242 research outputs found
van Ommen JR, Teuling M, Nijenhuis J, van Wachem BGM
No full textThe scaling rules using dimensionless groups are investigated using CFD simulations of two scaled fluidized beds with diameters of 15 and 36 cm. Three sets of dimensionless groups are used: the simplified set, the full set, and the full set extended with a dimensionless pressure group. Voidage and pressure data for the two scales are compared using different analysis techniques. The full set gives the largest differences between the two scales. The simplified set and the extended set perform better, but neither lead to complete similarity between the two scales
A state-dependent parameterization of saturated-unsaturated zone interaction
No full textThe relevance of groundwater as an important source of root zone moisture by means of capillary rise is increasingly being recognized. This is partly reflected in many current land surface schemes, which increasingly replace a one-way (i.e., downward) drainage of water by a two-way interaction flux between the root zone and a groundwater system. A fully physically correct implementation of this two-way saturated-unsaturated interaction flux requires transient simulations using the highly nonlinear Richards' equation, which is a computationally demanding approach. We test a classic simple approximation that computes the root zone¿groundwater interaction flux as the net effect of a downward drainage flux and an upward capillary rise flux against the Darcy equation for quasi steady state conditions. We find that for a wet root zone and/or shallow groundwater, the errors within this approximation are significant and of the same magnitude as the interaction flux itself. We present a new closed-form parameterization of the Darcy equation¿based fluxes that accounts both for root zone soil moisture and depth to the water table. Parameter values for this parameterization are listed for 11 different, widely applied soil texture descriptions. The high numerical efficiency of the proposed method makes it suitable for inclusion into demanding applications, e.g., a Monte Carlo framework, or high spatial resolution
Soil moisture monitoring for climate research: evaluation of a low cost sensor in the framework of the Swiss soil Expperiment (SwissSMEX) campaign
No full textSoil moisture measurements are essential to understand land surface–atmosphere interactions. In this paper we evaluate the performance of the low-cost 10HS capacitance sensor (Decagon Devices, United States) using laboratory and field measurements. Measurements with 10HS sensors of volumetric water content (VWC, Vol.%), integrated absolute soil moisture (millimeters) over the measured soil column, and the loss of soil moisture (millimeters) for rainless days are compared with corresponding measurements from gravimetric samples and time domain reflectometry (TDR) sensors. The field measurements were performed at two sites with different soil texture in Switzerland, and they cover more than a year of parallel measurements in several depths down to 120 cm. For low VWC, both sensor types present good agreement for laboratory and field measurements. Nevertheless, the measurement accuracy of the 10HS sensor reading (millivolts) considerably decreases with increasing VWC: the 10HS sensors tend to become insensitive to variations of VWC above 40 Vol.%. The field measurements reveal a soil type dependency of the 10HS sensor performance, and thus limited applicability of laboratory calibrations. However, with site-specific exponential calibration functions derived from parallel 10HS and TDR measurements, the error of the 10HS compared to the TDR measurements can be decreased for soil moisture contents up to 30 Vol.%, and the day-to-day variability of soil moisture is captured. We conclude that the 10HS sensor is appropriate for setting up dense soil moisture networks when focusing on medium to low VWC and using an established site-specific calibration function. This measurement range is appropriate for several applications in climate research, but the identified performance limitations should be considered in investigations focusing on humid conditions and absolute soil moistur
Hydrologic impacts of changing land use and climate in the Veneto lowlands of Italy
No full textThe Po valley in northern Italy is one of Europe’s largest and most anthropogenically-modified lowland areas, where intensifying climate and land transformation are increasingly causing water management problems. In this study, the Wageningen Lowland Runoff Simulator (WALRUS) is calibrated, validated, and applied to a reclaimed basin in the Veneto region (Italy) in order to assess the hydrologic impacts of land use and climate change scenarios. First-time model calibration for Mediterranean lowlands resulted in reasonable performance during the training year (NSE 0.77), but lower validation performance (NSE 0.53), while potential for improved calibration was limited by data availability. Scenario analysis covers the historical and future changes in land cover and climate throughout a century (1951–2060), based on aerial imagery analysis, hydrologic measurements, COSMO-CLM regional climate projections and demographics. WALRUS simulations illustrate how land use transformation (i.e. expanded built-up zones and a diminished drainage network) have a strong