Subsurface Water Flow Research Articles

Shallow groundwater nitrogen and denitrification in a newly afforested, subirrigated riparian buffer

Summary1. The EU ‘Nitrates Directive’ (Directive 91/676/EEC) and the WFD (Water Framework Directive 2000/60/EEC) introduced a series of measures designed to reduce and prevent water pollution caused or induced by nitrates from agricultural sources. Therefore, there is an urgent requirement to control the nitrate concentration in freshwater. The objective of this paper was to verify the potential capacity of a specifically designed afforested riparian zone in removing the excess of nitrogen from river water.2. A buffer zone was set with irrigation ditches, to produce a subsurface water flow carrying water from the study river through the buffer strip to drainage ditches. This experimental system enables the co‐occurrence of two main processes: vegetation/microbial nitrogen uptake and denitrification. Both in situ denitrification and denitrification potential were measured at different soil depths, and nitrogen removal of water passing through the buffer system was measured.3. After the first year, high removal rates (63–64%) of total nitrogen in water were recorded. The lowest rate of denitrification took place in the upper soil layer, while maximum denitrification occurred in the medium layer (40–55 cm). Denitrification occurred mainly in the first few metres of the irrigation ditches leading away from the river. The denitrification rates clearly increased from the second to the third year, with highest rates in summer and autumn. Denitrification potential indicated that carbon availability was the most limiting factor.4. Synthesis and applications. This study has demonstrated that nitrogen levels can be reduced in rivers by forcing water to circulate through afforested buffers. Nitrogen was removed both by plants and by microbial denitrification. Such activity can be supported by promoting anoxic conditions through appropriate water flow management. This could be achieved by creating semi‐natural floodplains where water flows can be efficiently managed as in a drained wetland.

Simulating unsteady flow, anabranching, and hyporheic dynamics in a glacial meltwater stream using a coupled surface water routing and groundwater flow model

Flooding affects ecosystems by transporting water and solutes across aquatic‐terrestrial interfaces, removing nutrient and organic substrate limitations, and spurring biogeochemical activity. Few studies have considered the influence of flooding on surface water–groundwater interactions. This research examines the temporally variable water storage and exchange in a stream in the McMurdo Dry Valleys (MDV) of Antarctica, where diel flood pulses occur due to glacial melt. Several MDV streams display truncated discharge peaks, suggesting water storage between the source glacier and the gauging station. We tested the hypothesis that stream braids and subsurface water storage contribute to the difference between glacial melt and stream outflow hydrographs by constructing a coupled surface water routing and subsurface water flow model. This model routes water into stream braids at high flows and allows this water to infiltrate and return to the stream via subsurface flow paths as flows recede. Our simulation demonstrates the importance of surface–subsurface water interactions in controlling the hydrograph shape. Maximum simulated discharge was sensitive to storage parameters including aquifer depth and the flooding threshold, while minimum discharge was sensitive to hydraulic conductivity. Subsurface storage volume varied by 38% over a diel cycle and stream‐subsurface exchange rates varied from 0 to 0.19 m3 h−1 m−1, with exchange from the stream to the subsurface during high flows, and vice versa at low flows. These results underscore how unsteady flow can increase hyporheic interactions and ecosystem productivity, and provide support for maintaining natural stream morphology and flow regimes.

Long-term Fate and Persistence of Oil from theExxon ValdezOil Spill: Lessons Learned or History Repeated 2011 International Oil Spill Conference, Portland, Oregon

Abstract As a continuation of the extensive shoreline studies undertaken since 1989, we conducted systematic surveys in 2007 and 2008 to evaluate the form, chemical composition, accessibility, and location of remaining subsurface oil (SSO) residues of Exxon Valdez oil on shorelines in Prince William Sound. We found SSO residues only in isolated patches, buried in low-permeability sediments protected from water-washing and erosion by surface boulder and cobble armoring. These SSO residues are sequestered and largely isolated from natural weathering processes that would result in their complete and rapid removal. SSO residues are not accessible or bioavailable to wildlife that forage on the shore. These findings confirm the lessons learned from all previous crude oil spills: 1) SSO residues can be sequestered for decades in intertidal sediments at locations where the subsurface water flow required for erosion, dissolution, and biodegradation of the oil is low. 2) Sequestration limits the exposure of biota t...

