Runoff Fluxes Research Articles

The processes and mechanism of failure and debris flow initiation for gravel soil with different clay content

Clay content can influence gravel soil mass failure and debris flow initiation significantly. But quantification of the clay content influence in the published literature is lacking. This paper describes the experiments carried out to explore soil mass failure and debris flow initiation with different clay contents. The whole process of gravel soil mass failure and debris flow initiation can be grouped into five phases: (i) infiltration excess runoff, (ii) fracture extending, (iii) initial slip, (iv) large-scale slip, (v) debris-flow triggering. In the experiments, when the clay content of soil mass is lower than 2.5%, no soil failure and debris flow are triggered. Only the soil masses with moderate clay contents (5–10%) show the clear processes of the five phases that need the shortest rainfall duration among different clay contents. Soil masses with high clay content (> 10%) and low clay content (2.5–5%) need longer rainfall duration to cover the five phases for partially triggering soil failure and debris flows. Clay content is closely linked with the soil failure and debris flow initiation under rainfall. The key mechanism can be grouped into two reasons: (i) the clay particles absorb the moisture and dilate to fill the soil pores, and the content of the clay has a direct link with the filled pores, (ii) the gravel soil masses with different clay content differ in their structure. For the low clay content soil mass group (2.5–5%), the coarse grains of the gravel soil form large pores and saturated clay particles are too few to block most soil pores. As a result, pore water pressure of soil mass is released easily in the raining process. Therefore, inner friction holds strong and soil mass is hard to mobilize into slip and debris flows. On the other hand, for soil masses with high (> 10%) clay contents, soil failure and debris flow are triggered under the aid of surface runoff flux to reduce the cohesion of the soil. Whereas, only for the soil masses with moderate clay contents (5–10%), large-scale soil failure and debris flow can be triggered within the shortest duration. In this case, the debris flow is of the form of “soil mechanics,” which is triggered after soil masses slip and liquefaction. The others are of the form of “hydraulic mechanics.” The experiments provide valuable evidence for quantifying clay content impact on gravel soil failure and debris flow initiation. Because they were carried out in a flume, the research results are applicable to the soil failure and debris flow initiation in bedrock gully situations.

Approach Using Active Groundwater Storage for Hydrologic Model Calibration in West-Central Florida

Hydrologic model calibration is always a challenging and tedious process especially for the calibration of complex models, which includes continuous hydrograph models, requires sophisticated calibration methods. The Hydrologic Simulation Program-FORTRAN (HSPF) is one of the popular and powerful time variable hydrologic models. However, in order to improve the assessment of hydrologic activities in shallow ground water settings, the model needs to be reliably calibrated for ground water contribution. Little guidance is provided in the literature concerning the manner of this contribution. In fact, the most common calibration of HSPF uses subjective parameter fitting and focuses on the attainment of statistical goodness of fit of runoff fluxes and water levels, ignoring ground water components. The goal of this research is using a different approach to calibrate HSPF with observed water table records. In this study, HSPF is applied on a small area in west-central Florida and calibrated by comparing active ground water storage to well elevation records in range land and forested land covers. The Nash-Sutcliffe efficiency and correlation coefficient computed using observed and simulated daily flows are 0.91 and 0.96 at Peace River, respectively, also with good fair results for other stations in the model domain. The study shows that improved calibration of the model can be achieved if active ground water storage and well records are compared for timing and magnitude of fluctuations.

Fluvial Fluxes of Water, Suspended Particulate Matter, and Nutrients and Potential Impacts on Tropical Coastal Water Biogeochemistry: Oahu, Hawai‘i

