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The Groundwater Ubiquity Score (GUS) is widely used to indicate the relative leachability of pesticides based on the soil half-life and the adsorption partition coefficient. In this manuscript, we derive mathematically the Theoretical Groundwater Ubiquity Score (TGUS) that, based on considerations of the preferential movement of pesticides to groundwater and a first-order pesticide degradation model, leads to a similar function as the GUS model. In the preferential flow model, movement to groundwater is fast, and the adsorption partition coefficient is thus not used for calculating the travel time to the groundwater (as it is in the advective-dispersive equation) but rather only determines the distribution of the pesticide between the water and soil phases. Both the GUS and TGUS models well predict the groundwater contamination of the originally studied pesticides for rainfall event(s) that caused pesticide leaching from 30 days after application. The theoretically derived Groundwater Ubiquity Score (TGUS) shows, in accordance with experimental evidence, that for leaching events shortly after spraying, the mass lost to (and resulting concentration in) groundwater is inversely related to the adsorption partition coefficient and not necessarily to the GUS index.

The highlands serve as water towers for the blue water in the surrounding area, with often insufficient rainfall for rainfed crops. The population is growing in the highlands, changing the hydrology and blue and green water availability. One of the changes that affect the highlands of Ethiopia is the rapidly expanding areas of small patches of eucalyptus on farmers fields. This manuscript aims to sort through the limited available literature and develop a water balance model as a starting point to discuss how hydrology is affected by increased acreage of eucalyptus in the highlands. We find some evidence in the literature that patches of eucalyptus during the dry phase may withdraw water from the subsoil up to twice the rate of evapotranspiration when the soil water is not limited. Since saturation excess generates surface runoff in the subhumid and humid highlands, water withdrawn during the dry phase should be made up before the area with eucalyptus becomes hydraulically active. The amount of water that can be removed from storage by eucalyptus is greater in areas with a long dry monsoon phase than in temperate climates for the same increase in trees. We also found, as expected, that the eucalyptus water balance model, as developed in this paper, was not valid for areas in semi-arid and arid regions. Soil crusting and water repellency might induce infiltration excess runoff. A tentative implication is that since most blue water is produced in the areas with the highest rainfall, eucalyptus in subhumid and humid monsoon regions will affect the blue water supply downstream but not the plant available green water although shading will decrease yield in the immediate surroundings of the eucalyptus trees.

Phosphonate‐based herbicides are extensively used in agriculture. Glyphosate was long considered immobile in soils due to strong complexation with metal oxides, but detection in agricultural effluents and surface waters points to glyphosate mobilization and transport. In particular, mechanisms of glyphosate adsorption and subsequent mobility from runoff‐prone soils enriched in quartz have not been fully elucidated. Here we used theoretical and experimental approaches to gain new insights on the soil mobility of glyphosate and two other phosphonate herbicides, glufosinate and fosamine. Molecular modeling simulations of optimized adsorbate complexes of the phosphonate herbicides with quartz and montmorillonite revealed Ca‐enhanced interactions for all three herbicides at both mineral interfaces, weak binding of Na‐complexed phosphonate herbicides on quartz, and more favorable interactions for glufosinate and fosamine with Na‐montmorillonite than for glyphosate. We conducted adsorption and desorption experiments to evaluate the mobility of glyphosate on quartz‐enriched agricultural soils. Consistent with our theoretical findings, adsorption experiments conducted with CaCl2resulted in greater than a twofold increase in glyphosate retention compared with experiments with NaCl. The desorption experiments revealed release of about 30–90% of the initially bound glyphosate into solution, whereby Ca in the desorption solution promoted glyphosate mobility compared with Na. Furthermore, in all conditions, the relative initial wetness of the agricultural soil was correlated positively with the enhanced glyphosate mobilization, consistent with field‐scale reports that herbicide applied to soils following rain events were prone to runoff. ■Herbicide adsorption mechanisms on minerals depends on compound chemical structures.■Only diffuse layer suspension occurs with a lack of surface adsorption on Na‐quartz.■Glufosinate and fosamine have stronger binding than glyphosate on Na‐montmorillonite.■Ca‐complexed herbicides have enhanced adsorption on both montmorillonite and quartz.■Experimental data with quartz‐enriched agricultural soils are consistent with simulation results.

Effective watershed planning requires an understanding of the hydrology. In the humid tropical monsoon climates and especially in volcanic highland regions such as the Ethiopian Highlands, the understanding of watershed processes is incomplete. The objective is to better understand the hydrology of the volcanic regions in the humid highlands by linking the hillslope processes with the discharge at the outlet. The Ene-Chilala watershed was selected for this study. The infiltration rate, piezometric water levels and discharge from two nested sub watersheds and at the watershed outlet were measured during a four-year period. Infiltration rates on the hillsides exceeded the rainfall intensity most of the time. The excess rain recharged a perched hillside aquifer. Water flowed through the perched aquifer as interflow to rivers and outlet. In addition, saturation excess overland flow was generated in the valley bottoms. Perched water tables heights were predicted by summing up the recharge over the travel time from the watershed divide. Travel times ranged from a few days for piezometers close to the divide to 40 days near the outlet. River discharge was simulated by adding the interflow from the upland to overland flow from the saturated valley bottom lands. Overland flow accounted only for one-fourth of the total flow. There was good agreement between predicted and observed discharge during the rain phase therefore the hillslope hydrologically processes were successfully linked with the discharge at the outlet.

Knowledge of the transport and retention behaviors of soft deformable particles on the microscale is essential for the design, evaluation, and application of engineered particle materials in the fields of energy, environment, and sustainability. Emulated convergent–divergent microchannels were constructed and used to investigate the transport and retention behaviors of soft deformable polyacrylamide microspheres at various conditions. Five different types of transport and retention patterns, i.e., surface deposition, smooth passing, direct interception, deforming remigration, and rigid blockage, are observed. Flow resistance variation characteristics caused by different patterns were quantitatively analyzed. Effects of flow rate, pore-throat size, particle size, and injection concentration on transport and retention patterns have been studied, and transport and retention pattern maps are presented and discussed.

While hydrological science has made great strides forward during the last 50 years with the advance of computing power and availability of satellite images, much is unknown about the sustainable development of water for irrigation, domestic use, and livestock consumption for millions of households in the developing world. Specifically, quantification of shallow underground water resources for irrigation in highland regions remains challenging. The objective is to better understand the hydrology of highland watersheds with sloping hillside aquifers. Therefore, we present a subsurface flow model for hillside aquifers with recharge that varied from day to day. Recharge to the aquifer was estimated by the Thornthwaite Mather procedure. A characteristic time was identified for travel time of water flowing from the upper part of the hillside to the river or well. Using the method of characteristics, we found that the height of shallow groundwater level can be predicted by determining the total recharge over the characteristic time divided by drainable porosity. We apply the model to farmer‐dug wells in the Ethiopian highlands using observed rainfall, potential evaporation, and a fitted travel time. We find that the model performs well with maximum water table heights being determined by the soil surface and minimum heights by the presence or absence of volcanic dikes downhill. Our application shows that unless the water is ponded behind a natural or artificial barrier, hillslope aquifers are unable to provide a continuous source of water during the long, dry season. This clearly limits any irrigation development in the highlands from shallow sloping groundwater. The population in the Ethiopian highlands, similar to other developing countries, is rapidly increasing. In order to have enough food during the year, most families grow their food crops during the rainy season. More food can be grown by irrigation during the dry season. We mathematically tested and observed data of eight wells as to whether shallow aquifers on the sloping lands can store sufficient water during the rainy season so it can be used for irrigation after the rains have stopped. We found that groundwater on sloping soils is only available during the whole year when volcanic‐derived subsurface barriers prevent flow downslope. A novel parsimonious model was developed for shallow hillslope groundwater tablesShallow groundwater depth in wells can be predicted from the recharge and the drainable porosityHillslope aquifers cannot supply water for irrigation at end of the dry monsoon phase unless a downhill obstruction to flow is present