Invited perspective: Why I am an optimist

I address a range of topics that provide the sociopolitical‐technological setting for my professional life. I discuss some influential features of post–World War II world geopolitics, landmark technological developments of that era, and the resulting follow‐up technologies that have made it possible to approach various problems in hydrology and water resources. I next address societal needs that have driven developments in hydrology and water resources engineering and follow with a discussion of the modern foundations of our science and what I think are the principal issues in hydrology. I pose three community challenges that when accomplished should advance hydrologic science: data network needs for improving the water budgets at all scales, characterizing subsurface water flow paths, and the information archiving and mining needs from instruments that will generate substantially richer data detail than have been used for most hydrologic work to the present. I then discuss several hydrologic and water resource risk‐based decision issues that matter to society to illustrate how such risks have been addressed successfully in the past. I conclude with a long‐term community “grand challenge,” the coupled modeling of the ocean‐atmosphere‐landform hydrologic cycle for the purpose of long–lead time hydrologic prediction.

Development and Application of an Integrated Hydrological Model for Lake Watersheds

Abstract Quantitative estimation of surface and subsurface water flow to lakes is important for lake resources management. The calculation of water volume into lakes is also an essential step for estimation of pollutant loads to lakes. This paper presents a mathematical model for coupled simulation of surface and subsurface water flows for lake watersheds, with emphasis on the modelling of several important aspects including groundwater level-soil water storage-surface runoff relation, river-groundwater interactions and lake-watershed hydrological linkage, which are all fundamental for an integrated simulation of hydrological processes for lake watersheds. Field case studies were also given as examples to test the model's suitability and capability. The model is grid-based spatially distributed to facilitate the discretisation of the high heterogeneity of land use types and soil properties. The model can be used as a useful tool to quantify components of hydrological processes for detailed watershed water balance analysis, and to assess impacts of watershed management strategies on the security of lake water resource

The Effect of Subsurface Water Flow on Herbicide Concentrations and Loads in the Stream Water

The Effect of Subsurface Water Flow on Herbicide Concentrations and Loads in the Stream Water

A Method for Installing Zero-Tension Pan and Wick Lysimeters in Soil

Zero-tension pan lysimeters and passive capillary fiberglass wick lysimeters are useful in determining water quality and volumetric aspects of subsurface water flow. A crucial aspect of installing pan and wick lysimeters beneath undisturbed soil is preventing cave-in of the tunnels in the soil that house the lysimeters. Equipment and a method were developed for installing zero-tension pan lysimeters and wick lysimeters in field plots, and this new method provides substantial structural strength of the tunnels. The dimensions of the upper horizontal surface of the pan lysimeters that were used were 0.280 0.430 m, and those of the wick lysimeters were 0.300 0.300 m, but the equipment can be modified to accommodate other lysimeter dimensions. The main equipment components are steel boxes which are pushed horizontally from a pit, into soil beneath the field plot, and a guiding frame that pushes the boxes using a hydraulic cylinder powered by a tractor hydraulic system. Following installation of each pair of steel boxes in soil, the soil from within the boxes was excavated, the upper lid of the lysimeter box was removed, and a lysimeter was placed within the box with the upper surface of the lysimeter directly contacting the soil. The equipment and method were used in the sandy loam soil of tall fescue (Fescue arundinacea Schreb.) pasture plots and the equipment and method worked well. The method also helps assure accurate lysimeter depth and levelness. The lysimeters were installed at a depth of 0.500 m beneath the soil surface. Costs and labor requirements for developing the equipment and using the method are presented. There was no evidence that the installation method altered the soil bulk density or the water flow paths above the lysimeters.

Characterization of active layer water contents in the McMurdo Sound region, Antarctica

AbstractThe liquid soil water contents in the seasonally thawed layer (active layer) were characterized from seven soil climate monitoring sites - four coastal sites from south to north (Minna Bluff, Scott Base, Marble Point and Granite Harbour), and inland sites from low to high altitude (Wright Valley, Victoria Valley and Mount Fleming). Mean water contents ranged from 0.013 m3 m-3 near the surface at Victoria Valley to 0.33 m3 m-3 near the ice-cemented layer at Granite Harbour. The coastal sites have greater soil water contents than the McMurdo Dry Valley and Mount Fleming sites, and moisture contents increase with depth in the active layer. The Wright Valley site receives very little infiltration from snowmelt, with none in most years. All other sites, except Mount Fleming, received between one and four wetting events per summer, and infiltrated water moved to greater depths (≈ 10–25 cm). The Scott Base and Granite Harbour sites are on sloping ground and receive a subsurface flow of water along the ice-cemented permafrost. Our findings indicate that water contents are low with very little recharge, are greatly influenced by the local microclimate and topography, and show no significant increasing or decreasing trend over 10 years of monitoring.

