Impact of various surface covers on water and thermal regime of Technosol
Introduction
Soil profiles and the ground beneath these profiles have recently been recognized as a popular source of energy for use in building heating (Adamovský et al., 2009, Brandl, 2006, Demir et al., 2009a, Demir et al., 2009b, Gonzalez et al., 2012, Koyun et al., 2009, Neuberger et al., 2014). Effective energy gathering using horizontal ground heat exchangers is controlled by the heat exchanger design and the heat regime around exchangers. Heat transport within the soil profile (i.e. heat accumulation during the warm season and heat discharge during the cold season) is coupled with water flow. Both transport processes are controlled by climatic conditions, soil properties, and character of surface cover. The effectiveness of heat discharge from the soil using heat exchangers may be assessed experimentally and mathematically. Previous studies mostly concentrated on the heat exchanger design (Brandl, 2006, Demir et al., 2009a, Fontaine et al., 2011, Koyun et al., 2009, Neuberger et al., 2014, Tarnawski et al., 2009). Few studies dealt with the impact of soil properties; e.g. porosity (Demir et al., 2009b), soil water content (Leong et al., 1998). Rezaei et al. (2012) studied the impact of insulating layer of recycled tires on the soil surface and they found that heat flux discharge from the soil during winter increased by 17%.
Horizontal ground heat exchangers are usually used in urban areas. Soil types, which were originally developed within the urban areas, are nowadays either heavily impacted by man or eventually transformed into Anthroposols (due to extensive soil cultivation) or Technosols (due to soil material redeposition and mixing with construction materials) (IUSS Working Group WRB, 2006, Rossiter, 2007). Common features of Technosols are a large heterogeneity due to mixing different soil materials, the presence of technogenic substances (like bricks, concrete, stones, steel, cables, piping, plastic and insulation materials), the way of these materials deposition and degree of consolidation, which makes it difficult to evaluate soil physical and chemical properties. There are some studies which attempt to evaluate the impact of particular substances on soil properties, e.g. influence of bricks on nutrient storage (Nehls et al., 2013), mobilization and transport of compounds (sulfate) from plaster and gypsum (Schonsky et al., 2013), weathering of technogenic substances (Howard and Olszewska, 2010), early pedogenic processes (Huotm et al., 2013, Séré et al., 2010), evolution of soil structure (Séré et al., 2012), or improvement of urban soils using various compost amendments (Cannavo et al., 2014, Chalhoub et al., 2013, Séré et al., 2008).
A frequent problem of urban soils is soil sealing (Scalenghe and Marsan, 2009, Wessolek, 2008), which increases runoff, decreases soil–water storage, and influences water quality, soil temperature regime, gas diffusion, biota, etc. On the other hand, in agricultural fields mulch can be applied to improve the soil water regime (i.e. to reduce water evaporation from the soil surface, etc.). Different materials can be employed to cover the soil surface, including gravel–sand (Wang et al., 2011, Xie et al., 2006, Xie et al., 2010), wheat straw (Dahiya et al., 2007), maize straw, biodegradable film or liquid film (Li et al., 2013), plastic film (Li et al., 2013, Wang et al., 2011; Wu et al., 2007; Yang et al., 2012), etc. Different covers also variably influence soil temperature, e.g. decreased temperatures and their oscillations due to mulching using organic material during the warm periods (Dahiya et al., 2007, Li et al., 2013, Wang et al., 2011) and due to sand and gravel mulching (Wang et al., 2011), negative correlation between soil temperatures and sand-gravel particle sizes (Xie et al., 2010), surface temperature increase when using plastic mulch (Wang et al., 2011, Yang et al., 2012, Xie et al., 2010), etc. Some of these materials are also used within urban areas, and they may be used to modify water and thermal regime around heat exchangers.
There is no study, which would evaluate different soil cover (which may appear above the heat exchanger) influence on water and thermal regime within the soil profile (particularly at the standard heat exchanger depths). Therefore the goal of this study was to evaluate the impact of different materials covering the soil surface, which can be used to optimize thermal conditions for effective soil heat utilization, on the water and thermal regime in the soil. The assessment was done mathematically using the program HYDRUS-1D. The programs HYDRUS-1D and HYDRUS 2D/3D (Šimůnek et al., 2008) are 1D, 2D and 3D models, which in many cases proved to be efficient tools for simulating water, heat, solute and gas processes in the vadose zone. HYDRUS models were applied for instance to the testing of a new heat sensor (Mortensen et al., 2006, Saito et al., 2007). They were also used to study the impact of temperature on water evapotranspiration (Dahiya et al., 2007, Deb et al., 2011, Saito et al., 2006, Schwartz et al., 2010, Zhao et al., 2010) and soil CO2 production (Buchner et al., 2008). The impact of a surface drip irrigation system on water and temperature distribution was measured and simulated by Wang et al. (2013). The model HYDRUS-1D was also used to test the effect of meteorological models on surface energy balance and soil temperature (Saito and Šimůnek, 2009).
The first aim was to calibrate the model HYDRUS-1D for simulating water and thermal regimes of a particular Technosol with grass cover. For this purpose soil water contents and temperatures were measured at different depths. Next aim was to perform numerical simulations with different materials (grass, bark chips mulch, sand, gravel, concrete paving) at the soil surface to document influence of different surface covers on temperatures within the soil profiles. For this purpose soil temperatures were measured 7 cm below the soil surface with different covers to: 1. prove experimentally influence of different surface covers on heat transfer at the soil surface, 2. use these data as top boundary conditions when simulating heat regimes within the soil profiles with different covers.
Section snippets
Experimental area, soil–water content and soil-temperature monitoring
The study was performed at the premises of VESKOM Ltd in Dolní Měcholupy, Prague, Czech Republic. The studied soil was defined as a Technosol (IUSS Working Group WRB, 2006, Rossiter, 2007). The soil profile was composed of two layers, the topsoil (thickness of 25 cm) and subsoil. Topsoil was created by redeposited topsoil material, which was removed before starting construction works at this area. Subsoil was created by subsoil material, which was redeposited during and after construction and
Soil water regime under the grass cover
Fig. 5 shows the average soil–water contents calculated from measured values at particular depths in 4 access tubes using the PR2 Soil Moisture Profile Probe. Standard deviations (error bars in this figure) show moderate variability of measured data. Besides of the impact of a natural spatial variability of soil properties on soil water regime within the study area, variability of measured soil water contents could be attributed to uneven precipitation distribution at the soil surface and
Conclusion
The model HYDRUS-1D proved to be an efficient tool for simulating soil water and thermal regimes within the soil profile to standard depths (120–180 cm) of horizontal ground heat exchangers installations. Measured temperatures close to the soil surface and simulated temperatures with HYDRUS-1D at various depths showed that different materials at the soil surface significantly influenced temperatures and their oscillations at all depths. The largest temperatures and their oscillations during the
Acknowledgements
Authors acknowledge the financial support of the Technology Agency of the Czech Republic (TA02020991) and supply of climatic data by Czech Meteorological Institute. Authors also thank to Christopher Ash for English language correction.
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