set in ML algorithms, the ability of ML to discover the inherent laws within the data is reduced. In this study, a hybrid framework was established to simulate ET in data-deficient areas. ET simulation results of a coupled model comprising the Budyko function and complementary principle (BC2021) were used as the target set of the random forest model, instead of using the flux station observation data. By combining meteorological and hydrological data, the monthly ET of the Inner Mongolia section of the Yellow River Basin (IMSYRB) was simulated from 1982 to 2020, and good results were obtained (R2 = 0.94, MAE = 3.82 mm/mon, RMSE = 5.07 mm/mon). Furthermore, the temporal and spatial variations in ET and the influencing factors were analysed. In the past 40 years, annual ET in the IMSYRB ranged between 241.38 mm and 326.37 mm, showing a fluctuating growth trend (slope = 0.80 mm/yr), and the summer ET accounted for the highest proportion in the year. Spatially, ET in the IMSYRB showed a regular distribution of high ET in the eastern region and low ET in the western area. The high ET value areas gradually expanded from east to west over time, and the area increased continuously, with the largest increase observed in the 1980s. Temperature, precipitation, and normalized difference vegetation index (NDVI) were found to be the most important factors affecting ET in the region and play a positive role in promoting ET changes. These results provide an excellent example of long-term and large-scale accurate ET simulations in an area with sparse flux stations.

Increasing Cloud Coverage Deteriorates Evapotranspiration Estimating Accuracy From Satellite, Reanalysis and Land Surface Models Over East Asia

AbstractAccurate estimation of evapotranspiration (ET) can be affected by cloud change, while cloud‐induced bias errors in ET change remain poorly understood. Here we evaluated seven ET data sets from satellite microwave and optical models, atmospheric reanalysis and land surface models over East Asia. Results showed that although the data sets estimated ET changes overall well under mild cloud increase, all of them had a systematic overestimation (by 73.8%–159.8%) and deteriorated capability of capturing ET change under heavy cloud increase, especially over dense vegetation with large cloud cover. Overestimated incident solar radiation under clouds contributed to the deteriorated performances of ET. Cloud‐induced change in radiation and its errors played larger role in summer ET change over densely vegetated lands, while those in air temperature and relative humidity were dominant jointly over grass and barren lands. These results can serve as a reference for improving ET models and their forcing inputs under clouds.

The estimation and partitioning of evapotranspiration in a coniferous plantation in subtropical China.

Accurate estimations of forest evapotranspiration (ET) and its components, transpiration (T) and evaporation (E), are important for deep understanding and predicting the responses of forest water cycles to climate change. In this study, the improved Shuttleworth-Wallace model (SWH) was applied to estimate ET, T, and E during 2003-2014 in a subtropical planation, and the modeled results were verified using in situ measurements by the eddy covariance technique, sap flow, and micro-lysimeter method. The study aimed to clarify whether it is feasible and reliable to use the SWH model to estimate and partition ET in forests. In addition, depending on the long-term data, the specific performances in modeling ET under different climatic backgrounds were investigated, and the underlying mechanisms were explored. The results verified that the SWH performed relatively well in the subtropical forest, and the modeled ET, T and E could track the seasonal variations, although overestimations were found in the peak seasons. However, the model was relatively weaker in estimating the interannual variabilities. It performed well in modeling ET in normal years but showed larger model residuals in years with obvious climatic anomalies. In the severe summer-drought (2003) and cold-spring (2005) years, the model greatly overestimated ET. It also overestimated ET in summer since 2010, which may be ascribed to the less dependency of ET on VPD induced by the more humid microclimate in forest accompanied with forest development. For the ET partitioning results, the modeled and measured E and T values were all in reasonable ranges. The possible reasons for underestimations (overestimations) of E and T by measurements (SWH model) were discussed. In this study, the data obtained using different methods and from different scales matched each other and could be cross validated, and the discussion on discrepancies would be beneficial for understanding the advantages and flaws of different methods and could be the basis for optimizing the measurement and model methods. In sum, this study verified that it is feasible to use the SWH model in forests and provided a basis for further improving and optimizing the modeled results under different climate backgrounds.

