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TECHNICAL REPORTS

Landscape and Watershed Processes

Phosphorus Loss from an Agricultural Watershed as a Function of Storm Size

^a Dep. of Crop, Soil and Environmental Sciences, 115 Plant Sciences Building, Univ. of Arkansas, Fayetteville, AR 72701
^b USDA-ARS, Pasture Systems and Watershed Management Research Unit, Building 3702, Curtin Road, University Park, PA 16802-3702
^c Centre for Sustainable Water Management, Lancaster Environment Centre, Lancaster Univ., Lancaster, LA1 4YQ, UK
^d USDA-ARS, Pasture Systems and Watershed Management Research Unit, Building 3702, Curtin Road, University Park, PA 16802-3702. Mention of trade names does not imply endorsement by the U.S. Government

^* Corresponding author (Sharpley{at}uark.edu).

Received for publication July 12, 2007.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
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 m³ 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.

Abbreviations: BMP, Best Management Practice • DP, dissolved reactive phosphorus • PP, particulate phosphorus • TP, total phosphorus • USEPA, U.S. Environmental Protection Agency

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
THE concept of phosphorus (P) loss risk assessment is based on the premise that most P exported from a watershed originates from a small part of the landscape close to the stream channel, which can be activated by frequent storm events. These hydrologically active areas contributing surface runoff or erosion to stream flow (i.e., transport factors) can be large sources of P when they are coincident with areas of high soil P or recent manure applications (i.e., source factors) (Gburek et al., 2000; Hart et al., 2004; Pionke et al., 2000). As a result, P-based remedial strategies focus on the underlying principle of targeting critical source areas and transport pathways (Sharpley et al., 2003; USDA and USEPA, 1999; USEPA, 2004).

Surface and subsurface transport pathways combine over the landscape to produce storm and base flow from a watershed (Gburek and Sharpley, 1998; Tomer et al., 2005). Base flow consists primarily of ground and shallow ground water, whereas storm flow is dominated by surface runoff (Ward and Trimble, 2003). Although storm flow generally comprises a fraction of total flow, it contributes most of the P exported from a watershed compared with base flow (Gburek and Folmar, 1999; Heathwaite and Dils, 2000). An historical survey of U.S. Geological Survey stream gages with hydrograph separation data (N = 14,366) revealed a mean base flow index of 0.50 (i.e., 50% of total flow was base flow), with 25% of all gages having base flow indices in excess of 0.67 (Wolock, 2003). Gages in the northeastern and mid-Atlantic regions had base flow indices comparable to the national average (mean = 0.48; N = 2072).

Considerably less information is available on the relative partitioning of P transport in storm and base flow, although several studies generalized that the major proportion of P exported from watersheds (>75%) occurs during large storms (Edwards and Owens, 1991; Pionke et al., 1999; Smith et al., 1991; Vanni et al., 2001). By measuring direct inputs of surface runoff and subsurface flow into a stream draining a pasture watershed (20 ha) in New Zealand for 3 yr (rainfall of 1010 mm yr^–1), Sharpley and Syers (1979) found that although base flow dominated stream flow (63%), only 12% of dissolved reactive P (DP) and 5% of total P (TP) was exported in base flow. Storm flow accounted for 53% of DP, 15% of particulate P (PP), and 23% of TP exported. Quantification of storm flow inputs of P from surface runoff plots adjacent to the stream channel enabled Sharpley and Syers (1979) to determine that stream bank and bed erosion was the largest source of PP (83%) exported from the watershed. Furthermpre, the release of DP from stream bank and bed material was estimated to account for 33% of stream DP exported (Sharpley and Syers, 1979).

