SPRING RUNOFF AND DRAINAGE N AND P LOSSES FROM HOG-MANURED CORN1

ABSTRACT: A two-year study was conducted to assess the effect of hog manure on the losses of nitrogen and phosphorus in runoff and drainage from grain-corn (Zea mays L.) plots, and the importance of spring versus annual loads. Treatments consisted of mineral N-P-K fertilizer applied at rates of 152 kg N ha^-1, 35 kg P ha^-1, and 86 kg K ha^-1; and hog (Sus scrofa domestica L.) manure applied preplant or post-emergence (six-to-eight leaf stage), at 152 kg N ha^-1, 39 kg P ha^-1, and 112 kg K ha^-1. The plots were rototilled (7 cm depth) in spring to incorporate fertilizer and preplant hog manure, and fall chisel-plowed (15 cm depth) to incorporate chopped corn residues. They were arranged in a completely randomized plot design. Flow volumes and nutrient levels in runoff and drainage waters were monitored year round but occurred mainly during the snowmelt (March 25-April 9), and post.snowmelt (April 10-May 13) periods. Of the total amount of water lost during snowmelt, 90 percent was in runoff, while 92 percent occurred as drainage in the post-snowmelt period. Sixty-five percent of the total annual volume of water lost was lost during these two periods as runoff and drainage. Treatments did not affect the annual snowmelt or post-snowmelt N and P loads. Total annual loads averaged 8.0 kg TKN ha^-1, 1.8 kg NH₄-N ha^-1, 43 kg NO₃-N ha^-1, 0.4 kg TP ha^-1, and 0.15 kg PO₄-P ha^-1. Spring (snowmelt and ost-snowmelt) runoff and drainage loads averaged 2.9 kg TKN ha^-1, 1.2 kg NH₄-N ha^-1, 18 kg NO₃-N ha^-1, 0.25 kg TP ha^-1, and 0.04 kg PO₄-P ha^-1, which were 40 percent to 70 percent of the yearly nutrient loads. Therefore, the hog manure management systems examined were of no greater threat to the environment than mineral fertilizers. However, spring N and P losses do represent an important part of the annual nutrient loss budget, even with conservation practices.     

SIMULATING NITROGEN LOSSES FROM AGRICULTURAL LAND: IMPLICATIONS FOR WATER QUALITY AND PROTECTION POLICY1

ABSTRACT: EPIC, a soil erosion/plant growth simulation model, is used to simulate nitrogen losses for 120 randomly selected and previously surveyed cropland sites. Simulated nitrogen losses occur through volatilization, surface water and soil runoff, subsurface lateral flow, and leaching. Physical and crop management variables explain a moderate but significant proportion of the variation in nitrogen losses. Site slope and tillage have offsetting effects on surface and ground water losses. Nitrogen applications in excess of agronomic recommendations and manure obtained off the farm and applied to the sites are significant contributors to nitrogen losses. Farm characteristics such as production of confined livestock, total manure nitrogen available, and farm income per cropland acre explain a relatively large portion of the variability in manure nitrogen applied to survey sites. The results help to identify farm characteristics that can be used to target nutrient management programs. Simulation modeling provides a useful tool for investigating variables which contribute to agricultural nitrogen losses.     

EVALUATION OF Ann AGNPS NITROGEN LOADING IN AN AGRICULTURAL WATERSHED1

ABSTRACT: Pollutants entering a water system can be very destructive to the health of that system. Best Management Practices (BMPs) are used to reduce these pollutants, but understanding the most effective practices is very difficult. Watershed models are an effective tool to aid in the decision‐making process of selecting the BMPs that are most effective in reducing the pollutant loading and are also the most cost effective. The Annualized Agricultural Nonpoint Source Pollution model (AnnAGNPS 2.0) is a technological tool that can be used to estimate watershed response to agricultural management practices. The main purpose of this paper is to test the performance of AnnAGNPS 2.0 on nitrogen loading using comparisons with measurements from the Deep Hollow watershed of the Mississippi Delta Management Systems Evaluation Area (MDMSEA) project. Previous work has demonstrated the capability of the model to simulate runoff and sediment. From sensitivity analyses in this study, initial nitrogen concentration in the soil and crop nitrogen uptake had the most impact on the nitrogen loadings. AnnAGNPS simulations of monthly nitrogen loadings are poor. However, statistical test (t‐test) showed that the predicted nitrogen loading is not significantly different from observed nitrogen loading at the 95 percent level of confidence.     

