Chemical transport from paired agricultural and restored prairie watersheds

A five-year record of streamflow and chemical sampling data was evaluated to assess the effects of large-scale prairie restoration on transport of NO3-N, Cl, and SO4 loads from paired 5,000-ha watersheds located in Jasper County, Iowa. Water quality conditions monitored during land use conversion from row crop agriculture to native prairie in the Walnut Creek watershed were compared with a highly agricultural control watershed (Squaw Creek). Combining hydrograph separation with a load estimation program, baseflow and stormflow loads of NO3-N, Cl, and SO4 were estimated at upstream and downstream sites on Walnut Creek and a downstream site on Squaw Creek. Chemical export in both watersheds was found to occur primarily with baseflow, with baseflow transport greatest during the late summer and fall. Lower Walnut Creek watershed, which contained the restored prairie areas, exported less NO3-N and Cl compared with upper Walnut Creek and Squaw Creek watersheds. Average flow-weighted concentrations of NO3-N exceeded 10 mg/L in upper Walnut Creek and Squaw Creek, but were estimated to be 6.6 mg/L in lower Walnut Creek. Study results demonstrate the utility of partitioning loads into baseflow and stormflow components to identify sources of pollutant loading to streams.     

Computing Atmospheric Nutrient Loads to the Chesapeake Bay Watershed and Tidal Waters

Application of integrated Chesapeake Bay models of the airshed, watershed, and estuary support air and water nitrogen controls in the Chesapeake. The models include an airshed model of the Mid‐Atlantic region which tracks the estimated atmospheric deposition loads of nitrogen to the watershed, tidal Bay, and adjacent coastal ocean. The three integrated models allow tracking of the transport and fate of nitrogen air emissions, including deposition in the Chesapeake watershed, the subsequent uptake, transformation, and transport to Bay tidal waters, and their ultimate influence on Chesapeake water quality. This article describes the development of the airshed model, its application to scenarios supporting the Chesapeake Total Maximum Daily Load (TMDL), and key findings from the scenarios. Key findings are that the atmospheric deposition loads are among the largest input loads of nitrogen in the watershed, and that the indirect nitrogen deposition loads to the watershed, which are subsequently delivered to the Bay are larger than the direct loads of atmospheric nitrogen deposition to Chesapeake tidal waters. Atmospheric deposition loads of nitrogen deposited in coastal waters, which are exchanged with the Chesapeake, are also estimated. About half the atmospheric deposition loads of nitrogen originate from outside the Chesapeake watershed. For the first time in a TMDL, the loads of atmospheric nitrogen deposition are an explicit part of the TMDL load reductions.     

Spatiotemporal variability of wet atmospheric nitrogen deposition to the Neuse River Estuary, North Carolina

Excessive nitrogen (N) loading to N-sensitive waters such as the Neuse River estuary (North Carolina) has been shown to promote changes in microbial and algal community composition and function (harmful algal blooms), hypoxia and anoxia, and fish kills. Previous studies have estimated that wet atmospheric deposition of nitrogen (WAD-N), as deposition of dissolved inorganic nitrogen (DIN: NO3-, NH3/NH4+) and dissolved organic nitrogen, may contribute at least 15% of the total externally supplied or "new" N flux to the coastal waters of North Carolina. In a 3-yr study from June 1996 to June 1999, we calculated the weekly wet deposition of inorganic and organic N at eleven sites on a northwest-southeast transect in the watershed. The annual mean total (wet DIN + wet organics) WAD-N flux for the Neuse River watershed was calculated to be 956 mg N/m2/yr (15026 Mg N/yr). Seasonally, the spring (March-May) and summer (June-August) months contain the highest total weekly N deposition; this pattern appears to be driven by N concentration in precipitation. There is also spatial variability in WAD-N deposition; in general, the upper portion of the watershed receives the lowest annual deposition and the middle portion of the watershed receives the highest deposition. Based on a range of watershed N retention and in-stream riverine processing values, we estimate that this flux contributes approximately 24% of the total "new" N flux to the estuary.     