Enhanced Impact of Vegetation on Evapotranspiration in the Northern Drought-Prone Belt of China

Evapotranspiration (ET) is an essential component of the land–atmosphere water cycle. In this work, the trend of ET and its dominant factors during 1982 to 2011 are investigated in the northern drought-prone belt of China (NDPB) based on five datasets, including the gridded FLUXNET, using the Pearson correlation and linear regression methods. Specially, we focus on the increasing contribution of vegetation in the change of ET. During 1982–2011, summer ET significantly increased at the rate of 0.33 mm/year (p &lt; 0.05) in the NDPB. However, similar to global-mean ET, the ET in NDPB also experienced a pronounced fluctuation during 1999 and 2002. The role of water supply differed remarkably before and after the fluctuation while the atmospheric demand maintained weak constraint on ET. Before the fluctuation (during 1982–2000), ET correlated significantly (p &lt; 0.01) and positively with soil moisture, indicating ET was primarily limited by water supply. However, their correlation weakened remarkably after the fluctuation when soil moisture decreased to the lowest level for the past thirty years, indicating that neither moisture supply nor atmospheric demand dominated the ET during this period. In contrast, vegetation leaf area index (LAI) maintained consistent significant (p &lt; 0.01) and positive correlation with ET before and after the fluctuation in the NDPB, and it reflected over 60% of the change in ET. Moreover, the LAI in NDPB increased by 19.6% which was more than double of the global-mean increase. The ET increase due to rising LAI offset the ET decrease due to reduction of soil moisture, and vegetation became the primary constraint on ET during 2001–2011. The expansion of vegetation may intensify the risk of drought and cause conflicting demands for water between the ecosystem and humans in the NDPB, especially in the case of weak summer monsoon.

A New Digital Twin for Climate Change Adaptation, Water Management, and Disaster Risk Reduction (HIP Digital Twin)

The paper analyzes the national DK-model hydrological information and prediction (HIP) system and HIP portal viewed as a ‘digital twin’ and how the introduction of real-time dynamic updating of the DK-model HIP simulations can make room for plug-in submodels with real-time boundary conditions made available from an HIP portal. The possible feedback to a national real-time risk knowledge base during extreme events (flooding and drought) is also discussed. Under climate change conditions, Denmark is likely to experience more rain in winter, more evapotranspiration in summer, intensified cloudbursts, drought, and sea level rise. These challenges were addressed as part of the Joint Governmental Digitalization Strategy 2016–2020 for better use and sharing of public data about the terrain, water, and climate to support climate adaptation, water management, and disaster risk reduction. This initiative included the development of a new web-based data portal (HIP portal) developed by the Danish Agency for Data Supply and Infrastructure (SDFI). GEUS delivered 5 terabytes of hydrological model data to the portal, with robust calibration methods and hybrid machine learning (ML) being key parts of the deliverables. This paper discusses the challenges and potentials of further developing the HIP digital twin with ‘plug-in digital twins’ for local river basins, including feedback to the national level.

Seasonal Variations of Ecosystem Water Use Efficiency and Their Responses to Climate Factors in Inner Mongolia of China

Ecosystem water use efficiency (eWUE) is a useful metric to examine the interactions between water and carbon cycles in ecosystems. To reveal the response and adaptation characteristics of different vegetation types within the context of global warming on a regional scale, the spatiotemporal characteristics and influencing factors of the seasonal eWUE of various vegetation types in Inner Mongolia from 2001 to 2020 were explored. Based on MODIS gross primary productivity (GPP), evapotranspiration (ET) data and meteorological data, in this study, we estimated eWUE in different seasons in Inner Mongolia and used trend analysis and correlation analysis methods to analyze the relationship between eWUE in spring, summer and autumn and the temperature–precipitation. From 2001 to 2020, in this region, the GPP and ET in spring, summer and autumn showed increasing trends. In addition, the growth rates of GPP and ET in spring and summer were higher than those in autumn. Under the combined effect of GPP and ET, eWUE in different seasons showed a significant decreasing trend (p &lt; 0.05)—this is ascribed to the extent of ET increasing more than GPP, especially in summer, with the most obvious decreasing rate. In terms of spatial trend, in spring and summer, there is a decreasing trend from northeast to southwest. The effects of precipitation and temperature on the eWUE in Inner Mongolia were mainly negatively correlated in the northeastern part of Inner Mongolia with higher altitudes during the spring and autumn seasons. In total, 95.096% of the total area had positive correlations between eWUE and temperature in spring. In summer, the region in which the WUE of the vegetation had an inverse relationship with both the temperature and the amount of precipitation was the largest compared to these regions in spring and autumn.

