ALBERT

Description: In our previous study, Faulkner et al. (2009) developed a Stokes-Darcy (SD) model for flow in a karst aquifer with a conduit bedded in matrix. Beavers-Joseph (BJ) interface condition was used to describe the matrix/conduit interface. Cao et al. (2010) studied the mathematical well-posedness of a coupled continuum pipe-flow (CCPF) model as well as convergence rates of its finite element approximation. In this study, to compare the SD model with the CCPF model, we use numerical analyses to validate finite element discretization methods for the two models. Using computational experiments, simulation codes implementing the finite element discretizations are then verified. Further model validation studies are based on the results of laboratory experiments. Comparing the results of computer simulations and experiments, we conclude that the SD model with the BJ interface condition is a valid model for conduit/matrix systems. On the other hand, the CCPF model with the value of the exchange parameter chosen within the range suggested in the literature perhaps does not result in good agreement with experimental observations. We then examine the sensitivity of the CCPF model with respect to the exchange parameter, concluding that, as has previously been noted, the model is highly sensitive for small values of the exchange parameter. However, for larger values, the model becomes less sensitive and, more important, also produces results that are in better agreement with experimental observations. This suggests that the CCPF model may also produce accurate simulation results, if one chooses larger values of the exchange parameter than those suggested in the literature. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Soil macropore networks are subsurface connected void spaces caused by processes such as fracture of soils, micro-erosion, and fauna burrows. Axial X-ray computed tomography (CT) scanning provides a convenient means of recording the spatial structure of soil macropore networks. The objective of this study were to (1) based on CT technique and GIS digitized image method, construction a new technique for tracing, visualizing and measuring the soil macropore networks; (2) investigate the effects of farming activities on soil macropore networks characteristics. Our technique using left-turning and nine-direction judgment methods, a combination of the layer-by-layer analysis method and the up-down tracking algorithm. The characteristics for the overall structure patterns of macropores, the spatial distribution of the macropore networks and each single macropore network can conveniently identify by our technique. Eight undisturbed soil columns from fields with two distinct land uses (under cultivation and not been cultivated) and four different depths (0-20 cm, 20-40 cm, 40-60 cm and 60-80 cm) were investigated. The soil columns were scanned using X-ray CT at a voxel resolution of 0.075 mm × 0.075 mm × 3.000 mm. Results indicate that farming activities can destroy the initial structure of macropores and those remaining are mainly small and medium-sized networks with lower extension and hydraulic conductivity. The network properties show a significant difference between upper and lower layer. The results can provide beneficial reference to further research centered on non-equilibrium flow prediction and chemical transport modeling in field soils. This article is protected by copyright. All rights reserved.

Description: The concentration and isotopic composition of nitrate were analyzed to improve an understanding of nitrate sources and transformation processes in a typical karstic agricultural field in the Houzhai catchment, Guizhou Province, Southwest China. The results revealed that no distinct spatial pattern of content and isotopic composition of nitrate exists in this karst catchment. Nitrate in surface stream (SFS) had slightly lighter isotopic composition and lower concentration compared to nitrate in subterranean stream (STS) during the dry season. Concentrations of SFS nitrate increased to concentrations similar to those of STS during the wet season. The isotopic values indicated that nitrate were mainly impacted by manure sources during the dry season, and influenced by a mix of chemical fertilizer and manure during the wet season. The denitrification rates were roughly estimated based on the isotopic compositions of nitrate after considering volatilization and ignoring assimilation. The calculated result showed approximately 1/5 of nitrate load was removed by denitrification in the catchment. Annual nitrate flux from the outlets accounted for 14.2% of applied total fertilizers used in the catchment, approximately 85% of total transported flux from the catchment in the wet season. Furthermore, chemical weathering processes were enhanced by using nitrogen fertilizer because liberated protons and enhanced HCO 3 − flux were produced through by nitrification. This article is protected by copyright. All rights reserved.

Description: The aim of this study is to understand the canopy interception of Qinghai spruce forest under conditions of different precipitation characteristics and canopy structures in the upper reach of Heihe River basin, northwestern China. Based on a continuous record covering our investigating period by an automatic throughfall collecting system, we analyzed the relationships between the canopy interception and the precipitation characteristics. Our results support the well-established exponential decay relationship between the gross precipitation and the interception percentage after the canopy is saturated. But, our results sufficiently illustrate a notable point that the variations in the interception percentage are almost independent from the variations in the gross precipitation before the canopy is saturated. Our examination into the relationship between the interception and the 10 min-average intensity of precipitation demonstrates a divergent relationship and the divergent relationship is bracketed by an upper “dry line” indicating that 100% of gross precipitation was intercepted before saturation and by a lower “wet line” suggesting that the actual canopy storage capacity reached the maximum and evaporation was the only component of the interception. To search for the relationship between canopy structures and interception, we grouped the canopy covers over the 90 throughfall-collecting tanks into 10 categories ranging from 0 (no cover) to 0.9 (nearly completely covered) and the corresponding canopy interception was calculated by subtracting the averaged throughfall of each canopy-cover category from the gross precipitation. The results show that the interception percentage increases faster with increasing canopy cover under intermediate rainfall conditions than that under heavy rainfall conditions. Unexpectedly, under light rainfall conditions the increasing rate of interception percentage with increasing canopy cover and also with increasing plant area index is not faster than that under the intermediate rainfall conditions simply because the tank-measured percentage of interception was extremely high at near-zero canopy cover conditions. Copyright © 2013 John Wiley & Sons, Ltd.