Hydrological variability and agricultural drainage ditch inorganic nitrogen reduction capacity

The application of inorganic nitrogen fertilizers on agricultural landscapes has the potential to generate concerns of environmental degradation at fine to coarse scales across the catchment and landscape. Inorganic nitrogen species (NO3*, NO2*, and NH3) are typically associated with subsurface flow processes; however, surface runoff from rainfall events in no-till agriculture with inorganic surface fertilizers might contribute to downstream eutrophication. Inorganic nitrogen reduction capacity of agricultural drainage ditches under no-till cotton was determined under natural, variable rainfall conditions in northern Mississippi. Monthly grab baseflow samples and storm-generated flow samples were variably sampled temporally within two experimental farm ditches over 2 yr. Inorganic nitrogen concentrations, in conjunction with Manning's equation and Natural Resources Conservation Service dimensionless hydrographs, provided individual water volumes per storm event and thus maximum effluent and outflow nitrogen loads. Base and stormflow regression results indicate drainage ditches reducing NO3* and NH3 over the length of the ditch for growing and dormant seasons. Overall, maximum storm loads of dissolved inorganic nitrogen (DIN) from the farm over the 2-yr sampling period accounted for 2.2% of the initial fertilizer application, of which 1.1% left the ditch (0.84 kg ha(-1) yr(-1)) (a 57% ditch reduction of DIN load over 2 yr). Long-term sampling incorporating data on application and loss of fertilizers and farm management will provide critical information for farmers and scientists on the potential of economic gains and downstream ecosystem eutrophication, respectively.     

Long-term effects of nitrogen fertilizer use on ground water nitrate in two small watersheds

Changes in agricultural management can minimize NO3-N leaching, but then the time needed to improve ground water quality is uncertain. A study was conducted in two first-order watersheds (30 and 34 ha) in Iowa's Loess Hills. Both were managed in continuous corn (Zea mays L.) from 1964 through 1995 with similar N fertilizer applications (average 178 kg ha(-1) yr(-1)), except one received applications averaging 446 kg N ha(-1) yr(-1) between 1969 and 1974. This study determined if NO3-N from these large applications could persist in ground water and baseflow, and affect comparison between new crop rotations implemented in 1996. Piezometer nests were installed and deep cores collected in 1996, then ground water levels and NO3-N concentrations were monitored. Tritium and stable isotopes (2H, 18O) were determined on 33 water samples in 2001. Baseflow from the heavily N-fertilized watershed had larger average NO3-N concentrations, by 8 mg L(-1). Time-of-travel calculations and tritium data showed ground water resides in these watersheds for decades. "Bomb-peak" precipitation (1963-1980) most influenced tritium concentrations near lower slope positions, while deep ground water was dominantly pre-1953 precipitation. Near the stream, greater recharge and mixed-age ground water was suggested by stable isotope and tritium data, respectively. Using sediment-core data collected from the deep unsaturated zone between 1972 and 1996, the increasing depth of a NO3-N pulse was related to cumulative baseflow (r2 = 0.98), suggesting slow downward movement of NO3-N since the first experiment. Management changes implemented in 1996 will take years to fully influence ground water NO3-N. Determining ground water quality responses to new agricultural practices may take decades in some watersheds.     

COMPARING SAMPLING SCHEMES FOR MONITORING POLLUTANT EXPORT FROM A DAIRY PASTURE1

ABSTRACT: Dairy cow pastures and feeding areas around barns can be a significant source of nonpoint source pollutants to nearby streams. To help document the significance of these sources, nutrient export in streamfiow from a 56.7-ha, mostly agricultural, watershed located in southwestern North Carolina was monitored from August 1994 to January 1996. Total nitrogen and phosphorus export rates from the upper, predominantly pasture, part of the watershed were 18.0 and 1.4 kg/ha/yr, respectively, as measured by weekly grab sampling and 18.7 and 4.9 kg/halyr, respectively, as measured from storm event monitoring. Nitrogen and phosphorus export rates for the area between the monitoring sites, which included overgrazed cow holding and feeding areas and farm buildings, were 376 and 86 kgfhalyr, respectively, for grab sampling and 351 and 160 kg/ha/yr, respectively, for storm event monitoring. To estimate the amount of reduction from nonpoint source controls necessary to effect a significant reduction in pollutant loading, statistical analyses of the load data were conducted. The analyses for the five pollutants monitored showed that total suspended solids would require the greatest reduction (34.6 percent for weekly grab and 33.6 percent for storm) in loading after the implementation of controls for statistical significance. Nitrate plus nitrite was found to require the least reduction (12.6 percent for weekly grab). Pollutant export rates computed from weekly grab samples and storm event samples used separately were compared to corresponding export rates computed from combining grab and storm event samples to assess the differences in monitoring schemes.     

