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  • 1
    Language: English
    In: Soil Science Society of America journal, 2011, Vol.75(2), pp.418-425
    Description: Although the multistep outflow (MSO) method is well suited for the estimation of soil hydraulic properties by inverse solution techniques, this method has not been widely adopted because it requires advanced instrumentation and is time consuming. The objective of this study was to develop a modified version of the multistep outflow technique that largely simplifies laboratory procedures and reduces costs and time. The numerical inversion procedures require applying user-friendly HYDRUS software to estimate fitting parameters for soil water retention and unsaturated hydraulic conductivity curves. Whereas values of saturated water content and saturated hydraulic conductivity must be measured independently, the remaining functional parameters are estimated using an inverse solution of a transient drainage experiment using multiple suction steps and a hanging water column, with drainage outflows measured during drainage. A comparison test showed that the simplified experiment without tensiometric measurements provided sufficient information in the parameter identification compared with a traditional pressure outflow experiment with tensiometric measurements for an Oso Flaco sand and a loamy sand field soil in the suction range of 0 to 17 kPa. ; p. 418-425.
    Keywords: Saturated Hydraulic Conductivity ; Sand ; Computer Software ; Coarse-Textured Soils ; Soil Water Retention ; Tensiometers ; Drainage ; Unsaturated Hydraulic Conductivity ; Water Content ; Instrumentation
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 2
    Language: English
    In: Journal of Hydrology, November 2015, Vol.530, pp.580-590
    Description: To minimize the number of soil water content ( ) measurements for estimation of field- or watershed-scale soil water storage, we present an analysis of time-stable soil water data across both measurement locations and soil depth intervals. The proposed analysis applies the time stability concept to select area-representative measurement locations, and assesses the potential for identifying the most time-stable depth interval (MTSD) using a minimal number of selected time-stable locations (MTSLs). For that purpose, we used a time series of 21 datasets, measured at 20 locations and 20 corresponding depth intervals down a 3-m soil profile, during a two-year period in the 38-ha study area of the Liudaogou watershed of the China Loess Plateau. After identifying the MTSLs, analysis of time stability of measurement depth intervals showed single soil water depth measurements at between 2 and 5 of the MTSLs were sufficient to determine the area-representative . The MTSD was determined to be about mid-way in the soil profile, irrespective of total soil profile depth measured. Confirmation of the time-stability analyses was done by comparing the representative estimations for the 38-ha sampling area with additional measurements across the 6.9 km watershed. The encouraging results of our analysis suggest that time stability analysis may be an effective way to assess large-scale soil water storage in arid and semi-arid regions.
    Keywords: Root-Zone Soil Water ; Time Stability ; Sampling Schemes ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 3
    Language: English
    In: Journal of Hydrology, 2015, Vol.530, p.580(11)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.jhydrol.2015.10.016 Byline: Dongli She, Wenjuan Zhang, Jan W. Hopmans, Luis Carlos Timm Abstract: * Identifies most time stable location (MTSL) and depth (MTSD) for study area soil water content (SWC). * Just one time-stable location sufficed to estimate the mean SWC of the study area. * Additional MTSLs were required to estimate the area's mean SWC based on the MTSD. * Identified MTSL's and MTSD's could be upscaled for SWC estimates at the watershed scale. Article History: Received 3 June 2015; Revised 27 August 2015; Accepted 5 October 2015 Article Note: (miscellaneous) This manuscript was handled by Corrado Corradini, Editor-in-Chief, with the assistance of Renato Morbidelli, Associate Editor
    Keywords: Soil Moisture
    ISSN: 0022-1694
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States, Dec 14, 2010, Vol.107(50), p.21263(8)
    Description: Increasing human appropriation of freshwater resources presents a tangible limit to the sustainability of cities, agriculture, and ecosystems in the western United States. Marc Reisner tackles this theme in his 1986 classic Cadillac Desert: The American West and Its Disappearing Water. Reisner's analysis paints a portrait of region-wide hydrologic dysfunction in the western United States, suggesting that the storage capacity of reservoirs will be impaired by sediment infilling, croplands will be rendered infertile by salt, and water scarcity will pit growing desert cities against agribusiness in the face of dwindling water resources. Here we evaluate these claims using the best available data and scientific tools. Our analysis provides strong scientific support for many of Reisner's claims, except the notion that reservoir storage is imminently threatened by sediment. More broadly, we estimate that the equivalent of nearly 76% of streamflow in the Cadillac Desert region is currently appropriated by humans, and this figure could rise to nearly 86% under a doubling of the region's population. Thus, Reisner's incisive journalism led him to the same conclusions as those rendered by copious data, modern scientific tools, and the application of a more genuine scientific method. We close with a prospectus for reclaiming freshwater sustainability in the Cadillac Desert, including a suite of recommendations for reducing region- wide human appropriation of streamflow to a target level of 60%. doi/ 10.1073/pnas.1009734108
    Keywords: Sustainable Development -- Analysis ; Sustainable Development -- Research ; Streamflow -- Analysis ; Streamflow -- Research ; Automobile Industry -- Analysis ; Automobile Industry -- Research ; Water Resources -- Analysis ; Water Resources -- Research
    ISSN: 0027-8424
    Source: Cengage Learning, Inc.