NITROGEN AND PHOSPHORUS CONCENTRATIONS IN FOREST STREAMS OF THE UNITED STATES1

ABSTRACT: Seventy to eighty percent of the water flowing in rivers in the United States originates as precipitation in forests. This project developed a synoptic picture of the patterns in water chemistry for over 300 streams in small, forested watersheds across the United States. Nitrate (NO₃^?) concentrations averaged 0.31 mg N/L, with some streams averaging ten times this level. Nitrate concentrations tended to be higher in the northeastern United States in watersheds dominated by hardwood forests (especially hardwoods other than oaks) and in recently harvested watersheds. Concentrations of dissolved organic N (mean 0.32 mg N/L) were similar to those of NO₃～, whereas ammonium (NH₄⁺) concentrations were much lower (mean 0.05 mg N/L). Nitrate dominated the N loads of streams draining hardwood forests, whereas dissolved organic N dominated the streams in coniferous forests. Concentrations of inorganic phosphate were typically much lower (mean 12 mg P/L) than dissolved organic phosphate (mean 84 mg P/L). The frequencies of chemical concentrations in streams in small, forested watersheds showed more streams with higher NO₃^? concentrations than the streams used in national monitoring programs of larger, mostly forested watersheds. At a local scale, no trend in nitrate concentration with stream order or basin size was consistent across studies.     

IMPLICATIONS OF ON‐GROUND NITROGEN LOADING AND SOIL TRANSFORMATIONS ON GROUND WATER QUALITY MANAGEMENT1

ABSTRACT: This paper presents a modeling approach based on a geographic information system (GIS) to estimate the variability of on‐ground nitrogen loading and the corresponding nitrate leaching to ground water. The methodology integrates all point and nonpoint sources of nitrogen, the national land cover database, soil nitrogen transformations, and the uncertainty of key soil and land use‐related parameters to predict the nitrate mass leaching to ground water. The analysis considered 21 different land use classes with information derived from nitrogen sources such as fertilizer and dairy manure applications, dairy lagoons, septic systems, and dry and wet depositions. Simulations were performed at a temporal resolution of one month to capture seasonal trends. The model was applied to a large aquifer of 376 square miles in Washington State that serves more than 100,000 residents with drinking water. The results showed that dairy manure is the main source of nitrogen in the area followed by fertilizers. It was also seen that nitrate leaching is controlled by the recharge rate, and there can be a substantial buildup of soil nitrogen over long periods of time. Uncertainty analysis showed that denitrification rate is the most influential parameter on nitrate leaching. The results showed that combining management alternatives is a successful strategy, especially with the use of nitrification inhibitors. Also, change in the land use pattern has a noticeable impact on nitrate leaching.     

NITROGEN AND PHOSPHORUS IN WATER AS RELATED TO ENVIRONMENTAL SEVI'ING IN NEBRASKA1

ABSTRACT: Spatial distributions of nitrogen and phosphorus in water were related to environmental setting as part of a regional water-quality assessment of the Central Nebraska Basins. The environmental settings (Sandhills, Loess Hills, Glaciated Area, and Platte Valley) were characterized by different concentrations of nitrogen and phosphorus in ground water and stream water. Statistically significant differences in nitrate concentrations in both ground-water and stream-water samples were related to regional distributions of cropland and rangeland. Nitrate concentrations were larger, especially in shallow ground water, in environmental settings dominated by cropland and associated fertilizer use than in settings dominated by rangeland. Similarly, total-nitrogen and nitrate concentrations were relatively large in selected streams draining primarily cropland. Comparative concentrations of phosphorus in stream water on the basis of environmental setting were similar to those of nitrogen, although the largest phosphorus concentrations probably relate to wastewater discharge into small streams. Nitrogen and phosphorus concentrations in much of the Platte River apparently reflected the quality of water entering the study unit from upstream and limited base-flow contributions from within the Platte Valley itself.     