Due to the importance of storm flow to watershed P loss, storm characteristics exert a strong control on the potential of P loss to occur. In landscapes prone to variable source area hydrology, the combination of antecedent moisture and storm characteristics have been linked to runoff-contributing areas (e.g., Needleman et al., 2004). However, little information is available documenting the effect of storm size on P loss, even though this information is fundamental to the concept of risk assessment and indexing P loss and to targeting watershed remediation efforts in general. This paper documents a 10-yr study of P loss from a mixed-land use watershed (FD-36) in south-central Pennsylvania as a function of storm and base flow (1997–2006). The effect of storm size on P export is determined.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Watershed Description
The study area is FD-36, a 39.5-ha sub-watershed of Mahantango Creek, which is a tributary of the Susquehanna River, the largest contributor of fresh water to the Chesapeake Bay (Fig. 1 ). The FD-36 watershed is typical of upland agricultural watersheds within the nonglaciated, folded and faulted, Appalachian Valley and Ridge Physiographic Province. Soils of the watershed are classified as Alvira (fine-loamy, mixed, mesic Aeric Fragiaquults), Berks (loamy-skeletal, mixed, active, mesic Typic Dystrudepts), Calvin (loamy-skeletal, mixed, active, mesic Typic Dystrudepts), Hartleton (loamy-skeletal, mixed, active, mesic Typic Hapludults), and Watson (fine-loamy, mixed, active, mesic Typic Fragiudults). Slopes within the watershed range from 1 to 20%.

View larger version (48K): [in this window] [in a new window]   Fig. 1. The FD-36 watershed, field boundaries, and field identification numbers.

For all stream water samples, a composited flow-weighted sample was filtered (0.45-µm Millipore membrane) within 24 h and stored at 4°C. The concentration of DP in stream flow was determined on a 0.45-µm filtered sample. The concentrations of total dissolved P and TP were determined on filtered and unfiltered samples, respectively, after digestion with a semi-micro Kjeldahl procedure and filtration (Whatman No. 40 filter paper) (Patton and Kryskalla, 2003). Phosphorus measurements were conducted in duplicate. Phosphorus in all stream water filtrates and neutralized Kjeldahl digests were determined by the colorimetric molybdenum-blue method of Murphy and Riley (1962). Solutions were neutralized using p-nitrophenol indicator (color change at pH 7.0) and drop-wise addition of 0.5 M H₂SO₄ or 1.0 M NaOH. Particulate P was calculated as the difference between TP and total dissolved P.

Stream flow at the watershed outlet was separated into storm flow and base flow using techniques dependent on storm characteristics (Hall, 1968). Storm flow is defined as starting when stage height at the flume increases by 2 cm within a 5-min sampling interval, which is sustained over 2 h. These storms can be categorized according to their return period or probability of occurrence based on long-term historical records (25–50 yr). For example, a storm having a 5-yr return period was determined based on an average for the long term, occurring once every 5 yr but not necessarily once within every 5-yr period. The following ranges of return periods were used to represent storm distribution frequencies for FD-36: <1, 1 to 3, 3 to 5, 5 to 10, and >10 yr return periods.

Phosphorus loss in each storm was calculated as the product of flow and mean flow-weighted P concentration of storm samples, with P loss in baseflow calculated as the product of flow between storms and baseflow P concentration obtained from monthly samples. Annual export of P from the watershed was determined as the sum of all storm and baseflow loss for each year (only April to October for 1997 to 2001). The return period for each storm during the study period was determined from peak flow of the stream during the storm (Flippo, 1977).

Statistical Analysis
Statistical analyses (t tests, means, and standard errors) were performed with SPSS v10.0 (SPSS, 1999). All r² values given are significant at the P < 0.05 level.

	Results and Discussion

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Phosphorus Loss in Storm and Base Flow
Annual rainfall varied from 706 to 1160 mm during the 10-yr study, with stream flow increasing as rainfall increased (Table 2 ). During the 2002 to 2006 sampling period, when annual stream flow was monitored, an average of 40% of rainfall left FD-36 as stream flow. For three tile-drained watersheds (106–481 km²) in Illinois, annual stream discharge averaged 30% of annual rainfall (Gentry et al., 2007). From 1997 to 2006, there were 248 storm-flow events, which annually comprised 246 to 2303 m³ ha^–1 discharge (Table 2). For the entire study period, mean annual storm flow (1038 m³ ha^–1) was less than base flow (2240 m³ ha^–1), with 32% of stream flow as storm flow and 68% as base flow (Table 2). Similarly, Tomer et al. (2005) found that during 25 yr of stream flow from two conventionally tilled corn watersheds (30–60 ha) in Treynor, Iowa, storm (34%) and base flows (66%) were similarly distributed to that in FD-36 (36 and 64%, respectively). In two adjacent conservation ridge till watershed in corn, base flow comprised a greater proportion of stream flow (84%) due to increased infiltration afforded by conservation tillage (Tomer et al., 2005). During the 25-yr record, stream flow from the conventional (24%) and conservation tillage (29%) watersheds were a similar proportion of annual rainfall (812 mm) as in FD-36 (35%) (Table 3 ). A similar proportion of base flow (63%), storm flow (37%), and partitioning of P forms exported from a pasture watershed (23 ha) in New Zealand over 3 yr was also found (Sharpley and Syers, 1979).