Soil erosion decreases soil fertility of the uplands and causes siltation of lakes and reservoirs; the lakes and reservoirs in tropical monsoonal African highlands are especially affected by sedimentation. Efforts in reducing loads by designing management practices are hampered by lack of quantitative data on the relationship of erosion in the watersheds and sediment accumulation on flood plains, lakes and reservoirs. The objective of this study is to develop a prototype quantitative method for estimating sediment budget for tropical monsoon lakes with limited observational data. Four watersheds in the Lake Tana basin were selected for this study. The Parameter Efficient Distributed (PED) model that has shown to perform well in the Ethiopian highlands is used to overcome the data limitations and recreate the missing sediment fluxes. PED model parameters are calibrated using daily discharge data and the occasionally collected sediment concentration when establishing the sediment rating curves for the major rivers. The calibrated model parameters are then used to predict the sediment budget for the 1994-2009 period. Sediment retained in the lake is determined from two bathymetric surveys taken 20 years apart whereas the sediment leaving the lake is calculated based on measured discharge and observed sediment concentrations. Results show that annually on average 34 t/ha/year of sediment is removed from the gauged part of the Lake Tana watersheds. Depending on the up-scaling method from the gauged to the ungauged part, 21 to 32 t/ha/year (equivalent to 24-38 Mt/year) is transported from the upland watersheds of which 46% to 65% is retained in the flood plains and 93% to 96% is trapped on the flood plains and in the lake. Thus, only 4-7% of all sediment produced in the watersheds leaves the Lake Tana Basin.

Core IdeasWe studied hotspots of nitrous oxide emission from perennial grasses on wet soil.Hotspots occurred in a limited range of soil moisture and temperature conditions.Hotspots recurred more frequently at specific places over a three year period. Hotspots of nitrous oxide (N2O) emission are thought to contribute substantially to annual emissions from agricultural soils. We observed N2O fluxes from fertilized and unfertilized C3and C4perennial grasses on a wet silt loam soil in New York, United States during the growing season in 2013, 2014, and 2015 using static chambers. Analysis of N2O hotspots within the research plots revealed that hotspots contributed between 34.3 and 39.1% of the total emissions, and constituted between 0.8% and 5.0% of all flux observations. Hotspots were more frequent and of greater magnitude in the fertilized treatments, and occurred when soil temperature was greater than 9.1°C and soil moisture was between about 40% and 80% water filled pore space (WFPS). A single chamber location in the fertilized switchgrass treatment was consistently a hotspot for N2O emission, suggesting that hotspots maintain a stable spatial pattern over extended periods. The maximum magnitude of N2O hotspot emission exhibited a relationship to soil temperature that is similar to that of the microbial growth rate constant.

Drainage of paved and unpaved roads has been implicated as a major contributor of overland flow and erosion in mountainous landscapes. Despite this, few watershed models include or have tested for the effect roads have on discharge and sediment loads. Though having a model is an important step, its proper application and attention to distinct landscape features is even more important. This study focuses on developing a module for drainage from a road and tests it on a nested watershed (Shanko Bahir) within a larger previously studied site (Debre Mawi) that receives overland flow contributions from a highly compacted layer of soil on an unpaved road surface. Shanko Bahir experiences a sub-humid monsoonal climate and was assessed for the rainy seasons of 2010, 2011, and 2012. The model chosen is the Parameter Efficient Distributed (PED) model, previously used where saturation-excess overland flow heavily influences discharge and sediment concentration variation, though infiltration-excess occasionally occurs. Since overland flow on unpaved surfaces emulates Hortonian flow, an adjustment to the PED model (the developed module) advances possible incorporation of both flow regimes. The modification resulted in similar modeling performance as previous studies in the Blue Nile Basin on a daily basis (NSE = 0.67 for discharge and 0.71 for sediment concentrations). Furthermore, the road while occupying a small proportion of the sub-watershed (11%) contributed importantly to the early discharge and sediment transport events demonstrating the effect of roads especially on sediment concentrations. Considerations for the dynamic erodibility of the road improved sediment concentration simulation further (NSE = 0.75). The results show that this PED modeling framework can be adjusted to include unpaved compacted surfaces to give reasonable results, but more work is needed to account for contributions from gullies, which can cause high influxes of sediment.

Engineered carbon‐based nanoparticles are increasingly used for environmental, industrial, and medical purposes. Thus, understanding their interaction with and transport through materials is important. We examine the impact of nanoparticle hydrophobicity and porous medium surface area on the transport of carbon nanoparticles through sand‐packed columns under saturated and unsaturated conditions. The fluorescent carbon nanoparticles (FCNs) used in this study, synthesized from citric acid and ethanolamine, exhibit synthesis‐temperature‐dependent hydrophobicity. To quantify the impact of hydrophobicity on retention, we use FCNs synthesized at four temperatures: 190°C (FCN190), 210°C (FCN210), 230°C (FCN230), and 250°C (FCN250). Several observations are noted. First, the more hydrophobic particles (FCN230 and FCN250) attain lower outlet concentrations and mass recovery than more hydrophilic particles (FCN190 and FCN210). For instance, while 77% of the FCN190 was recovered after passing through a fine‐sand‐packed column, only 23% of the FCN250 was recovered. Second, sand surface area significantly impacts FCN recovery. A 17‐fold increase in sand surface area yields a 30% decrease in the recovery of FCN210. Third, no significant difference in the mass recovery of the FCNs was observed between the unsaturated and saturated conditions, which is attributed to the small size of the FCNs relative to the water film thickness surrounding sand grains. Fourth, the particle transport model in HYDRUS‐1D successfully simulated FCN transport, showing approximately a tenfold increase in the attachment coefficient for hydrophobic FCNs. In summary, through experiments and modeling, we show that hydrophobicity is a major factor impacting FCN transport. The synthesis‐temperature‐dependent hydrophobicity reduces the transport of fluorescent carbon nanoparticles through sand‐packed columnsA two‐kinetic sites transport model describes the transport of fluorescent carbon nanoparticles in porous mediaFluorescent carbon nanoparticles are not trapped in the meniscus under unsaturated conditions at the tested moisture contents The synthesis‐temperature‐dependent hydrophobicity reduces the transport of fluorescent carbon nanoparticles through sand‐packed columns A two‐kinetic sites transport model describes the transport of fluorescent carbon nanoparticles in porous media Fluorescent carbon nanoparticles are not trapped in the meniscus under unsaturated conditions at the tested moisture contents