Modeling atmospheric nitrogen deposition and transport in the Chesapeake Bay watershed

Atmospheric deposition of nitrate nitrogen and ammonium nitrogen has been identified as a major factor in the decline of water quality in the Chesapeake Bay. Reports have indicated that atmospheric deposition may account for 25 to 80% of the total nitrogen load entering the bay. However, uncertainties exist regarding the accuracy of the atmospheric deposition inputs, nitrogen retention coefficients, and in-stream nutrient uptake rates used in these studies. This project was designed to reassess the potential inputs of atmospheric nitrogen deposition to the bay through the use of a high-resolution wet deposition model, improved wet and dry deposition and nutrient retention estimates, existing soils and land use data, and geographic information systems software. Model results indicate that the methods used in previous studies may overestimate the contribution of atmospheric nitrate and ammonium deposition to the Chesapeake Bay watershed (CBW). Wet and dry atmospheric nitrate and ammonium nitrogen deposition estimates to the CBW ranged from 52.7 to 141.9 and 41.9 to 60.1 million kg/yr, respectively, between 1984 and 1996. Dry and total atmospheric deposition loads to the watershed are substantially less than previous estimates. Estimates of the percent contribution of atmospherically deposited nitrogen to the Chesapeake Bay represent between 20 and 32% of the total nitrate and ammonium nitrogen load to the watershed from all nitrogen sources. While these estimates are lower than many other published estimates, regression analysis of model parameters, nitrogen retention coefficients, output, and measured in-stream nitrogen loads indicate that the calculated nitrogen loads may still be too high.     

ALAWAT: A spatially allocated watershed model for approximating stream,sediment, and pollutant flows in Hawaii,USA

The Ala Wai Canal Watershed Model (ALAWAT) is a planning-level watershed model for approximating direct runoff, streamflow, sediment loads, and loads for up to five pollutants. ALAWAT uses raster GIS data layers including land use, SCS soil hydrologic groups, annual rainfall, and subwatershed delineations as direct model parameter inputs and can use daily total rainfall from up to ten rain gauges and streamflow from up to ten stream gauges. ALAWAT uses a daily time step and can simulate flows for up to ten-year periods and for up to 50 subwatersheds. Pollutant loads are approximated using a user-defined combination of rating curve relationships, mean event concentrations, and loading/washoff parameters for specific subwatersheds, land uses, and times of year. Using ALAWAT, annual average streamflow and baseflow relationships and urban suspended sediment loads were approximated for the Ala Wai Canal watershed (about 10,400 acres) on the island of Oahu, Hawaii. Annual average urban suspended sediments were approximated using two methods: mean event concentrations and pollutant loading and washoff. Parameters for the pollutant loading and washoff method were then modified to simulate the effect of various street sweeping intervals on sediment loads.     

Agricultural drainage ditches mitigate phosphorus loads as a function of hydrological variability

Phosphorus (P) loading from nonpoint sources, such as agricultural landscapes, contributes to downstream aquatic ecosystem degradation. Specifically, within the Mississippi watershed, enriched runoff contributions have far-reaching consequences for coastal water eutrophication and Gulf of Mexico hypoxia. Through storm events, the P mitigation capacity of agricultural drainage ditches under no-till cotton was determined for natural and variable rainfall conditions in north Mississippi. Over 2 yr, two experimental ditches were sampled monthly for total inorganic P concentrations in baseflow and on an event-driven basis for stormflows. Phosphorus concentrations, Manning's equations with a range of roughness coefficients for changes in vegetative densities within the ditches, and discharge volumes from Natural Resources Conservation Service dimensionless hydrographs combined to determine ranges in maximum and outflow storm P loads from the farms. Baseflow regressions and percentage reductions with P concentrations illustrated that the ditches alternated between being a sink and source for dissolved inorganic P and particulate P concentrations throughout the year. Storm event loads resulted in 5.5% of the annual applied fertilizer to be transported into the drainage ditches. The ditches annually reduced 43.92 +/- 3.12% of the maximum inorganic effluent P load before receiving waters. Agricultural drainage ditches exhibited a fair potential for P mitigation and thus warrant future work on controlled drainage to improve mitigation capacity.     

