A dynamical system approach to soil salinity and sodicity

Soil salinity and sodicity impose severe constrains to agriculture, especially in arid and semi-arid regions, where good-quality water for irrigation is scarce. While detailed models have been proposed in the past to describe the dynamics of salt and sodium in the soil, they typically require cumbersome calculations and are not amenable to theoretical analysis. Here we present an analytical model for the dynamics of salinity and sodicity in the root zone. We determine the dependence of steady-state salinity and sodicity levels on irrigation water quality and derive the trajectories in the phase space. The only stationary solution the equations admit is a stable node. Through numerical integration and analysis of the eigenvalues of the derived two-dimensional system of equations, the slower time scale associated with sodification is quantified with respect to the faster time scale associated to salinization. The role of different cation exchange equations (Gapon and Vanselow conventions) are shown to be practically the same with regard to the phase-space dynamics and the time scales. The results can be applied in controlling for low levels of salinity and sodicity, and in planning remediation strategies that are timely and economical.     

Insights into groundwater salinization from hydrogeochemical and isotopic evidence in an arid inland basin

In the Manas River basin (MRB), groundwater salinization has become a major concern, impeding groundwater use considerably. Isotopic and hydrogeochemical characteristics of 73 groundwater and 11 surface water samples from the basin were analysed to determine the salinization process and potential sources of salinity. Groundwater salinity ranged from 0.2 to 11.91 g/L, and high salinities were generally located in the discharge area, arable land irrigated by groundwater, and depression cone area. The quantitative contributions of the evaporation effect were calculated, and the various groundwater contributions of transpiration, mineral dissolution, and agricultural irrigation were identified using hydrogeochemical diagrams and δD and δ¹⁸O compositions of the groundwater and surface water samples. The average evaporation contribution ratios to salinity were 5.87% and 32.7% in groundwater and surface water, respectively. From the piedmont plain to the desert plain, the average groundwater loss by evaporation increased from 7% to 29%. However, the increases in salinity by evaporation were small according to the deuterium excess signals. Mineral dissolution, transpiration, and agricultural irrigation activities were the major causes of groundwater salinization. Isotopic information revealed that river leakage quickly infiltrated into aquifers in the piedmont area with weak evaporation effects. The recharge water interacted with the sediments and dissolved minerals and subsequently increased the salinity along the flow path. In the irrigation land, shallow groundwater salinity and Cl^? concentrations increased but not δ¹⁸O, suggesting that both the leaching of soil salts due to irrigation and transpiration effect dominated in controlling the hydrogeochemistry. Depleted δ¹⁸O and high Cl^? concentrations in the middle and deep groundwater revealed the combined effects of mixing with paleo‐water and mineral dissolution with a long residence time. These results could contribute to the management of groundwater sources and future utilization programs in the MRB and similar areas.     

Impact of soil compaction and irrigation practices on salt dynamics in the presence of a saline shallow groundwater: An experimental and modelling study

Soil salt accumulation is a widespread problem leading to diminished crop yield and threatening food security in many regions of the world. The soil salinization problem is particularly acute in areas that lack adequate soil water drainage and where a saline shallow water table (WT) is present. In this study, we present laboratory-scale column experiments, extending over a period of more than 400 days that focus on the processes contributing to soil salinization. We specifically examine the combined impact of soil compaction, surface water application model and water quality on salt dynamics in the presence of a saline shallow WT. The soil columns (60 cm height and 16 cm diameter) were packed with an agricultural soil with bulk densities of 1.15 and 1.34 g/cm⁻³ for uncompacted and compacted layers, respectively, and automatically monitored for water content, salinity and pressure. Two surface water compositions are considered: fresh (deionized, DI) and saline water (~3.4 mS/cm). To assess the sensitivity of compaction on salt dynamics, the experiments were numerically modelled with the HYDRUS-1D computer program. The results show that the saline WT led to rapid salinization of the soil column due to capillarity, with the salinity reaching levels much higher than that at the WT. However, compaction layer provided a barrier that limited the downwards moisture percolation and solute transport. Furthermore, the numerical simulations showed that the application of freshwater can temporarily reverse the accumulation of salts in agricultural soils. This irrigation strategy can help, in the short-term, alleviate soil salinization problem. The soil hydraulic properties, WT depth, water quality, evaporation demand and the availability of freshwater all play a role in the practicability of such short-term solutions. The presence of a saline shallow WT would, however, rapidly reverse these temporary measures, leading to the recurrence of topsoil salinization.     