Denitrifying bioreactors have been suggested as effective best management practices to reduce nitrate and nitrite (NOx) in large‐scale agricultural tile drainage. This study combines experiments in flow‐through laboratory reactors with in situ continuous monitoring and experiments in a pair of field reactors to determine the effectiveness of reactors for small‐scale agriculture in New York. It also compares the use of a typical woodchip media with a woodchip and biochar mixture. Laboratory results showed linear increase in NOxremoval with both increased inflow concentration and increased residence time. Average removal of NOxin weekly monitoring of field reactors over the course of two growing seasons was 3.23 and 4.00 g N m−3d−1for woodchip and woodchip/biochar reactors, respectively. Removal of NOxduring two field experimental runs was similar to in situ monitoring and did not correlate with laboratory experiments. Factors that are uncontrollable at the field scale, such as temperature and inflow water chemistry, may result in more complex and resilient microbial communities that are less specialized for denitrification. Further study of other controlling variables, other field sites, and other parameters, including microbial communities and trace gas emissions, will help elucidate function and applicability of denitrifying bioreactors. Flow‐through lab reactors show a linear increase in NOxremoval with residence time.Lab reactors show a linear increase in NOxremoval at higher inflow NOxconcentration.Field reactors significantly reduce NOxoutflow from tile‐drained agriculture in NY.Field reactor experiment results do not follow linear inflow NOxtrend found in lab.Biochar amendments show no significant removal of phosphorus in field reactors.

Core IdeasWe monitored N2O fluxes from grasses on seasonally wet soil during spring thaw.We used three different statistical approaches to analyze trends in the fluxes.N2O emissions were elevated in wet areas recently converted to reed canarygrass. In temperate climates, a significant fraction of annual emissions of N2O from agricultural land can occur during soil thaw in late winter and early spring. The objective of this study was to determine the impact of land use change from long‐term fallow grassland to managed perennial grass crops on these thaw‐related N2O emissions and to identify field‐scale drivers that influence emissions. Using static chambers, we monitored mid‐afternoon N2O fluxes during the 2013 spring thaw from 27 March to 7 April, observing fallow grassland and second‐year reed canarygrass (Phalaris arundinaceae L., ‘Bellevue’) across a short topographical gradient. Soil temperature, soil moisture, and residual aboveground biomass were also observed, as were hourly air temperature and precipitation. Fluxes of N2O were generally low (−9.8 to 21.3 μg N2O‐N m−2h−1) except for one observation of 77.6 μg N2O‐N m−2h−1. Hot‐moment analysis, non‐parametric statistical tests, and ANOVA results showed that downslope positions converted to managed and fertilized grass had significantly higher N2O emissions than the fallow and upslope positions. We found that these downslope managed grass sites had mean soil moisture of 75.0% water‐filled pore space and less insulating residual aboveground biomass than the fallow grassland. Our results suggest that converting fallow grassland to managed perennial grass cropping systems for bioenergy or other uses could increase spring‐thaw N2O emissions in wetness‐prone areas.

In response to the continually increasing sediment concentrations in rivers and lakes, the Ethiopian government is leading an effort where farmers are installing soil and water conservation measures to increase infiltration and reduce erosion. This paper reports on findings from a four year study in the 95 ha Debre Mawi watershed where under the government led conservation works, mainly terraces with infiltration furrows were installed halfway in the period of observation. The results show that runoff volume decreased significantly after installation of the soil and water conservation practices but sediment concentration decreased only marginally. Sediment loads were reduced mainly because of the reduced runoff. Infiltration furrows were effective on the hillsides where rain water could infiltrate, but on the flat bottom lands that become saturated with the progress of the monsoon rain, infiltration was restricted and conservation practices became conduits for carrying excess rainfall. This caused the initiation of gullies in several occasions in the saturated bottomlands. Sediment concentration at the outlet barely decreased due to entrainment of loose soil from unstable banks of gullies in the periodically saturated bottom areas. Since most uphill drainage were already half filled up with sediments after two years, long term benefits of reducing runoff can only be sustained with continuous maintenance of uphill infiltration furrows.

Most rice culture in southern Asia occurs on relatively flat soils with seasonally high groundwater tables. Groundwater is often available for use during the dry season for irrigation. Bangladesh is a southern Asian country where dry season rice or boro irrigation is rapidly increasing, causing concern for possible groundwater depletion. This paper describes a simulation model developed to analyze the impacts that different irrigation management schemes have on groundwater levels. The model includes all aspects of the hydrological cycle, such as precipitation, irrigation, evapotranspiration, percolation, recharge, and river base flow. It consists of two linked models—one for the vadose and one for the groundwater zones. The model is checked for realism in northwest Bangladesh where four irrigation management schemes are also tested for their groundwater mining potential.

Carbofuran, alachlor, atrazine, bromide a tracer, and nitrate are applied to two tiledrained plots of com, one of which is conventionally tilled and another that is notill. Soil water samples are collected from the plots with suction lysimeters and groundwater samples are collected from wells and tile drains. The notill plot first showed low concentrations of atrazine and alachlor ranging from 0.1 to 0.4 gL below the root zone one month after application, but the conventionally tilled plot showed no pesticides below the root zone until late fall, when atrazine was detected at 0.4 gL. Dye studies indicated that, in the notill plots, some of the pesticide bypasses the root zone through surfaceconnected macropores, whereas in the conventionally tilled plot, most of the pesticide is adsorbed within the root zone as a result of greater contact with the soil. Concentrations of bromide which is not adsorbed in samples collected from wells and suction lysimeters do not differ between the two plots. Nitrate is found only in that part of the profile that remains unsaturated throughout the experiment.

The high-energy, highly collimated X-ray beam at the Cornell High Energy Synchrotron Source (CHESS) was used to provide rapid and high-resolution temporal and spatial density distributions in a consolidating saturated clay specimen. Porewater pressure at the undrained base of the clay was obtained by a porewater pressure transducer. Test results show good agreement between theoretical and experimental density distributions and indicate that synchrotron radiation is a promising and unique method to study consolidation.

Access to safe drinking water supply in Ethiopia is among the lowest in sub-Saharan Africa. While both governmental and non-governmental organizations have implemented water supply projects in recent years, many fail shortly after construction due to improper management. In this paper, we examine socio-economic, institutional and exogenous factors affecting household participation in the management of water supply systems for drinking purposes. A survey was carried out involving 16 water supply systems and 160 households within the Achefer area, in the Amhara region, Ethiopia. The results show that household contributions to water supply system management are positively and significantly affected by user participation during the project design and implementation, by advocacy provided by the project and by greater household income. Thus, for drinking water systems in rural areas to be sustainable, these factors should be included when planning water supply projects. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) emissions were monitored using the micrometeorological eddy covariance technique from manure‐fertilized cropland on a large dairy farm in New York State in 2006 to 2009. Nitrous oxide emissions demonstrated episodic behavior with intermittent short‐duration peak fluxes up to 39.7 mg N2O‐N m−2d−1, whereas most of background fluxes during the annual agricultural cycle were below 6.5 mg N2O‐N m−2d−1. This paper discusses temporal variability of measured N2O emissions using a “hot moment” approach. To identify and quantify peak events as potential hot moments and to determine whether or not they could be treated statistically as outliers, N2O daily fluxes were analyzed by the box plot method using multiple thresholds. Peak events exceeding outlier thresholds contributed up to 51% of cumulative annual N2O emissions, although they represented <7% of the total observation time. Individual N2O peaks were also categorized by their duration, as single day spikes and multiday events. The highest contributing instances were multiday N2O peaks during summer precipitation and early spring thaw, largely enhanced by manure fertilization. These high‐intensity emission events demonstrated repetitive seasonal responses to a combination of environmental factors and were therefore identified as hot moments. Abrupt rises in both temperature and soil moisture appeared to trigger major hot moments, whereas the availability of manure N controlled their magnitude. In the absence of strong correlations between time‐series of individual environmental factors and N2O emissions, the hot moment approach can be a promising tool for the integrated analysis of most significant N2O events in cultivated fields receiving manure applications.