The fact that storm flow contributes the major proportion of P exported from FD-36, despite being a minor contributor of water discharged, is consistent with findings from other watersheds. For instance, Sharpley and Syers (1979) showed that although base flow dominated stream flow, storm flow accounted for 53% of DP, 98% of PP, and 95% TP exported, although most of this was attributed to stream bank and bed erosion.

For the 10-yr study (1997–2006) in FD-36, DP concentrations in stream flow averaged 0.050 mg L^–1, and TP averaged 0.496 mg L^–1. For storm flow, DP and TP concentrations averaged 0.103 and 0.495 mg L^–1, respectively, whereas base flow concentrations of DP and TP were 0.026 and 0.057 mg L^–1, respectively. Similar trends in P transport during storm and base flow were found by Gentry et al. (2007) for three row crop (corn and soybean) watersheds in Illinois. Concentrations of DP and TP were greater during storm flow (>0.20 mg L^–1) than during base flow (<0.05 mg L^–1).

To put the P concentrations observed for FD-36 into a regional water quality perspective, values were compared with U.S. Environmental Protection Agency ambient water quality criteria recommendations for rivers and streams in Ecoregion XI, Level III–67, the Ridge and Valley ecoregion, in which FD-36 is located (USEPA, 2000). Based on the 25th percentile data for all seasons for 1990 to 1999, the reference TP concentration is 0.010 mg L^–1 (with observed values ranging from 0 to 1.388 mg L^–1). Average TP concentrations in stream (0.196 mg L^–1), storm (0.495 mg L^–1), and base flow (0.057 mg L^–1) were above ambient or reference concentrations for the Ridge and Valley ecoregion. Measured TP concentrations in all flow types were also greater than environmental thresholds for flowing water (0.05 mg L^–1) above which surface water eutrophication may be accelerated (Gibson et al., 2000).

Storm-Return Period and P Loss
There were 248 storm-flow events in FD-36 from 1997 through 2006. By definition, the number of storms in each category decreased as return period increased from <1 to >10 yr (Table 4 ). During the 10-yr study, 93% of storm flows occurring in FD-36 had a return period of 1 yr, whereas only 4% of the storms had a return period >3 yr.

View this table: [in this window] [in a new window]   Table 4. Amount of dissolved reactive phosphorus (DP), particulate phosphorus (PP), and total phosphorus (TP) exported in storm flow and percent of total storm P export from FD-36 during 1997 to 2006, as a function of storm-return period.

As storm-return period increased from 1 to 10 yr, there was a shift in the proportion of P transported as DP (24% of TP for 1-yr storms and 17% TP for >1-yr storms) and an increase in PP transport (76–83% of TP). Similarly, Gentry et al. (2007) observed that concentrations of PP were 2 to 5 times DP in the largest flow events during a 10-yr period for three row-cropped watersheds in Illinois. In contrast, in years without high discharge events, annual flow-weighted mean DP concentrations ranged from 27 to 50% of TP concentrations. The greater propensity for P transport as PP with an increase in storm size or return period reflects a combined increase in erosivity of overland and stream flow, as well as stream sediment resuspension, which can sequester DP during flow (Smith et al., 1991; McDowell and Wilcock, 2004). For instance, Sharpley et al. (1981) found that the DP concentration of runoff from grassed and cultivated watersheds decreased as suspended sediment concentration increased (R² = 0.80).

A similar predominance of TP loss in large storms was also observed by Udawatta et al. (2004) for several watersheds (1.7–4.4 ha) with clay-pan soils in corn–soybean rotation at the Greenly Memorial Research Center, MO (rainfall 920 mm yr^–1). From 1991 to 1997, the largest 5 of 66 storm flow events accounted for 27% of TP exported, which averaged 1.36 kg ha^–1 yr^–1 (Udawatta et al., 2004).

