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Development of Laboratory Experiments and Numerical Modeling Techniques to Quantify Vadose Zone Water Fluxes in Arid Regions

Pfletschinger, Heike (2012)
Development of Laboratory Experiments and Numerical Modeling Techniques to Quantify Vadose Zone Water Fluxes in Arid Regions.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication

Abstract

The quantification of vadose zone water fluxes in arid regions poses many difficulties due to low water input, coupled thermal and isothermal processes, spatial and temporal highly variable meteorological conditions and measurement deficits regarding spatial and temporal resolutions as well as measurement errors. Nevertheless, water flux processes in the vadose zone have to be understood and quantified as they govern rates of direct groundwater recharge. To quantify vadose zone water fluxes under controlled conditions, laboratory soil column experiments were developed that mimic atmospheric and soil water conditions as they can be expected in arid regions. The experimental setup allowed to measure water content and temperature distribution within a 92 cm deep soil profile in high temporal and spatial resolution. At the top of the column, a head space with controllable air stream, water input and applied temperature accounted for the simulation of changing atmospheric conditions. At the column bottom, temperature and outflow pressure were applied to obtain a temperature gradient within the column and water discharge under controlled pressure conditions. By applying different initial and boundary conditions, soil water dynamics and temperature distributions were studied for two different sands. Water content profiles, that were measured with a TDR “Taupe” cable, showed almost uniform infiltration fronts for steady-state experiments. Subsequent experimental runs indicated the high impact of irrigation amount and intensity on water infiltration, evaporation and redistribution within the sands. Obviously, only single irrigations exceeding potential evaporation and lasting long enough to infiltrate deeper than 20 cm, could account for discharge at the bottom of the column, depending on successively applied irrigations. According to the experiments, a numerical model was set up in Hydrus-1D, simulating coupled water, vapor and temperature fluxes in variably-saturated media (Šimunek et al., 2009). Hydraulic and thermal soil parameters, which are implemented into the model, were calibrated with experimental data of water content and temperature profiles at different times as well as transient water discharge and evaporation. Amongst the calibrated parameters, those controlling high saturated flow were less sensitive than those controlling evaporation and drainage, whereas highest sensitivities were obtained for the air entry pressure of the retention function of Brooks and Corey (1964). With the calibrated model, predictive scenario modeling was performed representing annual changing soil moisture conditions to identify parameters of primary importance for possible groundwater recharge in arid regions. The predictive modeling emphasized the high importance of single precipitation amounts on deep infiltration and percolation which can induce groundwater recharge. For annually low precipitation amounts, the residual water content of the ambient soil mainly determined percolation processes. Vapor fluxes, induced by temperature gradients, played a major role in total water fluxes under low saturated conditions. The laboratory experiments were a good tool for first estimates of vadose zone water fluxes under arid conditions and were essential for the model setup and calibration. Based on the calibrated model further predictions upon vertical water fluxes and deep percolation for critical meteorological conditions could be made. By this, the model offers a valuable tool for groundwater management issues, especially regarding smart field observation and measurement schemes and initial predictions on soil water states for expected future hydrological and microclimatological changes.

Item Type: Ph.D. Thesis
Erschienen: 2012
Creators: Pfletschinger, Heike
Type of entry: Primary publication
Title: Development of Laboratory Experiments and Numerical Modeling Techniques to Quantify Vadose Zone Water Fluxes in Arid Regions
Language: English
Referees: Schüth, Prof. Dr. Christoph ; Hinderer, Prof. Dr. Matthias
Date: 19 May 2012
Place of Publication: Darmstadt
Collation: 152 S.
Refereed: 17 April 2012
URL / URN: urn:nbn:de:tuda-tuprints-29866
Abstract:

