David E. Radcliffe

Soil structure development and preferential solute flow

Soil structure and preferential flow are thought to be highly related, but quantification of this relationship has remained problematic. A combination of experiments on two soils at local and field scale was used to study the relationships between the degree of structural development, saturated conductivities Ks, moisture release, breakthrough parameters, and observations of preferential flow at the field scale. The convection dispersion equation and mobile-immobile model were fitted to breakthrough curves, and flow channels were stained with dye. Well-developed structure was associated with high dispersivities, a high coefficient of variation of Ks, and low mobile water contents, low exchange coefficients, and a low dyed area. Field-scale observations matched the differences in structure and preferential flow parameters observed at the local scales. Local-scale preferential flow appeared to highly influence field-scale solute transport, indicating that this is an important local-scale process which is not dampened at the field scale. A conceptual model of subsoil structural development was expanded with physical parameters matching the structural development stage to predict the likelihood of preferential flow on the basis of the saturation regime of the horizon or pedon.

Diffuse phosphorus models in the United States and Europe: Their usages, scales, and uncertainties

Today there are many well-established computer models that are being used at different spatial and temporal scales to describe water, sediment, and P transport from diffuse sources. In this review, we describe how diffuse P models are commonly being used in the United States and Europe, the challenge presented by different temporal and spatial scales, and the uncertainty in model predictions. In the United States, for water bodies that do not meet water quality standards, a total maximum daily load (TMDL) of the pollutant of concern must be set that will restore water quality and a plan implemented to reduce the pollutant load to meet the TMDL. Models are used to estimate the current maximum daily and annual average load, to estimate the contribution from different nonpoint sources, and to develop scenarios for achieving the TMDL target. In Europe, the EC-Water Framework Directive is the driving force to improve water quality and models are playing a similar role to that in the United States, but the models being used are not the same. European models are more likely to take into account leaching of P and the identification of critical source areas. Scaling up to the watershed scale has led to overparameterized models that cannot be used to test hypotheses regarding nonpoint sources of P or transport processes using the monitoring data that is typically available. There is a need for more parsimonious models and monitoring data that takes advantage of the technological improvements that allow nearly continuous sampling for P and sediment. Tools for measuring model uncertainty must become an integral part of models and be readily available for model users.

Arsenate Displacement from Fly Ash in Amended Soils

Arsenic (As) is the biggest environment contaminant in most of the soils where fly ash is applied. Usually, it is not mobile and strongly adsorbed on to soil particles. However, in gypsum and phosphorus amended soils As may be much more mobile. A study in repacked columns was conducted to determine whether or not As becomes mobile when Ca(H2PO4)2and CaSO4are used as leaching solutions, and to compare the competitive interactions between PO4-AsO4and SO4-AsO4. Arsenic concentration in leachate was found to be approximately ten times greater when Ca(H2PO4)2was used to leach the columns as compared to CaSO4. A maximum concentration of 800 μg As L-1was found in the leachate in this case, which is much higher than the groundwater limit of 50 μg L-1for drinking water established by the United States Environmental Protection Agency. In fly ash, the portion of arsenate non-specifically adsorbed is believed to be much lower than that of specifically adsorbed. Sulfate anions were able to displace only non-specifically adsorbed arsenate. In this case the concentration of As in leachate was found to be within acceptable limits. On the other hand, phosphate can compete with arsenate for all available adsorption sites, non-specific and specific. Phosphate displacement of both forms of arsenates increases As mobility in both control and fly ash treatments.

Nutrient losses in surface and subsurface flow from pasture applied poultry litter and composted poultry litter

Over application of poultry litter may cause pollution of surface and ground water. Spatial variability in soil characteristics makes predictions difficult. Composting poultry litter could reduce the risk of pollution by creating more stable organic components. Three rates of poultry litter and compost (10 Mg ha-1 litter, 20 Mg ha-1 litter and 10 Mg ha-1 litter combined with 50 Mg ha-1 compost) to three watersheds under pasture. The watersheds were monitored for surface and subsurface flow. Nitrate-N concentrations in subsurface flow did not exceed the U.S. Environmental Protection Agency drinking water standard of 10 mg L-1. Soluble phosphorus concentrations in runoff were high, reaching a maximum of 8.5 mg L-1 under the compost treatment. These concentrations are generally lower than reported on smaller scale studies, which shows the need of studies at the correct scale.

Applicability of models to predict phosphorus losses in drained fields: a review.