Baseflow and storm runoff fluxes of water, suspended particulate matter (SPM), and nutrients (N and P) were assessed in conservation, urban, and agricultural streams discharging to coastal waters around the tropical island of Oahu, Hawai‘i. Despite unusually low storm frequency and intensity during the study, storms accounted for 8–77% (median 30%) of discharge, 57–99% (median 93%) of SPM fluxes, 11–79% (median 36%) of dissolved nutrient fluxes and 52–99% (median 85%) of particulate nutrient fluxes to coastal waters. Fluvial nutrient concentrations varied with hydrologic conditions and land use; land use also affected water and particulate fluxes at some sites. Reactive dissolved N:P ratios typically were ≥16 (the ‘Redfield ratio’ for marine phytoplankton), indicating that inputs could support new production by coastal phytoplankton, but uptake of dissolved nutrients is probably inefficient due to rapid dilution and export of fluvial dissolved inputs. Particulate N and P fluxes were similar to or larger than dissolved fluxes at all sites (median 49% of total nitrogen, range 22–82%; median 69% of total phosphorus, range 49–93%). Impacts of particulate nutrients on coastal ecosystems will depend on how efficiently SPM is retained in nearshore areas, and on the timing and degree of transformation to reactive dissolved forms. Nevertheless, the magnitude of particulate nutrient fluxes suggests that they represent a significant nutrient source for many coastal ecosystems over relatively long time scales (weeks–years), and that reductions in particulate nutrient loading actually may have negative impacts on some coastal ecosystems.

Hydrological properties of a double-layer soil cover system for uranium mining dumps in Eastern Germany

As an adequate rehabilitation strategy for less heavy metal contaminated and non acid-generating waste rock dumps of uranium mining in Eastern Germany a soil cover system with 0.8 m compacted loam sublayer and 0.2 m toplayer of compost and mineral soil (vol. 50%/vol. 50%) was suggested. However, at slope inclinations upto 1: 2.5 it was unclear whether this cover system would remain stable in case of high precipitation (100-year event) and to which degree water infiltration into the pile could be reduced. Within a six-year lysimeter study it was shown that after establishing permanent grassland surface, run-off and lateral water fluxes did not exceed 5% of annual precipitation, even at high rainfall. Moreover, depending on the exposure, water infiltration into the waste rock dump was reduced to 22% of annual precipitation (SE slope) and 32% (NE slope) with a linear correlation between annual rainfall and water fluxes. The hydrology of the double-layer covering was governed by the transpiration of the vegetation. Therefore, the cover system is quite advantageous under low precipitation conditions with predominating rainfall in the growing season and high potential evapotranspiration.

Research advances in nutrient runoff on sloping land in watersheds

High nitrate and phosphorus concentrations in runoff resulting from sloping lands (especially agricultural sloping lands) have direct consequences on the aquatic environment through eutrophication of surface water. This paper details the transfer mechanism of nutrients in soil on sloping lands influenced by natural and artificial factors. Results showed that the rainfall intensity had a small influence on nutrient concentrations in runoff, but a significant influence on the runoff flow on sloping lands. The slope length influenced the nutrient loss by soil erosion on areas that receive rainfall. The slope gradient influenced the nutrient loss by runoff flux and velocity on sloping land. As the slope gradient decreased, the nutrient loss decreased because of the increase in infiltration. The soil texture, porosity, and water content influenced the motion of soil water and the transfer and form of nutrients in soil, through oxidation and deoxidation. Vegetative coverage influenced the infiltration coefficient of rainwater into subsurface soil, and thus influenced the runoff flow velocity. Therefore, different sloping lands need to be managed in different ways. To better carry out best management practices in watersheds, a new management based on sloping lands needs to be further studied, including changing fertilization quantity, improving fertilization technology, adjusting vegetation coverage, and studying cultivation systems on different sloping lands within a watershed.

Sediment and nutrient losses from exotic Pinus plantation management operations under simulated rainfall