potential to increase discharge intensities from the catchment, mostly evident in summer peak flow (past −34%; future +48%). A historical scenario of combined land use and climate shows even stronger deviations from the present (annual discharge −19%; summer peak flow −45%), resulting from an observed increase in rainfall intensity and seasonality over the past 50 years. With drier future climate projections, however, the discharge response is moderate in the combined future scenario. Despite the non-optimal model calibration, the presented work in the Veneto region illustrates the directional impact of processes typical of anthropogenic lowlands. Particularly, the impact of observed land transformation seems to diminish the buffering and storage capacity of the catchment, thereby enhancing the hydrologic risks in modern times
Disentangling the response of forest and grassland energy exchange to heatwaves under idealized land–atmosphere coupling
No full textThis study investigates the difference in land–atmosphere interactions
between grassland and forest during typical heatwave conditions in order to
understand the controversial results of Teuling et al. (2010) (hereafter T10),
who found the systematic occurrence of higher sensible heat fluxes over
forest than over grassland during heatwaves. With a simple but accurate
coupled land–atmosphere model, we show that existing parametrizations are
able to reproduce the findings of T10 for normal summer and heatwave
conditions. Furthermore, we demonstrate the sensitivity of the coupled system
to changes in incoming radiation and early-morning temperature typical for
European heatwaves.
Our results suggest that the fast atmospheric control of stomatal resistance
can explain the observed differences between grassland and forest. The
atmospheric boundary layer has a buffering function therein: increases in
stomatal resistance are largely compensated for by increases in the potential
evaporation due to atmospheric warming and drying.
In order to disentangle the contributions of differences in several static
and dynamic properties between forest and grassland, we have performed a
virtual experiment with artificial land-use types that are equal to
grassland, but with one of its properties replaced by that of forest. From
these, we confirm the important role of the fast physiological processes that
lead to the closure of stomata. Nonetheless, for a full explanation of T10's
results, the other properties (albedo, roughness and the ratio of minimum
stomatal resistance to leaf-area index) play an important but indirect role;
their influences mainly consist of strengthening the feedback that leads to
the closure of the stomata by providing more energy that can be converted
into sensible heat. The model experiment also confirms that, in line with the
larger sensible heat flux, higher atmospheric temperatures occur over forest.
As our parametrization for stomatal resistance is empirical rather than
mechanical, our study stresses the demand for a better mechanistic
understanding of physiological processes in plants
A probabilistic climate change assessment for Europe
No full textGlobally, the impacts of climate change can vary across different regions. This study uses a probability framework to evaluate recent historical (1976–2016) and near-future projected (until 2049) climate change across Europe using Climate Research Unit and ensemble climate model datasets (under RCPs 2.6 and 8.5). A historical assessment shows that although the east and west of the domain experienced the largest and smallest increase in temperature, changes in precipitation are not as smooth as temperature. Results indicate that the maximum changes between distributions of the variables (temperature and precipitation) mainly occur at extreme percentiles (e.g., 10% and 90%). A group analysis of four subregions of Europe, namely east (G1), north (G2), west/south (G3), and UK/Ireland (G4), shows that G1 and G4 are expected to have the largest increase in temperature and precipitation extremes, respectively. Although maximum increases in temperature in G3 and G4 occur at larger percentiles, G1 and G2 experience maximum increases at both large and small percentiles. The maximum increase of precipitation over the study domain, however, occurs mainly at larger extremes. To quantify changes in the distribution of projection (2020–2049) relative to the historical reference (1976–2005), two measures are defined, namely a change in occurrences (KS statistic) and intensities at different quantiles (Δ). Results confirm that the temperature distribution tends to shift to higher temperatures. Changes in distribution and extremes of precipitation are spatially variable. Furthermore, extremes are expected to be intensified under RCP 8.5. The quantile analysis and defined measures reveal changes in the entire probability distribution, reflecting possible climate changes in the future.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Water ResourcesWater Managemen
Bivariate colour maps for visualizing climate data