Turning points in the impact of earlier green-up on evapotranspiration and gross primary productivity in a semi-arid grassland watershed

Warming-induced shifts in vegetation phenology affect ecosystem functions and the hydrological cycle. Research has so far focused on phenological trends, drivers and feedback across temperate and tropical forests, while their impacts on carbon assimilation and hydrological processes in temperate semi-arid grasslands remain poorly understood. Based on remote-sensing datasets from the Moderate Resolution Image Spectroradiometer (MODIS), we examined the trends in vegetation phenology, gross primary productivity (GPP) and evapotranspiration (ET) in the Chabagou watershed of the Loess Plateau, China, for 2001–2020. The results revealed that the start of the growing season (SOS) had advanced significantly between 2001 and 2020, at an average rate of –1.47 days yr−1 (p < 0.05), while the end of the growing season (EOS) had delayed non-significantly by about 0.04 days yr−1. SOS robustly regulated growing season length (GSL), which was extended by an average rate of 1.60 days yr−1. Regression analysis between phenology and two main climatic factors (temperature and precipitation) reveals that precipitation is the main climatic factor affecting SOS, while temperature plays a relatively important role in EOS delay in the study area. Consistent with the phenological trend, annual ET and GPP increased steadily from 2001 to 2020. Based on the Regional Hydro-Ecological Simulation System (RHESSys), we further investigated the net effects of phenological change on carbon and water fluxes. The simulation results indicated that, for our study site, the correlation between SOS and summer ET would transform from positive to negative once SOS had advanced to around 129 days of the year (DOY) (and 125 DOY for summer GPP). Summer ET and GPP increased as SOS advanced (and GSL extended) until the SOS reached the turning points, and decreased if the SOS (and GSL) advanced beyond the turning points. The mechanism of variations in water and carbon fluxes responding to phenology dynamics could be more complex than we acknowledged to date because the climate is now approaching turning points. We, therefore, suggest improving the dynamic phenologic models for better predicting trends of ecosystem carbon gain and water loss with ongoing climate change.

Wildfire controls on land surface properties in mixed conifer and ponderosa pine forests of Sierra Nevada and Klamath mountains, Western US

This study examines the post-fire biogeophysical and biochemical dynamics after several high-severity wildfires that occurred in mixed conifer and ponderosa pine forest types in the Sierra Nevada and Klamath Mountains regions between 1986 and 2017. We found a consistent pattern of reduced leaf area index (LAI) in the first year after fire, followed by gradual recovery over the subsequent 25 years. Recovery rate varied between forest types. For example, average summer LAI for 16-25 years post-fire was 88% of the pre-fire average for mixed conifers in the Sierra Nevada, 64% for ponderosa pine in the Sierra Nevada, and 83% for mixed conifers in Klamath Mountains (63, 35, and 64% in winter, respectively). The slower recovery of LAI in ponderosa pine could be due to poor species diversity and drier climate. Summer and winter albedo increased progressively until 12 years post-fire in Sierra Nevada, while it continued to increase until 25 years post-fire in Klamath Mountains. Ponderosa pine had the highest summer (0.148 ± 0.001) and winter (0.5 ± 0.0033) albedos. Post-fire changes in evapotranspiration (ET) and gross primary productivity (GPP) were consistent with the changes in LAI. Both summer and winter ET and GPP returned to pre-fire levels by 25 years after fire in mixed conifers of both regions, while the ET and GPP did not recover to pre-fire levels in ponderosa pine. Wildfires increased the land surface temperature (LST) immediately after fire in summer. This effect was significantly higher in mixed conifers of the Sierra Nevada (11 ± 0.03 °C) compared to Klamath Mountains (7 ± 0.01 °C). Our results suggest that reduced ET, consistent with less leaf area and its associated reduced evaporative cooling is the main factor controlling the immediate post-fire warming effect of wildfires in these regions. The findings reported here can be used to understand ecological responses to wildfire in these and nearby ecoregions as they represent mean historical behavior across multiple wildfire events.