Nitrate and sediment fluxes from a California rangeland watershed

Long-term water quality records for assessing natural variability, impact of management, and that guide regulatory processes to safeguard water resources are rare for California oak woodland rangelands. This study presents a 20-yr record (1981-2000) of nitrate-nitrogen (NO(3)-N) and suspended sediment export from a typical, grazed oak woodland watershed (103 ha) in the northern Sierra Nevada foothills of California. Mean annual precipitation over the 20-yr period was 734 mm yr(-1) (range 366-1205 mm yr(-1)). Mean annual stream flow was 353 mm y(-1) (range 87-848 mm yr(-1)). Average annual stream flow was 48.1 +/- 16% of precipitation. Mean annual NO(3)-N export was 1.6 kg ha(-1) yr(-1) (range 0.18-3.6 kg ha(-1) yr(-1)). Annual NO(3)-N export significantly (P < 0.05) increased with increasing annual stream flow and precipitation. Mean daily NO(3)-N export was 0.004 kg ha(-1) d(-1) (range 10(-5) to 0.55 kg ha(-1) d(-1)). Mean annual suspended sediment export was 198 kg ha(-1) yr(-1) (range 23-479 kg ha(-1) yr(-1)). There was a positive relationship (P < 0.05) between annual suspended sediment export, annual stream flow and precipitation. Mean daily suspended sediment export was 0.54 kg ha(-1) d(-1) (range 10(-4) to 155 kg ha(-1) d(-1)). Virtually no sediment was exported during the dry season. The large variation in daily and annual fluxes highlights the necessity of using long-term records to establish quantitative water quality targets for rangelands and demonstrates the difficulty of designing a water quality monitoring program for these ecosystems.     

Evaluation of the ADAPT model for simulating nitrogen dynamics in a tile-drained agricultural watershed in central Illinois

Assessing the accuracy of agronomic and water quality simulation models in different soils, land-use systems, and environments provides a basis for using and improving these models. We evaluated the performance of the ADAPT model for simulating riverine nitrate-nitrogen (NO3-N) export from a 1500-km2 watershed in central Illinois, where approximately 85% of the land is used for maize-soybean production and tile drainage is common. Soil chemical properties, crop nitrogen (N) uptake coefficient, dry matter ratio, and a denitrification reduction coefficient were used as calibration parameters to optimize the fit between measured and simulated NO3-N load from the watershed for the 1989 to 1993 period. The applicability of the calibrated parameter values was tested by using these values for simulating the 1994 to 1997 period on the same watershed. Willmott's index of agreement ranged from 0.91 to 0.97 for daily, weekly, monthly, and annual comparisons of riverine nitrate N loads. Simulation accuracy generally decreased as the time interval decreased. Willmott's index for simulated crop yields ranged from 0.91 to 0.99; however, observed crop yields were used as input to the model. The partial N budget results suggested that 52 to 72 kg N ha(-1) yr(-1) accumulated in the soil, but simulated biological N fixation associated with soybeans was considerably greater than literature values for the region. Improvement of the N fixation algorithms and incorporation of mechanisms that describe soybean yield in response to environmental conditions appear to be needed to improve the performance of the model.     

Estimating Nitrogen Loading to Ground Water and Assessing Vulnerability to Nitrate Contamination in a Large Karstic Springs Basin,Florida1

Abstract: A nitrogen (N) mass‐balance budget was developed to assess the sources of N affecting increasing ground‐water nitrate concentrations in the 960‐km² karstic Ichetucknee Springs basin. This budget included direct measurements of N species in rainfall, ground water, and spring waters, along with estimates of N loading from fertilizers, septic tanks, animal wastes, and the land application of treated municipal wastewater and residual solids. Based on a range of N leaching estimates, N loads to ground water ranged from 262,000 to 1.3 million kg/year; and were similar to N export from the basin in spring waters (266,000 kg/year) when 80‐90% N losses were assumed. Fertilizers applied to cropland, lawns, and pine stands contributed about 51% of the estimated total annual N load to ground water in the basin. Other sources contributed the following percentages of total N load to ground water: animal wastes, 27%; septic tanks, 12%; atmospheric deposition, 8%; and the land application of treated wastewater and biosolids, 2%. Due to below normal rainfall (97.3 cm) during the 12‐month rainfall collection period, N inputs from rainfall likely were about 30% lower than estimates for normal annual rainfall (136 cm). Low N‐isotope values for six spring waters (δ¹⁵N‐NO₃ = 3.3 to 6.3‰) and elevated potassium concentrations in ground water and spring waters were consistent with the large N contribution from fertilizers. Given ground‐water residence times on the order of decades for spring waters, possible sinks for excess N inputs to the basin include N storage in the unsaturated zone and parts of the aquifer with relatively sluggish ground‐water movement and denitrification. A geographical‐based model of spatial loading from fertilizers indicated that areas most vulnerable to nitrate contamination were located in closed depressions containing sinkholes and other dissolution features in the southern half of the basin.     

