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Coupled water, vapor, and heat movement in the vadose zone is a central process in many agricultural and engineering issues (Saito et al., 2006). Especially, in cold and arid regions, snowmelt or lateral water movement on frozen soil layers have a non-negligible influence on seasonal water balances. Frozen soil normally reduces infiltration capacity dramatically due to the blocking effects of ice lenses in soil pores. Consequently, lateral flow and snow melt may release huge quantities of water in spring and early summer, and cause substantial surface runoff (Lewkowicz and Kokelj, 2002; Bayard, et al., 2005). Furthermore, this will increase the risk for soil erosion and nutrient loss. However, although the former processes are recognized widely, the simulations of snow hydrology and soil freezing and thawing are rarely done due to limited data availability to parameterize or validate such models and lack of suitable models that describe the complex processes during phase changes. Generally, frozen soils are characterized by the coexistence of ice and water. Thus it is necessary to simulate the freezing and thawing process by specifically considering the phase change of ice and water at variable freezing points. Over the last few decades, two prevalent classes of soil freezing and thawing models, namely hydrodynamic models (e.g., Harlan, 1973; Flerchinger and Saxton, 1989) and frost heave models (e.g., Miller, 1980) have been developed. They mainly differed in the treatment of ice pressure, which is assumed to remain constant in hydrodynamic models and variable in the frost heave models (Hansson et al., 2004). As a result, the former offers accurate predictions under saturated conditions but limits to frost heave, which is reverse for the latter. Regarding the model’s availability in the unsaturated zone, it is customary to use the hydrodynamic model in soil science. Generally, water and energy balances are linked at the soil-atmosphere interface and controlled by climatic conditions and soil properties. In winter, snow cover results in low thermal conductivity and high albedo of soil surface, which greatly impacts soil thermal regime and microclimate. Within the soil, thermal gradient induces moisture transfer which, in turn, affects heat flow. Furthermore, soil heat and water regimes may be modified by different land uses 116
Chapter 6 Modeling of Coupled Water and Heat Transfer in Freezing and Thawing Soil due to differences in vegetation cover and soil conditions. Both result in altered heat and water transfer rates, which are further complicated by snow melt and soil freezing and thawing behaviors. Gerasimova et al. (1996) found that soil frost might create platy aggregates in loamy soils due to soil freezing and thawing processes. Further, a platy soil structure can also be generated by animal trampling during grazing in winter (Krümmelbein et al., 2006). Thus, lateral water movement on the upper frozen layer is expected to be high during soil thawing, as a frozen and platy-structured underlying soil horizon prevents infiltration (Lewkowicz and Kokelj, 2002). These established facts emphasize the need to better understand snowmelt and freezing and thawing processes. Currently, there are several numerical codes, e.g., SVAT (Jansson and Halldin, 1980), SHAW (Flerchinger and Saxton, 1989) and HYDRUS (Šimůnek et al., 1998), developed for simulating coupled water and heat flow in frozen soils. However, until now, the mutual interactions of water and heat flows in the frozen soil are limited in laboratory observation and theoretical analysis, and rarely considered in field applications. The present study addresses the field application of the hydrodynamic model HYDRUS-1D. In this new program, an extended freezing code is incorporated, which numerically solves coupled equations governing phase changes between water and ice and heat transport with a mass- and energy-conservative method (Hansson et al., 2004). Specifically, we will focus on following questions: (i) How well does HYDRUS-1D simulate soil water and temperature with and without “frozen soil module”? 2) How does the frozen soil module affect soil temperature, soil moisture and runoff simulations? 3) What is model sensitivity to reflect the role of land uses in soil water and heat fluxes? MATERIALS AND METHODS Experimental Site and Measurements Fieldwork was conducted at the grassland catchment of Xilin River (43 o 37′N, 116 o 42′E). Detailed descriptions of the experimental area were given in Zhao et al. (2008). The vegetation is a perennial rhizome grass, Leymus chinensis and Stipa gradis steppe. The mean annual precipitation is 343 mm. The annual mean 117
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SCHRIFTENREIHE Institut für Pflanz
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Aus dem Institut für Pflanzenernä
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I Table of contents I Table of cont
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controlling soil moisture are of pa
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Zusammenfassung Überweidung wird a
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Die Beweidung beieinflusst besonder
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II List of figures Fig. 2.1. Locati
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measured soil moisture in time stab
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III List of tables Table 2.1. Descr
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1.1 Background Chapter 1 Introducti
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Chapter 1 Introduction is also unce
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Chapter 1 Introduction water in spr
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Chapter 1 Introduction requires the
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Chapter 1 Introduction Krümmelbein
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Chapter 2 Spatial variability of so
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Chapter 3 Spatio-temporal variabili
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Chapter 3 Factors controlling spati
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Page 93 and 94: MRD(%) MRD(%) 100 80 60 40 20 0 -20
Page 101 and 102: Soil water content (%) 40 30 20 10
Page 105 and 106: Chapter 5 Modeling Grazing Effects
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Page 173 and 174: Curriculum Vitae M.Sc. Ying Zhao Of
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