Summary Rainfall simulation experiments were carried out to measure runoff and soil water fluxes of suspended solids, total nitrogen, total phosphorus, dissolved organic carbon and total iron from sites in Pinus plantations on the coastal lowlands of south-eastern Queensland subjected to various operations (treatments). The operations investigated were cultivated and nil-cultivated site preparation, fertilised site preparation, clearfall harvesting and prescribed burning; these treatments were compared with an 8-y-old established plantation. Flow-weighted mean concentrations of total nitrogen and total phosphorus in surface runoff from the cultivated and nil-cultivated site-preparation, clearfall harvest, prescribed burning and 8-y-old established plantation treatments were very similar. However, both the soil water and the runoff from the fertilised site preparation treatment contained more nitrogen (N) and phosphorus (P) than the other treatments — with 3.10 mg L−1 and 4.32 mg P L− (4 and 20 times more) in the runoff. Dissolved organic carbon concentrations in runoff from the nil-cultivated site-preparation and prescribed burn treatments were elevated. Iron concentrations were highest in runoff from the nil-cultivated site-preparation and 8-y-old established plantation treatments. Concentrations of suspended solids in runoff were higher from cultivated site preparation and prescribed burn treatments, and reflect the great disturbance of surface soil at these sites. The concentrations of all analytes were highest in initial runoff from plots, and generally decreased with time. Total nitrogen (mean 7.28, range 0.11–43.27 mg L−) and total phosphorus (mean 11.60, range 0.06–83.99 mg L−) concentrations in soil water were between 2 and 10 times greater than in surface runoff, which highlights the potential for nutrient fluxes in interflow (i.e. in the soil above the water table) through the general plantation area. Implications in regard to forest management are discussed, along with results of larger catchment-scale studies.

A comparison of solute fluxes and sources from glacial and non‐glacial catchments over contrasting melt seasons

AbstractSolute and runoff fluxes from two adjacent alpine streams (one glacial and one non‐glacial) were investigated to determine how the inorganic solute chemistry of runoff responded to seasonal and interannual changes in runoff sources and volume, and to differences in physical catchment properties. Intercatchment differences in solute composition were primarily controlled by differences in catchment geology and the presence of soils, whereas differences in total solute fluxes were largely dependent on specific discharge. The glacial stream catchment had higher chemical denudation rates due to the high rates of flushing (higher specific discharge). The non‐glacial Bow River had higher overall concentrations of solutes despite the greater prevalence of more resistant lithologies in this catchment. This is likely the result of both longer average water–rock contact times, and a greater supply of protons from organic soils and/or pyrite oxidation. Increases in snowpack depth/snowmelt runoff reduced the retention of nitrate in the Bow River catchment (i.e. increased nitrate export), probably by reducing net biological uptake, or by reducing the proportion of runoff that had contact with biologically active soil horizons that tend to remove nitrate.The two streams exhibited opposite solute flux responses to climate perturbations over three melt seasons (1998, 1999, and 2000). The 1998 El Niño event resulted in an unusually thin winter snowpack, and increased runoff and solute fluxes from the glacial catchment, but decreased fluxes from the Bow River catchment. Solute fluxes in the Bow River increased proportionally to discharge, indicating that increased snowmelt runoff in this catchment resulted in a proportional increase in weathering rates. In contrast, the proportional variation in solute flux in the glacial stream was only ∼70–80% of the variation in water flux. This suggests that increased ablation of glacier ice and the development of subglacial channels during the 1998 El Niño year apparently reduced the average water–rock contact time in the glacial catchment relative to seasons when the subglacial drainage system was primarily distributed in character. Copyright © 2005 John Wiley & Sons, Ltd.

Impact of DEM mesh size and soil map scale on SWAT runoff, sediment, and NO 3–N loads predictions

The accuracy of agricultural nonpoint source pollution models depends to a great extent on how well model input spatial parameters describe the relevant characteristics of the watershed. It is assumed that reducing the precision of spatial input parameters affects the simulation results of runoff and sediment yield from the entire watershed. However, there may be no significant increase in the accuracy of models, as a result of more precise topographic or soil information, which increase the input data collection and preparation. The objective of this study was to determine the impact of the mesh size of the digital elevation model, DEM (from 20 to 500 m) and the soil map scale (1/25,000; 1/250,000 and; 1/500,000 scale) within the Soil and Water Analysis Tool (SWAT) to simulate runoff, sediment, and NO 3–N loads at the outlet of an agricultural watershed. Results of the Lower Walnut Creek (21.8 km 2, central Iowa) showed that an upper limit to DEM mesh size of 50 m is required to simulate watershed loads. Decreasing the mesh size beyond this threshold does not substantially affect the computed runoff flux but generated prediction errors for nitrogen and sediment yields. Whatever the DEM mesh size considered, a detailed soil map has to be considered to accurately estimate the loads. Finally, the impacts of DEM mesh size and soil map scale on the modeling results are discussed in respect of the relevant characteristics of the watershed and included in SWAT.