In phosphorus‐limited freshwater systems, small increases in phosphorus (P) concentrations can lead to eutrophication. To reduce P inputs to these systems, various environmental and agricultural agencies provide producers with incentives to implement best management practices (BMPs). In this study, we examine both the water quality and economic consequences of systematically protecting saturated, runoff‐generating areas from active agriculture with selected BMPs. We also examine the joint water quality/economic impacts of these BMPs—specifically BMPs focusing on barnyards and buffer areas. Using the Variable Source Loading Function model (a modified Generalized Watershed Loading Function model) and net present value analysis (NPV), the results indicate that converting runoff‐prone agricultural land to buffers and installing barnyard BMPs are both highly effective in decreasing dissolved P loss from a single‐farm watershed, but are also costly for the producer. On average, including barnyard BMPs decreases the nutrient loading by about 5.5% compared with only implementing buffers. The annualized NPV for installing both buffers on only the wettest areas of the landscape and implementing barnyard BMPs becomes positive only if the BMPs lifetime exceeds 15 yr. The spatial location of the BMPs in relation to runoff producing areas, the time frame over which the BMPs are implemented, and the marginal costs of increasing buffer size were found to be the most critical considerations for water quality and profitability. The framework presented here incorporates estimations of nutrient loading reductions in the economic analysis, and is applicable to farms facing BMP adoption decisions.

Eddy covariance and static chambers are different‐scale methods for monitoring agricultural N2O that, when used together on heterogeneous agricultural landscapes, can help identify flux sources and sinks and evaluate the effect of management interventions on landscape‐scale N2O emissions. This study compared the N2O flux data obtained by eddy covariance and static chambers during a short‐term N2O measurement campaign from two adjacent agricultural treatments: alfalfa (Medicago sativaL.) and corn (Zea maysL.) fields. Wind direction data from micrometeorological observations were used to downscale the integrated eddy covariance N2O flux and estimate the treatment contributions. The N2O data from static chambers installed on each treatment were used to verify the partitioned eddy covariance fluxes. Both methods consistently showed greater emissions for the alfalfa field, which received more N fertilizer earlier in the growing season. Two methods were also compared with respect to the landscape‐integrated N2O flux measured at the eddy covariance mast location. Upscaling the chamber N2O fluxes was performed by totaling the contributions from individual chambers weighted toward the source area share associated with their field locations using a simple footprint model. The comparison of the chambers' total to the measured eddy covariance emissions showed a difference of 7 to 33% between the methods. The best agreement was observed when the integrated eddy covariance flux was associated with uniform wind direction and a homogeneous source area. The results suggest that localization of the flux source using wind directions and footprint information can help in comparing different‐scale N2O emissions.

Understanding the basic runoff processes in the Ethiopian highlands is vital for effective management and utilization of water resources and soil conservation planning. An important question for judging the effectiveness of conservation practices is whether runoff is affected by ecology (mainly type of crop) or topography (landscape). A study was conducted in the 113 ha watershed of Maybar, located in the highlands. This watershed has long‐term records of rainfall and discharge. To study runoff processes, piezometers were installed in eight transects up and down the slope. In addition, infiltration rates (measured earlier) were compared with rainfall intensities. The results show that the amount of runoff at the test plots was greater for cropland located on mild to intermediate slopes than for grasslands and woodlands on the steeper slopes. Water tables were closer to the surface on cropland for the mild to intermediate slopes than on grasslands and in woodlands for the steeper slopes. Thus, although water table depths and plot runoff were inconclusive on the type of runoff mechanisms, infiltration rates that were generally in excess of the rainfall rates imply that any ecological effect on the amount of surface runoff is small. This is because water infiltration is independent of crop type. Only in cases where the soil was saturated did runoff occur. Piezometer readings show that saturation occurs at the foot of the steep slopes and, therefore, it demonstrates that topographic processes are dominant. Ecology becomes important when infiltration rates are in the same order as the rainfall intensities. Copyright © 2010 John Wiley & Sons, Ltd.

Biochar land application can potentially be used for carbon sequestration, improving soil quality, and reducing non‐point source pollution. Understanding biochar mobility is important because its transport in soil greatly influences its stability, the dynamics of soil microbial communities and organic matter, and the movement of biochar‐associated contaminants. Here, the transport of biochar particles was studied in saturated and unsaturated sand columns by measuring breakthroughs of biochar pulse under three pH and two ionic strength (IS) levels. Breakthrough curves (BTCs) were fitted to a convection–dispersion model with kinetic and equilibrium deposition sites to estimate the key transport parameters (e.g. biochar deposition rate coefficients). Biochar retention was enhanced by lowering pH and increasing IS, corroborating the trends of fitted deposition rate coefficients. Under both saturated and unsaturated conditions, effluent mass recoveries decreased, respectively, by a factor of 6·6 or 15 when pH decreased from 10 to 4 at 10 mM IS, and by a factor of 1·4 or 3·9 when IS increased from 10 to 100 mM at pH 7. Biochar retention was greater in unsaturated media, implying that saturated flow elutes more biochar particles. The particles larger than 5·4% of median grain diameter were filtered out of suspension during passage through the media; whereas, the retention of smaller particles was clearly dependent on solution chemistry. Similar to other types of colloids, this study highlights the importance of pH, IS, particle size, and soil water saturation in controlling biochar movement by soil matrix flow. Copyright © 2010 John Wiley & Sons, Ltd.

The contribution of runoff from various land uses to stream channels in a watershed is often speculated and used to underpin many model predictions. However, these contributions, often based on little or no measurements in the watershed, fail to appropriately consider the influence of the hydrologic location of a particular landscape unit in relation to the stream network. A simple model was developed to predict storm runoff and the phosphorus (P) status of a perennial stream in an urban watershed in New York State using the covariance structure of runoff from different landscape units in the watershed to predict runoff in time. One hundred and twenty‐seven storm events were divided into parameterization (n= 85) and forecasting (n= 42) data sets. Runoff, dissolved P (DP), and total P (TP) were measured at nine sites distributed among three land uses (high maintenance, unmaintained, wooded), three positions in the watershed (near the outlet, midwatershed, upper watershed), and in the stream at the watershed outlet. The autocorrelation among runoff and P concentrations from the watershed landscape units (n= 9) and the covariance between measurements from the landscape units and measurements from the stream were calculated and used to predict the stream response. Models, validated using leave‐one‐out cross‐validation and a forecasting method, were able to correctly capture temporal trends in streamflow and stream P chemistry (Nash–Sutcliffe efficiencies, 0.49–0.88). The analysis suggests that the covariance structure was consistent for all models, indicating that the physical processes governing runoff and P loss from these landscape units were stationary in time and that landscapes located in hydraulically active areas have a direct hydraulic link to the stream. This methodology provides insight into the impact of various urban landscape units on stream water quantity and quality.

