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In het kader van KRW, ontstaat de wens om inzichten te verkrijgen in de trends van fysisch-chemische parameters, alsook inzichten in de effectiviteit van het waterbeheer. Het waterschap Aa en Maas beschikt over een uitgebreide database over fysisch-chemische en biologische gegevens (macrofauna, vissen, diatomeeën) die het mogelijk maken om de effectiviteit van beheersmaatregelen in kaart te brengen. In reeds uitgebrachte analyses in 2021-2022 zijn de effecten van beheermaatregelen op, alsook trends van, macrofauna en fysische-chemische parameters geanalyseerd. In de huidige notitie wordt een nadere analyse uitgevoerd onder een tweedelig projectvoorstel.

Waterschappen meten al decennia aan de waterkwaliteit van beken, kanalen en sloten. In een speciaal project lukte het waterschap Aa en Maas om met deze reguliere data (en een uitgekiende statistiek) conclusies te trekken over het effect van maaien en baggeren.

De effecten van klimaatverandering worden steeds beter voelbaar: extreme wateroverlast in 2016 en 2021 en extreem droge zomers in de tussenliggende jaren. Klimaatverandering heeft mede daardoor ook een groot effect op het leven in en onder water. Het realiseren van de ecologische doelen is sterk afhankelijk van het op orde krijgen van hydrologische randvoorwaarden in de zomerperiode: geen of beperkte droogval, voldoende doorstroming, voldoende grote stroomsnelheid en passende stromingsdynamiek. In de SGBP-plannen die Aa en Maas heeft opgesteld, is tot nu toe geen rekening gehouden met effecten van klimaatverandering, en ook is de realiseerbaarheid van de hydrologische randboorwaarden niet getoetst. Daarom is voor het beheergebied van Aa en Maas per KRW-waterlichaam verkend of de voor de ecologische doelen noodzakelijke hydrologische randvoorwaarden gerealiseerd kunnen worden bij zowel het huidig als toekomstig klimaat. Onderzocht is hoe het effect van klimaat via neerslag en verdamping leidt tot andere beekafvoeren en daarmee tot veranderingen in de hydrologische randvoorwaarden stroomsnelheid, watervoerendheid en fluctuatie in debieten. Er is geen rekening gehouden met het klimaateffect van een hogere watertemperatuur op het leven in de beek en ook is geen rekening gehouden met een wijziging in de beschibare hoeveelheid aanvoerwater als gevolg van klimaatverandering.

In beken zijn stromingminnende soorten in de droge periode 2018-2020 sterk afgenomen. In een aantal beken voeren waterschappen extra water aan om stroming te behouden en zo beekvissen te beschermen. Voor een meer structurele oplossing moeten beekdalen natuurlijker worden ingericht en grondwaterstromen hersteld.

De drie Brabantse waterschappen hebben met adviesbureau ATKB een analyse uitgevoerd naar de effecten van de afgelopen droge jaren op de visstand. Zowel de soortenrijkdom als het aantal vissen in de Brabantse beken blijken af te nemen. De visbeoordelingen voor de Europese Kaderrichtlijn Water dalen daardoor ook. Nadere analyses moeten inzicht geven in exacte oorzaken voor de achteruitgang, geconstateerde verschillen tussen waterschappen en de effecten van uitgevoerde maatregelen.

An analysis is presented of electric fields in thunderclouds using a recently proposed method based on measuring radio emission from extensive air shower events during thunderstorm conditions. This method can be regarded as a tomography of thunderclouds using cosmic rays as probes. The data cover the period from December 2011 till August 2014. We have developed an improved fitting procedure to be able to analyze the data. Our measurements show evidence for the main negative-charge layer near the −10° isotherm. This we have seen for a winter as well as for a summer cloud where multiple events pass through the same cloud and also the vertical component of the electric field could be reconstructed. On the day of measurement of some cosmic-ray events showing evidence for strong fields, no lightning activity was detected within 100 km distance. For the winter events, the top heights were between 5 and 6 km, while in the summer, typical top heights of 9 km were seen. Large horizontal components in excess of 70 kV/m of the electric fields are observed in the middle and top layers.

An analysis is presented of electric fields in thunderclouds using a recently proposed method based on measuring radio emission from extensive air shower events during thunderstorm conditions. This method can be regarded as a tomography of thunderclouds using cosmic rays as probes. The data cover the period from December 2011 till August 2014. We have developed an improved fitting procedure to be able to analyze the data. Our measurements show evidence for the main negative-charge layer near the −10° isotherm. This we have seen for a winter as well as for a summer cloud where multiple events pass through the same cloud and also the vertical component of the electric field could be reconstructed. On the day of measurement of some cosmic-ray events showing evidence for strong fields, no lightning activity was detected within 100 km distance. For the winter events, the top heights were between 5 and 6 km, while in the summer, typical top heights of 9 km were seen. Large horizontal components in excess of 70 kV/m of the electric fields are observed in the middle and top layers.

