William J. Gburek

Critical source area controls on water quality in an agricultural watershed located in the Chesapeake basin.

The importance of agricultural land use activities for supplying nutrients (N, P) to the Chesapeake Bay is examined and nutrient sources for a typical agricultural hill-land watershed within the Chesapeake Basin are identified and assessed. Based on up to 30 years of experimental and monitoring data, the outflow, N, and P exported from this Pennsylvania watershed is examined in terms of critical source areas. Most of the surface runoff and P export occurs from areas near the stream. About 90% of the algal-available P exported in outflow was generated during the largest 7 storms/year. In contrast, nearly all the nitrate (NO3) exported originated as subsurface flow entering the soil or ground water some distance from the stream, and mostly occurred during nonstorm flow periods. The NO3 export observed over the long term corresponds to the N excess computed by N balance obtained by farmer survey for agricultural land. By combining land use, hydrologic processes, watershed position, soil P status, and N balance information for agricultural land, the major source areas for P and N are predictable and identifiable. We apply these ideas and techniques to our research watershed and present the results as an example of this approach.

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.

Phosphorus leaching through intact soil columns before and after poultry manure application

Recent application of manure can increase phosphorus (P) loss from soil in subsurface flow (e.g., drainage water). This study investigated vertical leaching of P through two soils. Eight 30- and 50-cm-deep, intact soil columns (30-cm diameter) were collected. Columns were irrigated periodically (2.4 cm day−1) before and after surface application of poultry manure (85 kg total P ha−1), continuing for 11 weeks after the application. A dye tracer (FD&C blue No. 1) was used to identify the presence of active macropores at the bottom of each column, and to compare properties of undyed soil matrix material with dyed soil bordering active macropores. Before manure application, concentrations of total P (TP) in leachate did not exceed 0.57 mg L−1, with dissolved reactive P (DRP) a minor fraction of leachate TP (averaging 7%). Manure application resulted in significant increases in leachate P concentrations, with DRP averaging 72%of leachate TP. No significant differences in leachate DRP and TP concentrations were observed between 30- and 50-cm-deep columns or between soils, either before or after manure was applied, reflecting considerable variability in leachate P trends. In many columns, P concentrations in leachate peaked soon after manure application, with maximum DRP concentrations ranging from 1.1 to 11.2 mg L−1. In other columns, concentrations increased slowly over time, but maximum DRP concentrations were only 0.19 to 0.55 mg L−1. Different temporal trends in leachate P concentrations were unrelated to trends in flow. Increased P sorption saturation of soil bordering macropores in subsurface horizons, due to elevated Mehlich-3 P and depleted Mehlich-3 Al, points to the importance of macropores as preferential flow pathways for P. Results of this study highlight the significant, but temporally and spatially variable, nature of P leaching in manured soils.

Identifying critical sources of phosphorus export from agricultural watersheds

Surface runoff accounts for much of the phosphorus (P) input to and accelerated eutrophication of the fresh waters. Several states have tried to establish general threshold soil P levels above which the enrichment of surface runoff P becomes unacceptable. However, little information is available on the relationship between soil and surface runoff P, particularly for the northeastern United States. Further, threshold soil P criteria will be of limited value unless they are integrated with site potential for runoff and erosion. In response, the Natural Resource Conservation Service (NRCS) developed a P Index (PI), which ranks the vulnerability of fields as sources of P loss in runoff, based on soil P, hydrology, and land use. This study evaluated the relationship between soil and surface runoff P in a study watershed in central Pennsylvania. The relationship was then incorporated into the (PI), and its impact on the identification of critical source areas within the watershed was examined. Using simulated rainfall (6.5 cm h−1 for 30 min), the concentration of dissolved P in surface runoff (0.2–2.1 mg l−1) from soils was related (r2=0.67) to Mehlich-3 extractable soil P (30–750 mg kg−1). Using an environmentally based soil P threshold level of 450 mg kg−1 determined from the soil-runoff P relationship, the PI identified and ranked areas of the watershed vulnerable to P loss. The vulnerable areas were located along the stream channel, where areas of runoff generation and areas of high soil P coincide, and where careful management of P fertilizers and manure should be targeted.

Response of Stream Macroinvertebrates to Agricultural Land Cover in a Small Watershed

ABSTRACT We conducted a three-year study of the aquatic macroinvertebrate community of a rural, central-Pennsylvania, upland watershed (7.3 km2) with subwatersheds affected by a gradient of agricultural land use (87% forest to 96% managed agriculture). Macroinvertebrate samples were collected at 16 sites on first- to third-order streams within the watershed in May or June. Nitrate concentrations and habitat quality were also determined at these sites. The taxonomic richness of sensitive macroinvertebrates (mayflies and caddisflies) was lower in streams draining subwatersheds with high percentage agricultural land cover compared to low. Habitat quality decreased as percentage agricultural land cover increased. Nitrate concentration increased with percentage agricultural land cover for the three years of study but was only correlated with habitat quality in one year. Our data suggest that a high percentage of agricultural land cover reduced the number of sensitive macroinvertebrate taxa and produced a macroinvertebrate community composition that reflected altered stream habitat.