Understanding the Non‐Linear Response of Summer Evapotranspiration to Clouds in a Temperate Forest Under the Impact of Vegetation Water Content

AbstractUnderstanding the cloud impact on forest evapotranspiration (ET) is crucial for studying the interaction of vegetation‐cloud‐atmosphere. Combining long‐term (2003–2010) satellite passive microwave observations and in‐situ measurements, a non‐linear response of canopy‐scale ET to cloud increase was found at a temperate forest in Northeast China. As cloud increased, an initial enhancement (4%–10%) in ET occurred under less cloudy sky, while a significant reduction (&gt;20%) in ET occurred under more cloudy sky. The phenomenon existed under both high and low vegetation water content (VWC) indicated by satellite microwave emissivity difference vegetation index (EDVI). Analysis showed that this was the combined effect from the enhancement (5%–30%) in evaporative fraction (EF) and the reduction (5%–50%) in net radiation under cloud increase. Decoupling analysis based on coefficients (ρ) of path analysis model showed that enhanced EF (ρ &gt; 0.61) rather than radiation (ρ &lt; 0.47) dominated the ET enhancement under less cloudy sky, while the control of reduced radiation became stronger (ρ &gt; 0.63) and could not be compensated by increased EF (ρ &lt; 0.48) under more cloudy sky. EF enhancement under clouds was strongly correlated with the decline in canopy resistance (Rs) which was dominated by vapor pressure deficit (VPD). Higher VWC increased ET via reducing Rs and enlarging EF. This positive effect of VWC was more noticeable under less cloudy sky. Associated mechanisms could be related to the dynamic controls of plant physiology and environmental conditions induced by VWC and clouds. This study highlighted the dynamic effect of clouds and VWC on forest ET and improved our knowledge of vegetation‐cloud interactions.

Evapotranspiration trends and variability in southeastern South America: The roles of land‐cover change and precipitation variability

AbstractSoutheastern South America is subject to considerable precipitation variability on seasonal to decadal timescales and has undergone very heavy land‐cover changes (LCCs) since the middle of the past century. The influence of local LCC and precipitation as drivers of regional evapotranspiration (ET) long‐term trends and variability remains largely unknown in the region. Here, ensembles of stand‐alone dynamic global vegetation models (DGVMs) with different atmospheric forcings are used to disentangle the influence of those two drivers on austral summer ET from 1950 to 2010. This paper examines the influence of both the El Niño‐Southern Oscillation (ENSO) and the dipole‐like first‐mode of southeastern South American precipitation variability (EOF1) on regional ET. We found that in the lower La Plata Basin, ET was driven by precipitation variability and showed a positive summer trend. Moreover, the region showed marked seasonal anomalies during El Niño and La Niña summers but mainly during EOF1 phases. On the contrary, in the upper La Plata Basin, LCCs forced the negative summer ET trend and particularly reduced the summer anomalies of the late 1990s, a period of ENSO and EOF1‐positive phases. In the South Atlantic Convergence Zone region, the high ET uncertainty across ensemble members impeded finding robust results, which highlights the importance of using multiple DGVMs and atmospheric forcings instead of relying on single model/forcing results.

Complex High‐ and Low‐Flow Networks Differ in Their Spatial Correlation Characteristics, Drivers, and Changes