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  • 5
    Language: English
    In: Soil Science Society of America Journal, 2015, Vol.79(4), p.1059(14)
    Description: The dual-probe heat-pulse (DPHP) method is attractive for measuring soil thermal properties and volumetric water content. The purpose of this study was to develop and test a DPHP sensor having rigid probes made from thick-walled stainless steel tubing (2.38-mm outside diameter). The probes of this sensor are much more resistant to deflection than those of conventional DPHP sensors, decreasing measurement error caused by probe deflection during insertion into the soil. Laboratory experiments were conducted across a wide range of saturation levels with glass beads and three soils of different textures. For inferring soil properties from the proposed sensor, we applied the recently developed identical cylindrical perfect conductors (ICPC) model instead of the infinite line source (ILS) model that is typically used. The ICPC model improves solution for heat transport through the probeusoil system by accounting for the heat capacity and radius of the probes. Our results show a root mean square error of 1.4% volumetric water content and elimination of the measurement bias typically encountered with DPHP measurements. We conclude that the improved sensor, in combination with the ICPC model, provides a general, soil-independent water content estimate that is especially suitable for field soil water content monitoring because of its robust design with rigid probes. Because of its simplicity and measurements independent of soil type, we propose the presented DPHP method as an excellent alternative to other available measurement techniques for soil water content.
    Keywords: Soils – Research ; Soil Moisture – Research ; Volumetric Instruments – Usage
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 6
    Language: English
    In: Agriculture, Ecosystems and Environment, 01 February 2017, Vol.238, pp.67-77
    Description: Little work has been done on nitrous oxide (N O) emissions from nitrogen-fertigation systems, despite the greater degrees of control allowed over fertilizer N fate. More fertigation users are adopting high-frequency (HF) N application schedules to improve nitrogen use efficiency and reduce nitrate (NO ) leaching. The possibility is raised that this practice may also lower N O emissions, through effects on soil N concentrations and soil microbial populations. At the same time, NO -based N-fertilizers are frequently selected for use in HF systems, where they should be more immediately available to plants than ammoniacal fertilizers. This choice of N-source is likely to affect N O emissions. We monitored surface emissions of N O in an almond orchard ( [Mill.] DA Webb) in Belridge, California. Fertigation treatments were 4x year (Standard) urea ammonium nitrate (UAN), 20x year (HF) UAN and 20x year (HF) CaNO + KNO . Estimated surface emissions were HF UAN 〉 Standard UAN 〉 HF NO3, with differences only significant between the HF treatments, where HF UAN emitted 2.0 times the N O seen from HF NO3. Net production was also monitored by depth in the soil using sampling tubes and Fickian diffusion calculations. UAN typically had highest N O production at 10–15 cm depth, while N O was generally reduced to N below 20 cm in all treatments. Differences were seen in the distribution of NH and NO on the soil exchange complex and in soil solution, with data from 60 cm suggesting that leaching hazards could be greater from Standard UAN than from HF NO3. Multiple linear regression of N O production with predictors had the best fit at 15 cm, where extractable NH , WFPS and temperature together accounted for an adjusted R of 0.68. Persistent soil microbial changes were seen in denitrification capacity, with HF UAN = HF NO3 〉 Standard UAN, while 3% O assays suggested high contributions of N O from nitrifier denitrification in the fertigation context. High-frequency fertigation with ammoniacal fertilizers did not mitigate N O emissions, but nitrate-based fertilizers did, suggesting that N sources for fertigation warrant careful study.