High temporal resolution of ion fluxes in semi-natural ecosystems – gain of information or waste of resources?

Monitoring programs of ion concentrations and fluxes in semi-natural ecosystems are confronted with the task to gain as much information as possible with simultaneously minimizing costs and efforts. The aim of this study was (i) to assess how much of the heterogeneity of solution concentrations is lost because of temporal integration of measurements and (ii) to estimate the error in ion fluxes due to temporal integration. High resolution measurements (daily interval) of ion concentrations (sulfate, nitrate, chloride, pH and EC) in throughfall, soil solutions and runoff at the catchment Lehstenbach (Fichtelgebirge, Northeast Bavaria, Germany) were compared over a two year period with the reference monitoring program (biweekly measurement interval). Evaluation of the maximum temporal heterogeneity of ion concentrations in throughfall, soil solution and runoff (expressed as minimum, maximum, median and 25–75% percentile) did not result in an overall higher heterogeneity of the high resolution measurements compared to the reference program. The calculation of runoff fluxes from the reference data (biweekly concentration) resulted in significant errors of up to 25% for time periods < 1 year (high resolution data was considered the "true" value and set as 100%). However, errors became minor (< 10%) if longer time periods were considered. The suitability of different interpolation methods to up-scale biweekly concentration data for the calculation of runoff fluxes was evaluated in this study. We concluded for the monitoring programs at the Lehstenbach catchment that a biweekly measurement interval seemed to be suitable to capture the heterogeneity of ion concentrations and fluxes (and thus temporal trends). In comparison, high resolution measurements with a daily measurement interval were higher in cost, work and time resources and had a relatively low information gain. While the introduced methods are applicable in all monitoring programs, conclusions on temporal resolution of measurements are most likely not valid for systems where ion concentrations have a low autocorrelation length (e.g., agricultural or urban systems with nitrate or pesticide treatment; tropical systems with extreme temperature or hydrological events).

Inputs and outputs of mercury from terrestrial watersheds: a review

This review focuses on mercury (Hg) inputs and outputs in temperate and boreal terrestrial systems. It covers deposition via throughfall and litterfall, whose sum (ca. 38 µg m–2 a–1) is greater than that via precipitation (ca. 10 µg m–2 a–1). Outputs considered include volatilization, soil sequestration, and streamflow. The former is highly uncertain, but the mean rate (11 ng m–2 h–1) over a growing season is equivalent to about 32 µg m–2 a–1. Modern rates of soil sequestration (ca. 5 µg m–2 a–1) and streamflow fluxes (ca. 2 µg m–2 a–1) balance the annual budget. The majority of the uncertainty in the budget is related to volatilization. Nonetheless, a large fraction of atmospheric Hg is likely a product of continuing deposition and volatilization. Watershed characteristics related to streamflow fluxes of both Hg and methylmercury (MeHg) are discussed. Both runoff concentration and flux of Hg are weakly and inversely related to watershed size. Dissolved organic carbon (DOC) and particulates are important carriers of Hg; watershed activities that affect either affect Hg flux. Runoff flux of MeHg is skewed with about 80% of observations less than 0.15 µg m–2 a–1. Although there is no pattern of MeHg flux with watershed size, there is a strong positive relationship between flux and wetland area. Wetlands are a site of MeHg production and their presence increases water residence time; both increase MeHg flux. Concentrations of MeHg in streamflow from watersheds with wetlands are near the current water quality criterion, and effective control measures in those watersheds appear problematic.Key words: deposition, non-point pollution, methylmercury, wetlands, dissolved organic carbon (DOC).