Most phosphorus (P) modeling studies of water quality have focused on surface runoff loses. However, a growing number of experimental studies have shown that P losses can occur in drainage water from artificially drained fields. In this review, we assess the applicability of nine models to predict this type of P loss. A model of P movement in artificially drained systems will likely need to account for the partitioning of water and P into runoff, macropore flow, and matrix flow. Within the soil profile, sorption and desorption of dissolved P and filtering of particulate P will be important. Eight models are reviewed (ADAPT, APEX, DRAINMOD, HSPF, HYDRUS, ICECREAMDB, PLEASE, and SWAT) along with P Indexes. Few of the models are designed to address P loss in drainage waters. Although the SWAT model has been used extensively for modeling P loss in runoff and includes tile drain flow, P losses are not simulated in tile drain flow. ADAPT, HSPF, and most P Indexes do not simulate flow to tiles or drains. DRAINMOD simulates drains but does not simulate P. The ICECREAMDB model from Sweden is an exception in that it is designed specifically for P losses in drainage water. This model seems to be a promising, parsimonious approach in simulating critical processes, but it needs to be tested. Field experiments using a nested, paired research design are needed to improve P models for artificially drained fields. Regardless of the model used, it is imperative that uncertainty in model predictions be assessed.

Modeling Phosphorus in the Environment

Basic Approaches Modeling Phosphorus Movement from Agriculture to Surface Waters, A. Sharpley Modeling Runoff and Erosion in Phosphorus Models, M.L. Wolfe Modeling Phosphorus in Runoff: Basic Approaches, M.L. Cabrera Basic Approaches to Modeling Phosphorus Leaching, N.O. Nelson and J.E. Parsons Phosphorus Transport in Streams: Processes and Modeling Considerations, B.E. Haggard and A.N. Sharpley Uncertainty Estimation in Phosphorus Models, K. Beven, T. Page, and M. McGechan Models Phosphorus Modeling in Soil and Water Assessment Tool (SWAT) Model, I. Chaubey, K.W. Migliaccio, C.H. Green, J.G. Arnold, and R. Srinivasan Modeling Phosphorus with Hydrologic Simulation Program-Fortran, D.E. Radcliffe and Z. Lin Phosphorus Modeling in the Annualized Agricultural Nonpoint Source Pollution (AnnAGNPS) Model, Y. Yuan, R.L. Bingner, and I. Chaubey Answers-2000: A Nonpoint Source Pollution Model for Water, Sediment, and Phosphorus Losses, F. Bouraoui and T.A. Dillaha Watershed Ecosystem Nutrient Dynamics - Phosphorus (WEND-P) Models, R.L. Kort, E.A. Cassell, and S.G. Aschmann Modeling Phosphorus with the Generalized Watershed Loading Functions (GWLF) Model, E.M. Schneiderman Phosphorus Indices, Best Management Practices, and Calibration Data Phosphorus Indices, J. Weld and A. Sharpley Challenges to Using and Implementing Phosphorus Indices in Nutrient Management Planning: an MMP Perspective, P. Hess, B. Eisenhauer, and B. Joern Quantifying the Effects of Phosphorus Control Best Management Practices, M.W. Gitau and T.L. Veith Small Watershed Data Collection to Support Phosphorus Modeling, D. Harmel and B. Haggard Modeling in the Future Suggestions to Improve Modeling of Phosphorus, D.E. Radcliffe and M.L. Cabrera Index

Spatial resolution of soil data and channel erosion effects on SWAT model predictions of flow and sediment

Water quality modeling efforts for developing total maximum daily loads often use geographical information system data of varying quality in watershed-scale models and have shown varying impacts on model results. Several streams in the southern Piedmont are listed for sediment total maximum daily loads. The objective of this study was to test the effect of spatial resolution of soil data on the SWAT (Soil and Water Assessment Tool) model predictions of flow and sediment and to calibrate the SWAT model for a watershed dominated by channel erosion. The state soil geographic (STATSGO) database mapped at 1:250,000 scale was compared with the soil survey geographic (SSURGO) database mapped at 1:12,000 scale in an ArcSWAT model of the North Fork Broad River in Georgia. Model outputs were compared for the effect of soil data before calibration using default model parameters as calibration can mask the effect of soil data. The model predictions of flow and sediment by the two models were similar, and the differences were statistically insignificant (α = 0.05). These results were attributed to the similarity in key soil property values in the two databases that govern stream flow and sediment transport. The two models after calibration had comparable model efficiency in simulating stream flow and sediment loads. The calibrated models indicated that channel erosion contributed most of the suspended sediment in this watershed. These findings indicate that less detailed soil data can be used because more time, effort, and computational resources are required to set up and calibrate a model with more detailed soil data, especially in a larger watershed.

