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Abbreviations: ET, evapotranspiration; Tp, transpiration; E, evaporation; Ks, saturated hydraulic conductivity; RMSE, root mean square error; AWHC, available water holding capacity. Movement of soil water and heat plays a key role in the whole water and energy balance in many agricultural issues, especially in arid or semi-arid regions. Since the 1970s, many numerical solutions for the water and heat flow equations were increasingly used (Feddes et al., 1988; de Jong and Bootsma, 1996; Saito et al., 2006). A vast variety of soil water models exist ranging from simple water balance calculations to complex mechanistic models based on Richards’ equation. Richards’ equation, which usually describes unsaturated water flow, requires the two basic hydraulic functions: water retention curve θ(h) and hydraulic conductivity K(h) (Butters and Duchateau, 2002). This approach is only valid for rigid and non-interacting pore walls if laminar flow is assumed, thus detailed and in situ measurements are normally necessary for accurate simulation considering the spatial variability of soil properties. 86 Land management is important to heat and water transfer by altering plant or residue architecture and soil functions (Flerchinger et al., 2003). It is essential to quantify and predict management effects on soil properties in order to assess their consequences on plant production and the environment. Many studies have addressed the effects of land management (tillage, wheel-traffic compaction, grazing, and crop residue management) on soil properties (Green et al., 2003). However, few studies have dealt with the consequences of these practices on water and heat fluxes (Chung and Horton, 1987; Ndiaye et al., 2007). Especially, considering the stress-dependent changes of the environment, an adequate account of water flow processes based on hydrological models is lacking due to the complexity of the soil system (Peth and Horn, 2006; Ndiaye et al., 2007), although there are widely applicable hydrological models like SHAW (Flerchinger and Saxton, 1989), HYDRUS (Šimůnek et al., 1998) and SWAT (Neitsch et al., 2002). In the steppe of Inner Mongolia, grazing is a widely used land management
Chapter 5 Modeling Grazing Effects on Coupled Water and Heat Fluxes in Inner Mongolia Grassland system. In recent years, this steppe has been reported to be severely deteriorated because of heavy grazing (Chen and Wang, 2000). Heavy grazing accompanied with animal trampling normally has detrimental effects on soil properties (Greenwood and McKenzie, 2001; Zhao et al., 2007). Particularly in the topsoil, soil deformation is characterized by a decrease of pore volume and altered pore size distribution, which both affect water and air conductivities (Willat and Pullar, 1983; Krümmelbein et al., 2006), and soil water retention characteristics (Martinez and Zinck, 2004; Kutilek et al., 2006). The disturbance of soil structure also leads to lower water infiltrability and thus increases the risks for soil erosion and nutrient depletion. Therefore, these processes may cause a low water storage capacity and the loss of soil fertility, consequently decrease the grassland productivity (Christensen et al., 2004). Under the prevailing semi-arid climatic condition in Inner Mongolia, plant available water plays a key role for the sustainable development of steppe ecosystems. Therefore, it is necessary to <strong>und</strong>erstand the effects of grazing on water-related mechanisms and water budgets. Some studies have shown that grazing increases evaporation and decreases transpiration caused by an increase in bare gro<strong>und</strong> area and a decrease in biomass production (Bremer et al., 2001; Chen et al., 2007). However, to which extent grazing affects evapotranspiration and its partitioning into transpiration and evaporation is still unclear (Leenhardt et al., 1995). Another deficiency is that interception by the plant canopy and residue is normally ignored despite its potentially large contribution to the water budget. Moreover, it is widely recognized that the movement of water and heat is closely coupled, but their mutual interactions are rarely considered in practical applications (Saito et al., 2006). For instance, the effect of heat transport on water flow is often neglected mainly because of a lack of comprehensive data to fully parameterize coupled water and heat flow models. In this paper, a comprehensive dataset including soil, plant and meteorological measurements from the project “MAGIM” (Matter fluxes in grasslands of Inner Mongolia as influenced by stocking rate; 87
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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 6 Modeling of Coupled Water
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Chapter 7 General discussion and co
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References Chapter 7 General discus
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8. ACKNOWLEDGMENTS Firstly I would
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Curriculum Vitae M.Sc. Ying Zhao Of
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Schriftenreihe des Instituts Neuere
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