A screening model-based study of transport fluxes and fate of airborne mercury deposited onto catchment areas

Dynamics of airborne mercury deposited onto catchment areas is investigated within the framework of a simulation model. Model results show that, for a particular atmospheric deposition rate, significant interannual variability in mercury transport flux in catchments is caused by climatology and corresponding differences in catchment soil loss rates; in comparison to the normal year, runoff flux increased by a factor of 2–3 for the wet year (rainfall 35% above normal) while for the dry year (rainfall 18% below normal) runoff flux decreased by factors of 5–7. The interaction of parameters describing soil type, topography and vegetation cover causes variability in both transport and emission fluxes among catchments; as soil loss rate increases by a factor of 5 due to variations in these parameters among the examined catchments, annual average transport flux increases by a factor of 3; and annual average emission flux of mercury (as Hg 0) from soil to the atmosphere decreases by a factor of 2 due to the decreased levels of soil mercury associated with catchment soil loss increases. Seasonal variability of transport flux is associated with seasonal changes in precipitation and soil loss rates while seasonal changes of emission flux are primarily due to changes in soil moisture regime and temperature. Although modeled results are consistent with observational data from previous studies, they must be interpreted in a relative sense due to the screening-level character of this study.

Fluxes and Trends of Nitrogen and Sulphur Compounds at Integrated Monitoring Sites in Europe

The International Cooperative Programme on Integrated Monitoring (ICP IM) is part of the effects monitoring strategy of the UN/ECE Convention on Long-Range Transboundary Air Pollution. We calculated input-output budgets and trends of N and S compounds, base cations and hydrogen ions for 22 forested ICP IM catchments/plots across Europe. The site-specific trends were calculated for deposition and runoff water fluxes and concentrations using monthly data and non-parametric methods. The reduction in deposition of S and N compounds, caused by the new Gothenburg Protocol of the Convention, was estimated for the year 2010 using atmospheric transfer matrices and official emissions. Statistically significant downward trends of SO4, NO3 and NH4 bulk deposition (fluxes or concentrations) were observed at 50% of the ICP IM sites. Implementation of the new UN/ECE emission reduction protocol will further decrease the deposition of S and N at the ICP IM sites in western and northwestern parts of Europe. Sites with higher N deposition and lower C/N-ratios clearly showed an increased risk of elevated N leaching. Decreasing SO4 and base cation trends in output fluxes and/or concentrations of surface/soil water were commonly observed at the ICP IM sites. At several sites in Nordic countries decreasing NO3 and H+ trends (increasing pH) were also observed. These results partly confirm the effective implementation of emission reduction policy in Europe. However, clear responses were not observed at all sites, showing that recovery at many sensitive sites can be slow and that the response at individual sites may vary greatly.

Surface and subsurface nitrate flow pathways on a watershed scale.

Determining the interaction and impact of surface runoff and subsurface flow processes on the environment has been hindered by our inability to characterize subsurface soil structures on a watershed scale. Ground penetrating radar (GPR) data were collected and evaluated in determining subsurface hydrology at four small watersheds in Beltsville, MD. The watersheds have similar textures, organic matter contents, and yield distributions. Although the surface slope was greater on one of the watersheds, slope alone could not explain why it also had a nitrate runoff flux that was 18 times greater than the other three watersheds. Only with knowledge of the subsurface hydrology could the surface runoff differences be explained. The subsurface hydrology was developed by combining GPR and surface topography in a geographic information system. Discrete subsurface flow pathways were identified and confirmed with color infrared imagery, real-time soil moisture monitoring, and yield monitoring. The discrete subsurface flow patterns were also useful in understanding observed nitrate levels entering the riparian wetland and first order stream. This study demonstrated the impact that subsurface stratigraphy can have on water and nitrate (NO3-N) fluxes exiting agricultural lands, even when soil properties, yield distributions, and climate are similar. Reliable protocols for measuring subsurface fluxes of water and chemicals need to be developed.

A catchment‐based approach to modeling land surface processes in a general circulation model: 2. Parameter estimation and model demonstration

The viability of a new catchment‐based land surface model (LSM) developed for use with general circulation models is demonstrated. First, simple empirical functions, tractable enough for operational use in the LSM, are established that faithfully capture the control of topography on the subgrid variability of soil moisture and the surface water budget, as predicted by theory. Next, the full LSM is evaluated off‐line. Using forcing and validation data sets developed for the Project for Intercomparison of Land‐Surface Parameterization Schemes Phase 2c, the minimally calibrated model is shown to reproduce observed evaporation and runoff fluxes successfully in the Red‐Arkansas River Basin. A complementary idealized study that employs the range of topographic variability seen over North America demonstrates that the simulated surface water budget does vary strongly with topography, which can, by itself, induce variations in annual runoff as high as 20%.