Nitrate exported in drainage waters of two sprinkler-irrigated watersheds

Nitrate contamination of surface waters has been linked to irrigated agriculture across the world. We determined the NO3-N loads in the drainage waters of two sprinkler-irrigated watersheds located in the Ebro River basin (Spain) and their relationship to irrigation and N management. Crop water requirements, irrigation, N fertilization, and the volume and NO3-N concentration of drainage waters were measured or estimated during two-year (Watershed A; 494 irrigated ha) and one-year (Watershed B; 470 irrigated ha) study periods. Maize (Zea mays L.) and alfalfa (Medicago sativa L.) were grown in 40 to 60% and 15 to 33% of the irrigated areas, respectively. The seasonal irrigation performance index (IPI) ranged from 92 to 100%, indicating high-quality management of irrigation. However, the IPI varied among fields and overirrigation occurred in 17 to 44% of the area. Soil and maize stalk nitrate contents measured at harvest indicated that N fertilizer rates could be decreased. Drainage flows were 68 mm yr(-1) in Watershed A and 194 mm yr(-1) in Watershed B. Drainage NO3-N concentrations were independent of drainage flows and similar in the irrigated and nonirrigated periods (average: 23-29 mg L(-1)). Drainage flows determined the exported mass of NO3-N, which varied from 18 (Watershed A) to 49 (Watershed B) kg ha(-1) yr(-1), representing 8 (Watershed A) and 22% (Watershed B) of the applied fertilizer plus manure N. High-quality irrigation management coupled to the split application of N through the sprinkler systems allowed a reasonable compromise between profitability and reduced N pollution in irrigation return flows.     

Phosphorus loss from an agricultural watershed as a function of storm size

Phosphorus (P) loss from agricultural watersheds is generally greater in storm rather than base flow. Although fundamental to P-based risk assessment tools, few studies have quantified the effect of storm size on P loss. Thus, the loss of P as a function of flow type (base and storm flow) and size was quantified for a mixed-land use watershed (FD-36; 39.5 ha) from 1997 to 2006. Storm size was ranked by return period (<1, 1-3, 3-5, 5-10, and >10 yr), where increasing return period represents storms with greater peak and total flow. From 1997 to 2006, storm flow accounted for 32% of watershed discharge yet contributed 65% of dissolved reactive P (DP) (107 g ha(-1) yr(-1)) and 80% of total P (TP) exported (515 g ha(-1) yr(-1)). Of 248 storm flows during this period, 93% had a return period of <1 yr, contributing most of the 10-yr flow (6507 m(3) ha(-1); 63%) and export of DP (574 g ha(-1); 54%) and TP (2423 g ha(-1); 47%). Two 10-yr storms contributed 23% of P exported between 1997 and 2006. A significant increase in storm flow DP concentration with storm size (0.09-0.16 mg L(-1)) suggests that P release from soil and/or area of the watershed producing runoff increase with storm size. Thus, implementation of P-based Best Management Practice needs to consider what level of risk management is acceptable.     

Environmental impact of irrigation in La Violada District (Spain): II. Nitrogen fertilization and nitrate export patterns in drainage water

Fertilizer leaching affects farm profitability and contributes to nonpoint-source pollution of receiving waters. This work aimed to establish nitrate nitrogen export from La Violada Gully in relation to nitrogen fertilization practices in its basin (La Violada Gully watershed, VGW, 19,637 ha) and especially in La Violada Irrigation District (VID, 5282 ha). Nitrogen (N) fertilization in VID (and VGW) was determined through interviews with local farmers for the hydrologic years 1995 and 1996 and NO3-N load in the gully was monitored from 1995 to 1998. The N fertilizer applied in VGW was 2175 Mg in 1995 and 2795 Mg in 1996. About 43% was applied in VID (945 Mg in 1995 and 1161 Mg in 1996). The most fertilized crop was corn: 398 kg N ha-1 (665 Mg) in 1995 and 453 kg N ha-1 (911 Mg) in 1996. Nitrogen fertilization was higher than N uptake for irrigated crops, especially for corn and rice. Nitrate N load in La Violada Gully averaged 427.4 Mg yr-1. Seventy-five percent of the exports took place during the irrigation season (321.8 Mg). During the non-irrigation season maximum NO3-N loads (3.1 Mg NO3-N d-1) were found after heavy rains following the N side-dressing of wheat in the rain-fed area of VGW (February). During the irrigation season NO3-N load was determined by outflow from the district (caused by irrigation) and to a lesser extent by changes in NO3 concentration (caused by fertilization), showing peaks in April (pre-sowing corn N fertilization and first irrigations) and June to August (highest irrigation months and corn side-dress N applications, maximum 6.3 Mg NO3-N d-1 in July). Adjusting N fertilization to crops' needs, improving irrigation efficiencies, and better scheduling N fertilization and irrigation in corn could reduce N export from VID.     