Calibration and Validation of ADAPT and SWAT for Field‐Scale Runoff Prediction1

Abstract: The pollutant reduction possible with a given agricultural best‐management practice (BMP) is complex and site‐specific. Water‐quality models can evaluate BMPs, but model results are often limited by the lack of calibrated parameters for a given BMP. This study calibrated runoff prediction of two models (ADAPT and SWAT) for individual field plots having one till and two no‐till management practices. The factors used for runoff calibration were curve number II (CN_II) and saturated hydraulic conductivity (Ksat) for ADAPT, and CN_II, Ksat, and available water capacity for SWAT. Results were evaluated using coefficient of determination (R²), Nash‐Sutcliffe efficiency (E_f), root‐mean square error, median‐based E_f, and sign tests. Results indicated that for ADAPT, the best‐fit CN_II was 66 for the NT/SB (no‐till plot with surface‐broadcast fertilizer) treatment, 68 for the NT/DB (no‐till with deep‐banded fertilizer) treatment, and 70 for the tilled plot, whereas for SWAT the best‐fit CN_II was much higher, 86, for all treatments. Neither agreed with the textbook CN_II, 78, for sorghum in silty clay loam soil. The best‐fit model parameters for both runoff calibration phases had excellent correlation to monthly totals and moderate correlation to individual events.     

MODELING THE DISTRIBUTION OF DIFFUSE NITROGEN SOURCES AND SINKS IN THE NEUSE RIVER BASIN OF NORTH CAROLINA,USA1

This study quantified nonpoint source nitrogen (NPS‐N) sources and sinks across the 14,582 km² Neuse River Basin (NRB) located in North Carolina, to provide tabular data summaries and graphic overlay products to support the development of management approaches to best achieve established N reduction goals. First, a remote sensor derived, land cover classification was performed to support modeling needs. Modeling efforts included the development of a mass balance model to quantify potential N sources and sinks, followed by a precipitation event driven hydrologic model to effectively transport excess N across the landscape to individual stream reaches to support subsequent labeling of transported N values corresponding to source origin. Results indicated that agricultural land contributed 55 percent of the total annual NPS‐N loadings, followed by forested land at 23 percent (background), and urban areas at 21 percent. Average annual N source contributions were quantified for agricultural (1.4 kg/ha), urban (1.2 kg/ha), and forested cover types (0.5 kg/ha). Nonpoint source‐N contributions were greatest during the winter (40 percent), followed by spring (32 percent), summer (28 percent), and fall (0.3 percent). Seasonal total N loadings shifted from urban dominated and forest dominated sources during the winter, to agricultural sources in the spring and summer. A quantitative assessment of the significant NRB land use activities indicated that high (greater than 70 percent impervious) and medium (greater than 35 percent impervious) density urban development were the greatest contributors of NPS‐N on a unit area basis (1.9 and 1.6 kg/ha/yr, respectively), followed by row crops and pasture/hay cover types (1.4 kg/ha/yr).     

NEAR‐STREAM LANDUSE EFFECTS ON STREAMWATER NUTRIENT DISTRIBUTION IN AN URBANIZING WATERSHED1

ABSTRACT: We investigated spatial and temporal relationships among surface and subsurface watershed attributes and stream nutrient concentrations in urbanizing Johnson Creek watershed in northern Oregon. We sampled stream water at eight urban and five nonurban locations from March 1998 through December 1999. We sampled eight wells distributed over the two primary aquifers in the watershed. Using a Geographic Information System (GIS), percentages of landuse attributes within a radius of 30, 91, and 152 m from each sample site were quantified. We analyzed relationships between (1) nutrient concentrations and percentage cover of different landuse attributes, and (2) nutrient concentrations and underlying hydrologic units. We did not find a significant relationship between ground water chemistry and stream water chemistry. We found elevated levels of phosphorus (P) concentrations correlated with urban landuse, while higher nitrogen (N) concentrations were correlated with nonurban (primarily agricultural) landuse. We concluded that elevated levels of N in nonurban areas of Johnson Creek watershed were associated with agricultural practices. We further concluded that urban development factors such as increases in storm drains, dry wells, and impermeable surfaces may be responsible for higher input of P to the stream in urbanizing areas of the Johnson Creek watershed.     