Phosphorus flux from agricultural landscapes to surface waters may cause eutrophication. In the northeastern United States, P transport largely depends on P sorption of soils in variable source areas or in land treatment systems. Soil redox fluctuation commonly occurs in these areas. Nevertheless, the effect of soil redox on P sorption has been variable in the literature. This study investigated P sorption of an organic‐rich northeastern glaciated silt loam (Langford) under air‐dried, field‐wet, and reduced conditions using batch P sorption experiments. Additionally, the influence of farm wastewater on soil P sorption was studied. Major results indicated that soil reduction increased the maximum amount of P that can be sorbed (Smax) and decreased the aqueous P concentration at which P sorption and desorption are equal (EPC0), both determined from a modified Langmuir isotherm model. The slightly reduced field‐wet soils had no significant difference in Smaxdue to limited soil reduction. Using the diluted wastewater as the sorption solution matrices instead of 0.01 mol L−1KCl solution, the soils generally exhibited greater Smaxand lower EPC0except for the EPC0of a reduced surface soil, implying more complex P sorption in the field. Identified P sorption mechanisms include phosphate precipitation, ligand exchange with organic matter, and adsorption onto Fe hydroxides. Transformation of Fe compounds during soil reduction is primarily responsible for the changes in soil P sorption.

The phosphorus (P) sorption isotherm experiment is a widely used tool in environmental applications for assessing soil's vulnerability to P loss to runoff or drainage. The sorbed legacy P (S0) (i.e., the P retained in soils from previous P applications) participates in sorption processes but cannot readily be determined in a sorption experiment. Thus, it is important to accurately estimate S0for P-enriched soils (e.g., the soils that heavily receive fertilizer, manure, farm wastewater, or sewage sludge). Two curve-fitting procedures (i.e., one-step method and two-step method) with Langmuir models have been used to estimate S0and other sorption parameters, including the P sorption maxima (Smax), the bonding energy constant (k), and the zero-sorption equilibrium concentration (EPC0). This study evaluated these two methods on 16 samples of Langford, Volusia, and Mardin channery silt loam soils at surface (0-8 cm) and subsurface (61-91 cm) in New York. The results indicate that the two methods agreed well in estimating P sorption maxima, and the estimates of kwere close. The S0estimates by the two methods had a good agreement for surface soils but a poor agreement for subsurface soils, which may be of little concern because of small S0of subsurface soils. Although the one-step method yielded greater EPC0estimates, the EPC0estimates by the two methods had an excellent linear correlation for P-enriched surface soils, suggesting that both methods could work equally if only the relative magnitudes of EPC0among soils are needed. Overall, both methods are acceptable to fit the Langmuir isotherms.

In the Northeast US, saturation excess is the most dominant runoff process and locations of runoff source areas, typically called variable source areas (VSAs), are determined by the available soil water storage and the landscape topographic position. To predict runoff generated from VSAs some water quality models use the Soil Conservation Service Curve Number equation (SCS‐CN), which assumes a constant initial abstraction of rainfall is retained by the watershed prior to the beginning of runoff. We apply a VSA interpretation of the SCS‐CN runoff equation that allows the initial abstraction to vary with antecedent moisture conditions. We couple this modified SCS‐CN approach with a semi‐distributed water balance model to predict runoff, and distribute predictions using a soil topographic index for the Town Brook watershed in the Catskill Mountains of New York State. The accuracy of predicted VSA extents using both the original and the modified SCS‐CN equation were evaluated for 14 rainfall‐runoff events through a comparison with average water table depths measured at 33 locations in Town Brook from March–September 2004. The modified SCS‐CN equation captured VSA dynamics more accurately than the original equation. However, during events with high antecedent rainfall VSA dynamics were still under‐predicted suggesting that VSA runoff is not captured solely by knowledge of the soil water deficit. Considering the importance of correctly predicting runoff generation and pollutant source areas in the landscape, the results of this study demonstrate the feasibility of integrating VSA hydrology into water quality models to reduce non‐point source pollution. Copyright © 2009 John Wiley & Sons, Ltd.

Although water quality problems associated with agricultural nonpoint source (NPS) pollution have prompted the rapid and widespread adoption of a variety of so called “best management practices” (BMPs), there have been few realistic efforts to assess their combined effectiveness in reducing NPS pollution. This study used the Variable Source Loading Function (VSLF) model, a distributed watershed model, to simulate phosphorus (P) loading from an upstate New York dairy farm before and after the implementation of a suite of BMPs. With minimal calibration, the model calculates the dissolved P (DP) losses from impervious surfaces (e.g., barnyards), the plant/soil complex, field‐applied manure, and loads associated with baseflow conditions. The simulated DP loads agreed well with measured loads for both the pre‐BMP and post‐BMP periods. More importantly, results showed that BMPs reduced DP loads by 35%, which is over half of the expected reduction if all manure was removed from the watershed, i.e., ∼50% reduction. The model results indicate that had no BMPs been installed DP loads would be ∼37% greater than observed at the watershed outlet. The most effective BMPs were those that disassociated pollutant loading areas from areas prone to generating runoff, i.e., hydrologically sensitive areas. By contrast, attempts to reduce P content in manure were somewhat less effective. This study demonstrates that a combination of distributed, mechanistic modeling and long‐term monitoring provides better insights into the effectiveness of water quality protection efforts than either individually.

Pathogen contamination of waterways is a serious concern in dairy farming areas where livestock waste is applied to agricultural fields. As an alternative, a biodrying composting system dries collected livestock waste, reduces the strong odors, and has been proposed as a means of reducing, and even eliminating pathogens present in the waste. Therefore, the survival of pathogens in a biodrying composting system was investigated. Dairy farm livestock waste was piled in a biodrying storage shed where forced aeration and natural decomposition processes heated a major portion of the waste pile to temperatures exceeding 55°C. Ascaris suumeggs were used as the surrogate species and inoculated into special chambers and placed at three different elevations at different intervals along the length of the pile. Control chambers were stored in water at 4°C in the laboratory. Both compost and control chambers were removed at Day 4, 8, 12, 16, and 20. The eggs were extracted from the chamber medium and analyzed for viability. No viable eggs were recovered from any of the chambers removed from the compost pile, while ≥90% viability was observed in the control chambers. High temperatures and continued drying were the major contributing factors to the inactivation of the helminth eggs. The complete inactivation of A. suumeggs by the biodrying process encourages the storage and treatment of manure to high temperatures and reduced moisture conditions before field spreading to reduce the risk of harmful pathogens contaminating waterways and potential drinking water supplies.

