M. Todd Walter

A GIS-based variable source area hydrology model

Effective control of nonpoint source pollution from contaminants transported by runoff requires information about the source areas of surface runoff. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runoff in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation-excess overland flow (i.e., surface runoff), and interflow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of New York State and observed stream flow hydrographs and soil moisture measurements were compared to model predictions. Stream flow prediction during non-winter periods generally agreed with measured flow resulting in an average r2 of 0·73, a standard error of 0·01 m3/s, and an average Nash-Sutcliffe efficiency R2 of 0·62. Soil moisture predictions showed trends similar to observations with errors on the order of the standard error of measurements. The model results were most accurate for non-winter conditions. The model is currently used for making management decisions for reducing non-point source pollution from manure spread fields in the Catskill watersheds which supply New York Citys drinking water. Copyright

Increasing Evapotranspiration from the Conterminous United States

Abstract Recent research suggests that evapotranspiration (ET) rates have changed over the past 50 years; however, some studies conclude ET has increased, and others conclude that it has decreased. These studies were indirect, using long-term observations of air temperature, cloud cover, and pan evaporation as indices of potential and actual ET. This study considers the hydrological cycle more directly and uses published precipitation and stream discharge data for several large basins across the conterminous United States to show that ET rates have increased over the past 50 years. These results suggest that alternative explanations should be considered for environmental changes that previously have been interpreted in terms of decreasing large-scale ET rates.

Phosphorus transport into subsurface drains by macropores after manure applications: Implications for best manure management practices

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.

Linking the pacific decadal oscillation to seasonal stream discharge patterns in Southeast Alaska

Abstract This study identified and examined differences in Southeast Alaskan streamflow patterns between the two most recent modes of the Pacific decadal oscillation (PDO). Identifying relationships between the PDO and specific regional phenomena is important for understanding climate variability, interpreting historical hydrological variability, and improving water-resources forecasting. Stream discharge data from six watersheds in Southeast Alaska were divided into cold-PDO (1947–1976) and warm-PDO (1977–1998) subsets. For all watersheds, the average annual streamflows during cold-PDO years were not significantly different from warm-PDO years. Monthly and seasonal discharges, however, did differ significantly between the two subsets, with the warm-PDO winter flows being typically higher than the cold-PDO winter flows and the warm-PDO summer flows being typically lower than the cold-PDO flows. These results were consistent with and driven by observed temperature and snowfall patterns for the region. During warm-PDO winters, precipitation fell as rain and ran-off immediately, causing higher than normal winter streamflow. During cold-PDO winters, precipitation was stored as snow and ran off during the summer snowmelt, creating greater summer streamflows. The Mendenhall River was unique in that it experienced higher flows for all seasons during the warm-PDO relative to the cold-PDO. The large amount of Mendenhall River discharge caused by glacial melt during warm-PDO summers offset any flow reduction caused by lack of snow accumulation during warm-PDO winters. The effect of the PDO on Southeast Alaskan watersheds differs from other regions of the Pacific Coast of North America in that monthly/seasonal discharge patterns changed dramatically with the switch in PDO modes but annual discharge did not.

Estimating basin-wide hydraulic parameters of a semi-arid mountainous watershed by recession-flow analysis

Insufficient sub-surface hydraulic data from watersheds often hinders design of water resources structures. This is particularly true in developing countries and in watersheds with low population densities because well-drilling to obtain the hydraulic data is expensive. The objective of this study was to evaluate the applicability of ‘Brutsaert’ recession flow analysis to steeper and more arid watersheds than those that were previously used. Using daily streamflow data (1962 ‐ 1992), a modified version of the analysis was used to estimate the subsurface hydraulic parameters of four semi-arid, mountainous watersheds (204 ‐ 764 km 2 ) near Oaxaca, Mexico. In this analysis, a dimensionless recession curve (DRC) was translated to best-fit observed recession flow (Q) data. The basin-wide hydraulic parameters were directly related to the magnitude of the translation used to fit the DRC to the data. One unique aspect of this study was too few high flow data to confidently fit the DRC to the data via previously published protocols. We, thus, proposed using three different approaches for translating the DRC in order to establish a range for the hydraulic parameters. Our analyses predicted a narrow range of basin-wide hydraulic parameters that were near regionally measured values and consistent within commonly published values for similar geology, suggesting that the Brutsaert method is applicable to arid, mountainous basins like those used here. This method potentially provides a costeffective alternative to traditional geohydrological field methods for determining groundwater parameters. q 2003 Elsevier B.V. All rights reserved.

Application of SMR to modeling watersheds in the Catskill Mountains

Very few hydrological models commonly used in watershed management are appropriate for simulating the saturation excess runoff. The Soil Moisture Routing model (SMR) was developed specifically to predict saturation excess runoff from variable source areas, especially for areas where shallow interflow controls saturation. A recent version of SMR was applied to two rural catchments in the Catskill Mountains to evaluate its ability to simulate the hydrology of these systems. Only readily available meteorological, topographical, and landuse information from published literature and governmental agencies was used. Measured and predicted streamflows showed relatively good agreement; the average Nash–Sutcliffe efficiency for the two watersheds were R2=72% and R2=63%. Distributed soil moisture contents and the locations of hydrologically sensitive areas were also predicted well.