EFFECTS OF AGRICULTURAL NUTRIENT MANAGEMENT ON NITROGEN FATE AND TRANSPORT IN LANCASTER COUNTY PENNSYLVANIA1

ABSTRACT: Nitrogen inputs to, and outputs from, a 55-acre site in Lancaster County, Pennsylvania, were estimated to determine the pathways and relative magnitude of loads of nitrogen entering and leaving the site, and to compare the loads of nitrogen before and after the implementation of nutrient management. Inputs of nitrogen to the site were manure fertilizer, commercial fertilizer, nitrogen in precipitation, and nitrogen in ground-water inflow; and these sources averaged 93, 4, 2, and 1 percent of average annual nitrogen additions, respectively. Outputs of nitrogen from the site were nitrogen in harvested crops, loads of nitrogen in surface runoff, volatilization of nitrogen, and loads of nitrogen in ground-water discharge, which averaged 37, less than 1, 25, and 38 percent of average annual nitrogen removals from the site, respectively. Virtually all of the nitrogen leaving the site that was not removed in harvested crops or by volatilization was discharged in the ground water. Applications of manure and fertilizer nitrogen to 47.5 acres of cropped fields decreased about 33 percent, from an average of 22,700 pounds per year (480 pounds per acre per year) before nutrient management to 15,175 pounds of nitrogen per year (320 pounds per acre per year) after the implementation of nutrient management practices. Nitrogen loads in ground-water discharged from the site decreased about 30 percent, from an average of 292 pounds of nitrogen per million gallons of ground water before nutrient management to an average of 203 pounds of nitrogen per million gallons as a result of the decreased manure and commercial fertilizer applications. Reductions in manure and commercial fertilizer applications caused a reduction of approximately 11,000 pounds (3,760 pounds per year; 70 pounds per acre per year) in the load of nitrogen discharged in ground water from the 55-acre site during the three-year period 1987–1990.     

Sources and Delivery of Nutrients to the Northwestern Gulf of Mexico from Streams in the South‐Central United States1

Rebich, Richard A., Natalie A. Houston, Scott V. Mize, Daniel K. Pearson, Patricia B. Ging, and C. Evan Hornig, 2011. Sources and Delivery of Nutrients to the Northwestern Gulf of Mexico From Streams in the South‐Central United States. Journal of the American Water Resources Association (JAWRA) 47(5):1061‐1086. DOI: 10.1111/j.1752‐1688.2011.00583.x Abstract: SPAtially Referenced Regressions On Watershed attributes (SPARROW) models were developed to estimate nutrient inputs [total nitrogen (TN) and total phosphorus (TP)] to the northwestern part of the Gulf of Mexico from streams in the South‐Central United States (U.S.). This area included drainages of the Lower Mississippi, Arkansas‐White‐Red, and Texas‐Gulf hydrologic regions. The models were standardized to reflect nutrient sources and stream conditions during 2002. Model predictions of nutrient loads (mass per time) and yields (mass per area per time) generally were greatest in streams in the eastern part of the region and along reaches near the Texas and Louisiana shoreline. The Mississippi River and Atchafalaya River watersheds, which drain nearly two‐thirds of the conterminous U.S., delivered the largest nutrient loads to the Gulf of Mexico, as expected. However, the three largest delivered TN yields were from the Trinity River/Galveston Bay, Calcasieu River, and Aransas River watersheds, while the three largest delivered TP yields were from the Calcasieu River, Mermentau River, and Trinity River/Galveston Bay watersheds. Model output indicated that the three largest sources of nitrogen from the region were atmospheric deposition (42%), commercial fertilizer (20%), and livestock manure (unconfined, 17%). The three largest sources of phosphorus were commercial fertilizer (28%), urban runoff (23%), and livestock manure (confined and unconfined, 23%).     

AVGWLF‐Based Estimation of Nonpoint Source Nitrogen Loads Generated Within Long Island Sound Subwatersheds1

Abstract: The Generalized Watershed Loading Functions (GWLF) model and its ArcView interface (AVGWLF) were used to estimate and examine the components of the total nitrogen (TN) nonpoint source (NPS) load generated within New York and Connecticut (CT) watersheds surrounding Long Island Sound (LIS, the Sound). The majority of data used as model inputs were generally available from online sources, and the work involved an overall calibration to streamflow and TN data in accordance with generic guidelines recommended in the GWLF manual. The GWLF model performance for three calibration and two validation watersheds in CT was compared with results of a detailed model, Hydrological Simulation Program in Fortran, developed in a previous study. The results of the application illustrate the usefulness of the relatively simpler, less parameter‐intensive GWLF model in performing exploratory loading analysis in preparation for adaptive nutrient management in the LIS watersheds. The presented methodology is valuable for identification of priority watersheds for NPS pollution reduction and also for planning‐level evaluation of best management practices to achieve the desired reductions. It is estimated that ground‐water base flow may be the largest pathway for NPS TN to the Sound, contributing about 54% of the total NPS TN load, a finding with significant implications for LIS total maximum daily load reduction scenarios. In addition to ground water, septic systems are estimated to contribute about 17% of the total load, with the remaining TN load being mostly runoff from urban (17%), agricultural (5%), and low impact (e.g., forest) areas (6%).     