Epigenetic salt accumulation and water movement in the active layer of central Yakutia in eastern Siberia

Observations of soil moisture and salt content were conducted from May to August at Neleger station in eastern Siberia. Seasonal changes of salt and soil moisture distribution in the active layer of larch forest (undisturbed) and a thermokarst depression known as an alas (disturbed) were studied. Electric conductivity EC_e of the intact forest revealed higher concentrations that increased with depth from the soil surface into the active layer and the underlying permafrost: 1 mS cm^?1 at 1·1 m, to 2·6 mS cm^?1 at 160 cm depth in the permafrost. However, a maximum value of 5·4 mS cm^?1 at 0·6 m depth was found in the dry area of the alas. The concentration of ions, especially Na⁺, Mg²⁺, Ca²⁺, SO₄^2? and HCO₃^? in the upper layers of this long‐term disturbed site, indicates the upward movement of ions together with water. A higher concentration of solutes was found in profiles with deeper seasonal thawing. The accumulation of salts in the alas occurs from spring through into the growing season. The low concentration of salt in the surface soil layers appears to be linked to leaching of salts by rainfall. There are substantial differences between water content and electric conductivity of soil in the forest and alas. Modern salinization of the active layer in the alas is epigenetic, and it happens in summer as a result of spring water collection and high summer evaporation; the gradual salt accumulation in the alas in comparison with the forest is controlled by the annual balance of water and salts in the active layer. Present climatic trends point to continuous permafrost degradation in eastern Siberia increasing the risk of surface salinization, which has already contributed to changing the landscape by hindering the growth of forest. Copyright © 2006 John Wiley & Sons, Ltd.     

Hydrological and salinity impacts of a bio‐drainage strategy application in the Yizre'el Valley,Israel

The bio‐drainage–commercial forestry strategy was applied in five plots in the Yizre'el Valley, northern Israel, to evaluate the hydrological and salinity impacts of eucalypt plantations. Each plot contained a mix of 11 selected eucalyptus species/ecotypes. Two plots (Nahalal and Genigar), representing the two extreme waterlogging/salinity conditions in the valley, were selected for in‐depth monitoring over a 10‐year period to assess the likely environmental improvement through bio‐drainage. Despite impressive growth rates of genetically improved Eucalyptus camaldulensis in the year‐round waterlogged, slightly saline Nahalal site (650 mm annual rainfall), the water uptake by the trees was insufficient to control the rising water table caused by excessive water inputs, both natural and human. In the more saline, alkaline and drier Genigar plot (450 mm annual rainfall), where rainfall is the only water input, the ground water dropped to below 3 m from soil surface in the fourth year after planting, i.e. deeper than the adjacent ground water levels. Both sites showed appreciable rise in wells that penetrated the 3‐ to 4‐m confining layer. The 10‐year salinity (EC) trend of the top layer in Nahalal varied because the drainage was limited by the positive water balance and the above‐average spells of dry winters. In and below the 4 m deep layer the EC remained below 1·5 dS m^?1 throughout the entire 10‐year study. The last EC measurement, taken in 2003, showed values not higher than 4 dS m^?1 throughout the 6 m soil profile. In Genigar, there was significant leaching of salts from the top layer (1 m) during the 9‐year monitoring period, but recently a salts ‘bulge’ was gradually developed in the 1–5 m strata indicating that the expected downward movement of leached salts was impeded by the 3–4 m deep low‐permeability clayey layer that lies over a coarser, far more conductive and notably confined layer, which leads to a perched water body. The last EC measurement at the end of 2003 showed a maximum value of 5·5 dS m^?1 at 3 m depth. No signs of tree stress were observed in either site, at any soil depth during the 10 years of monitoring. Theoretical considerations do not support the hypothesis that there would be a fatal long‐term accumulation of salts in the root zone. The Israeli experience has shown that the bio‐drainage technique can effectively lower a shallow water table and reverse salinity trends, provided that the overall water balance is negative, i.e. that the water inputs match the water use by the tree plantation and local drainage characteristics. However, the rate of improvement of the hydraulic, salinity, sodicity and soil physical properties is site specific. Excess fresh water inputs into the plantation, although they create waterlogging conditions, supply unlimited water to the trees, which, in turn, show exceptional growth rates, with usable commercial value. Copyright © 2007 John Wiley & Sons, Ltd.     