Cold electron runaway means that free electrons in gases are accelerated by electric fields from eV energies to energies above tens of keV where they can be accelerated further. To run away, the electrons need to overcome a barrier at intermediate energies where they can lose much energy in collisions. When they have reached the runaway regime, they can produce high-energy radiation by bremsstrahlung that can be detected as (terrestrial) gamma ray flashes. When can thermal electrons from active discharges like streamers and leaders reach the runaway regime? The deterministic approach to this question is based on an energy-dependent electron friction that has to be overcome by electric acceleration. Taking the stochastic nature of the electron molecule collisions into account, we find (1) that the classical friction curve in the energy regime up to 1 keV does not characterize the mean electron energy, but rather, it seems to approximate the upper limit of the electron energy distribution and (2) that electrons can “tunnel” through the barrier when the field is close to 3 MV/m, below the so-called cold runaway threshold (or critical field) of approximately 26 MV/m in air at standard temperature and pressure. (3) This is only true in the bulk perspective where the electron liberation and attachment in a given electric field is taken into account in the continuously refreshing electron ensemble. In a flux simulation that follows individual electrons as long as they are free, electron attachment reduces electron runaway very strongly in air, differently to what we observe in nitrogen.

Lightning streamers and leaders need thermal electrons to initiate, but free electrons are extremely rare in the wet air of a thundercloud. Here we analyze the probabilities that high electron densities occur in extensive air showers. We argue that relevant air showers are created by cosmic particles with energies between 1015 and 1017 eV impinging onto our atmosphere. We simulate a large number of air showers and perform a stochastic analysis of their results. We present the available densities of thermal electrons as a function of altitude, time interval, and considered area, while neglecting effects of local electric fields. We find that free electron densities at altitudes between 5 and 13 km can reach values of order 103 cm−3, but only in shower cores with a radius on a centimeter scale. Above 6 km, the availability of extreme free electron densities decreases significantly with increasing altitude. Recent measurements by Rison et al. (2016, https://doi.org/10.1038/ncomms10721) indicate that several streamers must have been triggered simultaneously during discharge inception, and we suggest that an extensive air shower could have been the trigger. Rison's measurements show further that the streamers are laterally separated by several tens of meters, so they must have been triggered by electron densities as low as 1 cm−3. Such low electron densities demand a stochastic approach to streamer initiation near hydrometeors.

During thunderstorm conditions the radio footprint (intensity as well as polarization) of cosmic ray showers deviates strongly from those measured during fair weather. We have been able to interpret this pattern in terms of atmospheric electric fields. We see that even when there is no thunderstorm activity within 100 km distance within an hour of the event, the radio footprint may be strongly affected.

Lightning mapping technology has proven instrumental in understanding lightning. In this work we present a pipeline that can use lightning observed by the LOw-Frequency ARray (LOFAR) radio telescope to construct a 3-D map of the flash. We show that LOFAR has unparalleled precision, on the order of meters, even for lightning flashes that are over 20km outside the area enclosed by LOFAR antennas (approximate to 3,200km(2)), and can potentially locate over 10,000 sources per lightning flash. We also show that LOFAR is the first lightning mapping system that is sensitive to the spatial structure of the electrical current during individual lightning leader steps.

The emerging field of high-energy atmospheric physics studies how high-energy particles are produced in thunderstorms, in the form of terrestrial γ-ray flashes and γ-ray glows (also referred to as thunderstorm ground enhancements). Understanding these phenomena requires appropriate models of the interaction of electrons, positrons and photons with air molecules and electric fields. We investigated the results of three codes used in the community-Geant4, GRanada Relativistic Runaway simulator (GRRR) and Runaway Electron Avalanche Model (REAM)-to simulate relativistic runaway electron avalanches (RREAs). This work continues the study of Rutjes et al. (2016), now also including the effects of uniform electric fields, up to the classical breakdown field, which is about 3.0 MV m-1 at standard temperature and pressure.We first present our theoretical description of the RREA process, which is based on and incremented over previous published works. This analysis confirmed that the avalanche is mainly driven by electric fields and the ionisation and scattering processes determining the minimum energy of electrons that can run away, which was found to be above ≈ 10 keV for any fields up to the classical breakdown field.To investigate this point further, we then evaluated the probability to produce a RREA as a function of the initial electron energy and of the magnitude of the electric field. We found that the stepping methodology in the particle simulation has to be set up very carefully in Geant4. For example, a too-large step size can lead to an avalanche probability reduced by a factor of 10 or to a 40 % overestimation of the average electron energy. When properly set up, both Geant4 models show an overall good agreement (within ≈ 10 %) with REAM and GRRR. Furthermore, the probability that particles below 10 keV accelerate and participate in the high-energy radiation is found to be negligible for electric fields below the classical

