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On Dew and Micrometeorology in an Arid Coastal Ecosystem

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On Dew and Micrometeorology in an Arid Coastal Ecosystem On dew and micrometeorology in an arid coastal ecosystem Bert G. Heusinkveld Promotor: Prof. dr. A.A.M. Holtslag Hoogleraar in de meteorologie Leerstoelgroep Meteorologie en Luchtkwaliteit, Wageningen Universiteit Co-promotor: Dr. ir. A.F.G. Jacobs Universitair hoofddocent Leerstoelgroep Meteorologie en Luchtkwaliteit Wageningen Universiteit Samenstelling promotiecommissie: Prof. dr. A. Kaplan Hebrew University, Jerusalem, Israel Prof. dr. T. Foken University of Bayreuth, Germany Prof. dr. ir. B.J.J.M. v.d. Hurk Universiteit Utrecht, KNMI, De Bilt Prof. dr. ir. S.E.A.T.M. van der Zee Wageningen Universiteit Dit onderzoek is uitgevoerd binnen de onderzoeksschool Productie Ecologie. On dew and micrometeorology in an arid coastal ecosystem Bert G. Heusinkveld Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit, Prof. dr. M.J. Kropff, in het openbaar te verdedigen op woensdag 4 juni 2008 des namiddags te vier uur in de Aula CIP-data Koninklijke Bibliotheek, Den Haag On dew and micrometeorology in an arid coastal ecosystem Heusinkveld, B.G., 2008 PhD thesis, Wageningen University, The Netherlands With references – with summaries in English and Dutch ISBN 978-90-8504-932-6 Voorwoord Dankbaar en blij dat het proefschrift klaar is wil ik beginnen met een voorwoord om al die mensen nog eens naar voren te halen die op een of andere manier hebben bijgedragen tot deze mijlpaal. Promotor Bert Holtslag wil ik danken voor de inhoudelijke discussies en de stimulering die daarvan uitging. Mijn co-promotor Adrie Jacobs wil ik bedanken voor zijn toegewijde begeleiding en zijn altijd bijzonder positieve instelling. Adrie, wat hebben we eigenlijk veel samen beleefd, of moet ik zeggen overleefd? Als ik zo terugkijk, dan begon het eigenlijk met zo’n reisje op de Rijn, maar dan wel hartje winter en hoogste waterstand. Ja, om een system beter te leren kennen moeten soms extremen opgezocht worden. Ook in het onderzoek ging dat op, want we zijn uiteindelijk een droge woestijn in gegaan voor het dauw onderzoek. Dat we daarbij ook bij nacht en ontij onze metingen moesten verrichten maakte het ook weer avontuurlijk. Het onderzoek in de Negev woestijn was niet mogelijk geweest zonder de enorme toewijding en motivatie van mijn Israelische vriend Simon Berkowicz. Simon, you are a fantastic friend and your enthousiasm has always pushed the research forward. Your Jewish mother instinct has helped me through hard times! Aron Kaplan, thank you for providing the experimental site and support. Eyal Sachs, you logistics support and humor was very much appreciated. Ariel Cohen and Shy Ron, thank you for watching after us and helping out during noctural measurements, shalom! Jos van Dam je hebt me geweldig geholpen met het bepalen van de vochtkarakteristieken! Peter Felix-Henningson möchte ich danken für die Luftnahe Bodenschicht Beratung. Wat een geweldige werksfeer bij MAQ. Gerrie van den Brink, je was altijd een steun en toeverlaat. Kees Stigter, wat heb ik een respect voor jouw werk in Afrika gekregen. Kees, je hebt me het belang van wetenschappelijk onderzoek naar soms vanzelfsprekende landbouwmeteorologische methoden laten zien. Kees van den Dries, dank dat je altijd klaar staat voor allerlei hulp, ook als het niet met een computer te maken heeft! Frits Antonijsen, je was altijd een steun en toeverlaat tijdens de veldexperimenten. Willy Hillen, wat kun je mooie dingen maken uit een blokje metaal of kunststof. Joost Nieveen, dank voor jouw bijdrage in het opstarten van dit STW (Stichting Technische Wetenschappen) project. En hierbij wil ik dan ook tevens technologiestichting STW danken voor de sponsoring van dit onderzoek. Arjan van Dijken, je hebt een grote bijdrage geleverd aan het fundamenteel uitwerken en opzetten van de software voor de verwerking van turbulentie metingen en jouw discussies worden zeer gemist. Henk de Bruin, dankzij jou heb ik veel geleerd van de vele internationale veldexperimenten. Wim Kohsiek, ik genoot altijd van de discussies over instrumentele ontwikkeling. Rushdi M. M. El-Kilani, thanks for the interesting i discussions and salam aleikum, we miss you! Krijn Paaimans, ik zal niet gauw onze muggensafari vergeten! Jordi Villa, Leo Kroon, Arnold Moene, Wouter Meijninger, Oscar Hartogensis, Bas van de Wiel, Gert Jan Steeneveld, Bert van Hove, Roy Wichink Kruit wil ik danken voor de prettige samenwerking. Joel Schröter, vielen Dank für die Interesse an meiner Arbeit! Peter Hofschreuder, jouw inbreng verlevendigt de groep. Maarten Krol zet je strijd voort voor betere luchtkwaliteit ook op de vakgroep. Chiel van Heerwaarden, dank voor het inwijden in de vele fiets cols in Limburg. Verder wil ik mijn vrienden bij