Preferential flow is implicated as one of the causes of groundwater contamination with agrochemicals. Although preferential flow is a natural process, on turfgrass surfaces, it may be enhanced by mechanical aeration that creates surface perforations similar to macropores. Aeration effect on pesticide mobility is not well known, and therefore, the objective of this paper is to visualize the infiltration flow paths from aerated turfgrass systems as influenced by soil moisture and sand content. Forty-eight lysimeters located in a greenhouse were established to simulate greens constructed of sand or existing sandy clay soil. The movement of blue dye was observed under two moisture regimes for three aeration practices (shallow hollow tines, deep drill, and high-pressure water injection) and a nonaerated control. After the dye was applied, stained cross sections taken at 5-cm intervals were traced on transparent sheets and digitized. Analyzing the dye patterns revealed three types of flow matric flow, preferential flow through the man-made aeration macropores, and fingered flow due to hydrophobicity. Hydrophobicity induced by the grass cover caused ponding which caused water either to infiltrate in the man-made macropores or formed fingers especially when infiltration through the macropores was limited. Hydrophobicity was most severe for the dry treatments. Wet sand was not hydrophobic and water infiltrated as matric flow. In all cases, the dry sandy clay soils exhibited more preferential flow than the sand, independent of aeration treatment. The results show that the best time to apply agrochemicals on cultivated turfgrass sites is shortly after irrigation or on wet soils to reduce the sod hydrophobicity. However, this was not the case for deep-drill application on sandy clay soil.

Mechanistic, predictive equations for phosphorus (P) transport in runoff from manure‐applied fields constitute a critical knowledge gap for developing nonpoint‐source pollution models. We derived two simple equations to describe the P release from animal manure during a rainfall event—one based on first‐order P desorption kinetics and one based on second‐order kinetics. The manure characteristics needed in the two kinetic equations are the maximum amount of water‐extractable phosphorus (WEP) and a characteristic desorption time. Water‐extractable P can be measured directly but currently the characteristic time can only be obtained by fitting experimental data. In addition, we evaluated two models usually used to estimate P loss from soil, the Elovitch equation and power function, both of which relate P loss to time. The models were tested against previously published data of P release from different manures under laboratory conditions. All equations fit the data well. Of the two kinetic equations, the second‐order model showed better agreement with the data than the first‐order model; for example, maximum relative differences between the model results and measured data were 2.6 and 4.7%, respectively. The characteristic times varied between 20 min for dairy manure and almost 100 min for poultry manure. The characteristic time did not appear to change with flow rate but decreased with smaller manure aggregates. The parameters for power‐function relationships could not be related to measured manure characteristics. These results provide the first step to process‐based approximations for predicting P release from manure with time during rainfall shortly after land application, when P losses are the greatest.

Modeling the transport of chemicals is challenging due to the presence of preferential flow paths and the input data needed to describe these paths. We propose a simple equation that can predict the breakthrough of solutes without excessive data requirements under steady state flow conditions. In our generalized preferential flow model (GPFM), the soil is conceptually divided into a distribution zone and a conveyance zone. The distribution zone acts as a linear reservoir resulting in an exponential loss of solutes through preferential flow paths from this zone. In the conveyance zone, the transport of solutes is described with the convective‐dispersive (CD) equation. Input data required are the apparent water content of the distribution zone, and solute velocities and dispersivities in the conveyance zone. The model is tested with chloride breakthrough curves (BTCs) resulting from three sets of experiments performed with steady state artificial acid rainfall at different intensities applied sequentially using duplicate columns filled with undisturbed soil or sand both exhibiting preferential flow. The model is able to describe the breakthrough of solutes with physically meaningful parameters through coarse sand with fingered flow and through undisturbed soil cores. In coarse sand, water and solutes flow only through the fingered flow paths in the conveyance zone. In undisturbed columns there is both flow through the matrix and preferential flow paths, and two velocities in the conveyance zone are needed to simulate the breakthrough curves.

Fire is commonly used to establish and maintain pastures in the Amazon. Fire is also known to induce soil water repellency but few published data exist for the humid tropics. The objectives of this study were to characterise the intensity and spatial variability of water repellency on previously burned pasture soils in the Amazon, and its effect on the nutrient status of the forage grass Brachiaria brizantha (Hochst.) Stapf. Surface soils of pastures and forests in north-western Mato Grosso, Brazil, were found to exhibit soil water repellency using MED and WDPT tests. Soil water repellency was found only within 0?0.01 m of the mineral soil surface, with soil below 0.01 m found to be hydrophilic in all cases.  Spatial variability of repellency was high for both pasture and forest soils. For pasture soils, soil water repellency was strongest on recently burned pastures, which exhibited some extremely high values (MED > 8 m). Repellency decreased rapidly with time following burning. Increasing soil water repellency was associated with decreasing N: P ratios of B. brizantha above-ground biomass (r2 = 0.66, P = 0.004). These findings indicate that soil water repellency and pasture productivity are inversely related. Since pasture abandonment fuels continued deforestation, disrupting the processes causing pasture degradation may lead to more sustainable land use in the Amazon.

A few hydrophobic grains in otherwise hydrophilic sand render the soil hydrophobic and can completely alter the flow of water through unsaturated sands. In this paper we examine whether percolation theory can explain the phenomenon. Percolation theory has been used to describe the dependence of large-scale flow phenomena on heterogeneities found at the pore scale and should, therefore, be able to explain the water flow behaviour in hydrophobic soil. We show that the theory is valid, in general, for a hydrophilic soil into which a small but increasing fraction of highly hydrophobic grains is mixed. However, the application of percolation theory is limited by the complex interactions of matric potential and contact angle effects due to the introduction of hydrophobic particles.

Due to geochemical processes, peat soils often have elevated concentrations of trace elements, which are gradually released following drainage for agriculture. Our objectives were to use incubation temperatures to vary microbial activity in two metalliferous peats (M7 acidic peat and M3 neutral peat) from the Elba, New York region, and to use periodic leaching to assess the extent of trace element release from these soils. Dried soils were mixed with glass beads to maintain aeration, moistened, and incubated at 4, 16, 28, and 37°C in 10‐cm‐diameter × 8‐cm‐tall columns. Five incubation–leaching cycles were performed, each consisting of 7.3 d of incubation (28 d for the final cycle) followed by 16 h of leaching with synthetic acid rain at 2.5 mm h−1Microbial activity was determined initially and after the final leaching by measuring C mineralization following glucose stimulation. Cumulative respiration results were ranked 28 > 16 > 4 > 37°C, with M7 acidic peat respiration values greater than M3 neutral peat at each temperature. Initial leachate pH levels were between 2 and 4, with acidification less pronounced and shorter‐lived for the M3 peat. Leachate S, dissolved organic carbon (DOC), NO3–N, and trace elements declined with successive leachings (rebounding slightly in the final M3 leachate), with concentrations typically greater in the M7 leachate. Elemental losses followed the same general ranking (28 > 16 > 4 > 37°C); losses at 28°C were 15 to 22% for As, Cd, Ni, and Zn from the M7 peat; losses from M3 were comparable only for Cu (1%) and Ni (19%). The correlation of respiration with S, DOC, and trace elements losses indicates that microbial processes mediated the release of trace elements in both peat soils. Neutral M3 peat pH levels limited losses of most analytes.