AbstractHydrologic extremes such as floods and droughts are often spatially related, which increases management challenges and potential impacts. However, these spatial relationships in high and low flows are often overlooked in risk assessments and we know little about their differences and origins. Here, we ask how spatial relationships of both types of hydrologic extremes and their potential hydro‐meteorological drivers differ and vary by season. We propose lagged upper‐ and lower‐tail correlation as a measure of extremal dependence for temporally ordered events to build complex networks of high and low flows. We compare complex networks of overall, low and high flows, determine hydro‐meteorological drivers of these networks, and map past changes in spatial relationships using a large‐sample data set in Central Europe. Our network comparison shows that low flows are correlated more strongly and over longer distances than high flows and high‐ and low‐flow networks are strongest in spring and weakest in summer. Our driver analysis shows that high‐flow dependence is most strongly governed by precipitation in winter and evapotranspiration in summer while low‐flow dependence is most strongly governed by snowmelt in winter and evapotranspiration in fall. Finally, our change analysis shows that changes in connectedness (i.e., the number of catchments a catchments shows strong flow correlations with) vary spatially and are mostly positive for high flows. We conclude that spatial flow correlations are considerable for both high and particularly low flows as a result of a combination of spatially related hydro‐meteorological drivers whose importance varies by extreme type and season.

River runoff in Switzerland in a changing climate – changes in moderate extremes and their seasonality

Abstract. Future changes in river runoff will impact many sectors such as agriculture, energy production, or ecosystems. Here, we study changes in the seasonality, frequency, and magnitude of moderate low and high flows and their time of emergence. The time of emergence indicates the timing of significant changes in the flow magnitudes. Daily runoff is simulated for 93 Swiss catchments for the period 1981–2099 under Representative Concentration Pathway 8.5 with 20 climate model chains from the most recent transient Swiss Climate Change Scenarios. In the present climate, annual low flows typically occur in the summer half-year in lower-lying catchments (&lt;1500 m a.s.l.) and in the winter half-year in Alpine catchments (&gt;1500 m a.s.l.). By the end of the 21st century, annual low flows are projected to occur in late summer and early autumn in most catchments. This indicates that decreasing precipitation and increasing evapotranspiration in summer and autumn exceed the water contributions from other processes such as snowmelt and glacier melt. In lower-lying catchments, the frequency of annual low flows increases, but their magnitude decreases and becomes more severe. In Alpine catchments, annual low flows occur less often and their magnitude increases. The magnitude of seasonal low flows is projected to decrease in the summer half-year in most catchments and to increase in the winter half-year in Alpine catchments. Early time of emergence is found for annual low flows in Alpine catchments in the 21st century due to early changes in low flows in the winter half-year. In lower-lying catchments, significant changes in low flows emerge later in the century. Annual high flows occur today in lower-lying catchments in the winter half-year and in Alpine catchments in the summer half-year. Climate change will change this seasonality mainly in Alpine catchments with a shift towards earlier seasonality in summer due to the reduced contribution of snowmelt and glacier melt in summer. Annual high flows tend to occur more frequent, and their magnitude increases in most catchments except some Alpine catchments. The magnitude of seasonal high flows in most catchments is projected to increase in the winter half-year and to decrease in the summer half-year. However, the climate model agreement on the sign of change in moderate high flows is weak.

The Weierbach experimental catchment in Luxembourg: A decade of critical zone monitoring in a temperate forest ‐ from hydrological investigations to ecohydrological perspectives

AbstractThe Weierbach experimental catchment (0.45 km2) is the most instrumented and studied sub‐catchment in the Alzette River basin in Luxembourg. Within the last decade, it has matured towards an interdisciplinary critical zone observatory focusing on a better understanding of hydrological and hydro‐geochemical processes. The Weierbach catchment is embedded in an elevated sub‐horizontal plateau, characterized by slate bedrock and representative of the Ardennes Massif. Its climate is semi‐marine, with precipitation being rather evenly distributed throughout the year. Base flow is lowest from July to September, essentially due to higher losses through evapotranspiration in summer. The regolith is composed of Devonian slates, overlaid by Pleistocene slope deposits and entirely covered by forest with 70% deciduous and 30% coniferous trees. Since 2009, the Weierbach has been extensively equipped for continuously monitoring water fluxes and physico‐chemical parameters within different compartments of the critical zone. Additionally, these compartments are sampled fortnightly at several locations to analyze δ18O and δ2H isotopic composition of water including rainfall, throughfall, soil water, groundwater and streamwater. This ongoing monitoring and sampling programme is used for answering pressing questions related to fundamental catchment functions of water infiltration, storage, mixing and release in forest ecosystems. A recently started research line aims at investigating interactions between forest eco‐hydrosystems with the atmosphere and understanding how catchments will respond to a non‐stationary climate.