The quantification of vadose zone water fluxes in arid regions poses many difficulties due to low water input, coupled thermal and isothermal processes, spatial and temporal highly variable meteorological conditions and measurement deficits regarding spatial and temporal resolutions as well as measurement errors. Nevertheless, water flux processes in the vadose zone have to be understood and quantified as they govern rates of direct groundwater recharge. To quantify vadose zone water fluxes under controlled conditions, laboratory soil column experiments were developed that mimic atmospheric and soil water conditions as they can be expected in arid regions. The experimental setup allowed to measure water content and temperature distribution within a 92 cm deep soil profile in high temporal and spatial resolution. At the top of the column, a head space with controllable air stream, water input and applied temperature accounted for the simulation of changing atmospheric conditions. At the column bottom, temperature and outflow pressure were applied to obtain a temperature gradient within the column and water discharge under controlled pressure conditions. By applying different initial and boundary conditions, soil water dynamics and temperature distributions were studied for two different sands. Water content profiles, that were measured with a TDR “Taupe” cable, showed almost uniform infiltration fronts for steady-state experiments. Subsequent experimental runs indicated the high impact of irrigation amount and intensity on water infiltration, evaporation and redistribution within the sands. Obviously, only single irrigations exceeding potential evaporation and lasting long enough to infiltrate deeper than 20 cm, could account for discharge at the bottom of the column, depending on successively applied irrigations. According to the experiments, a numerical model was set up in Hydrus-1D, simulating coupled water, vapor and temperature fluxes in variably-saturated media (Šimunek et al., 2009). Hydraulic and thermal soil parameters, which are implemented into the model, were calibrated with experimental data of water content and temperature profiles at different times as well as transient water discharge and evaporation. Amongst the calibrated parameters, those controlling high saturated flow were less sensitive than those controlling evaporation and drainage, whereas highest sensitivities were obtained for the air entry pressure of the retention function of Brooks and Corey (1964). With the calibrated model, predictive scenario modeling was performed representing annual changing soil moisture conditions to identify parameters of primary importance for possible groundwater recharge in arid regions. The predictive modeling emphasized the high importance of single precipitation amounts on deep infiltration and percolation which can induce groundwater recharge. For annually low precipitation amounts, the residual water content of the ambient soil mainly determined percolation processes. Vapor fluxes, induced by temperature gradients, played a major role in total water fluxes under low saturated conditions. The laboratory experiments were a good tool for first estimates of vadose zone water fluxes under arid conditions and were essential for the model setup and calibration. Based on the calibrated model further predictions upon vertical water fluxes and deep percolation for critical meteorological conditions could be made. By this, the model offers a valuable tool for groundwater management issues, especially regarding smart field observation and measurement schemes and initial predictions on soil water states for expected future hydrological and microclimatological changes.