Neutron emissions with different durations have been observed during thunderstorms. These neutrons can be produced by microsecond to millisecond fast Terrestrial Gamma-ray Flashes correlated with lightning, or by Gamma-ray Glows lasting several seconds to minutes. In both cases, the neutrons are produced through a photonuclear reaction of gamma rays in the energy range of 10 to 30 MeV with nuclei of air molecules. Here we present simulations of gamma-ray beams propagating downward from different source altitudes. In our analysis the primary photons with energies between 10 and 30 MeV are separated into four energy intervals, each of 5 MeV width. From these data, arbitrary spectra of primary photons and of their products can be composed. Our results indicate that the neutrons are created essentially along the trajectory of the primary photons and that they reach ground within a transversal area of radius below 500 m. This lateral spreading is dominated by neutron diffusion due to collisions with air molecules. A secondary longer lasting photon pulse at sea level is predicted as well by our simulations. We have introduced this Terrestrial Gamma-ray Flash afterglow already in (Rutjes et al. 2017, https://doi.org/10.1002/2017GL075552). It is due to neutron capture by air molecules, and it has recently been observed by Bowers et al. (2017, https://doi.org/10.1002/2017GL075071) and Enoto et al. (2017, https://doi.org/10.1038/nature24630).

Lightning mapping technology has proven instrumental in understanding lightning. In this work we present a pipeline that can use lightning observed by the LOw-Frequency ARray (LOFAR) radio telescope to construct a 3-D map of the flash. We show that LOFAR has unparalleled precision, on the order of meters, even for lightning flashes that are over 20 km outside the area enclosed by LOFAR antennas (∼3,200 km2), and can potentially locate over 10,000 sources per lightning flash. We also show that LOFAR is the first lightning mapping system that is sensitive to the spatial structure of the electrical current during individual lightning leader steps.

Contains fulltext : 173504.pdf (publisher's version ) (Open Access)

Contains fulltext : 173575.pdf (publisher's version ) (Open Access)

Contains fulltext : 173530.pdf (publisher's version ) (Open Access)

Contains fulltext : 173585.pdf (publisher's version ) (Open Access)

Thunderstorms are known to create terrestrial gamma ray flashes (TGFs) which are microsecond-long bursts created by runaway of thermal electrons from propagating lightning leaders, as well as gamma ray glows that possibly are created by relativistic runaway electron avalanches (RREA) that can last for minutes or more and are sometimes terminated by a discharge. In this work we predict a new intermediate thunderstorm radiation mechanism, which we call TGF afterglow, as it is caused by the capture of photonuclear neutrons produced by a TGF. TGF afterglows are milliseconds to seconds long; this duration is caused by the thermalization time of the intermediate neutrons. TGF afterglows indicate that the primary TGF has produced photons in the energy range of 10-30 MeV; they are nondirectional in contrast to the primary TGF. Gurevich et al. might have reported TGF afterglows in 2011.

We show that LOFAR can be used as a lightning mapping array with a resolution that is orders of magnitude better than existing arrays. In addition the polarization of the radiation can be used to track the direction of the stepping discharges.

Radio detection as well as modeling of cosmic rays has made enormous progress in the past years. We show this by using the subtle circular polarization of the radio pulse from air showers measured in fair weather conditions and the intensity of radio emission from an air shower under thunderstorm conditions.

We present a simple and robust strategy for the selection of sampling points in uncertainty quantification. The goal is to achieve the fastest possible convergence in the cumulative distribution function of a stochastic output of interest. We assume that the output of interest is the outcome of a computationally expensive nonlinear mapping of an input random variable, whose probability density function is known. We use a radial function basis to construct an accurate interpolant of the mapping. This strategy enables adding new sampling points one at a time, adaptively. This takes into full account the previous evaluations of the target nonlinear function. We present comparisons with a stochastic collocation method based on the Clenshaw-Curtis quadrature rule, and with an adaptive method based on hierarchical surplus, showing that the new method often results in a large computational saving.

We present measured radio emission from cosmic-ray-induced air showers under thunderstorm conditions. We observe for these events large differences in intensity, linear polarization and circular polarization from the events measured under fair-weather conditions. This can be explained by the effects of atmospheric electric fields in thunderclouds. Therefore, measuring the intensity and polarization of radio emission from cosmic ray extensive air showers during thunderstorm conditions provides a new tool to probe the atmospheric electric fields present in thunderclouds.