Impacts of Land‐Cover Change on Suspended Sediment Transport in Two Agricultural Watersheds1

Schilling, Keith E., Thomas M. Isenhart, Jason A. Palmer, Calvin F. Wolter, and Jean Spooner, 2011. Impacts of Land‐Cover Change on Suspended Sediment Transport in Two Agricultural Watersheds. Journal of the American Water Resources Association (JAWRA) 47(4):672‐686. DOI: 10.1111/j.1752‐1688.2011.00533.x Abstract: Suspended sediment is a major water quality problem, yet few monitoring studies have been of sufficient scale and duration to assess the effectiveness of land‐use change or conservation practice implementation at a watershed scale. Daily discharge and suspended sediment export from two 5,000‐ha watersheds in central Iowa were monitored over a 10‐year period (water years 1996‐2005). In Walnut Creek watershed, a large portion of land was converted from row crop to native prairie, whereas in Squaw Creek land use remained predominantly row crop agriculture. Suspended sediment loads were similar in both watersheds, exhibiting flashy behavior typical of incised channels. Modeling suggested that expected total soil erosion in Walnut Creek should have been reduced 46% relative to Squaw Creek due to changes in land use, yet measured suspended sediment loads showed no significant differences. Stream mapping indicated that Walnut Creek had three times more eroding streambank lengths than did Squaw Creek suggesting that streambank erosion dominated sediment sources in Walnut Creek and sheet and rill sources dominated sediment sources in Squaw Creek. Our results demonstrate that an accounting of all sources of sediment erosion and delivery is needed to characterize sediment reductions in watershed projects combined with long‐term, intensive monitoring and modeling to account for possible lag times in the manifestation of the benefits of conservation practices on water quality.     

PCN BASED METAL PARTITIONING IN URBAN SNOWMELT,RAINFALL/RUNOFF,AND RIVER FLOW SYSTEMS1

ABSTRACT: Drawing an analogy between the popular Soil Conservation Service curve number (SCS‐CN) method based infiltration and metal sorption processes, a new partitioning curve number (PCN) approach is suggested for partitioning of heavy metals into dissolved and particulate bound forms in urban snowmelt, rainfall/runoff, and river flow environments. The parameters, the potential maximum desorption, ψ, and the PCN analogous to the SCS‐CN parameters S and CN, respectively, are introduced. Under the condition of snowmelt, PCN (or ψ) is found to generally rely on temperature, relative humidity, pH, and chloride content; during a rainstorm, ψ is found to depend on the alkalinity and the pH of the rainwater; and in the river flow situation, PCN is found to generally depend on the temperature, pH, and chloride content. The advantage of using PCN instead of the widely used partitioning parameter, K_d, is found to lie in the PCN's efficacy to distinguish the adsorption (or sorption) behavior of metals in the above snowmelt, rainfall/runoff, and river flow situations, analogous to the hydrological behavior of watersheds.     

Streambank Soil and Phosphorus Losses Under Different Riparian Land‐Uses in Iowa1