There is a greater amount of flow and P loss per event as storm size (i.e., the return period) increases (Table 4). For instance, mean DP loss increased from 2 to 98 g ha^–1 event^–1 for the 1- and 10-yr storms, respectively. For PP, the respective mean loss increased from 18 to 491 g ha^–1 event^–1, and for TP it increased from 11 to 589 g ha^–1 event^–1 (Table 4). An appreciably greater export of P in large flow events is also evident in watersheds in the Corn Belt region of the USA (Gentry et al., 2007; Borah et al., 2003; Royer et al., 2006). The overall increase in P loss with storm size has direct relevance to P loss risk assessment (the P Index concept) and P-based management of nutrients within a watershed to minimize the potential for P loss.

There was a general increase in P concentration of storm flow with larger storm events or return period (Fig. 2 ). The mean DP concentration of >10-yr return period storms (0.156 mg L^–1) between 1997 and 2006 was significantly greater than means for <1-yr return-period storms (0.080 mg L^–1) (Fig. 2). A similar trend was found for PP and TP, with concentrations averaging 0.787 and 0.943 mg L^–1, respectively, for >10-yr storms and 0.284 and 0.372 mg L^–1 for <1-yr storms. As storm size increases, flow and event P losses increase. The increase in mean concentration of P forms in flow with return period shown in Fig. 2 suggests that there is a greater propensity for P release from soil and/or that different areas of the watershed contribute runoff to storm flow. Furthermore, as storm return period increases, it is not simply more runoff occurring from the same contributing watershed area; rather, hydrologically active runoff-producing zones may increase with storm size.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Relationship between storm-flow return period and mean dissolved phosphorus (DP), particulate phosphorus (PP), and total phosphorus P (TP) concentration for FD-36 during 1997 to 2006 (all regressions were significant at P > 0.05). For each form, concentration points assigned different letters were significantly different (P < 0.05) as determined by Tukey's studentized range test.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

Large infrequent storm flow events have the potential to carry large amounts of P from a watershed. Indeed, there were only two storm flow events with greater than a 10-yr return period during 1997 to 2006. However, during these two events, appreciably more DP (98 g ha^–1 event^–1) and TP (589 g ha^–1 event^–1) was exported from FD-36 than in storms with shorter return periods (2–31 DP and 11–164 g TP ha^–1 event^–1). Over the 10-yr study, however, the largest storms (>10-yr return period) only comprised about 20% of P exported. Thus, designing conservation practices that minimize the risk of P loss from more frequent low intensity storms would likely be a cost-effective strategy. Such measures focus on decreasing DP loss, which was found to be the dominant P form in more frequent low-intensity storms and could include management of the rate, timing, and method of P application. For example, carefully matching the crop P requirements of expected crop yields with incorporated or subsurface-injected fertilizer and manure application, when the likelihood of intensive rainfalls is less, would decrease the source of P available to be transported during surface runoff. Structural conservation practices or Best Management Practices (BMPs), such as edge-of-field vegetative buffers, which are designed to filter particulates, are less effective at decreasing the transport of DP (Sims and Kleinman, 2005). In FD-36, conservation practices designed to 5-yr return period storms would address flow events contributing almost 77% of the P exported over this 10-yr watershed study.

Data presented in this paper suggest that the area of a watershed contributing runoff to P export likely increases with storm size or return period. This has important implications to watershed management of P. For example, P-based BMPs need to be applied to an increasingly large area of the watershed to minimize the risk of P loss. Thus, at some stage, BMP implementation and remedial strategies in general need to address what level of risk management is acceptable. This needs to be weighed against an increased area of land that may require more restrictive P-based management to affect a P loss reduction. Finally, with the large variation in P export found with storm size and the unpredictability with which extreme rainfall and associated storm flow events occur, a minimum of 5 yr is likely needed to reliably assess the effect of P-based conservation measures.

	ACKNOWLEDGMENTS

 
We acknowledge the assistance of Todd Strohecker, Mike Reiner, and Terry Troutman for maintenance of flumes and sampling equipment in FD-36 and for collection of flow data and stream flow samples. We also thank Mary Kay Lupton, Joan Weaver, Charles Montgomery, and Paul Spock for analysis P forms in water samples. The mention of trade names does not imply endorsement.

	NOTES

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