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Simulations of water, heat, and solute transport in partially frozen soils
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Land and Water Resources Engineering. (Soil physics)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Experiments for soil freezing/thawing were conducted in two seasonally frozen agricultural fields in northern China during 2011/2012 and 2012/2013 wintertime, respectively. Mass balance was checked based on measured data at various depths. Simulation work was conducted by combining CoupModel with Monte-Carlo sampling method to achieve parameter sets with equally good performance. Uncertainties existed in both measurements and model due to complexity in freezing/thawing processes as well as in surface energy partitioning. Parameters related to surface radiation and soil frost were strongly constrained with datasets available in two sites combining multi-criterion on outputs. Simulated soil heat process were better described than soil water processes given the data obtained for calibration. Model performance was improved with consideration of solute effects on freezing point depression. More detailed solute transport processes in CoupModel needed to be improved by taking more processes such as diffusion and expulsion into consideration based on more precise experimental results, to reduce uncertainty in model. Generally, combination of measurement with process-based model and Monte-Carlo sampling method provided an approach for understanding of solute transport as well as its influences on soil freezing/thawing in cold arid agricultural regions. Incorporating more detailed descriptions of processes for frozen soil in the model can be justified if uncertainties in measurements can be reduced by introducing of high-precision novel technologies.

Keyword [en]
Frozen soil; Solute; Uncertainty; Freezing point; Salinization
National Category
Environmental Sciences related to Agriculture and Land-use
Identifiers
URN: urn:nbn:se:kth:diva-184103OAI: oai:DiVA.org:kth-184103DiVA: diva2:914372
Note

QC 20160404

Available from: 2016-03-23 Created: 2016-03-23 Last updated: 2016-04-04Bibliographically approved
In thesis
1. Coupled processes in seasonally frozen soils: Merging experiments and simulations
Open this publication in new window or tab >>Coupled processes in seasonally frozen soils: Merging experiments and simulations
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Soil freezing/thawing is of importance in the transport of water, heat and solute, with coupled effects. Due to complexity in soil freezing/thawing, uncertainty could be influential in both experimentation and simulation work in frozen soils. Solute and water in frozen soil could reduce the freezing point, resulting in uncertainty in simulation water, heat and solute processes as well as in estimation of frozen soil evaporation. High salinity and groundwater level could result in high soil evaporation during wintertime. Seasonal courses in energy and water balance on surface have shown to be influential to soil water and heat dynamics, as well as in salt accumulation during wintertime. Water and solute accumulated during freezing period resulted in high evaporation during thawing period and enhanced surface salinization. Diurnal changes in surface energy partitioning resulted in significant cycle of freezing/thawing as well as in evaporation/condensation in surface layer, which could in turn affect atmosphere. Uncertainties in experiments and simulations were detectable in investigation of seasonally frozen soils with limited methods and simplified representations of reality in two agricultural fields in northern China. Soil water and solute contents have shown to be more uncertain than soil temperatures in both measurements and simulations. The combination of experiments with process-based model (CoupModel) has proven to be useful in understanding freezing/thawing processes and in identification of uncertainty, when Monte-Carlo based methods were used for evaluation of simulations. Correlations between parameters and model performance indices needed to be taken into account carefully in calibration of the process-based model. Parameters related to soil hydraulic processes and surface energy processes were more sensitive when using different datasets for calibration. In using multiple model performance indicators for multi-objective evaluation, the trade-offs between them have shown to be a source of uncertainty in calibration. More proper representations of the reality in model (e.g., soil hydraulic and thermal properties) and more detailed measurements (e.g., soil liquid water content and solute concentration) as input would be efficient in reducing uncertainty. Relationships between groundwater, soil and climate change would be of high interest for better understanding of cold regions water and energy balance.

