626 research outputs found

    Improved Calibration Functions of Three Capacitance Probes for the Measurement of Soil Moisture in Tropical Soils

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    Single capacitance sensors are sensitive to soil property variability. The objectives of this study were to: (i) establish site-specific laboratory calibration equations of three single capacitance sensors (EC-20, EC-10, and ML2x) for tropical soils, and (ii) evaluate the accuracy and precision of these sensors. Intact soil cores and bulk samples, collected from the top 20 and 80 cm soil depths at five locations across the Upper Mākaha Valley watershed, were analyzed to determine their soil bulk density (ρb), total porosity (θt), particle size distribution, and electrical conductivity (EC). Laboratory calibration equations were established using soil packed columns at six water content levels (0–0.5 cm3 cm−3). Soil bulk density and θt significantly varied with sampling depths; whereas, soil clay content (CC) and EC varied with sampling locations. Variations of ρb and θt at the two depths significantly affected the EC-20 and ML2x laboratory calibration functions; however, there was no effect of these properties on calibration equation functions of EC-10. There was no significant effect of sampling locations on the laboratory calibration functions suggesting watershed-specific equations for EC-20 and ML2x for the two depths; a single watershed-specific equation was needed for EC-10 for both sampling depths. The laboratory calibration equations for all sensors were more accurate than the corresponding default equations. ML2x exhibited better precision than EC-10, followed by EC-20. We conclude that the laboratory calibration equations can mitigate the effects of varying soil properties and improve the sensors’ accuracy for water content measurements

    Performance Assessment of Electromagnetic Soil Water Sensors in Different Soil Textural, Temperature, and Salinity Conditions

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    Determination of accurate and continuous measurements of volumetric water content (θv) is extremely valuable for irrigation management and other agronomic decisions. Lately, electromagnetic (EM) sensors are being widely used to monitor θv continuously which also offer the benefits of ease of installation, fewer regulatory and safety concerns, and cost effectiveness. However, the accuracy of parameters [soil temperature, electrical conductivity (ECa), dielectric permittivity (εra), and θv] reported by EM sensors need to be evaluated for them to be utilized for agricultural water management. In the current study, the accuracy of a wide range of EM sensors was evaluated over field and laboratory conditions. The performance of eight EM sensors (TDR315, CS655, HydraProbe2, 5TE, EC5, CS616, Field Connect, AquaCheck), was analyzed through a field study in a loam soil. In addition, performance assessment of two improved and recently developed EM sensors (TDR315 and CS655) was done in a laboratory over different soil type, temperature, and salinity conditions. For the field study, the reported temperature and ECa difference among the sensors were within 1°C and 1 dS m-1, respectively. Among the single-sensor probes, the range of depth-combined (0.15 and 0.76 m) RMSD for factory calibration varied from 0.039 m3 m-3 (5TE) to 0.157 m3 m-3 (CS616). Regression calibrations improved θv accuracy substantially beyond factory calibrations and the betterment in θv accuracy gained by using offset calibrations was smaller and less consistent than the improvements gained by using regression calibrations. For the laboratory study, the models for estimation of θv at hot (35°C) and cold (23.9°C) temperature were not significantly different from each other (two-tail p-value within 0.1387 and 0.7231) for TDR315 and CS655 sensors. The models for no salinity and added salinity were significantly different from each other (two-tail p-value within 2.2 × 10-16 and 0.005). It was found that CS655 and TDR315 calibration varied with soil type and the relationship of the calculated coefficients (quadratic, linear, and intercept) for CS655 and TDR315 sensors across each soil type were investigated with respect to their clay content. Based on external validation of the relationships of TDR315 and CS655 sensors with the clay content, it was found that soil type has a noteworthy effect on the performance of CS655, but not TDR315 sensors. Future work aiming to test the developed universal calibration would strengthen the claims of this study and may signal new opportunities. Advisor: Daran Rudnic

    Embeddable Soil Moisture Content Sensor based on Open–end Microwave Coaxial Cable Resonator