Sources of salinity in ground water from Jericho area, Jordan Valley

One of the major problems in the lower Jordan Valley is the increasing salinization (i.e., chloride content) of local ground water. The high levels of salinity limit the utilization of ground water for both domestic and agriculture applications. This joint collaborative study evaluates the sources and mechanisms for salinization in the Jericho area. We employ diagnostic geochemical fingerprinting methods to trace the potential sources of the salinity in (1) the deep confined subaquifer system (K2) of Lower Cenomanian age; (2) the upper subaquifer system (K1) of Upper Cenomanian and Turonian ages; and (3) the shallow aquifer system (Q) of Plio-Pleistocene ages. The chemical composition of the saline ground water from the two Cenomanian subaquifers (K1 and K2) point to a single saline source with Na/Cl approximately 0.5 and Br/Cl approximately 7 x 10(-3). This composition is similar to that of thermal hypersaline spring that are found along the western shore of the Dead Sea (e.g., En Gedi thermal spring). We suggest that the increasing salinity in both K1 and K2 subaquifers is derived from mixing with deep-seated brines that flow through the Rift fault system. The salinization rate depends on the discharge volume of the fresh meteoric water in the Cenomanian Aquifer. In contrast, the chemical composition of ground water from the Plio-Pleistocene Aquifer shows a wide range of Cl- (100-2000 mg/L), Na/Cl (0.4-1.0), Br/Cl (2-6 x 10(-3)), and SO4/Cl (0.01-0.4) ratios. These variations, together with the high SO4(2-), K+, and NO3- concentrations, suggest that the salinity in the shallow aquifer is derived from the combination of (1) upconing of deep brines as reflected by low Na/Cl and high Br/Cl ratios; (2) leaching of salts from the Lisan Formation within the Plio-Pleistocene Aquifer, as suggested by the high SO4(2-) concentrations; and (3) anthropogenic contamination of agriculture return flow and sewage effluents with distinctive high K+ (80 mg/L) and NO3- (80 mg/l) contents and low Br/Cl ratios (2 x 10(-3)). Our data demonstrates that the chemical composition of salinized ground water can be used to delineate the sources of salinity and hence to establish the conceptual model for explaining salinization processes.     

Nutrient budget of a montane evergreen broad‐leaved forest at Ailao Mountain National Nature Reserve,Yunnan, southwest China

Hydrological fluxes and associated nutrient budget were studied during a 2 year period (1998–99) in a montane moist evergreen broad‐leaved forest at Ailao Mountain, Yunnan. Water samples of rainfall, throughfall, and stemflow, and of surface runoff, soil water, and stream flow were collected bimonthly to determine the concentration and fluxes of nutrients. Soil budgets were determined from the difference between precipitation input (including nutrient leaching from canopy) and output via runoff and drainage. The forest was characterized by low canopy interception and surface runoff, and high percolation and stream flow. Concentrations of nutrients were increased in throughfall and stemflow compared with precipitation. Surface runoff and drainage water had higher nutrient concentrations than precipitation and stream water. Total nitrogen and NH₄⁺‐N concentrations were higher in soil water than stream water, whereas K⁺, Ca²⁺, and Mg²⁺ concentrations were lower in the former than the latter. Annual nutrient fluxes decreased with soil depth following the pattern of water flux. Annual losses of most nutrient elements via stream flow were less than the corresponding inputs via throughfall and stemflow, except for calcium, for which solute loss was greater than the inputs via precipitation. Leaching losses of that element may be compensated by weathering. Losses of nitrogen, phosphorus, potassium, magnesium, sodium, and sulphur could be replaced through atmospheric inputs. Copyright © 2002 John Wiley & Sons, Ltd.     