Abstract [zh]

土壤冻融过程对于水热及溶质的运移具有十分重要的影响,并对于寒旱区水文过程的研究有着深远意义。在冻土中,溶质的存在导致冰点降低,改变土壤冻融规律,进而影响冻融土壤水热运移。冻融土壤地表水分及能量平衡的季节性变化规律对土壤水热运移及盐分的积累影响较大。同时,土壤冻融的水热平衡日变化规律对表层土壤水热过程影响较为强烈,并进而影响地表的气象变化。试验研究表明,高溶质含量及浅埋深地下水条件为地表的蒸发提供了便利条件,因为高溶质含量土壤冰点降低,同一负温条件下的液态含水量增大,为蒸发提供了可利用水分;而浅埋深地下水对冻融期水盐的表聚提供的方便,进而有助于融化期地表水分的大量蒸发及下层土层水分的大量向上补给。例如,当地下水初始埋深设置在1.5 m时,对于初始含盐量分别为0.2%,0.4% 和0.6% g/g的冻融试验组,冻融期累积蒸发量分别为51.0,96.6和114.0 mm。同样的增加趋势在其它初始地下水埋深设置试验组里也被验证,且初始地下水埋深越浅,累积蒸发量也越大。对于利用有限的试验数据及简化的数值模型对冻融土壤水热及溶质研究,由于试验及模型的不确定性,会造成结果的不确定性。而通过利用不确定性分析的方法将试验结果与数值模型结合起来可以较好地理解土壤冻融过程及处理不确定性,并进而为改进试验方法及完善数值模型提供参考。模型不确定分析结果表明,模型参数之间,及参数与模型模拟效果评价因子之间存在较强的相关性,会造成模拟结果的不确定性。而不同模拟方法的结果对比表明,在进行冻融土壤水热及溶质模拟时,建立更为完善的考虑更为详细的过程的数值模型可以提高模型的模拟效率,减小模拟结果的不确定性。同时,试验数据的不确定性也显示出了对模拟结果的显著影响。精确的试验数据及更为科学的试验方法有助于减少模拟结果的不确定性。在减少模拟结果的不确定性上也有重要作用。同时,由于寒区水文过程的复杂性及在气候变化过程中的重要性,有必要进一步开展寒区地下水,土壤水热盐与气候变化关系的研究,以便于制定更为合理的寒区水资源管理策略。

Abstract [sv]

Frysning och tining är av betydelse för kopplade flöden av vatten, värme och lösta ämnen i mark. Komplexiteten i sambanden mellan lösta ämnen, ofruset vatten och fryspunkten skapar en osäkerhet vid simuleringar av processer för både vatten, värme och lösta ämnen i marken samt för avdunstningen från markytan. Årtidsberoende mönster i energi- och vattenbalansen för markytan påverkar värme- och vattendynamiken i marken samt ackumulering av salter under vintern. Dygnsvariationer i energibalansens uppdelning vid markytan ger upphov till frysning/tining samt avdunstning och kondensation i ytliga lager som har återkopplingar också till tillstånden i atmosfärens ytskikt. Osäkerheter i både experiment och i simuleringar spårades i undersökningar av säsongstyrd frysning av mark i två provincer av norra Kina. Begräningar i metodik och förenklingar av naturens komplexitet kunde klargöras. Kombinationen av experiment och processbaserad modellering med CoupModel var lyckosam för föreståelsen av frysning/tining och kunde klargöra osäkerhet med hjälp av Monte Carlo teknik. Korrelation mellan parametrar och prestanda hos modellen var en viktig del av kalibreringsproceduren. En förbättrad processbeskrivning av marken och minskad parameterosäkerhet kan erhållas om också mer detaljerade mätningar inbegrips i framtida studier. Sambanden mellan grundvatten, mark och klimatförändringar är av största intresse för en bättre kunna beskriva kalla regioners vatten- och energibalanser.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. xiv, 44 p.
Series
TRITA-LWR. PHD, ISSN 1650-8602 ; 2016:03
Keyword
Frozen soil, Coupled transport, Energy balance, Uncertainties, Merged study, 冻土;水热盐运移;耦合关系;水热平衡;不确定性
National Category
Environmental Sciences related to Agriculture and Land-use
Research subject
Land and Water Resources Engineering
Identifiers
urn:nbn:se:kth:diva-184104 (URN)978-91-7595-918-4 (ISBN)
Public defence
2016-04-28, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160329

Available from: 2016-04-04 Created: 2016-03-23 Last updated: 2016-04-04Bibliographically approved

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