Morphologic and hydraulic properties of soils with water restrictive horizons in the Georgia Coastal Plain

Hydraulic and morphological properties of soils from a 0.36-ha site in the Georgia Coastal Plain were evaluated. Objectives included characterizing the morphological and hydraulic properties ofmajor horizons in soils with plinthite, determining the extent of preferential flow, and relating flow/transport parameters derived from breakthrough curve analyses to morphological properties. These soils have developed from Miocene aged sediments and are classified in fine-loamy, siliceous, thermic families of Plinthaquic, Aquic, Arenic Plinthic, Plinthic, and Typic Kandiudults. Morphological evidence indicates that BC horizons are restrictive to vertical percolation of drainage water. Methylene blue dye staining, K sat and breakthrough curves (analyzed by two region/MIM using CXTFIT) were measured on 15-cm-diameter undisturbed cores to determine the effects ofargillic horizons, argillic horizons with plinthite, and subjacent BC horizons on hydraulic properties of the soils. K sat for seven sampled pedons averaged 1.6 X 10 -2 , 1.1. X 10 -2 , and 3.8 X 10 -3 mm s -l for Bt, Btv, and BC horizons, respectively. Dye staining and model output (MIM) indicate a greater degree of preferential flow with depth and subsequent less mobile area contributing to water flow. Analyses of the flowpaths indicate water is translocated in regions with relatively higher porosity that also contain a higher proportion of coarser pores. Micrographs indicate that flowpaths are associated with bio-pores and areas ofbetter aggregation in the Bt horizons and structural voids in the BC horizons. BC horizons in these soils are less permeable because ofincreased clay content, differences in pore characteristics, and less cross-sectional area contributing to flow.

Modeling Phosphorus in the Lake Allatoona Watershed Using SWAT: II. Effect of Land Use Change

Lake Allatoona is a large reservoir northeast of metropolitan Atlanta, GA, threatened by excessive algal growth. We used the calibrated Soil and Water Assessment Tool (SWAT) models developed in our companion paper to estimate the annual P load to Lake Allatoona in 1992 and in 2001 after significant changes occurred in land use. Land use data in 1992 and 2001 from the Multi-Resolution Land Characteristics (MRLC) Consortium showed that forest land use decreased during this period by about 20%, urban land use increased by about 225%, and pasture land uses increased by about 50%. Simulation results showed that the P load to Lake Allatoona increased from 176.5 to 207.3 Mg, which were 87.8% and 103.1%, respectively, of the total P (TP) annual cap (201 Mg) set by the Georgia Environmental Protection Division (GAEPD) for discharge into Lake Allatoona. In the early 1990s, the greatest sources of the TP load to Lake Allatoona (and their percentages of the total load) were pasture (33.6%), forest (27.5%), and point sources (25.0%). Urban land uses contributed about 6.0% and row-crop agriculture contributed about 6.8%. A decade later, the greatest two TP sources were pasture (52.7%) and urban (20.9%) land uses. Point-source P loads decreased significantly to 11.6%. Permit limits on poultry processing plants reduced the point-source P loads, but increasing urban and pasture land uses increased nonpoint sources of P. To achieve further reductions in the P load to Lake Allatoona, contributions from pasture and urban nonpoint sources will need to be addressed.

Beyond the urban gradient: barriers and opportunities for timely studies of urbanization effects on aquatic ecosystems

Abstract Many studies have shown that streams degrade in response to urbanization in the watershed. These studies often are based on use of biotic and abiotic variables to measure stream health across a gradient of land cover/land use. The results of these studies can be applied to other urban systems, but often fail to provide a mechanistic understanding of the urban impact, in part, because of the nature of the experimental design. We analyzed the advantages and disadvantages of using environmental gradient studies to further understanding of urban stream systems. We also evaluated alternative experimental design approaches, including best management practice monitoring, long-term watershed studies, paired-watershed studies, and before–after control–impact studies, which could be used to complement the gradient approach. We illustrate these theoretical discussions with an urban paired-watershed case study in the Etowah watershed in northern Georgia. Our goal is to move experimental designs in a direction that will further our mechanistic understanding of the effects of existing urbanization on aquatic ecosystems and will provide opportunities to evaluate stream responses to environmentally sensitive urban land cover.