Ibex vermelden, want tijdens het klimmen wordt steeds weer veel nieuwe energie opgedaan. Johan Koelman, wat een geweldige boulderproblemen hebben we al opgelost. Het benodigde doorzettingsvermogen daarvoor blijkt ook te werken voor het afronden van het proefschrift. Toru Furukubo you are a very good friend, sayonara, and see you soon! Ook niet te vergeten zijn de mooie en soms stormachtige zeiltochten met Marcel Wokke en de Aqua’s. Saakje Hazenberg, thanks for the relaxing long distance ski touring weekends! Hans Kleingeld, je hebt me een tijdje moeten missen op het water vanwege mijn schrijfwerk, maar binnenkort hoop ik het speeden weer met je te kunnen delen! Tenslotte wil ik mijn lieve ouders en broer Robbie bedanken voor de steun en toeverlaat. ii Abstract This study investigated intriguing aspects of dew within a sandy arid ecosystem situated in the NW Negev desert, Israel. The goal was to quantify dew formation and evaporation processes through sensor design, field measurements and modelling. To do this, two new sensors were developed. The first sensor is an automated microlysimeter for measurements of water content changes within the upper soil layer. This sensor uses a sensitive inexpensive Load Cell to provide daily records of dew input and evaporation. Dew amounts reached almost 0.4 mm day-1 but were found to depend strongly on the soil type. The second sensor developed was a remote optical surface wetness sensor that was designed and tested in the laboratory and in the Negev desert. The novelty of the sensor is that it measures surface wetness directly at the interface of the atmosphere, it is non-destructive and allows for repetitive measurements on the same sample, and it can operate outdoors without being affected by direct sunlight. The sensor was found to be capable of measuring leaf water content and near soil surface water content. Data obtained for an Anabasis articulata bush showed that leaves change their leaf water content during the evening. In general, errors in flux measurements can lead to difficulties in closing the energy balance at the surface, especially if turbulent flux or soil heat flux measurements are underestimated. Soil heat flux was assessed during the field experiments using a new approach. By placing the soil heat flux sensor near the surface, depth corrections are no longer needed. It is shown that the large soil heat fluxes in hot desert regions are very important in energy balance studies. The soil heat flux measurements led to a very good closure of the surface energy balance and highlighted the importance for careful soil heat flux measurements.An analysis of dew formation conditions revealed that the actual dew is greater than the calculated potential dew. The dew formation process seems to be enhanced by the extreme dryness of the soil. The dry soil pores reduce vapour pressure and increase the downward vapour flux. This is demonstrated by using a Penman model and incorporating the vapour pressure reduction of the soil surface. The dew amount that is available within a soil is limited by the soil physical properties. The term FAD “Free Available Dew” is introduced to define the amount of water available for biological activity. The FAD would be higher for a biological crust with cyanobacteria than for vascular plant roots. The FAD for cyanobacteria is around 20% of total dew. To improve our understanding of the distribution and flow of dew water within a sandy soil covered with a biological crust, a multilayer soil model was constructed. The model was tested with the microlysimeter data and gravimetric soil moisture profile measurements. Diurnal soil moisture variation was found to be iii confined to the top 15 mm. The FAD is available in the upper half of a 4 mm thick biological crust only and limited to < 25% of the day. The possibility of harvesting dew from an artificial flat surface within the interdune was investigated by constructing a dew collector and analyzing the measurements. This resulted in a detailed dew model to simulate dew yield in general. Dew yields of 0.30 mm day-1 are possible in the NW Negev desert at a location 40 km from the coast at the end of the dry season. The artificial flat surface tested here provided 0.10 mm day-1. Modelled yields were found to be larger near the coastline. Open path gas analyzers and ultrasonic anemometers have been used to measure water vapour fluxes. Dew can affect the performance of open path gas analyzers and thus affect data quality in an arid environment. A field experiment showed that additional sensor heating can avoid dew problems and yet greatly improve the quality of flux measurements. A detailed energy balance model of the gas analyzer window housings is presented so that existing datasets can be filtered for dew contamination events on the windows. Additional
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