Sodicity effects on hydrological processes of sodic soil slopes under simulated rainfall

Soil salinization can occur in many regions of the world. Soil sodicity affects rainfall‐runoff relationships and related erosion processes considerably. We investigated sodicity effects on infiltration, runoff and erosion processes on sodic soil slopes for two soils from China under simulated rainfall. Five sodicity levels were established in a silt loam and a silty clay with clay contents of 8.5% and 46.0%, respectively. The soils, packed in 50 cm × 30 cm × 15 cm flumes at two slope gradients (22° and 35°), were exposed to 60 min of simulated rainfall (deionized water) at a constant intensity of 125 mm h^?1. Results showed that, for both soils, increasing soil sodicity had some significant effects on hydrological processes, reducing the infiltration coefficient (pr = ?0.69, P  < 0.01) and the quasi‐steady final infiltration rate (pr = ?0.80, P  < 0.01), and increasing the mean sediment loss (pr = 0.39, P  < 0.05); however, it did not significantly affect the cumulative rainfall to ponding (P  > 0.05). Moreover, increasing sodicity significantly increased the Reynolds number and the stream power (pr = 0.78 and 0.66, P  < 0.01, respectively) of the runoff, decreased Manning roughness and Darcy–Weisbach coefficient (pr = ?0.52 and ?0.52, P  < 0.05, respectively), but did not significantly affect the mean flow velocity, mean flow depth, Froude number and hydraulic shear stress. Stream power was shown to be the most sensitive hydraulic variable affecting sediment loss for both soils. Furthermore, as sodicity increased, the values of critical stream power decreased for both the silt loam (R ² = 0.29, P  < 0.05) and the silty clay (R ² = 0.49, P  < 0.05). The findings of this study were applied to a real situation and identified some negative effects that can occur with increasing sodicity levels. This emphasized the importance of addressing the influences of soil sodicity in particularly high risk situations and when predicting soil and water losses.     

Vulnerability of three spatially variable soil groups for solute leaching

Solute leaching in unsaturated soil is influenced by the variability in hydraulic functions (water retention and conductivity) that govern the flow process. Variability in measured soil hydraulic functions of a coarse-, medium- and fine-textured soil group was quantified with the scaling theory of similar media. Solute leaching in these soils was calculated with Monte Carlo simulation assuming, successively, hydraulic conductivity, K, volumetric water content, 0, and pressure head, h, to be constant. In addition to variability in hydraulic functions, variability in the solute retardation factor was also taken into account. To examine this effect five solutes were considered: a conservative solute (chloride), a non-retarded solute subject to decay (nitrate), a retarded solute that does not decay (cadmium) and two organic solutes which are retarded but have different sorption and decay parameters (the pesticide atrazine and a chlorinated hydrocarbon). The numerical results obtained with Monte Carlo simulation were in a number of instances verified with analytical solutions. The three soil groups distinguished showed considerable differences in vulnerability for leaching of the five solutes, emphasizing the importance of the effect of variability in soil hydraulic functions when studying solute leaching. Numerical and analytical results showed good agreement. Therefore, in relatively simple situations analytical solutions are attractive. However, in complicated situations, analytical solutions are cumbersome and numerical solutions are the only realistic alternative.     

新疆艾比湖湿地土壤水盐空间变异性分析

为揭示艾比湖湿地土壤退化程度空间分布特征,在离艾比湖湖滨5~15 km,绕湖一周160 km范围内,以湖心质点为中心,将艾比湖划分为东北、东南、西南、西北4个区域,采用传统统计学和地统计学相结合的方法对表层(0~20 cm)土壤盐分、含水量与p H的空间分异特征进行研究.结果表明:绕湖一周不同区域的土壤盐分均属中等变异强度;土壤含水量在西北部属强变异性,而东北、东南和西南部均属中等变异强度;土壤p H在不同区域内均属弱变异强度.绕湖一周除西北部土壤盐分的半方差理论模型较符合球状模型外,其它区域土壤盐分、含水量和p H均符合高斯模型;受结构性因素影响,不同区域土壤盐分、含水量和p H均具有较强的空间相关性;西南部土壤盐分、含水量和p H的Moran's I系数比其它区域的波动大,表明空间相关性较强.艾比湖湿地常年大风、干旱、缺水及沙化盐化的自然因素与引水围堰、种植耐盐碱植物的人为活动造成了采样区表层土壤盐分、含水量和p H的空间分布多呈现不规则条带状格局.艾比湖湿地土壤以盐土为主,重度盐化土次之,土壤盐渍化日益严重.     