Abstract: Phosphorus and sediment are major nonpoint source pollutants that degrade water quality. Streambank erosion can contribute a significant percentage of the phosphorus and sediment load in streams. Riparian land‐uses can heavily influence streambank erosion. The objective of this study was to compare streambank erosion along reaches of row‐cropped fields, continuous, rotational and intensive rotational grazed pastures, pastures where cattle were fenced out of the stream, grass filters and riparian forest buffers, in three physiographic regions of Iowa. Streambank erosion was measured by surveying the extent of severely eroding banks within each riparian land‐use reach and randomly establishing pin plots on subsets of those eroding banks. Based on these measurements, streambank erosion rate, erosion activity, maximum pin plot erosion rate, percentage of streambank length with severely eroding banks, and soil and phosphorus losses per unit length of stream reach were compared among the riparian land‐uses. Riparian forest buffers had the lowest streambank erosion rate (15‐46 mm/year) and contributed the least soil (5‐18 tonne/km/year) and phosphorus (2‐6 kg/km/year) to stream channels. Riparian forest buffers were followed by grass filters (erosion rates 41‐106 mm/year, soil losses 22‐47 tonne/km/year, phosphorus losses 9‐14 kg/km/year) and pastures where cattle were fenced out of the stream (erosion rates 22‐58 mm/year, soil losses 6‐61 tonne/km/year, phosphorus losses 3‐34 kg/km/year). The streambank erosion rates for the continuous, rotational, and intensive rotational pastures were 101‐171, 104‐122, and 94‐170 mm/year, respectively. The soil losses for the continuous, rotational, and intensive rotational pastures were 197‐264, 94‐266, and 124‐153 tonne/km/year, respectively, while the phosphorus losses were 71‐123, 37‐122, and 66 kg/km/year, respectively. The only significant differences for these pasture practices were found among the percentage of severely eroding bank lengths with intensive rotational grazed pastures having the least compared to the continuous and rotational grazed pastures. Row‐cropped fields had the highest streambank erosion rates (239 mm/year) and soil losses (304 tonne/km/year) and very high phosphorus losses (108 kg/km/year).     

NUTRIENT AND β17‐ESTRADIOL LOSS IN RUNOFF WATER FROM POULTRY LITTERS1

ABSTRACT: A main water quality concern is accelerated eutrophication of fresh waters from nonpoint source pollution, particularly nutrient transport in surface runoff from agricultural areas and confined animal feeding operations. This study examined nutrient and β₁₇‐estradiol concentrations in runoff from small plots where six poultry litters were applied at a rate of about 67 kg/ha of total phosphorus (TP). The six poultry litter treatments included pelleted compost, pelleted litter, raw litter, alum (treated) litter, pelleted alum litter, and normal litter (no alum). Four replicates of the six poultry litter treatments and a control (plots without poultry litter application) were used in this study. Rainfall simulations at intensity of 50 mm/hr were conducted immediately following poultry litter application to the plots and again 30 days later. Composite runoff samples were analyzed for soluble reactive phosphorus (SRP), ammonia (NH₄), nitrate (NO₃), TP, total nitrogen (TN) and β₁₇‐estradiol concentrations. In general, poultry litter applications increased nutrient and β₁₇‐estradiol concentrations in runoff water. Ammonia and P concentrations in runoff water from the first simulation were correlated to application rates of water extractable NH₄ (R²= 0.70) and P (R²= 0.68) in the manure. Results suggest that alum applications to poultry litter in houses in between flocks is an effective best management practice for reducing phosphorus (P) and β₁₇‐estradiol concentrations in runoff and that pelleted poultry litters may increase the potential for P and β₁₇‐estradiol loss in runoff water. Inferences regarding pelleted poultry litters should be viewed cautiously, because the environmental consequence of pelleting poultry litters needs additional investigation.     

Using Spatially Detailed Water‐Quality Data and Solute‐Transport Modeling to Support Total Maximum Daily Load Development1

Walton‐Day, Katherine, Robert L. Runkel, and Briant A. Kimball, 2012. Using Spatially Detailed Water‐Quality Data and Solute‐Transport Modeling to Support Total Maximum Daily Load Development. Journal of the American Water Resources Association (JAWRA) 48(5): 949‐969. DOI: 10.1111/j.1752‐1688.2012.00662.x Abstract: Spatially detailed mass‐loading studies and solute‐transport modeling using OTIS (One‐dimensional Transport with Inflow and Storage) demonstrate how natural attenuation and loading from distinct and diffuse sources control stream water quality and affect load reductions predicted in total maximum daily loads (TMDLs). Mass‐loading data collected during low‐flow from Cement Creek (a low‐pH, metal‐rich stream because of natural and mining sources, and subject to TMDL requirements) were used to calibrate OTIS and showed spatially variable effects of natural attenuation (instream reactions) and loading from diffuse (groundwater) and distinct sources. OTIS simulations of the possible effects of TMDL‐recommended remediation of mine sites showed less improvement to dissolved zinc load and concentration (14% decrease) than did the TMDL (53‐63% decrease). The TMDL (1) assumed conservative transport, (2) accounted for loads removed by remediation by subtracting them from total load at the stream mouth, and (3) did not include diffuse‐source loads. In OTIS, loads were reduced near their source; the resulting concentration was decreased by natural attenuation and increased by diffuse‐source loads during downstream transport. Thus, by not including natural attenuation and loading from diffuse sources, the TMDL overestimated remediation effects at low flow. Use of the techniques presented herein could improve TMDLs by incorporating these processes during TMDL development.     