Improving surface–subsurface water budgeting using high resolution satellite imagery applied on a brownfield

The estimation of surface–subsurface water interactions is complex and highly variable in space and time. It is even more complex when it has to be estimated in urban areas, because of the complex patterns of the land-cover in these areas. In this research a modeling approach with integrated remote sensing analysis has been developed for estimating water fluxes in urban environments. The methodology was developed with the aim to simulate fluxes of contaminants from polluted sites. Groundwater pollution in urban environments is linked to patterns of land use and hence it is essential to characterize the land cover in a detail. An object-oriented classification approach applied on high-resolution satellite data has been adopted. To assign the image objects to one of the land-cover classes a multiple layer perceptron approach was adopted (Kappa of 0.86). Groundwater recharge has been simulated using the spatially distributed WetSpass model and the subsurface water flow using MODFLOW in order to identify and budget water fluxes. The developed methodology is applied to a brownfield case site in Vilvoorde, Brussels (Belgium). The obtained land use map has a strong impact on the groundwater recharge, resulting in a high spatial variability. Simulated groundwater fluxes from brownfield to the receiving River Zenne were independently verified by measurements and simulation of groundwater-surface water interaction based on thermal gradients in the river bed. It is concluded that in order to better quantify total fluxes of contaminants from brownfields in the groundwater, remote sensing imagery can be operationally integrated in a modeling procedure.

Infiltration in Arid Environments: Spatial Patterns between Subsurface Heterogeneity and Water‐Energy Balances

Transient simulations of an arid mountain system were conducted using a fully integrated model of subsurface and overland water flow and the land surface energy balance (ParFlow). An hourly atmospheric time series was constructed and used to force a two‐dimensional model containing detailed, stochastic descriptions of subsurface heterogeneity for alluvial and fractured units. Two cases were simulated for multiple years of simulation: a dry case based on a year with below‐average precipitation, and a wet case with above‐average precipitation. For each case, four realizations of hydraulic conductivity were simulated, along with a homogeneous domain and a domain with lower fracture density. Detailed analysis of water balances from each simulation indicated that recharge (change in subsurface water storage) over multiple years was significant even for the dry case. The fractured model of subsurface heterogeneity, along with episodic infiltration, combined to create localized regions of fully saturated (perched) water. This was shown to be even more substantial in the wet case. By contrast, homogeneous simulations did not exhibit this behavior and estimated lower recharge than the heterogeneous simulations. Geospatial statistics were subsequently used to evaluate correlations in land‐energy fluxes. The land‐energy fluxes exhibited clear spatial correlation and were influenced by both the shallow and deeper soil structures. This indicates that land–atmosphere fluxes may provide an integrated measure of subsurface heterogeneity. Finally, the vertical spatial structure of soil saturation in the fractured system was shown to exhibit multiscale behavior.

Modeling shallow landslides and river bed variation associated with extreme rainfall-runoff events in a granitoid mountainous forested catchment in Japan

In this study, we developed an algorithm to predict the long-term dynamic behavior of shallow landslides (< 1.0 m depth) based on physical processes. Shallow landslides and rainfall-runoff processes from a mountain area were calculated using the kinematic wave method to quantify subsurface and surface water flow, calculate sediment yield, and conduct infinite-length slope stability analysis. Debris flow, bedload, suspended load, and wash load were considered in the proposed model. Moreover, we simulated the presence of dams and sand-erosion control devices in the stream channel using a non-uniform flow sub-model in which energy loss near a dam is expressed by an energy-conservation equation. Sand-erosion control is important to controlling the amount of sediment in a dam catchment and the stream channel. The proposed method was validated by applying it to the Yahagi Dam catchment in Japan. Comparison of observed and calculated landslides indicated that the model reproduces the general characteristics of sediment transport dynamics reasonably well. The results showed 7.0 million m 3 of total sediment inflow to the dam, 2.6 million m 3 (approximately 37% of the total sediment flux) of sediment deposition behind the dam, and 4.4 million m 3 (approximately 63% of total sediment flux) of sediment discharge (mostly wash load) from the dam. A survey measured 2.4 million m 3 of sediment deposition behind the dam, which compared to our calculated value of 2.6 million m 3 gives an error between simulated and observed values of 7%.