    Keywords: Agriculture ; Environmental Sciences
    ISSN: 0167-8809
    E-ISSN: 1873-2305
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  • 7
    Language: English
    In: Agriculture, Ecosystems and Environment, 2010, Vol.138(3), pp.355-355
    Keywords: Agriculture ; Environmental Sciences
    ISSN: 0167-8809
    E-ISSN: 1873-2305
    Source: ScienceDirect Journals (Elsevier)
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  • 8
    In: New Phytologist, November 2011, Vol.192(3), pp.653-663
    Description: • Despite the importance of rhizosphere properties for water flow from soil to roots, there is limited quantitative information on the distribution of water in the rhizosphere of plants. • Here, we used neutron tomography to quantify and visualize the water content in the rhizosphere of the plant species chickpea (Cicer arietinum), white lupin (Lupinus albus), and maize (Zea mays) 12 d after planting. • We clearly observed increasing soil water contents (θ) towards the root surface for all three plant species, as opposed to the usual assumption of decreasing water content. This was true for tap roots and lateral roots of both upper and lower parts of the root system. Furthermore, water gradients around the lower part of the roots were smaller and extended further into bulk soil compared with the upper part, where the gradients in water content were steeper. • Incorporating the hydraulic conductivity and water retention parameters of the rhizosphere into our model, we could simulate the gradual changes of θ towards the root surface, in agreement with the observations. The modelling result suggests that roots in their rhizosphere may modify the hydraulic properties of soil in a way that improves uptake under dry conditions.
    Keywords: Extent Of Rhizosphere ; Modelling ; Neutron Tomography ; Rhizosphere Hydraulic Properties ; Root Water Uptake ; Soil Moisture Profile ; Water Distribution
    ISSN: 0028-646X
    E-ISSN: 1469-8137
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  • 9
    In: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 12/2011, Vol.4(4), pp.748-753
    Description: We are interested in Ground Penetrating Radar (GPR) as a geophysical tool useful for determining the depth of the ground water table (GWT) and for monitoring shallow water infiltration in sandy soils. At hydrostatic equilibrium, the water content distribution in a homogeneous unsaturated soil down to the water saturated zone depends on the soil water retention function. A classical way to fit retention curve data is to use the van Genuchten continuous model. Using Finite Difference Time Domain simulations, we study the sensitivity of the GPR signal reflected by a van Genuchten type transition to the hydraulic parameters. We show a power type relationship between the reflected signal amplitude and the slope of the soil retention curve. Furthermore, for simulating GPR reflection data acquired above a transition from unsaturated to saturated soil, geophysicists often approximate the soil water retention curve by a piece-wise linear model. We test the validity of such an approximation depending on the frequency of the radar signal and the abruptness of the retention curve. We illustrate our results with high resolution GPR data (1600 MHz) acquired above a fluctuating water table in a sand column at the laboratory scale.
    Keywords: Geology;
    ISSN: 1939-1404
    E-ISSN: 21511535
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  • 10
    Language: English
    In: Ecological Modelling, 2009, Vol.220(4), pp.505-521
    Description: Plant root water and nutrient uptake is one of the most important processes in subsurface unsaturated flow and transport modeling, as root uptake controls actual plant evapotranspiration, water recharge and nutrient leaching to the groundwater, and exerts a major influence on predictions of global climate models. In general, unsaturated models describe root uptake relatively simple. For example, root water uptake is mostly uncompensated and nutrient uptake is simulated assuming that all uptake is passive, through the water uptake pathway only. We present a new compensated root water and nutrient uptake model, implemented in HYDRUS. The so-called root adaptability factor represents a threshold value above which reduced root water or nutrient uptake in water- or nutrient-stressed parts of the root zone is fully compensated for by increased uptake in other soil regions that are less stressed. Using a critical value of the water stress index, water uptake compensation is proportional to the water stress response function. Total root nutrient uptake is determined from the total of active and passive nutrient uptake. The partitioning between passive and active uptake is controlled by the a priori defined concentration value . Passive nutrient uptake is simulated by multiplying root water uptake with the dissolved nutrient concentration, for soil solution concentration values below . Passive nutrient uptake is thus zero when is equal to zero. As the active nutrient uptake is obtained from the difference between plant nutrient demand and passive nutrient uptake (using Michaelis–Menten kinetics), the presented model thus implies that reduced passive nutrient uptake is compensated for by active nutrient uptake. In addition, the proposed root uptake model includes compensation for active nutrient uptake, in a similar way as used for root water uptake. The proposed root water and nutrient uptake model is demonstrated by several hypothetical examples, for plants supplied by water due to capillary rise from groundwater and surface drip irrigation.
    Keywords: Root Water Uptake ; Nutrient Uptake ; Compensated Uptake ; Unsaturated Water Flow ; Numerical Model ; Hydrus ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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