Pools and fluxes of mercury and methylmercury in two forested catchments in Germany

Mercury (Hg) and methylmercury (CH 3Hg +) are global pollutants, but little information is available on rates of atmospheric input, distribution and mobility in soils and catchments of central Europe. The objectives of this study were to investigate input and output fluxes of these compounds in a deciduous and a coniferous catchment in NE Bavaria (Germany), and to estimate pools and mobility of total Hg (Hg tot) and CH 3Hg + at the catchment scale. Bulk precipitation, throughfall, litterfall and runoff were collected biweekly from April 1998 to April 1999. Several soil profiles were sampled to estimate pools of Hg compounds in the catchments. In both catchments highest contents of Hg tot were found in the Oa layer of the forest floor (up to 500 ng g −1) and the soil storage of Hg tot calculated for a soil depth of 60 cm was approximately 890 g ha −1 in the coniferous and 190 g ha −1 in the deciduous catchment. Highest contents of CH 3Hg + in upland soils were observed in the Oi layer of the forest floor, and soil storage of CH 3Hg + was 4.35 g ha −1 in the coniferous and 0.59 g ha −1 in the deciduous catchment. The annual total deposition of Hg tot (total deposition not measured directly but calculated from throughfall and litterfall) was 541 mg ha −1 year −1 in the coniferous and 618 mg ha −1 year −1 in the deciduous catchment. Total deposition rates of CH 3Hg + were 3.5 and 2.6 mg ha −1 year −1. The contribution of litterfall to the total deposition of Hg tot and CH 3Hg + was 55% in the deciduous catchment. In the coniferous catchment, the contribution of litterfall to total deposition was only 29% for Hg tot, but 55% for CH 3Hg +. By far the largest proportion of the deposited CH 3Hg + and Hg tot remained in the catchments (85% in the coniferous, 95% in the deciduous). As compared to remote Swedish catchments, deposition and output via runoff of Hg tot were higher, but deposition and output of CH 3Hg + were lower in our catchments. In contrast to other studies, the annual budget revealed no differences in the mobility between the two species at the catchment scale. However, temporal patterns of the runoff fluxes and converse gradients of CH 3Hg + and Hg tot contents in the forest floor indicated differences in mobility on shorter time scales.

Direct Atmospheric Inputs versus Runoff Fluxes of Mercury to the Lower Everglades and Florida Bay

Age-dated sediments from the lower Everglades and Florida Bay provide a record of inputs of excess Hg from direct atmospheric input versus runoff. Direct atmospheric fluxes of excess Hg to sediments in the lower Everglades and Florida Bay, calculated using a mass balance model for excess 210Pb, currently average 24 ± 9 μg m-2 yr-1 and are comparable with recent results from bulk atmospheric deposition. In contrast, present-day runoff fluxes of excess Hg to area sediments are variable, ranging from about 4−160 μg m-2 yr-1. The runoff flux now carries 60−80% of the total flux of excess Hg to the sediments in areas near river sloughs but less than 20% of the total flux of excess Hg in more remote areas of Florida Bay. These results show the greater importance of runoff relative to direct atmospheric deposition for Hg inputs to many areas of the lower Everglades and immediately adjacent Florida Bay. Thus, the choice of future water management strategies can play an important role in controlling Hg inputs to t...