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    In This Paper, We Propose and Demonstrate a Novel Corrosion-Resistant, Embeddable Open-End Coaxial Cable Soil Moisture Sensor. This Microwave Resonator is Constructed using Two Reflectors Along the Coaxial Line. the First Reflector is a Metal Post at the Signal Input End, Short-Circuiting the Inner Conductor to the Outer Conductor. the Second Reflector Comprises a Welded Metal Plate Parallel to the Open-End of the Coaxial Line, Maintaining a Fixed Gap. a Moisture-Sensitive Polyvinyl Alcohol (PVA) Film is Inserted into This Gap. the Resonance Frequency of the Open-End Coaxial Cable Resonator is Highly Dependent on the Fringe Capacitance, Which Varies with Soil Moisture Levels. as Such, Tracking Resonance Frequency Changes Allows for Correlation with Soil Moisture Fluctuations. We Provide a Detailed Discussion of the Embeddable Open-End Microwave Coaxial Cable Resonator (EOE-MCCR) Mathematical Model and a Proof of Concept for Soil Moisture Measurement. the Demonstration Experiments Investigate Soil Moisture Content Ranging from 4% to 24%. the Prototype Device Exhibits a Soil Moisture Measurement Sensitivity of 0.76MHz% for Soil Moisture between 4% and 10%, and 1.44MHz% for Soil Moisture between 10% and 24%. the Soil Moisture Sensor Presented Here is Robust, Easy to Manufacture, Chemically Resistant, Low-Cost, and Suitable for Long-Term Applications and Potential Industrial Uses. This Innovative Sensor is Ideal for Sensing Applications in Harsh Environments, Advancing the Field of Chemical Trace Sensing

    Comparison of techniques for measuring the water content of soil and other porous media

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    The measurement of water in soil on a potential, gravimetric or volumetric basis is considered, with studies concentrating on the measurement of water by dielectric and neutron moderation methods. The ability of the time-domain reflectometry technique to measure water content simultaneously at different spatial locations is an important advantage of the technique. The reported apparent dielectric by the TRASE� time-domain reflectometer and Pyelab time-domain reflectometry systems is sensitive to change in extension cable length. In some soil, e.g. a commercial sand, the response to increasing extension length of extension cable is linear. For other soil a linear response occurs for certain lengths of cable at different moisture contents. A single model accounting for clay content, extension cable length, time-domain reflectometry system, probe type and inherent moisture conditions explained 62.2 % of variation from the control (0 m extension) cable. The extension cable causes a decrease in the returning electromagnetic-wave energy; leading to a decline in the slope used in automatic end-point determination. Calibration for each probe installation when the soil is saturated, and at small water contents is recommended. The ability of time-domain reflectometry, frequency-domain and neutron moderation techniques in measuring soil water content in a Brown Chromosol is examined. An in situ calibration, across a limited range of water contents, for the neutron moderation method is more sensitive to changing soil water content than the factory supplied 'universal' calibration. Comparison of the EnviroSCAN� frequency-domain system and the NMM count ratio indicates the frequency-domain technique is more sensitive to change in soil water conditions. The EnviroSCAN� system is well suited to continuous profile-based measurement of soil water content. Results with the time-domain reflectometry technique were disappointing, indicating the limited applicability of time-domain reflectometry in profile based soil water content measurement in heavy-textured soil, or soil with a large electrical conductivity. The method of auguring to a known depth and placement of the time-domain reflectometry probe into undisturbed soil is not recommended. A time-domain reflectometry system is adapted for in situ measurement of water in an iron ore stockpile. The laboratory calibration for water content of the processed iron ore compares favourably to a field calibration. In the field study, the 28 m extension cable used to connect the probes to the time-domain reflectometry affected the end-point determination of the time-domain reflectometry system. To account for this, 0.197 should be subtracted from the reported apparent dielectric before calculation of volumetric moisture content

    Comparison of techniques for measuring the water content of soil and other porous media

    Get PDF
    The measurement of water in soil on a potential, gravimetric or volumetric basis is considered, with studies concentrating on the measurement of water by dielectric and neutron moderation methods. The ability of the time-domain reflectometry technique to measure water content simultaneously at different spatial locations is an important advantage of the technique. The reported apparent dielectric by the TRASE� time-domain reflectometer and Pyelab time-domain reflectometry systems is sensitive to change in extension cable length. In some soil, e.g. a commercial sand, the response to increasing extension length of extension cable is linear. For other soil a linear response occurs for certain lengths of cable at different moisture contents. A single model accounting for clay content, extension cable length, time-domain reflectometry system, probe type and inherent moisture conditions explained 62.2 % of variation from the control (0 m extension) cable. The extension cable causes a decrease in the returning electromagnetic-wave energy; leading to a decline in the slope used in automatic end-point determination. Calibration for each probe installation when the soil is saturated, and at small water contents is recommended. The ability of time-domain reflectometry, frequency-domain and neutron moderation techniques in measuring soil water content in a Brown Chromosol is examined. An in situ calibration, across a limited range of water contents, for the neutron moderation method is more sensitive to changing soil water content than the factory supplied 'universal' calibration. Comparison of the EnviroSCAN� frequency-domain system and the NMM count ratio indicates the frequency-domain technique is more sensitive to change in soil water conditions. The EnviroSCAN� system is well suited to continuous profile-based measurement of soil water content. Results with the time-domain reflectometry technique were disappointing, indicating the limited applicability of time-domain reflectometry in profile based soil water content measurement in heavy-textured soil, or soil with a large electrical conductivity. The method of auguring to a known depth and placement of the time-domain reflectometry probe into undisturbed soil is not recommended. A time-domain reflectometry system is adapted for in situ measurement of water in an iron ore stockpile. The laboratory calibration for water content of the processed iron ore compares favourably to a field calibration. In the field study, the 28 m extension cable used to connect the probes to the time-domain reflectometry affected the end-point determination of the time-domain reflectometry system. To account for this, 0.197 should be subtracted from the reported apparent dielectric before calculation of volumetric moisture content