While the rapid transport of nonadsorbed chemicals is relatively well understood, preferential transport of adsorbable chemicals including P requires further study. Our objective was to characterize subsurface P transport in glaciated soils of the northeastern USA. Large intact columns from five soils with differing structures were subjected in duplicate to both synthetic acid rainfall at a low rate simulating natural rainfall, and ponded water for observing saturated flow. The rain was enriched with inorganic and organic P after baseline conditions were established. Drainage water P concentrations were measured and adsorption isotherms were determined. Type of flow was characterized using Cl breakthrough curves and visualizations of blue dye patterns. Baseline P concentrations in drainage water were 0.02 to 0.04 mg L−1At low flow rates, P appeared in the drainage water soon after application of either inorganic or organic P for the silt loam soil (firm, moderate coarse prismatic parting to moderate medium subangular blocky structure) in which preferential flow paths carried most of the water flow. In contrast, the soils in which matrix type flow dominated (weak fine granular or weak medium subangular blocky structures) had little or no increase in drainage water P. However, under ponded conditions all soils exhibited preferential flow and rapid P breakthrough. Elevated P concentration in the drainage water could not be explained by the P adsorption strength with the possible exception of the sandy loam soil, where the outflow P concentration was consistently low. Variation in flow rate in conjunction with soil structure satisfactorily explained elevated concentrations of dissolved inorganic and organic P in drainage water from surface applied P.

Non-aqueous phase liquids enter the vadose zone as a result of spills or leaking underground storage facilities, thus contaminating groundwater resources. Measuring the contaminant concentrations is important in assessing the risk to human health and the environment and to develop effective remediation. This research presents the development and application of the light transmission method (LTM) for three-phase flow systems, aimed at investigating unstable fingered flow in a soil-air-oil-water system. The LTM uses the hue and intensity of light transmitted through a slab chamber to measure fluid content, since total liquid content is a function of both hue and light intensity. Evaluation of the LTM is obtained by comparing experiments with LTM and synchrotron X-rays. The LTM captures the spatial resolution of the fluid contents and can provide new insights into rapidly changing, two-phase and three-phase flow systems. Application of the LTM as a visualization technique for environmental and physical phenomena is noted. Visualization by LTM of groundwater remediation by surfactants as well as visualization of model cluster growth and fractal dimensions was also explored.

Watersheds, in many parts of the world, consist of sloping soils with a dense subsoil at shallow depth. Very few measurement techniques exist for realistically determining hydraulic properties in situ on these hillside soils. A hillslope infiltrometer, open at the bottom, top, and downhill sides, was developed that can measure the vertical saturated conductivity of the horizon sequence and lateral hydraulic conductivity of the horizon most conductive to water. The infiltrometer was tested on the steeply sloping soils in Honduras. Increasing rainfall intensities were applied and lateral flow rate was measured in several troughs at the downslope end of the hillslope infiltrometer. Vertical infiltration rates for the horizon sequence and lateral conductivity were calculated at each rainfall rate after steady state conditions were established. The hillslope infiltrometer proved useful in the characterization of subsurface flow on the steep slopes in Honduras.

Land application of liquid manure can result in nutrient enrichment of subsurface drainage effluent when conditions promote leaching or macropore flow. This contamination is most likely to occur when precipitation follows manure application closely and may cause environmental impacts to receiving waters. Field and column studies were initiated in New York to investigate the impact of manure applications on phosphorus (P) transport through the soil into subsurface drains. Field studies evaluated tile effluent contamination from liquid manure under wet and dry antecedent soil moisture conditions (year 1) and under disk and plow tillage practices (year 2). In year 1, liquid dairy manure was broadcast on the surface and the field was then irrigated. Though the tile drains in the wet plots flowed much earlier and in greater volume than the drains in the dry plots, both wet and dry plots produced similar average peak total phosphorus (TP) concentrations. Irrigation 6 days later produced similar tile discharges, but the peak TP concentrations were about one-third of the earlier values. Cumulative TP loss was significantly higher from wet than dry plots. In year 2, manure was tilled into the soil via one-pass disking or plowing before irrigation commenced. The disking did not incorporate the manure into the soil as effectively as did plowing and exhibited one order of magnitude higher effluent TP concentrations and cumulative TP loss. The timing of P transport in tile effluent relative to the tile flow is consistent with macropore transport as the primary mechanism moving TP through the soil. Column studies utilizing packed soil and artificial macropores were used to examine further the role of macropore size on P sorption to pore walls. Dissolved P was added directly to the macropore, and the effluent from the macropore showed that soluble P may be transported through macropores 1 mm or greater with negligible P sorption to pore walls. In the absence of macropores, no measurable P was transported through the soil columns. Consequently, high P concentrations observed in the tile drain effluent soon after manure application during the field studies can be attributed to macropore transport processes. Even small continuous macropores are potential pathways. Plowing-in manure apparently disturbs these macropores and promotes matrix flow, resulting in greatly reduced P concentrations in the drainage effluent.

The understanding of simple laminar flow in tubes has often been used to interpret the more complicated flow in porous media. A study of the motion of two immiscible liquids in closed tubes with relatively large diameter (> 0.3 cm i.d), was conducted in order to examine the influence of wetting and nonwetting liquids on the flow behavior. The results indicate that the wetting properties of the fluids with regard to the tube wall have a major efffect on the formation and motion of long bubbles. A physically based model was used to predict the velocity and the conditions for no motion of bubbles and drops in tubes. These results were used to interpret the nature of oil and water flow in porous media. Experiments in which the wetting liquid was displaced by the nonwetting, or vice versa, were conducted by injecting the displacing liquid at a constant flux at the center of a two-dimensional chamber saturated with the displaced liquid. The influence of wetting-nonwetting characteristics on the quantity of liquid displaced, the shape of the interface between the two liquids, and the interpretation of the no motion radius in a closed tube to the case of a porous medium are discussed. It would appear that the no motion radius gives a good indication of the minimum width of a nonwetting penetrating finger and the maximum width of nonwetting ganglia left by drainage.

Preferred movement of chloride and nitrate during runoff events following nutrient application was studied experimentally on 109 m long fields with a hard pan at shallow depth near Ithaca, NY. Chloride and nitrate applications were surface‐applied and incorporated at 36 and 69 m uphill from the collection point. All of the applied chemicals, when surface‐applied on saturated soil, were completely lost in the surface runoff and interflow water. When incorporated, at most 60% of the mass applied was lost. The fraction lost appeared to be related to the initial moisture content of the soil. After a chemical application, the concentration in both runoff and interflow was initially high and then decreased exponentially. The concentrations in the runoff water, interflow, and macropores were approximately equal throughout a runoff event.

Pesticides are found in groundwater sooner than commonly is predicted with traditional management models. These models do not account for preferential flow and assume uniform biodegradation. For more accurate modeling, the interaction of preferential flow and nonuniform biodegradation needs to be addressed. In this paper, we tested techniques to measure the rate of biodegradation in preferential flow paths by continuously applying p‐nitrophenol (PNP) to the surface of packed soil columns that contained two types of preferential flow paths. The first type was an open macropore in a uniform soil matrix (macropore column), and the second type was a channel of coarser soil in a finer soil matrix (channel columns). Breakthrough curves were obtained from separate small areas of the macropore column with fiberglass wicks to collect and separate macropore flow from soil matrix flow. Breakthrough curves were also obtained from the channel columns for comparison to curves from homogeneous columns. Average biodegradation rates were calculated from the breakthrough curves. The PNP biodegradation occurred in both matrix and macropore flow regions of the macropore column, but the time and rate of biodegradation differed in the two flow regions, as well as between the channel columns and the homogeneous columns. The PNP was degraded rapidly despite short travel times through the path of preferential flow, indicating that rapid biodegradation may occur in macropores and other preferential flow paths. The data provide a basis for improved modeling of fate and transport of chemicals through vadose soil to groundwater.