Patterns of contaminant transport in a layered fractured aquifer

We investigated patterns of contaminant transport within the layered and fractured aquifer of a 7.3-km2 upland agricultural watershed in east-central Pennsylvania, USA. Geometry and hydraulic properties of the aquifer had been characterized by field testing and model calibration. These results were extended to simulate flow pathways and patterns of contaminant transport in both areal and cross-section formats within the watershed. The analyses indicated that the ground water flow system at the larger watershed scale is comprised of smaller units of subsurface flow which are self-contained at the scale of first- or second-order streams. For this scale subwatershed or larger, contaminant inputs to ground water from the mix of land use within the subwatershed should translate directly to the quality of nonstorm streamflow. For illustration, recharge water quality from typical land-use distributions were combined with a simple model of contaminant transport to simulate nitrate concentration patterns in ground water in a cross-section format. Land use in the vicinity of the drainage divides between streams was found to control ground water quality within the deeper layers of the aquifer, while land use over the remainder of the watershed area affected water quality only within the shallower layers of the aquifer. Streamflow nitrate data collected during a baseflow survey on the watershed were examined in context of these simulations and found to support the conclusions. Results of the study demonstrate the potential for localized contamination of ground water and nonstorm streamflow by agricultural land use, as well as the potential for managing stream quality and minimizing contamination within targeted zones of the ground water by controlling land use position.

Cannonsville Reservoir and Town Brook watersheds: Documenting conservation efforts to protect New York City's drinking water

The Cannonsville Reservoir watershed is a major component of the unfiltered New York City water supply system. The voluntary, incentive-based Watershed Agricultural Program is a collaborative effort among producers, federal, state, and local organizations to address the problem of phosphorus loading effects on water quality through implementation of whole-farm plans. The effectiveness of selected conservation practices, including stream-bank fencing, precision feeding, and the use of cover crops with silage corn (Zea mays L.) are being evaluated. Simulation models have been developed and improved to evaluate the effectiveness of individual conservation practices and better assess animal agriculture and manure management practices. Conservation practices implemented through the Watershed Agricultural Program are resulting in lower phosphorus loading from nonpoint sources in the watershed. Future efforts need to identify the most cost-effective conservation practices and extend our knowledge of watershed quality protection beyond the boundaries of the Cannonsville Reservoir watershed.

Baseflow nitrate in relation to stream order and agricultural land use.

Management of agricultural nonpoint-source pollution continues to be a challenge because of spatial and temporal variability. Using stream order as an index, we explored the distribution of nitrate concentration and load along the stream network of a large agricultural watershed in Pennsylvania-the East Mahantango Creek Watershed and two of its sub-watersheds. To understand nitrate concentration variation in the stream water contributed from ground water, this study focused on baseflow. Impacts of agricultural land use area on baseflow nitrate in the stream network were investigated. Nitrate concentration showed a general decreasing trend with increasing stream order based on stream order averaged values; however, considerable spatial and temporal variability existed within each snapshot sampling. Nitrate loads increased with stream order in a power function because of the dominant effect of stream flow rate over the nitrate concentration. Within delineated sub-watersheds based on stream orders, positive linear functions were found between agricultural land use area percentage and the baseflow nitrate concentration and between agricultural drainage area and the nitrate load. The slope of the positive linear regression between the baseflow nitrate concentration and percent agricultural land area seems to be a valuable indicator of a watersheds water quality as influenced by agricultural practices, watershed size, and specific physiographic setting. Stream order seems to integrate, to a certain degree, the source and transport aspects of nonpoint-source pollution on a yearly averaged basis and thus might provide a quick estimate of the overall trend in baseflow nitrate concentration and load distribution along complex stream networks in agricultural watersheds.

Production and Feeding Strategies for Phosphorus Management on Dairy Farms in New York

Whole-farm simulation was used to evaluate the long-term effects of changes in feeding, cropping, and other production strategies on phosphorus loading and the economics of two farms in southeastern New York. Reducing the level of dietary P fed and maximizing the use of farm-grown forage provided a long-term P balance along with an increase in farm profitability.

Optimizing Best Management Practice Selection to Increase Cost-effectiveness

With Best Management Practices (BMPs) being used increasingly to control losses of major agricultural pollutants to surface waters, establishing the effectiveness of these practices has become important. A methodology was developed for determining cost-effective watershed scenarios. This technique combines three existing tools: a genetic algorithm (GA), a watershedlevel nonpoint source model (Soil and Water Assessment Tool, SWAT), and a BMP assessment tool. The GA combines initial pollutant loadings from SWAT with literature-based pollution reduction efficiencies provided by the assessment tool and BMP costs appropriate to the study area to determine cost-effective watershed scenarios. The methodology was successfully applied to a 300-ha farm within the Cannonsville Reservoir watershed in New York. The Cannonsville Reservoir is phosphorous (P) restricted, and planners are implementing BMPs to reduce P loading to the reservoir. The optimal scenario for the farm, under the presented methodology, achieved a cost-effectiveness of 0.6 kg dissolved P reduction per dollar spent. Additionally, the methodology determined alternative scenarios which met the pollution reduction criteria cost-effectively.