Description: A one-dimensional, two-layer solute transport model is developed to simulate chemical transport process in an initially unsaturated soil with ponding water on the soil surface before surface runoff starts. The developed mathematical model is tested against a laboratory experiment. The infiltration and diffusion processes are mathematically lumped together and described by incomplete mixing parameters. Based on mass conservation and water balance equations, the model is developed to describe solute transport in a two-zone layer, a ponding runoff zone and a soil mixing zone. The two-zone layer is treated as one system to avoid describing the complicated chemical transport processes near the soil surface in the mixing zone. The proposed model was analytically solved, and the solutions agreed well with the experimental data. The developed experimental method and mathematical model were used to study the effect of the soil initial moisture saturation on chemical concentration in surface runoff. The study results indicated that, when the soil was initially saturated, chemical concentration in surface runoff was significantly (two orders of magnitude) higher than that with initially unsaturated soil, while the initial chemical concentrations at the two cases were of the same magnitude. The soil mixing depth for the initially unsaturated soil was much larger than that for the initially saturated soil, and the incomplete runoff mixing parameter was larger for the initially unsaturated soil. The higher the infiltration rate of the soil, the greater the infiltration-related incomplete mixing parameter. According to the quantitative analysis, the soil mixing depth was found to be sensitive for both initially unsaturated and saturated soils, and the incomplete runoff mixing parameter was sensitive for initially saturated soil but not for the initially unsaturated soil; the incomplete infiltration mixing parameter behaved just the opposite. Some suggestions are made for reducing chemical loss from runoff. © 2010 John Wiley & Sons, Ltd.

Description: Soil macropore networks are subsurface connected void spaces caused by processes such as fracture of soils, micro-erosion and fauna burrows. Axial X-ray computed tomography (CT) scanning provides a convenient means of recording the spatial structure of soil macropore networks. The objective of this study were to (1) based on CT technique and GIS digitized image method, construction a new technique for tracing, visualizing and measuring the soil macropore networks and (2) investigate the effects of farming activities on soil macropore networks characteristics. Our technique uses left-turning and nine-direction judgment methods, a combination of the layer-by-layer analysis method and the up-down tracking algorithm. The characteristics for the overall structure patterns of macropores, the spatial distribution of the macropore networks and each single macropore network can be conveniently identified by our technique. Eight undisturbed soil columns from fields with two distinct land uses (under cultivation and not been cultivated) and four different depths (0-20, 20-40, 40-60 and 60-80cm) were investigated. The soil columns were scanned using X-ray CT at a voxel resolution of 0.075×0.075×3.000mm. Results indicate that farming activities can destroy the initial structure of macropores, and those remaining are mainly small-sized and medium-sized networks with lower extension and hydraulic conductivity. The network properties show a significant difference between upper and lower layer. The results can provide beneficial reference to further research centered on non-equilibrium flow prediction and chemical transport modeling in field soils. © 2013 John Wiley & Sons, Ltd.

Description: The aim of this study is to understand the canopy interception of Qinghai spruce forest under conditions of different precipitation characteristics and canopy structures in the upper reach of Heihe River basin, northwestern China. On the basis of a continuous record covering our investigating period by an automatic throughfall-collecting system, we analysed the relationships between the canopy interception and the precipitation characteristics. Our results support the well-established exponential decay relationship between the gross precipitation and the interception percentage after the canopy is saturated. But our results sufficiently illustrate a notable point that the variations in the interception percentage are almost independent from the variations in the gross precipitation before the canopy is saturated. Our examination into the relationship between the interception and the 10-min average intensity of precipitation demonstrates a divergent relationship, and the divergent relationship is bracketed by an upper 'dry line' indicating that 100% of gross precipitation was intercepted before saturation and by a lower 'wet line' suggesting that the actual canopy storage capacity reached the maximum and evaporation was the only component of the interception. To search for the relationship between canopy structures and interception, we grouped the canopy covers over the 90 throughfall-collecting tanks into ten categories ranging from 0 (no cover) to 0.9 (nearly completely covered), and the corresponding canopy interception was calculated by subtracting the averaged throughfall of each canopy-cover category from the gross precipitation. The results show that the interception percentage increases faster with increasing canopy cover under intermediate rainfall conditions than that under heavy rainfall conditions. Unexpectedly, under light rainfall conditions the increasing rate of interception percentage with increasing canopy cover and also with increasing plant area index is not faster than that under the intermediate rainfall conditions simply because the tank-measured percentage of interception was extremely high at near-zero canopy cover conditions. © 2013 John Wiley & Sons, Ltd.

Description: The concentration and isotopic composition of nitrate were analyzed to improve an understanding of nitrate sources and transformation processes in a typical karstic agricultural field in the Houzhai catchment, Guizhou Province, Southwest China. The results revealed that no distinct spatial pattern of content and isotopic composition of nitrate exists in this karst catchment. Nitrate in surface stream (SFS) had slightly lighter isotopic composition and lower concentration compared with nitrate in subterranean stream (STS) during the dry season. Concentrations of SFS nitrate increased to concentrations similar to those of STS during the wet season. The isotopic values indicated that nitrate were mainly impacted by manure sources during the dry season and influenced by a mix of chemical fertilizer and manure during the wet season. The denitrification rates were roughly estimated based on the isotopic compositions of nitrate after considering volatilization and ignoring assimilation. The calculated result showed that approximately one fifth of nitrate load was removed by denitrification in the catchment. Annual nitrate flux from the outlets accounted for 14.2% of applied total fertilizers used in the catchment, approximately 85% of total transported flux from the catchment in the wet season. Furthermore, chemical weathering processes were enhanced by using nitrogen fertilizer because liberated protons and enhanced HCO3- flux were produced through by nitrification. © 2014 John Wiley & Sons, Ltd.