The transport of pesticides and other chemicals through macropores has been widely observed and predicting it is a challenge. This article considers a simplified two‐layer model, similar to overland flow models in which the processes of adsorption and desorption are separated. For the layer near the surface, or the mixing layer, the solute concentration in the layer is equal to that in the percolating water (including preferentially moving water). In the lower profile, the flow is partitioned between matrix and preferential flow. The solute concentration of the matrix flow is characterized by the soil condition near the outlet point, whereas the preferential flow is represented by the solute concentration in the mixing layer. The closed form equation, exhibiting exponentially decreasing macropore flow solute concentrations, is tested against solute breakthrough curves using three independent sets of experimental data. The predicted depths of mixing between 5 and 25 cm are physically realistic and the closed form is shown to reproduce the form of experimental data, particularly under conditions of significant macropore flow. Although highly simplified, the physically based model yields a framework for predicting solute concentration for preferentially moving water.

Wetting-front instability, or gravity-driven fingering, can occur during vertical infiltration. Previous studies found that fingers persist over long periods of constant infiltration and in subsequent infiltration cycles. We present a physically based theory to explain finger persistence. The theory is tested using a new technique in which moisture contents are visualized by the transmission of light through unsaturated sand, recorded with a video camera, and then analyzed by video digitizing computer hardware. Theory and experiments show that hysteresis in the moisture characteristic relationship explains the persistence of fingers in time.

IrrigationRehab is intended to produce an understanding of irrigation system rehabilitation by simulating the many tasks and problems likely to be encountered during a rehabilitation effort. Using theRehab software and the accompanying slide set, participants are challenged to formulate a rehabilitation plan considering changes that have occurred in a community since its irrigation system was constructed. African and Asian versions are available.

Water‐repellent soils occur all over the world and affect both groundwater pollution and crop yield. The finger‐like wetting patterns in these soils have many similarities with unstable wetting fronts in coarse‐grained sandy soils. Our objectives were to study the water movement in water‐repellent sand and to examine how the theory for unstable wetting fronts applies to water‐repellent sands. Infiltration experiments, in which moisture content and matric potentials were measured, were carried out in slab chambers with identical sands but with different levels of water repellence. Soil water characteristics were determined in separate experiments. Infiltration in the hydrophilic soil resulted in a uniform and horizontal front. All water‐repellent sands showed a fingered flow pattern. For negative water‐entry values, water infiltrated without delay. For positive values, water entered the soil only after the depth of the ponded water equaled or exceeded the water‐entry pressure, which increased with increasing repellency. The finger widths predicted with the unstable flow theory agreed rather well with the observed values. In general, the research showed that the wetting patterns of water‐repellent sands depended directly on the soil water characteristic curve. This implies that the type of wetting front and risk to groundwater pollution can be predicted based on laboratory‐measured soil hydraulic properties.

Plant roots and soil biota require O2to function normally. When the soil O2is depleted, it must be replaced by O2from the atmosphere. The transport occurs primarily by gaseous diffusion. Although O2diffusion has been researched in homogeneous soils, and in tight clay soils with cracks, little is known about the effects of macropores on O2transport in typical agricultural soils. In this study we examined the impact of large pores on O2concentration in a sandy loam soil. Experiments were carried out with homogeneous and simulated‐macropore columns with a steady‐state rainfall of 3 cm d‐1in which the O2concentration in the soil matrix and in the macropore was measured at 15‐cm vertical intervals. The diameter of each column was 20 cm and the height was 105 cm. Groundwater depth was systematically varied between 30 and 105 cm. Soil water content did not vary with distance above the water table for the 30‐, 60‐, and 90‐cm groundwater depths. While both the soil water for the entire column and the O2concentration from the surface to the 30‐cm depth was the same for the homogeneous and macropore columns, the O2penetrated deeper in the macropore column and was 4% greater below the 30‐cm depth in the macropore column than for the homogeneous column.

Minimization of the concentration and mobility of trace metals is a primary concern when considering the land application of wastewater sludges. The effects of pelletization/drying, composting, incineration, and N‐Viro chemical stabilization on composition and mobility of trace metals and P were compared. One day's production of dewatered digested sludge (Syracuse, NY) was used as the sole feedstock so that observed differences would solely be a result of the process used. Mobility was determined using the toxic characteristic leaching procedure (TCLP). Analyte concentrations were essentially constant during dewatered sludge production with mean values (mg kg−1TS) of 5.6 Cd, 10.7 Co, 130 Cr, 587 Cu, 49.7 Mo, 35.8 Ni, 26 880 P, 132 Pb, and 545 Zn. Concentrations in pellets were similar to dewatered sludge, but were reduced in composted sludge and N‐Viro product by the addition of amendments. Incineration concentrated all analytes except Cd and Pb, which experienced volatilization losses. The mode of processing had widely ranging effects on mobilities. The TCLP‐mobile fraction (percent of total) of analytes in dewatered sludge was <3% except Ni (7.1%), P (6.0%), and Zn (11.2%). Composting slightly increased Cd mobility, but reduced that of Ni and P. Pelletization increased the mobility of Cd (7.9%), Cu (3.8%), Ni (15.4%), and Zn (15%). The N‐Viro process substantially increased mobilities of Cu (43%), Mo (50%), and Ni (24%) at elevated pH. Incineration increased mobilities of Cd and Mo, but reduced Ni, P, and Zn mobility. These results, with those of longer term in situ studies, can be used to guide the selection of sludge processes that minimize potential metal and P mobility.

Wick samplers have the potential to improve solute sampling in the vadose zone, but more information is needed about the effect of soil-wick sampling interface disturbances on measured fluxes and solute concentrations. The objective of this paper is to examine how the wick sampler alters the matric potential, streamlines, and solute concentrations in the native soil. Both theoretical analysis and experimental studies were performed. An equation for travel time was developed that included the effect of changes in moisture content in the soil when a wick sampler is installed. Experimental measurements of pressure head near the soil-wick interface were taken, and a travel time moment analysis of narrow solute pulses was conducted for several steady flow experiments. Model and theory agreed well and showed that even when the capillary length α−1is similar in the soil and wick, dissimilarity of the hydraulic conductivity and cross-section flow area may significantly affect soil moisture content above the soilwick interface and on the solute pulse travel time. It was also found that in many cases solute pulse travel time was affected more by the pressure head changes that occurred at the soil-wick interface than by the flow through the wick.

The purpose of this research was to develop a computer model to simulate water and heat movement in the soil under freezing conditions. The model was based on the theory of coupled heat and mass transport, which was derived from the theory of irreversible thermodynamics. A finite difference scheme was used to solve the coupled‐fluxes equations in space. The difference scheme used only first‐order derivatives to facilitate the simulation of steep gradients of hydraulic potential at the freezing front. The simulated results were compared against two sets of laboratory experiments in which soil columns were suddenly cooled below freezing at one end. Thermal and hydraulic soil properties used in the simulation model were either obtained from independent measurements on the soil or by well‐established estimating procedures. The simulated results agreed well with the observed values in the first experiment, and in the second experiments the simulated data followed the trend, but a too low ice content was predicted and the freezing front prediction lagged behind. Overall it seems that although the model in its present form is limited to nonheaving conditions, the theory of irreversible thermodynamics is a promising approach to simulate heat and moisture transfer in partly frozen soils.