An approximate analytical solution for describing surface runoffand sediment transport over hillslope

Soil and water loss from farmland causes&nbsp;land degradation&nbsp;and water pollution, thus continued efforts are needed to establish mathematical model for quantitative analysis of relevant processes and mechanisms. In this study, an approximate analytical solution has been developed for overland flow model and sediment transport model, offering a simple and effective means to predict overland flow and erosion under natural rainfall conditions. In the overland flow model, the flow regime was considered to be transitional with the value of parameter&nbsp;&beta;&nbsp;(in the kinematic wave model) approximately two. The change rate of unit discharge with distance was assumed to be constant and equal to the runoff rate at the outlet of the plane. The excess rainfall was considered to be constant under uniform rainfall conditions. The overland flow model developed can be further applied to natural rainfall conditions by treating excess rainfall intensity as constant over a small time interval. For the sediment model, the recommended values of the runoff erosion calibration constant (cr) and the splash erosion calibration constant (cf) have been given in this study so that it is easier to use the model. These recommended values are 0.15 and 0.12, respectively. Comparisons with observed results were carried out to validate the proposed analytical solution. The results showed that the approximate analytical solution developed in this paper closely matches the observed data, thus providing an alternative method of predicting runoff generation and sediment yield, and offering a more convenient method of analyzing the quantitative relationships between variables. Furthermore, the model developed in this study can be used as a theoretical basis for developing runoff and&nbsp;erosion control&nbsp;methods.</span

Boundary layer theory description of solute transport in soil

Solute transport through soil may affect groundwater and surface water quality. The convection-dispersion equation has been widely used to describe soil solute transport processes. Dispersion coefficient (D) and retardation factor (R) are two important transport parameters in the model. Earlier researchers used boundary layer methods to estimate the soil solute transport parameters based on assumptions of parabolic or cubic polynomial concentration profiles. In this article, boundary layer theory is used to calculate solute concentration distributions and the boundary layer distances, based on assumptions of quartic and quintic concentration profiles. Equations describing soil solute concentration profiles are evaluated by comparing them with an exact solution. Equations for estimating D and R are developed, and the effects of instrument sensitivity on determining D and R are analyzed. The results indicate that all of the boundary layer equations may be used to estimate soil solute transport parameters, but the instrument sensitivity required by each equation is different. Therefore, instrument sensitivity should be evaluated before selecting the boundary layer equation used to estimate D and R

Effect of surface stone cover on sediment and solute transport on the slope of fallow land in the semi-arid loess region of northwestern China

In the semi-arid loess region of northwestern China, use of stone and gravel as mulch has been an indigenous farming technique for improving crop production for over 300 years. However, systematic studies on the effects of stone covers on soil and water conservation have been rarely conducted, except for a few investigations and documentations on the stone cover effects on erosion and solute transport in such a highly erodible loess region. Materials and methods We experimentally examined the effects of surface stone cover on sediment erosion and solute transport using the water-scouring method on sloping land in a semi-arid region in China, which had been left fallow with alfalfa (Medicago sativa) for 3 years. All covered stones rested on the soil surface, and none were partly or completely embedded in the soil surface layer. Stone cover percentages were classified into three groups: 0% (no stone cover, the control treatment), 5.1%, and 20.8%. Two sizes of stones, SCA (7.6 x 7.6 cm) and SCB (18.4 x 18.4 cm), were used in the treatment of 5.1% stone cover. A dye method was used to measure flow velocities in the experiments. Each stone treatment had one replicate. Results and discussion The surface cover by stones influenced soil erosion processes, runoff generation, and solute transport. Runoff rate and sediment yield decreased as stone cover percentages increased from zero (no stone cover) to 20.8%. The effect of stone sizes on the runoff was not significant, whereas stone size type SCA caused lower sediment yield than SCB at the same stone cover percentage of 5.1%. Likewise, water flow velocity and the Froude numbers also decreased with increasing stone cover percentage. The Manning roughness increased with increasing stone cover percentage, ranging from 0.0296 to 0.0579 m(-1/3) s. But the Reynolds numbers among different stone cover percentages and sizes remained nearly the same with a small variation from 483 to 486. Conclusions The study implied that stone cover percentage and size have important influences on sediment and solute concentration in runoff. Surface-covering stones reduced the velocity of runoff, increased surface roughness, decreased sediment yield in runoff, and consequently reduced the quantities of solute release from soil surface