The role of ground water in arid/semiarid ecosystems, Northwest China

Ground water plays an important role in water supply and the ecology of arid to semiarid areas such as Northwest China, where the landscape is fragile due to frequent drought in the past few decades. This paper discusses the role of ground water in these ecosystems, including the effect of condensation water and water table depth on the growth of plants and degree of soil salinity. The paper also discusses the controlling process for land desertification and soil salinization in Northwest China. Water table depth is a key factor controlling the water balance, ground water flow, and salt transport in the vadose zone. The suitable water table depth for vegetation growth, which can prevent land desertification and soil salinization, is within a range of 2 to 4 m; the optimal depth is approximately 3 m. As examples, changes in ecosystems owing to water resources development in Tarim and Manas basins, Xinjiang, China, are discussed.     

Biogeochemical Characterizations and Reclamation Strategies of Saline Sodic Soil in Northeastern China

Soil salinity and sodicity is considered one of the most import impediments to agricultural development in Northeast China. The contents of TP and TK decrease with soil depth and high coefficients of variation were found in TOC, AN, and AP. Mean EC in the 0–50 cm soil layers ranged from 0.61 to 0.89 dS m^?1 and the average soluble ion concentrations in the topsoil (0–10 cm) were approximately 11.38 mmol L^?1 for Na⁺, 1.21 mmol L^?1 for Ca²⁺, and 0.40 mmol L^?1 for Mg²⁺. High SAR existed in the layers 10–50 cm, indicating the studied soil was bearing low salinity in the top layer and high sodic layer in the subsurface. The soil presented strong alkali reactions all through the profile with pH over 9.5. To improve and utilize saline sodic soil rationally, several strategies were put forward based on long‐term field studies and demonstration works. The results implied that ameliorating with sand, applying farm yard manure, regenerating salt tolerant grasses and leaching with groundwater, and growing rice were effective measures for improving physical and chemical qualities of saline sodic soil.     

Simulation of field‐scale water flow and bromide transport in a cracked clay soil

Single domain models may seriously underestimate leaching of nutrients and pesticides to groundwater in clay soils with shrinkage cracks. Various two‐domain models have been developed, either empirical or physically based, which take into account the effects of cracks on water flow and solute transport. This paper presents a model concept that uses the clay shrinkage characteristics to derive crack volume and crack depth under transient field conditions. The concept has been developed to simulate field average behaviour of a field with cracks, rather than flow and transport at a small plot. Water flow and solute transport are described with basic physics, which allow process and scenario analysis. The model concept is part of the more general agrohydrological model SWAP, and is applied to a field experiment on a cracked clay soil, at which water flow and bromide transport were measured during 572 days. A single domain model was not able to mimic the field‐average water flow and solute transport. Incorporation of the crack concept considerably improved the simulation of water content and bromide leaching to the groundwater. Still deviations existed between the measured and simulated bromide concentration profiles. The model did not reproduce the observed bromide retardation in the top layer and the high bromide dispersion resulting from water infiltration at various soil depths. A sensitivity analysis showed that the amounts of bromide leached were especially sensitive to the saturated hydraulic conductivity of the top layer, the solute transfer from the soil matrix to crack water flow and the mean residence time of rapid drainage. The shrinkage characteristic and the soil hydraulic properties of the clay matrix showed a low sensitivity. Copyright © 2000 John Wiley & Sons, Ltd.     

Finite-element analysis of the transport of water and solutes in title-drained soils

The finite-element method based on a Galerkin technique was used to formulate the problem of simulating the two-dimensional (cross-sectional) transient movement of water and solute in saturated or partially saturated nonuniform porous media. The numerical model utilizes linear triangular elements. Nonreactive, as well as reactive solutes whose behaviour can be described by a distribution coefficient or first-order reaction term were considered. The flow portion of the model was tested by comparison of the model results with experimental and finite-difference results for transient flow in an unsaturated sand column and the solute transport portion of the model was tested by comparison with analytical solution results. The model was applied to a hypothetical case involving movement of water and solutes in tile-drained soils. The simulation results showed the development of distinct solute leaching patterns in the soil as drainage proceeded. Although applied to a tile drainage problem in this study, the model should be equally useful in the study of a wide range of two-dimensional water and solute migration problems.     