Atrazine leaching from biochar-amended soils

The herbicide atrazine is used extensively throughout the United States, and is a widespread groundwater and surface water contaminant. Biochar has been shown to strongly sorb organic compounds and could be used to reduce atrazine leaching. We used lab and field experiments to determine biochar impacts on atrazine leaching under increasingly heterogeneous soil conditions. Application of pine chip biochar (commercially pyrolyzed between 300 and 550 °C) reduced cumulative atrazine leaching by 52% in homogenized (packed) soil columns (p=0.0298). Biochar additions in undisturbed soil columns did not significantly (p>0.05) reduce atrazine leaching. Mean peak groundwater atrazine concentrations were 53% lower in a field experiment after additions of 10 t ha(-1) acidified biochar (p=0.0056) relative to no biochar additions. Equivalent peat applications by dry mass had no effect on atrazine leaching. Plots receiving a peat-biochar mixture showed no reduction, suggesting that the peat organic matter may compete with atrazine for biochar sorption sites. Several individual measurement values outside the 99% confidence interval in perched groundwater concentrations indicate that macropore structure could contribute to rare, large leaching events that are not effectively reduced by biochar. We conclude that biochar application has the potential to decrease peak atrazine leaching, but heterogeneous soil conditions, especially preferential flow paths, may reduce this impact. Long-term atrazine leaching reductions are also uncertain.

Funneled flow mechanisms in layered soil: field investigations

Abstract The movement of water and potential pollutants in the vadose zone of fluvial deposits is often difficult to predict because fine-over-coarse layers may behave as capillary barriers, funneling water and dissolved solutes into concentrated preferential flow paths. Capillary barriers have been studied in laboratory experiments and by mathematical analysis with well-defined boundaries but little is known about water and solute movement in naturally layered soils. This paper demonstrates how naturally occurring capillary barriers affect water and solute movement. Experiments were carried-out in a river valley near Amherst, MA and on a prehistoric beach near Cornell University in Ithaca, NY. Dye tracer and chloride were initially applied near the soil surface, followed by intermittent rainfall over several weeks. After 2–6 weeks the blue dye, soil water content, and chloride concentration distributions in the soil profile were examined by excavating a trench and photographing and sampling the exposed soil face. At both sites, the infiltrating water was diverted and flowed laterally over a relatively coarse layer. At the Cornell site, flow broke through the fine–coarse interface into the coarse layer at a few points along the barrier. The primary breakthrough at the Cornell site occurred at a horizontal section of the interface, 2 m down-slope from the point of dye application. The capillary diversions generally agreed with current theory, however, the flow patterns were difficult to predict accurately without more detailed information about the soil layering characteristics.

Coupling terrestrial and atmospheric water dynamics to improve prediction in a changing environment

Humans have profoundly influenced their environment. It has been estimated that nearly one-third of the global land cover has been modified while approximately 40% of the photosynthesis has been appropriated. As the interface between the subsurface and the atmosphere is altered, it is imperative that we understand the influence this alteration has in terms of changing regional and global climates. Land surface heterogeneity is sometimes a principal modulator of local and regional climates and, as such, there are potential aggregation and teleconnection effects ranging in scales from soil pores to the general atmospheric circulation when the land surface is altered across a range of scales. The human fingerprint on land surface processes is critical and must also be accounted for in the discourse on land-atmosphere coupling as it pertains to climate and global change as well as local processes such as evapotranspiration and streamflow. It is at this pivotal interface where hydrologists, atmospheric scientists and ecologists must understand how their disciplines interact and influence each other.Fluxes across the land-surface directly influence predictions of ecological processes, atmospheric dynamics, and terrestrial hydrology. However, many simplifications are made in numerical models when considering terrestrial hydrology from the view point of the atmosphere and visa-versa. While this may be a necessity in the current generation of operational models used for forecasting, it can create obstacles to the advancement of process understanding. These simplifications can limit the numerical prediction capabilities on how water partitions itself throughout all phases of the water cycle. The feedbacks between terrestrial and atmospheric water dynamics are not well understood or represented by the current generation of operational land-surface and atmospheric models. This can lead to erroneous spatial patterns and anomalous temporal persistence in land-atmosphere exchanges and atmospheric water cycle predictions. Cross-disciplinary efforts are needed not only to identify but also to quantify feedbacks between terrestrial and atmospheric water at appropriate spatiotemporal scales. This is especially true as today’s young scientists set their sights on improving process understanding and prediction skill from both research and operational models used to describe such linked systems.In recognition of these challenges, a junior faculty and early career scientist forum was recently held at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado with the intent of identifying and characterizing feedback interactions, and their attendant spatial and temporal scales, important for coupling terrestrial and atmospheric water dynamics. The primary focus of this forum is on improved process understanding, rather than operational products, as the possibility of incorporating more realistic physics into operational models is computationally prohibitive. We approached the subject of improved predictability through better process understanding by focusing on the following three framework questions described and discussed below.

Evaluating Urban Pollutant Buildup/Wash-Off Models Using a Madison, Wisconsin Catchment

Buildup/wash-off (BUWO) models are widely used to estimate pollutant export from urban and suburban watersheds. Here, we propose that the mass of washed-off particulate during a storm event is insensitive to the time between storm events (the traditional predictor of particulate accumulation in BUWO models). Our analysis employed USGS data of total suspended solids and discharge data for nonsnow events in a 9.4-km 2 suburban catchment in Madison, Wis. Kinetic energy of rainfall was calculated using National Weather Service NEXRAD radar reflectivity. A regression analysis found that storm event runoff volume and rainfall kinetic energy explained 81% of the variability in event particulate load; volume alone explained 69% of the variability in event loads. Time between storm events was not significant. Additionally, we simulated storm event particulate loads using a BUWO model and a model assuming a constant mass available for wash-off. Both models produced very similar predictions over a range of parameterizations, suggesting that buildup models could perhaps be simplified under many circumstances.