Chloride transport in undisturbed soil columns of the loess Plateau

In soils containing preferential flow paths, both water and solute can move preferentially, bypassing much of the soil matrix. The object of this study was to examine the effect of preferential solute transport in Changwu (loamy soil) soil and Ansai soil (sandy soil) containing macroporosity. Miscible displacement experiments were conducted with 5 undisturbed soil columns (19.45 cm diameter, 43.5 cm long). Breakthrough curves (BTC's) of Chloride were measured under water-saturated steady flow conditions. The data were simulated using three conceptual models. The results show that two-flow region model described the preferential solute transport much better than the two-region model and the convection dispersion equation (CDE), especially there were humps in the tailing side. Moreover, distinct double peaks were apparent with the increase of pore water velocity in a loamy soil column. In addition, high pore water velocity and small mass transfer coefficient between the two-flow regions enhanced the development of double BTC peaks

Analytical Method for Estimating Soil Hydraulic Parameters from Horizontal Absorption

Soil hydraulic properties are required to quantitatively simulate water and chemical transport processes in the vadose zone and groundwater with various numerical models. Most methods for determination of soil hydraulic properties are time consuming and expensive, which limits the application of the methods. The objective of this study was to develop an analytical method based on an assumption of exponential flux distribution to determine soil hydraulic parameters. Using this method, parameters of the Brooks-Corey model and exponential water diffusivity can be easily estimated from cumulative infiltration and wetting length vs. time in horizontal absorption experiments. The analytical method was tested and improved using 19 numerical soils in a wide range of textures. The results indicated that all the Brooks-Corey model parameters estimated by the improved method were very close to the simulated values. In addition, the water content, soil tension, and water flux distributions of three typical soils (clay, loam, and sand soils) estimated by the approximate solution of the improved method agreed well with those calculated by the HYDRUS-1D software as well as cumulative infiltration data. Therefore, although a large number of experiments are still required to test the method, it provides a simple approach to determine the hydraulic parameters of soils in a wide range of textures, providing a very good approximate solution to the problem of horizontal absorption

Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

Soil erosion and nutrient losses with surface runoff in the loess plateau in China cause severe soil quality degradation and water pollution. It is driven by both rainfall impact and runoff flow that usually take place simultaneously during a rainfall event. However, the interactive effect of these two processes on soil erosion has received limited attention. The objectives of this study were to better understand the mechanism of soil erosion, solute transport in runoff, and hydraulic characteristics of flow under the simultaneous influence of rainfall and shallow clear-water flow scouring. Laboratory flume experiments with three rainfall intensities (0, 60, and 120 mm h(-1)) and four scouring inflow rates (10, 20, 30, and 40 l min(-1)) were conducted to evaluate their interactive effect on runoff. Results indicate that both rainfall intensity and scouring inflow rate play important roles on runoff formation, soil erosion, and solute transport in the surface runoff. A rainfall splash and water scouring interactive effect on the transport of sediment and solute in runoff were observed at the rainfall intensity of 60 mm h(-1) and scouring inflow rates of 20 l min(-1). Cumulative sediment mass loss (Ms) was found to be a linear function of cumulative runoff volume (Wr) for each treatment. Solute transport was also affected by both rainfall intensity and scouring inflow rate, and the decrease in bromide concentration in the runoff with time fitted to a power function well. Reynolds number (Re) was a key hydraulic parameter to determine erodability on loess slopes. The Darcy-Weisbach friction coefficients (f) decreased with the Reynolds numbers (Re), and the average soil and water loss rate (M(1)) increased with the Reynolds numbers (Re) on loess slope for both scenarios with or without rainfall impact. Copyright (C) 2010 John Wiley &amp; Sons, Ltd