The MANAGE database: nutrient load and site characteristic updates and runoff concentration data

The "Measured Annual Nutrient loads from AGricultural Environments" (MANAGE) database was developed to be a readily accessible, easily queried database of site characteristic and field-scale nutrient export data. The original version of MANAGE, which drew heavily from an early 1980s compilation of nutrient export data, created an electronic database with nutrient load data and corresponding site characteristics from 40 studies on agricultural (cultivated and pasture/range) land uses. In the current update, N and P load data from 15 additional studies of agricultural runoff were included along with N and P concentration data for all 55 studies. The database now contains 1677 watershed years of data for various agricultural land uses (703 for pasture/rangeland; 333 for corn; 291 for various crop rotations; 177 for wheat/oats; and 4-33 yr for barley, citrus, vegetables, sorghum, soybeans, cotton, fallow, and peanuts). Across all land uses, annual runoff loads averaged 14.2 kg ha(-1) for total N and 2.2 kg ha(-1) for total P. On average, these losses represented 10 to 25% of applied fertilizer N and 4 to 9% of applied fertilizer P. Although such statistics produce interesting generalities across a wide range of land use, management, and climatic conditions, regional crop-specific analyses should be conducted to guide regulatory and programmatic decisions. With this update, MANAGE contains data from a vast majority of published peer-reviewed N and P export studies on homogeneous agricultural land uses in the USA under natural rainfall-runoff conditions and thus provides necessary data for modeling and decision-making related to agricultural runoff. The current version can be downloaded at http://www.ars.usda.gov/spa/manage-nutrient.     

Efficacy of Bacteroides measurements for reducing the statistical uncertainty associated with hydrologic flow and fecal loads in a mixed use watershed

This paper presents an analysis of the occurrence and uncertainty of source-specific Bacteroides and Escherichia coli in a stream in a mixed land-use watershed with human, cattle, and wildlife fecal inputs located in a karstic geologic region during baseflow conditions. The objectives of the study were to evaluate the occurrence, hydrologic significance, and source of fecal mass in the stream using assays for total Bacteroides (AllBac) and bovine-specific Bacteroides (BoBac), and then to compare these measurements with E. coli densities and loads. Samples were collected during baseflow conditions over several months at seven different main channel sites in the Stock Creek watershed, a 49.3 km2 basin located in Knoxville, TN (USA). We determined instantaneous loads for total fecal loads, bovine fecal loads, and E. coli from measured flow rates and the representative Bacteroides fecal masses and/or E. coli densities. The study indicated a strong correlation between total fecal load (kg d(-1)), bovine fecal load (kg d(-1)), E. coli load rate (CFU d(-1)), 7-d antecedent precipitation, and turbidity. The various datasets were used to establish parameter correlations and spatial dependencies throughout the watershed. The data analysis demonstrated two prevalent patterns throughout the watershed: (i) a runoff-dominated transport and occurrence; and (ii) potential groundwater-dominated transport and occurrence.     