High chloride concentrations in the soil and groundwater under an oak hedge in the West of France: an indicator of evapotranspiration and water movement

Chloride is a major anion in soil water and its concentration rises essentially as a function of evapotranspiration. Compared to herbaceous vegetation, high transpiration rates are measured for isolated trees, shelterbelts or hedgerows. This article deals with the influence of a tree hedge on the soil and groundwater Cl^? concentrations and the possibility of using Cl^? as an indicator of transpiration and water movements near the tree rows. Cl^? concentrations were measured over 1 year at different depths in the unsaturated zone and in the groundwater along a transect intersecting a bottomland oak hedge. We observed a strong spatial heterogeneity of Cl^? concentrations, with very high values up to 2 g l^?1 in the unsaturated zone and 1·2 g l^?1 in the upper part of the groundwater. This contrasts with the low and homogeneous concentrations (60–70 mg l^?1) in the deeper part of the groundwater. Cl^? accumulation in the unsaturated zone at the end of the vegetation season allows us to identify the active root zone extension of trees. In winter, upslope of the tree row, downwards leaching partly renews the soil solution in the root zone, while the slow water movement under the trees or farther downslope results in Cl^? accumulation and leads to a salinization of the soil and groundwater. This salinization is of the same order as experimental conditions produce negative effects on oak seedlings. The measurement of Cl^? concentrations in the unsaturated zone under tree rows at the end of the vegetation season would indicate whether certain topographic, pedological or climatic conditions are likely to favour a strong salinization of the soil, as observed in the present study. Copyright © 2009 John Wiley & Sons, Ltd.     

Percolation losses in paddy fields with a dynamic soil structure: model development and applications

The hydraulic characteristics of the plough pan of paddy fields provide continuous ponding conditions during the growing season and control the water use efficiency in wet rice production. Its saturated hydraulic conductivity K_s, however, exhibits a large spatiotemporal variability as a consequence of a highly dynamic soil structure involving temporary shrinkage cracks. Water flow through the earthen bunds surrounding the fields further contributes to the uncertainty in water flux calculations. The objective of this study was to develop a simple deterministic model with stochastic elements (‘PADDY‐FLUX’) for depiction of deep percolation, and to assess the effect of different water management scenarios on percolation in two channel command areas. Darcy's law is used as the fundamental equation for water flow calculations with the ponding water depth h as a time‐dependent variable. Flux uncertainty is estimated by a Monte‐Carlo‐type implementation. K_s is treated as a random variable of a bimodal probability density function (PDF), which is the weighted sum of two Gaussian PDFs (accounting for a matrix and a preferential flow domain). The weighing factor α is a function of h, reflecting an increasing risk for preferential flow situations after desiccation and the development of shrinkage cracks. Under‐bund percolation is calculated using transfer functions. The results demonstrate that percolation losses increase in the following order: continuous soil saturation < continuous flooding (CF) < mid‐season drainage and intermittent irrigation (MD + II) < mid‐season drainage and continuous flooding. The bunds contribute up to 54 and 17% to total fluxes under CF and MD + II, respectively. Preferential water fluxes are responsible for the major part of water losses as soon as desiccation causes the formation of shrinkage cracks. As a conclusion, continuous soil saturation should be promoted as the least water‐intensive irrigation regime, while intermittent irrigation is recommended only in case that irreversible shrinkage cracks have already developed. Copyright © 2010 John Wiley & Sons, Ltd.     

Salinization of a fresh palaeo-ground water resource by enhanced recharge

Deterioration of fresh ground water resources caused by salinization is a growing issue in many arid and semi-arid parts of the world. We discuss here the incipient salinization of a 10(4) km2 area of fresh ground water (<3,000 mg/L) in the semiarid Murray Basin of Australia caused by widespread changes in land use. Ground water 14C concentrations and unsaturated zone Cl soil water inventories indicate that the low salinity ground water originated mainly from palaeo-recharge during wet climatic periods more than 20,000 years ago. However, much of the soil water in the 20 to 60 m thick unsaturated zone throughout the area is generally saline (>15,000 mg/L) because of relatively high evapotranspiration during the predominantly semiarid climate of the last 20,000 years. Widespread clearing of native vegetation over the last 100 years and replacement with crops and pastures leads to enhancement of recharge rates that progressively displace the saline soil-water from the unsaturated zone into the ground water. To quantify the impact of this new hydrologic regime, a one-dimensional model that simulates projected ground water salinities as a function of depth to ground water, recharge rates, and soil water salt inventory was developed. Results from the model suggest that, in some areas, the ground water salinity within the top 10 m of the water table is likely to increase by a factor of 2 to 6 during the next 100 years. Ground water quality will therefore potentially degrade beyond the point of usefulness well before extraction of the ground water exhausts the resource.     