Computational benefits using artificial intelligent methodologies for the solution of an environmental design problem: saltwater intrusion

Artificial Neural Networks (ANNs) comprise a powerful tool to approximate the complicated behavior and response of physical systems allowing considerable reduction in computation time during time-consuming optimization runs. In this work, a Radial Basis Function Artificial Neural Network (RBFN) is combined with a Differential Evolution (DE) algorithm to solve a water resources management problem, using an optimization procedure. The objective of the optimization scheme is to cover the daily water demand on the coastal aquifer east of the city of Heraklion, Crete, without reducing the subsurface water quality due to seawater intrusion. The RBFN is utilized as an on-line surrogate model to approximate the behavior of the aquifer and to replace some of the costly evaluations of an accurate numerical simulation model which solves the subsurface water flow differential equations. The RBFN is used as a local approximation model in such a way as to maintain the robustness of the DE algorithm. The results of this procedure are compared to the corresponding results obtained by using the Simplex method and by using the DE procedure without the surrogate model. As it is demonstrated, the use of the surrogate model accelerates the convergence of the DE optimization procedure and additionally provides a better solution at the same number of exact evaluations, compared to the original DE algorithm.

Modeling Runoff Generation in a Small Snow‐Dominated Mountainous Catchment

Snowmelt in mountainous areas is an important contributor to river water flows in the western United States. We developed a distributed model that calculates solar radiation, canopy energy balance, surface energy balance, snow pack dynamics, soil water flow, snow–soil–bedrock heat exchange, soil water freezing, and lateral surface and subsurface water flow. The model was applied to describe runoff generation in a subcatchment of the Dry Creek Experimental Watershed near Boise, ID. Calibration was achieved by optimizing the soil water field capacity (a trigger for lateral subsurface flow), lateral saturated soil hydraulic conductivity, and vertical saturated hydraulic conductivity of the bedrock. Validation results show that the model can successfully calculate snow dynamics, soil water content, and soil temperature. Modeled streamflow for the validation period was underestimated by 53%. The timing of the streamflow was captured reasonably well (modeling efficiency was 0.48 for the validation period). The model calculations suggest that 50 to 53% of the yearly incoming precipitation in the subcatchment is consumed by evapotranspiration. The model results further suggest that 34 to 36% of the incoming precipitation is transformed into deep percolation into the bedrock, while only 11 to 16% is transformed into streamflow.

A new model for coupling surface and subsurface water flows: With an application to a lagoon

Summary Built upon the work of Yuan et al. (Yuan, D., Lin, B., Falconer, R.A., 2008. Simulating moving boundary using a linked groundwater and surface water flow model. Journal of Hydrology 349, 524–535), we present in this note an integrated, vertically averaged numerical model based on an unstructured finite volume/finite difference method for simulating coupled surface and subsurface water flows in a coastal hydrological system. The modelled domain was divided into a surface water flow layer and a groundwater flow layer, with the former overlying the latter across the interface. In contrast to the work of Yuan et al. (2008) , the governing equations for describing these two different flows were discretised using an unstructured triangular mesh and then assembled into a single set of linear algebraic equations for computing the solutions. A semi-implicit Eulerian–Lagrangian method was used to deal with the momentum convection term of the surface flow equation. In this method, the constraint on the model’s time step imposed by the Courant–Friedrichs–Lewy condition for stable solutions is relaxed. The model was validated against the results of a previous experiment on the interaction of surface and subsurface flows in a coastal lagoon. This showed that the new model retained the same degree of accuracy in its predictions even with time steps increased by two orders of magnitude from those allowed in the model of Yuan et al. (2008) . The resulting increase of computational efficiency will enable model applications to large scale hydrological systems commonly encountered in the coastal zone.