    Aquametry in Agrophysics

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    Soil water availability in agriculturally used wetlands of East Africa

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    Wetlands contribute increasingly to the food needs and income generation of the rural population in East Africa. The transformation of wetlands into farmland has gained momentum due to a growing food demand, upland shortages and increasing climate variability. A wetland's agricultural production potential is largely determined by its soil fertility as well as the water availability in the rooting zone of crops. In this Dissertation, I assessed the spatial and temporal soil water availability, and related moisture status to hydrogeomorphological conditions and types of land use. Based on an initial classification of wetlands in the region, the present work focussed on two representative agriculturally used wetlands. Tegu is an inland valley wetland of about 10 ha in the humid tropical highlands (1720 m a.s.l.) south of Mount Kenya, Kenya. Malinda is a floodplain located in the plains of Mkomazi River in the sub-humid tropical lowlands (360 m a.s.l.) at the western leeward side of the Usambara Mountains, Tanzania. While both sites are mainly used for crop production, unused portions remain under natural wetland vegetation. Hydrometeorological state variables, comprising Frequency Domain Reflectometry (FDR)-based soil moisture contents, groundwater levels and stream discharge, were monitored between 2009 and 2011. Thereby, different landscape positions and land use types in the wetlands were covered. A site-specific calibration procedure for the FDR system yielded a higher measurement accuracy for the Tegu site than that achieved using the manufacturer's calibration function. Groundwater level and the duration of soil saturation were closely related to the landscape position along and across the Tegu inland valley, thus determining the suitability of specific locations within the wetland for crop cultivation. In the bowl-shaped valley head with no pronounced stream channel, the cultivation of forage grass (Pennisetum purpureum) prevailed. In the floodplain-like lower section with an extensive network of drainage/irrigation channels, taro (Colocasia esculenta) was widespread and generally cultivated in association or rotation with upland crops, mainly maize (Zea mays) and vegetables. The Tegu valley bottom is agriculturally used although soil moisture contents are often too high, which bears the risk of crop failure due to flooding. The application of a semi-distributed rainfall-runoff model yielded an overestimation of the stream discharge during the long dry season of the hydrological year 2010/2011, indicating the impact of water abstraction for irrigation. In the Malinda floodplain, soil moisture measurements were conducted for a) natural vegetation, b) dry season grazing land, c) rainfed and irrigated rice plots, and d) upland crop fields. The shrink-swell-behaviour of the Vertisols impeded the application of the FDR sensor, as well as the determination of hydraulic soil properties. The length of the growing period as derived from HYDRUS-1D daily soil water balances ranged from 115 days in the fringe areas to 243 days in areas with prevailing shallow groundwater tables. Thus, agricultural land use and crop production potential, as determined from the period of crop-specific soil water storage, depend largely on the dominant water source (groundwater, surface water). The outcomes of the Dissertation can guide appropriate land use planning for a sustainable wetland use reconciling the increasing demand for agricultural products with the need to respect environmental concerns and to preserve ecosystem services.Verfügbarkeit von Bodenwasser in landwirtschaftlich genutzten Feuchtgebieten Ostafrikas Feuchtgebiete leisten einen zunehmend wichtigen Beitrag zur Ernährungssicherung und der Erwirtschaftung von Einkommen für die ländliche