Trend assessment of deposition runoff water fluxes and water chemistry at European ICP Integrated Monitoring sites

Trend assessment of deposition runoff water fluxes and water chemistry at European ICP Integrated Monitoring sites

Nutrient Movement Due to Overland Flow in Managed Native Eucalyptus Forests, Southeastern Australia

A rainfall simulator study of total nutrient movement due to overland flow on forest skid trails and general harvesting areas (GHA) in the native eucalypt forests of southeastern Australia, confirmed that the disturbed and compacted areas were the dominant sources of sediment-bound pollutants. Low runoff production and low sediment availability on the rough surfaces of the GHA reduced total nutrient yields. On the more recently disturbed and burnt sites, the solution pathway may have been more dominant due to runoff coming in contact with surface ash and charcoal. We also recorded significant reductions in the total nutrient yields as runoff from the skid trail was delivered to the adjacent hillslope via a cross bank (water bar). This was due primarily to coarse-grained sediment deposition at the base of the banks. Nutrient concentrations in the eroded sediment at the cross bank outlets, however, remained 9 times higher than the in-situ surface soils, due to the preferential delivery of fine aggregate material to the adjacent hillslope. Further reductions in total nutrient yields occurred as overland flow passed through the GHA, illustrating the effectiveness of vegetative filtering in reducing runoff volumes and sediment fluxes. Temporal trends in the data highlighted significant reductions in total nutrient movement on both areas of disturbance as sites regenerated over a period of ∼ 5 years. Water pollution abatement practices in these environments should aim to locate skid trails to maximise the redistribution of runoff and sediment from highly disturbed areas to more vegetated parts of the hillslope, and by appropriate cross bank spacing, reduce the volume of overland flow carrying sediment and nutrients.

A modeling analysis of factors influencing mass balance components of airborne deposited mercury in terrestrial landscapes

A modeling analysis of factors influencing deposited airborne mercury mass balance in terrestrial landscapes is presented. Computer simulations are performed with a recently developed model for various sets of conditions. Preliminary estimates based on variability and sensitivity analysis of simulations are compatible with the currently ‐available knowledge of mercury terrestrial cycling. Atmospheric deposition of divalent mercury (Hg(II)) is the main source for mercury in uncontaminated terrestrial systems. Mercury mass balance is characterized by the intensive exchange of mercury between atmosphere and ground surface. Atmospheric deposition, weather conditions, hydrologic transport, Hg(H) reduction and landscape scale characteristics are the most important factors regulating conditions that have impacts on mercury mass balance components. Deposited mercury displays a strong tendency to be remobilized into the atmosphere as elemental mercury (Hg°) formed by the reduction of divalent mercury in the surface soil; atmospheric flux (emission flux) is found to exceed, in general, mercury transport flux from soil, ranging from 35 to 90% of the atmospheric deposition. Total mercury surface runoff flux varies from 2 to 60% of the atmospheric deposition. Leaching flux is a minor component of mercury mass balance, ranging from 3 to 16% of atmospheric deposition. Transformation processes such as methylation and demethylation, although not important for total mercury flux, dominate the amount of methyl mercury in the soil and strongly affect methyl mercury fluxes with significant implications for biotic uptake and bioaccumulation. Limitations of the modeling analysis are also discussed.

Soil hydrological response under simulated rainfall in the Dehesa land system (Extremadura, SW Spain) under drought conditions

Soil hydrology was investigated in the Guadelperalon experimental watershed in order to determine the influence of land use and vegetation cover on runoff and infiltration within the Dehesa land system. Five soil–vegetation units were selected: (1) tree cover, (2) sheep trials, (3) shrub cover, (4) hillslope grass and (5) bottom grass. The results of the simulated rainfall experiments performed at an intensity of 56·6 mm h−1 during one hour on plots of 0·25 m2, and the water drop penetration time test indicate the importance of water repellency in the Dehesa land system under drought conditions. Low infiltration rates (c. 9–44 mm h−1) were found everywhere except at shrub sites and in areas with low grazing pressure. Soil water repellency greatly reduced infiltration, especially beneath Quercus ilex canopies, where fast ponding and greater runoff rates were observed. The low vegetation cover as a consequence of a prolonged drought and grazing pressure, in conjunction with the soil water repellency, induces high runoff rates (15–70 per cent). In spite of this, macropore fluxes were found in different locations, beneath trees, on shrub-covered surfaces, as well as at sites with a dominance of herbaceous cover. Discontinuity of the runoff fluxes due to variations in hydrophobicity causes preferential flows and as a consequence deeper infiltration, especially where macropores are developed. © 1998 John Wiley & Sons, Ltd.