Sources of small‐scale variation in the invertebrate communities of headwater streams

Summary1. Our objective was to investigate the associations between benthic macro‐invertebrate communities and environmental factors described at three spatial scales: local, reach and catchment. Differences in these associations, because of local topography, were determined by categorising sites into those having a large or small ‘reach contributing area’ (RCA), which is the lateral area of land contributing surface and subsurface flow of water, nutrients, organic and inorganic materials to a stream reach independent of catchment size and upstream contributions, and created by local topography.2. Twenty‐three sites were sampled in the Lake Nipigon Forest in north‐western Ontario. Local variables included hydrological metrics and stream morphology characteristics. Terrestrial reach and catchment variables included forest characteristics, topography and geomorphology. Invertebrates were identified and classified into functional feeding guilds. Canonical correspondence analysis and redundancy analysis were used to determine which variables contributed most to macro‐invertebrate community structure and how the RCA influenced those associations.3. Overall, local‐scale variables explained most variation in taxonomic and functional community structure (51.6 and 59.1%). Catchment‐scale variables explained more variation in the community (43.9 and 43.5%) than reach‐scale variables. In sites with a large RCA, the reach‐scale variables accounted for almost 10% more variability in taxonomic structure than the catchment‐scale variables, and in sites with a small RCA, catchment‐scale variables explained almost 23% more variation in feeding guild structure than reach‐scale variables.4. Topography was important at both the reach and the catchment scales. Variables at the reach scale most associated with invertebrate community tended to be related to forest cover, whereas important variables at the catchment scale were related to topography, wetlands and lakes.5. The difference in associations between invertebrate communities and environmental factors based on RCA indicates the influence of local topography on the linkage between aquatic habitats and the surrounding terrestrial environment. Understanding how these linkages affect aquatic communities can help to develop more effective predictive models of invertebrate community and more prescriptive conservation and management strategies for small streams.

A numerical simulation of landslide-prone slope in Himalayan region—a case study

Stability of slope will always a problem due to its geodynamic nature of Himalayan region. The area investigated belongs to the Lower Siwalik formation, which is prone to failure due to presence of various weak planes or structures present in the rock mass. Frequent landslides causing loss of life, property, disturbance of morphological, surface and subsurface water flow pattern. A huge landslide in the Amiyan area occurred in 1999 and blocked the Gaula River which caused loss of agricultural land and other properties as well as waterlogged upstream side. The debris of Amiyan slope contains soil and sandstones of various types. This article deals with instability analysis of slopes of the Amiyan area, near Kathgodam, Nainital, Uttarakhand. This area experiences a number of local as well as regional slides. Extensive field surveys have been carried out to understand the geological detail. Laboratory experiments have been conducted to determine the various physicomechanical properties of rock mass. These properties have been used as input parameters for the numerical simulation. A number of researchers have used two-dimensional numerical models to simulate the slide area. Three-dimensional slope stability studies provide a better understanding of the mechanism of failure as well as zone of influence. The computed deformations and stress distribution, along the failure surface, have been compared with the field measurements and found to be in good agreement with field observations. In this case, finite difference method has been applied on the stability analysis of Amiyan slope. The study indicates that the slope is vulnerable and needs proper protection.

A physically based FVM watershed model fully coupling surface and subsurface water flows

A sophisticated modeling approach for simulating-coupled surface and subsurface flows in a watershed is presented. The watershed model developed is a spatially distributed physically based model of composite dimension, consisting of 3-D variably saturated groundwater flow submodel, 2-D overland flow submodel and 1-D river flow submodel. The 3-D subsurface flow is represented by the complete Richards equation, while the 2-D and 1-D surface flows by the diffusive approximations of their complete dynamic equations. For piecewise integration of these equations, the finite volume method (FVM) is employed assuming unknown variables such as the water depth and the pressure head to be volume-averaged state ones. Problem plane geometry is meshed with the unstructured cells of triangular shape which conforms to external as well as internal irregular boundaries such as those between 1-D and 2-D flows. A cell size controlling scheme, referred to as quasi-adaptive meshing scheme, is introduced to keep the local discretization errors caused by topographic elevation gradient even over the entire-meshed geometry. Performance of the model is tested through its practical application to a rugged intermountain watershed. Tuning the values of the three key parameters ensures successful calibration of the model. Once the model is so calibrated, it could reproduce satisfactory runoff response to any rainfall event. Expansion and shrinkage of the contributing area importantly affecting the direct runoff, caused by the vicissitude of rainfall during its total duration, are well reproduced, like what the commonly accepted runoff theory argues. It is thus concluded that the model developed could serve as a powerful watershed simulator usable for investigating and assessing the hydrological aspect of a watershed.