WATERSHED OPTIMIZATION OF AGRICULTURAL BEST MANAGEMENT PRACTICES: CONTINUOUS SIMULATION VERSUS DESIGN STORMS1

ABSTRACT: Nonpoint source (NPS) models and expert opinions are often used to prescribe best management practices (BMPs) for controlling NPS pollution. An optimization algorithm (e.g., a genetic algorithm, or GA) linked with a NPS model (e.g., Annualized AGricultural Nonpoint Source pollution model, or AnnAGNPS), can be used to more objectively prescribe BMPs and to optimize NPS pollution control measures by maximizing pollutant reduction and net monetary return from a watershed. Pollutant loads from design storms and annual loads from a continuous simulation can both be used for optimizing BMP schemes. However, which strategy results in a better solution (in terms of providing water quality protection) for a watershed is not clear. The specific objective of the study was to determine the differences in watershed pollutant loads, in an experimental watershed in Pennsylvania, resulting from optimization analyses performed using pollutant loads from a series of five 2‐yr 24‐hr storm events, a series of five 5‐yr 24‐hr storm events, and cumulative pollutant loads from a continuous simulation of five years of weather data. For each of these three different event alternatives, 100 near optimal solutions (BMP schemes) were generated. Sediment (Sed), sediment nitrogen (SedN), dissolved N (SolN), sediment organic carbon (SedOC), and sediment phosphorus (SedP) loads from a different five‐year period (an evaluation period) suggest that the optimal BMP schemes resulting from the use of annual cumulative pollutant loads from a continuous simulation of five years of weather data provide smaller cumulative NPS pollutant loads at the watershed outlet.     

Tillage, cropping systems, and nitrogen fertilizer source effects on soil carbon sequestration and fractions

Quantification of soil carbon (C) cycling as influenced by management practices is needed for C sequestration and soil quality improvement. We evaluated the 10-yr effects of tillage, cropping system, and N source on crop residue and soil C fractions at 0- to 20-cm depth in Decatur silt loam (clayey, kaolinitic, thermic, Typic Paleudults) in northern Alabama, USA. Treatments were incomplete factorial combinations of three tillage practices (no-till [NT], mulch till [MT], and conventional till [CT]), two cropping systems (cotton [Gossypium hirsutum L.]-cotton-corn [Zea mays L.] and rye [Secale cereale L.]/cotton-rye/cotton-corn), and two N fertilization sources and rates (0 and 100 kg N ha(-1) from NH(4)NO(3) and 100 and 200 kg N ha(-1) from poultry litter). Carbon fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). Crop residue varied among treatments and years and total residue from 1997 to 2005 was greater in rye/cotton-rye/cotton-corn than in cotton-cotton-corn and greater with NH(4)NO(3) than with poultry litter at 100 kg N ha(-1). The SOC content at 0 to 20 cm after 10 yr was greater with poultry litter than with NH(4)NO(3) in NT and CT, resulting in a C sequestration rate of 510 kg C ha(-1) yr(-1) with poultry litter compared with -120 to 147 kg C ha(-1) yr(-1) with NH(4)NO(3). Poultry litter also increased PCM and MBC compared with NH(4)NO(3). Cropping increased SOC, POC, and PCM compared with fallow in NT. Long-term poultry litter application or continuous cropping increased soil C storage and microbial biomass and activity compared with inorganic N fertilization or fallow, indicating that these management practices can sequester C, offset atmospheric CO(2) levels, and improve soil and environmental quality.     

Dry Weather Water Quality Loadings in Arid,Urban Watersheds of the Los Angeles Basin,California, USA1

Abstract: Dry weather runoff in arid, urban watersheds may consist entirely of treated wastewater effluent and/or urban nonpoint source runoff, which can be a source of bacteria, nutrients, and metals to receiving waters. Most studies of urban runoff focus on stormwater, and few have evaluated the relative contribution and sources of dry weather pollutant loading for a range of constituents across multiple watersheds. This study assessed dry weather loading of nutrients, metals, and bacteria in six urban watersheds in the Los Angeles region of southern California to estimate relative sources of each constituent class and the proportion of total annual load that can be attributed to dry weather discharge. In each watershed, flow and water quality were sampled from storm drain and treated wastewater inputs, as well as from in‐stream locations during at least two time periods. Data were used to calculate mean concentrations and loads for various sources. Dry weather loads were compared with modeled wet weather loads under a range of annual rainfall volumes to estimate the relative contribution of dry weather load. Mean storm drain flows were comparable between all watersheds, and in all cases, approximately 20% of the flowing storm drains accounted for 80% of the daily volume. Wastewater reclamation plants (WRP) were the main source of nutrients, storm drains accounted for almost all the bacteria, and metals sources varied by constituent. In‐stream concentrations reflected major sources, for example nutrient concentrations were highest downstream of WRP discharges, while in‐stream metals concentrations were highest downstream of the storm drains with high metals loads. Comparison of wet vs. dry weather loading indicates that dry weather loading can be a significant source of metals, ranging from less than 20% during wet years to greater than 50% during dry years.