Bevölkerung in Ostafrika. Die Umwandlung von Feuchtgebieten in Ackerland wird durch den wachsenden Bedarf an Nahrungsmitteln, die Verknappung an Flächen für den Trockenfeldbau und eine zunehmende Veränderlichkeit des Klimas beschleunigt. Neben der Bodenfruchtbarkeit bestimmt dabei die Wasserverfügbarkeit in der Wurzelzone von Ackerfrüchten das landwirtschaftliche Produktionspotential von Feuchtgebieten. In der vorliegenden Promotionsarbeit wurde die räumliche und zeitliche Verfügbarkeit von Bodenwasser untersucht und in Beziehung zu hydrogeomorphologischen Bedingungen und Landnutzungstypen gesetzt. Die Arbeit wurde in zwei repräsentativen landwirtschaftlich genutzten Feuchtgebieten durchgeführt, deren Auswahl auf einer umfassenden Kategorisierung von Feuchtgebieten in der Region basierte. Tegu ist ein Inlandtal im tropisch-feuchten Hochland (1720 m NN) südlich des Mount Kenya in Kenia mit einer Fläche von etwa 10 ha. Die Malinda Überschwemmungsfläche befindet sich in den Ebenen des Mkomazi Flusses im tropischen sub-humiden Tiefland (360 m NN) an der westlichen, leewärtigen Seite der Usambara Berge in Tansania. Beide Standorte werden überwiegend ackerbaulich genutzt, wobei auch noch weitgehend ungestörte Restflächen mit natürlicher Feuchtgebietsvegetation existieren. Hydrometeorologische Zustandsgrößen, einschließlich FDR (Frequency Domain Reflectometry)-basierter Bodenwassergehalte, Grundwasserflurabstände und Durchfluss, wurden zwischen 2009 und 2011 gemessen. Dabei wurden verschiedene Reliefpositionen und Landnutzungstypen innerhalb der Feuchtgebiete unterschieden. Eine standortspezifische Kalibrierung des FDR-Sensors lieferte eine höhere Messgenauigkeit für das Inlandtal als die Kalibrierungsfunktion des Herstellers. Grundwasserflurabstand und die Dauer der Bodensättigung standen in Beziehung zu der Reliefposition im Längs- und Querprofil des Inlandtales und bestimmten damit die Standorteignung für ackerbauliche Nutzung. In dem kesselförmigen Talkopf ohne ausgeprägtes Flussbett herrschte der Anbau von Futtergras (Pennisetum purpureum) vor. Der untere Talabschnitt, der einer Überschwemmungsebene ähnelt, war durch ein ausgeprägtes Netz an Be- und Entwässerungskanälen gekennzeichnet und wurde vorwiegend für den Anbau von Taro (Colocasia esculenta), üblicherweise im Mischanbau oder in einer Fruchtfolge mit Trockenlandkulturen (Zea mays) sowie diversen Gemüsearten genutzt. Für diese Art der Landnutzung ist der Boden allerdings verbreitet zu feucht, was das Risiko von Überstau-bedingten Ernteausfällen mit sich bringt. Die Anwendung eines semi-distributiven Niederschlags-Abfluss Modells ergab eine Überschätzung des Abflusses während der langen Trockenzeit des hydrologischen Jahres 2010/2011, was auf den Einfluss der Entnahme von Bewässerungswasser zurückzuführen war. In der Malinda Überschwemmungsebene wurden Bodenfeuchtemessungen an Standorten mit a) natürlicher Vegetation, b) Beweidung während der Trockenzeit, c) Nassreis im Regen-abhängigen sowie im Bewässerungsanbau, und d) dem Anbau von Trockenlandkulturen durchgeführt. Das Schwellungs-Schrumpfungs-Verhalten der Vertisole erschwerte den Einsatz des FDR-Sensors ebenso wie die Ermittlung bodenhydraulischer Kenngrößen. Die Länge der Wachstumsperiode wurde basierend auf Ergebnissen der täglichen Bodenwasserhaushaltsbilanzierung in HYDRUS-1D bestimmt und lag zwischen 115 Tagen im Randbereich der Ebene und 243 Tagen in Gebieten mit geringen Grundwasserflurabständen. Das landwirtschaftliche Nutzungspotential, welches über die verfügbare Dauer Kulturabhängiger Bodenwasservorräte ermittelt wurde, hing in hohem Maße von der Art der Wasserquelle ab (Grund-/Oberflächenwasser). Die Ergebnisse der Dissertation können bei der Planung einer nachhaltigen Feuchtgebietsnutzung herangezogen werden, welche sowohl dem zunehmenden Bedarf an landwirtschaftlichen Produkten wie auch dem Naturschutz und dem Erhalt ökosystemarer Dienstleistungen Rechnung trägt
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