The Regionalization of National‐Scale SPARROW Models for Stream Nutrients1

Schwarz, Gregory E., Richard B. Alexander, Richard A. Smith, and Stephen D. Preston, 2011. The Regionalization of National‐Scale SPARROW Models for Stream Nutrients. Journal of the American Water Resources Association (JAWRA) 47(5):1151‐1172. DOI: 10.1111/j.1752‐1688.2011.00581.x Abstract: This analysis modifies the parsimonious specification of recently published total nitrogen (TN) and total phosphorus (TP) national‐scale SPAtially Referenced Regressions On Watershed attributes models to allow each model coefficient to vary geographically among three major river basins of the conterminous United States. Regionalization of the national models reduces the standard errors in the prediction of TN and TP loads, expressed as a percentage of the predicted load, by about 6 and 7%. We develop and apply a method for combining national‐scale and regional‐scale information to estimate a hybrid model that imposes cross‐region constraints that limit regional variation in model coefficients, effectively reducing the number of free model parameters as compared to a collection of independent regional models. The hybrid TN and TP regional models have improved model fit relative to the respective national models, reducing the standard error in the prediction of loads, expressed as a percentage of load, by about 5 and 4%. Only 19% of the TN hybrid model coefficients and just 2% of the TP hybrid model coefficients show evidence of substantial regional specificity (more than ±100% deviation from the national model estimate). The hybrid models have much greater precision in the estimated coefficients than do the unconstrained regional models, demonstrating the efficacy of pooling information across regions to improve regional models.     

Nutrient and Sediment Production,Watershed Characteristics,and Land Use in the Tahoe Basin,California‐Nevada1

Abstract: In efforts to control the degradation of water quality in Lake Tahoe, public agencies have monitored surface water discharge and concentrations of nitrogen, phosphorus, and suspended sediment in two separate sampling programs. The first program focuses on 20 watersheds varying in size from 162 to 14,000 ha, with continuous stream gaging and periodic sampling; the second focuses on small urbanized catchments, with automated sampling during runoff events. Using data from both programs, we addressed the questions (1) what are the fluxes and concentrations of nitrogen and phosphorus entering the lake from surface runoff; (2) how do the fluxes and concentrations vary in space and time; and (3) how are they related to land use and watershed characteristics? To answer these questions, we calculated discharge‐weighted average concentrations and annual fluxes and used multiple regression to relate those variable to a suite of GIS‐derived explanatory variables. The final selected regression models explain 47‐62% of the variance in constituent concentrations in the stormwater monitoring catchments, and 45‐72% of the variance in mean annual yields in the larger watersheds. The results emphasize the importance of impervious surface and residential density as factors in water quality degradation, and well‐developed soil as a factor in water quality maintenance.     

DETERMINING NUTRIENT EXPORT COEFFICIENTS AND SOURCE LOADING UNCERTAINTY USING IN‐STREAM MONITORING DATA1

ABSTRACT: Over a three‐year period, flow and nutrients were monitored at 13 sites in the upper North Bosque River watershed in Texas. Drainage areas above sampling sites differed in percent of dairy waste application fields, forage fields, wood/range, and urban land area. A multiple regression approach was used to develop total phosphorus (TP) and total nitrogen (TN) export coefficients for the major land uses in these heterogeneous drainage areas. The largest export coefficients were associated with dairy waste application fields followed by urban, forage fields, and wood/range. An empirical model was then established to assess nutrient contribution by major sources using developed export coefficients and point source loadings from municipal wastewater treatment. This model was verified by comparison of estimated loadings to measured in‐stream data. Monte Carlo simulation techniques were applied to provide an uncertainty analysis for nutrient loads by source, based on the variance associated with each export coefficient. The largest sources of nutrients contributing to the upper North Bosque River were associated with dairy waste application fields and forage fields, while the greatest relative uncertainty in source contribution was associated with loadings from urban and wood/range land uses.