Soil desiccation for Loess soils on natural and regrown areas

In the Loess Plateau, soil desiccation has become a serious problem for forest and grass vegetation. Soil desiccation leads to the formation of a dried soil layer (DSL). This paper presents the results of research carried out in the central part of the Loess Plateau. The objective of the research was to produce a statistically supported set of indicators for evaluating soil desiccation of forestlands, to present a heuristic idea for soil desiccation and to supply scientific support for replacing farmland with forest or grass in the Loess Plateau and other regions of China. Here, we suggest that more attention should be paid to soil desiccation and its effects on the ecosystem of the region in the future. The results showed that natural Quercus liaotungensis forestlands (NQF) retained more water content than regrown Robinnia pseudoscacia forestlands (RRF). Significant DSLs were formed in the RRF but not in the NQF. A possible reason for no formation of DSL in NQF could be due to the presence of an arbor-shrub-herb stand structure and large humus and litter accumulation, which increased the natural forest's (NF) adaptability to the environmental conditions. Soil water content in the north-facing slope was significantly larger than in the south-facing slope. DSLs formed in the 0-500 cm layer of the south-facing slope. When slope gradient was greater than 25 degrees, soil water content deceased sharply and showed significant difference compared with 9 degrees, 15 degrees and 20 degrees (P &lt; 0.05). So, we conclude that plant species, aspect and slope angle could be the predicators for the formation of DSLs. The analysis on soil physical properties of 0-60 cm layer indicated that plant species, aspect and slope angle also have significant effects on bulk density, porosity, plant-available capacity, and hydraulic conductivity, especially in the 0-20 and 20-40 cm layers. In the NQF and RRF with north-facing slope, soil physical properties were improved. (C) 2008 Elsevier B.V. All rights reserved

Evaluation of the Aqua Crop model for simulating the impact of water deficits and different irrigation regimes on the biomass and yield of winter wheat grown on China's Loess Plateau

Accurate models of crop growth are important for evaluating the effects of water deficits on crop yield or productivity. AquaCrop was developed by the FAO (Food and Agricultural Organization) of the United Nations to simulate yield responses to changes in the supply of water. The objectives of this study were to evaluate the model's ability to simulate winter wheat performance under full and deficit water conditions on China's Loess Plateau and to study the effect of different irrigation scenarios on wheat yield. The model's output was compared to experimental data collected between 2006 and 2011 at the Changwu Agri-ecological Station on the Loess Plateau. The model accurately estimated the soil water content of the root zone as well as the biomass and grain yields of winter wheat. When simulating the soil water during the 2008-2009 growing season, the calculated values of r(2), RMSE, ME, and the d-index were 0.98, 8.4 mm, 0.98, and 0.99 for no irrigation; 0.95, 14.4 mm, 0.93, and 0.98 for double irrigation; 0.88, 22.9 mm, 0.68, and 0.90 for triple irrigation; and 0.93, 17.5 mm, 0.75, and 0.9 for quadruple irrigation, respectively. For the grain yield, the r2 values for the model's outputs under the single irrigation, double irrigation, triple irrigation, and quadruple irrigation treatments were 0.80, 0.98, 0.99, and 0.77, respectively. Comparing to no irrigation the highest increases in grain yield were observed for scenarios in which irrigation was applied during the over-wintering and turning green stages. Moreover, the simulations indicated that under double irrigation regimes, water can be withheld during over-wintering and either turning green or stem elongation without greatly reducing yields. The minimum amounts of irrigation water required to achieve high WUE in wet, normal and dry years were 225, 150 and 150 mm, respectively. (C) 2013 Elsevier B.V. All rights reserved