Alternative Abstract:
Alternative abstract Language

Die Quantifizierung von Wasserflüssen in der vadosen Bodenzone arider Gebiete ist mit verschiedenen Fehlerquellen behaftet. Schwierigkeiten bestehen aufgrund der generell sehr niedrigen Bodenwassergehalte und Niederschlagsmengen, gekoppelten thermischen und isothermen Prozessen, räumlich und zeitlich hoch variablen meteorologischen Konditionen und Defiziten in Messmethoden für die den Konditionen entsprechenden Messkampagnen. Da Prozesse in der vadosen Bodenzone Raten direkter Grundwasserneubildung bestimmen, ist es jedoch notwendig, diese zu verstehen und zu quantifizieren. Zur vereinfachten Quantifizierung von vertikalen Wasserflüssen in ungesättigtem Boden unter kontrollierten Bedingungen wurden Labor-Bodensäulenversuche entwickelt, die die Simulation arider klimatischer Bedingungen ermöglichten. Innerhalb des Versuchsaufbaus wurden hochaufgelöste Wassergehalts- und Temperaturprofile in einer 92 cm tiefen Bodensäule gemessen. Über der Bodensäule wurde eine atmosphärische Randbedingung mit kontrollierbarem Luftstrom mit definierter Eingangsfeuchtigkeit, Beregnung und regelbarer Temperatur in einem 5 cm hohen Luftraum vorgegeben. Am Säulenfuß wurde ebenfalls die Temperatur geregelt. Eine Saugplatte mit angelegtem konstantem Unterdruck sorgte für einen kontrollierbaren Wasserfluss aus der Säule. Versuche wurden mit zwei verschiedenen Sanden und ändernden Eingangs- und Randbedingungen durchgeführt. Mehrere Beregnungs- und Trocknungsversuche zeigten den starken Einfluss von Beregnungsmenge und –intensität auf Infiltrationstiefen, Evaporationsmengen und der Wasserredistribution innerhalb des Bodenprofils. Beregnungen führten hierbei nur bei einer Infiltrationstiefe von mindestens 20 cm zu Durchfluss, wobei die Menge des Durchflusses sowohl von der Anfangssättigung als auch von dem weiteren Versuchsverlauf abhing. Entsprechend des Versuchsaufbaus wurde ein numerisches Modell in Hydrus-1D aufgebaut. Das Programm Hydrus-1D berechnet gekoppelte Wasser-, Wasserdampf- und Temperaturflüsse in variabel gesättigten porösen Medien (Šimunek et al., 2009). Hydraulische und thermische Bodeneigenschaften, die in dem Modell parametrisiert sind, wurden mit Daten der Säulenversuche von Sättigungsprofilen, Temperaturprofilen sowie Evaporations- und Durchflussmengen kalibriert. Innerhalb der kalibrierten Parameter zeigten die hydraulischen Parameter, die Mengen von Evaporation und Durchfluss in niedrig gesättigten Bereichen bestimmen, und insbesondere der Lufteintrittspunkt im hydraulischen Retentionsmodell von Brooks und Corey (1964) die höchsten Sensitivitäten. Mit dem kalibrierten Modell wurden weitere Szenarien modelliert. Hierbei wurden wechselnde Bodenwassergehaltszustände innerhalb typischer Jahresgänge arider Gebiete untersucht. Weiterhin wurden Modellläufe zur Bestimmung der Einflussgrößen einzelner Eingangs- und Randparameter auf mögliche Grundwasserneubildungsraten durchgeführt. Als größter Einfluss für tiefe Infiltrationsmengen und mögliche Grundwasserneubildung wurde die Niederschlagsmenge einzelner Regenereignisse bestimmt. Bei geringen jährlichen Niederschlagsmengen beeinflusste hauptsächlich der residuale Bodenwassergehalt die berechneten Perkolationsprozesse. Bei niedrigen Wassersättigungen dominierten thermische Wasserdampfflüsse die gesamte Wasserflussmenge. Die Laborexperimente dienten der ersten Untersuchung von Wasserflüssen in der vadosen Bodenzone arider Gebiete. Für die Modellkalibrierung lieferten sie zuverlässige und hochaufgelöste Daten. Mit dem kalibrierten Modell konnten weitere Untersuchungen zu Einflussfaktoren auf Wasserflüsse in der vadosen Zone durchgeführt werde. Das Modell kann somit für Fragen des Grundwassermanagements in ariden Gebieten eingesetzt werden. Insbesondere kann es als Planungsinstrument für die Durchführung von Messkampagnen entsprechend errechneter Sensitivitäten herangezogen werden. Zusätzlich können weitere Modellläufe Bodensättigungen und Perkolationssraten für zukünftige hydrologische und klimatische Veränderungen oder Extreme abschätzen.

German
Classification DDC: 500 Science and mathematics > 550 Earth sciences and geology
Divisions: 11 Department of Materials and Earth Sciences > Earth Science > Hydrogeology
11 Department of Materials and Earth Sciences > Earth Science
11 Department of Materials and Earth Sciences
Date Deposited: 30 May 2012 09:12
Last Modified: 05 Mar 2013 10:01
PPN:
Referees: Schüth, Prof. Dr. Christoph ; Hinderer, Prof. Dr. Matthias
Refereed / Verteidigung / mdl. Prüfung: 17 April 2012
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