Comparison of soil salinity and solute transport for different cultivated soil types in northeastern Egypt / Comparaison de la salinité du sol et du transport de solutés pour différents types de sols cultivés du nord-est de l'Egypte

Abstract

The soil salinity distribution and solute transport properties of three different soil types were investigated and compared within a project area in northeastern Egypt. For this purpose, dye tracer experiments and salinity sampling were carried out. The resulting salinity maps showed that the soil salinity in the cultivated western site of the project area is 8–10 times higher than that in the cultivated eastern site. However, the cultivated soil displayed significantly lower salinity with higher uniformity as compared to the uncultivated soil. The preferential flow phenomenon was less apparent in the cultivated soil. This is mainly due to tillage which disrupts the structure of the soil so that deep cracks are no longer connected to the soil surface. This reduces the risk for groundwater contamination through preferential flow. The study showed that careful and continuous monitoring of the salinity status is needed now and in the future.     

The effect of beech stemflow on spatial patterns of soil solution chemistry and seepage fluxes in a mixed beech/oak stand

Stemflow of beech (Fagus sylvatica L.) represents a significant input of water and elements to the soil and might influence the spatial patterns and the rate of seepage fluxes at the stand scale. We investigated the soil solution chemistry at different depths and distances from the stem and the element fluxes with stemflow, throughfall and seepage in proximal and distal stem areas of a 130‐year‐old beech/oak forest in Steigerwald (northern Bavaria, Germany). The proximal stem area (in total 286 m² ha⁻¹) was defined as a 1 m², 60 cm deep cylinder around the beech stem. Seepage fluxes were calculated by a soil hydrological model for 1996 using measured soil matrix potentials and tree xylem flow data for calibration. Stemflow represented 6·6% of the annual soil water input. With the exception of H⁺ fluxes, less than 10% of the total element fluxes with throughfall and stemflow reached the soil via stemflow. The volume‐weighted concentrations of H⁺, K⁺ and SO₄²⁻ in stemflow were higher than those in throughfall, while other elements had similar concentrations. Soil solution K⁺ concentrations decreased with stem distance, but the Na⁺, Mg²⁺, Cl⁻ and SO₄²⁻ concentrations increased. Gradients for other elements were not statistically significant. Stemflow had a strong influence on the spatial patterns of element fluxes with seepage. The water fluxes through the soil of the proximal stem areas at a depth of 60 cm contributed 13·5% to the total seepage at the stand scale. Proximal to the stems about 20% of total seepage for K⁺, Mn²⁺, Alⁿ⁺, dissolved organic N and dissolved organic C were concentrated, but only 8–10% for Na⁺, Mg²⁺ and Ca²⁺. The loss of acid‐neutralizing capacity calculated from the flux balance was about four times higher proximal to the stems compared with distal areas, indicating high rates of soil acidification proximal to the stems. Our results confirm the concept of a microsite around beech stems, characterized by high element and water fluxes in comparison with distal stem areas. Calculations of seepage fluxes and element budgets in beech stands have to consider the spatial heterogeneity of fluxes induced by stemflow. Copyright © 2000 John Wiley & Sons, Ltd.     

Solute transport and water content measurements in clay soils using time domain reflectometry

Abstract

Clayey and saline soils have been shown to be problematic for time domain reflectometry (TDR) measurements. This study presents some of these problems and discusses solutions to them. Thirteen solute transport experiments were carried out in three undisturbed soil columns of swelling clay soil from Tunisia, labelled S1, S2, and S3 respectively. The columns were collected at three different physiographical regions within a catchment. Water fluxes ranged from 1.2 to 7.2 cm day^?1. The large solute transport heterogeneity and large tailing indicated that preferential flow was most pronounced in S1. The preferential flow took place in voids between structural elements and in wormholes. In S3, preferential flow was also evident, but not to the same extent as in S1. In S2, the solute transport was more uniform with little preferential flow. The heterogeneity of the solute transport increased with the water flux in S1 and to a smaller extent in S3, whereas it remained constant in S2. In a previous dye experiment in the field, preferential flow in cracks was observed at those sites where S1 and S3 were collected. In the column experiments, preferential flow in these cracks was less due to the higher initial water content compared to the dye experiments, indicating that the desiccation cracks were closed by the swelling clay.