No full textThe increasing availability of gridded, high-resolution, multivariate climatological data sets calls for innovative approaches to visualize inter-variable relations. In this study, we present a methodology, based on properties of common colour schemes, to plot two variables in a single colour map by using a two-dimensional colour legend for both sequential and diverging data. This is especially suited for climate data as the spatial distribution of the relation between different variables is often as important as the distribution of variables individually. Two example applications are given to illustrate the use of the method: one that shows the global distribution of climate based on observed temperature and relative humidity, and the other showing the distribution of recent changes in observed temperature and precipitation over Europe. A flexible and easy-to-implement method is provided to construct different colour legends for sequential and diverging data
Technical note: Towards a continuous classification of climate using bivariate colour mapping
No full textClimate is often defined in terms of discrete classes. Here I use bivariate colour mapping to show that the global distribution of Köppen-Geiger climate classes can largely be reproduced by combining the simple means of two key states of the climate system (i.e. air temperature and relative humidity). This allows for a classification that is not only continuous in space, but can be applied at and transferred between timescales ranging from days to decades
Evaporation from a large lowland reservoir-observed dynamics and drivers during a warm summer
No full textWe study the controls on open water evaporation of a large lowland reservoir in the Netherlands. To this end, we analyse the dynamics of open water evaporation at two locations, Stavoren and Trintelhaven, at the border of Lake IJssel (1100ĝ€¯km2); eddy covariance systems were installed at these locations during the summer seasons of 2019 and 2020. These measurements were used to develop data-driven models for both locations. Such a statistical model is a clean and simple approach that can provide a direct indication of (and insight into) the most relevant input parameters involved in explaining the variance in open water evaporation, without making a priori assumptions regarding the process itself. We found that a combination of wind speed and the vertical vapour pressure gradient can explain most of the variability in observed hourly open water evaporation. This is in agreement with Dalton's model, which is a well-established model often used in oceanographic studies for calculating open water evaporation. Validation of the data-driven models demonstrates that a simple model using only two variables yields satisfactory results at Stavoren, with R2 values of 0.84 and 0.78 for hourly and daily data respectively. However, the validation results for Trintelhaven fall short, with R2 values of 0.67 and 0.65 for hourly and daily data respectively. Validation of the simple models that only use routinely measured meteorological variables shows adequate performance at hourly (R2Combining double low line0.78 at Stavoren and R2Combining double low line0.51 at Trintelhaven) and daily (R2Combining double low line0.82 at Stavoren and R2Combining double low line0.87 at Trintelhaven) timescales. These results for the summer periods show that open water evaporation is not directly coupled to global radiation at the hourly or daily timescale. Rather a combination of wind speed and vertical gradient of vapour pressure is the main driver at these timescales. We would like to stress the importance of including the correct drivers of open water evaporation in the parametrization in hydrological models in order to adequately represent the role of evaporation in the surface-Atmosphere coupling of inland waterbodies.Water Resource
Global multimodel analysis of drought in runoff for the second half of the twentieth century
No full textDuring the past decades large-scale models have been developed to simulate global and continental terrestrial water cycles. It is an open question whether these models are suitable to capture hydrological drought, in terms of runoff, on global scale. A multi-model ensemble analysis was carried out to evaluate if ten of such large-scale models agree on major drought events during the second half of the 20th century. Time series of monthly precipitation, monthly total runoff from ten global hydrological models, and their ensemble median have been used to identify drought. Temporal development of area in drought for various regions across the globe was investigated. Model spread was largest in regions with low runoff and smallest in regions with high runoff. In vast regions, correlation between runoff drought derived from the models and meteorological drought was found to be low. This indicated that models add information to the signal derived from precipitation and that runoff drought cannot directly be determined from precipitation data alone in global drought analyses with a constant aggregation period. However, duration and spatial extent of major drought events differed between models. Some models showed a fast runoff response to rainfall, which led to deviations from reported drought events in slowly responding hydrological systems. By using an ensemble of models, this fast runoff response was partly overcome and delay in drought propagating from meteorological drought to drought in runoff was included. Finally, an ensemble of models also allows to consider uncertainty associated with individual model structures
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