Crude Oil at the Bemidji Site: 25 Years of Monitoring, Modeling, and Understanding

The fate of hydrocarbons in the subsurface near Bemidji, Minnesota, has been investigated by a multidisciplinary group of scientists for over a quarter century. Research at Bemidji has involved extensive investigations of multiphase flow and transport, volatilization, dissolution, geochemical interactions, microbial populations, and biodegradation with the goal of providing an improved understanding of the natural processes limiting the extent of hydrocarbon contamination. A considerable volume of oil remains in the subsurface today despite 30 years of natural attenuation and 5 years of pump-and-skim remediation. Studies at Bemidji were among the first to document the importance of anaerobic biodegradation processes for hydrocarbon removal and remediation by natural attenuation. Spatial variability of hydraulic properties was observed to influence subsurface oil and water flow, vapor diffusion, and the progression of biodegradation. Pore-scale capillary pressure-saturation hysteresis and the presence of fine-grained sediments impeded oil flow, causing entrapment and relatively large residual oil saturations. Hydrocarbon attenuation and plume extent was a function of groundwater flow, compound-specific volatilization, dissolution and biodegradation rates, and availability of electron acceptors. Simulation of hydrocarbon fate and transport affirmed concepts developed from field observations, and provided estimates of field-scale reaction rates and hydrocarbon mass balance. Long-term field studies at Bemidji have illustrated that the fate of hydrocarbons evolves with time, and a snap-shot study of a hydrocarbon plume may not provide information that is of relevance to the long-term behavior of the plume during natural attenuation.

Unsaturated 2D modelling of subsurface water flow in the coarse-grained porous matrix of a green roof

Summary The analysis of the interactions between hydrologic processes and green roof installations in the urban environment requires to improve the understanding of the unsaturated water flow in the coarse-grained porous media usually employed in such technological solutions. In order to base our research on experimental evidences, a monitoring program was carried out at the green roof experimental site of the University of Genoa (Italy). A continuous acquisition system is available at the experimental site for rainfall intensity and subsurface water flow measurements at 1 min resolution. Soil water content is also measured at the same resolution in time by four TDR probes located along a vertical profile. The SWMS_2D model, based on Richards’ law and the Van Genuchten–Mualem functions, is here applied to simulate the variably saturated flow within the green roof system. The model was calibrated and validated using rainfall–runoff events observed at the experimental site. The calibration and validation strategy involved comparing predicted and measured outflow hydrographs. The model adequately reproduces the hydrographs, as demonstrated by the limited relative percentage deviations obtained for the total discharged volume, the peak flow and the hydrograph centroid. Furthermore the predicted water content closely matches the observed one at various depths along the vertical profile where measurements are available, thus confirming that the model correctly describes the variably saturated flow field within the green roof. The mechanistic model, based on a single porosity approach, here employed to describe the variably saturated flow within the thin stratigraphy, is demonstrated to suitably describe the hydrologic performance of a green roof, which is typically realized with high hydraulic conductivity and coarse-grained porous media.

Soil Moisture and Metolachlor Volatilization Observations over Three Years

A 3-yr study was conducted to focus on the impact of surface soil water content on metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide) volatilization from a field with different surface soil water regimes created by subsurface water flow paths. Metolachlor vapor fluxes were measured at two locations within the field where local meteorological and soil conditions were relatively constant, except for surface soil water content, which differed significantly. Surface soil water content at the two sites differed in response to the presence of subsurface flow pathways. Detailed soil moisture observations over the duration of the study showed that for the first 2 yr (2004 and 2005), surface soil water contents at the dry location (V1) were nearly half those at the wetter location (V2). Cumulative metolachlor vapor fluxes during 2004 and 2005 at V1 were also about half that at V2. In the third year (2006), early-season drought conditions rendered the soil water content at the two locations to be nearly identical, resulting in similar metolachlor volatilization losses. Analysis of infrared soil surface temperatures suggests a correlation between surface soil temperatures and metolachlor volatilization when soils are wet (2004 and 2005) but not when the soils are dry (2006). Field-averaged metolachlor volatilization losses were highly correlated with increasing surface soil water contents (r(2) = 0.995).