HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in JEQ Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (9) Citing Articles via Google Scholar Google Scholar Articles by Zheng, F.-L. Articles by Norton, L. D. Search for Related Content PubMed PubMed Citation Articles by Zheng, F.-L. Articles by Norton, L. D. Agricola Articles by Zheng, F.-L. Articles by Norton, L. D. Related Collections Vadose Zone Processes and Chemical Transport Surface Water Quality Surface Hydrology Soil Hydrology Water Pollution

TECHNICAL REPORTS

Surface Water Quality

Effects of Near-Surface Hydraulic Gradients on Nitrate and Phosphorus Losses in Surface Runoff

^a State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation, CAS and MWR, Northwestern Sci-Tech University of Agriculture and Forestry, 26 Xinong Road, Yangling, Shaanxi, 712100 China
^b USDA-ARS National Soil Erosion Research Laboratory, 275 South Russell Street, Purdue University, West Lafayette, IN 47907-2077

^* Corresponding author (flzh{at}ms.iswc.ac.cn)

Received for publication December 22, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS CONCLUSIONS REFERENCES

 
Phosphorous (P) and nitrogen (N) in runoff from agricultural fields are key components of nonpoint-source pollution and can accelerate eutrophication of surface waters. A laboratory study was designed to evaluate effects of near-surface hydraulic gradients on P and N losses in surface runoff from soil pans at 5% slope under simulated rainfall. Experimental treatments included three rates of fertilizer input (control [no fertilizer input], low [40 kg P ha^–1, 100 kg N ha^–1], and high [80 kg P ha^–1, 200 kg N ha^–1]) and four near-surface hydraulic gradients (free drainage [FD], saturation [Sa], artesian seepage without rain [Sp], and artesian seepage with rain [Sp + R]). Simulated rainfall of 50 mm h^–1 was applied for 90 min. The results showed that near-surface hydraulic gradients have dramatic effects on NO₃–N and PO₄–P losses and runoff water quality. Under the low fertilizer treatment, the average concentrations in surface runoff from FD, Sa, Sp, and Sp + R were 0.08, 2.20, 529.5, and 71.8 mg L^–1 for NO₃–N and 0.11, 0.54, 0.91, and 0.72 mg L^–1 for PO₄–P, respectively. Similar trends were observed for the concentrations of NO₃–N and PO₄–P under the high fertilizer treatment. The total NO₃–N loss under the FD treatment was only 0.01% of the applied nitrogen, while under the Sp and Sp + R treatments, the total NO₃–N loss was 11 to 16% of the applied nitrogen. These results show that artesian seepage could make a significant contribution to water quality problems.

Abbreviations: C, control fertilizer treatment (no fertilizer input) • L, low fertilizer treatment (40 kg P ha^–1, 100 kg N ha^–1) • H, high fertilizer treatment (80 kg P ha^–1, 200 kg N ha^–1) • FD, free drainage hydraulic gradient • Sa, saturation hydraulic gradient • Sp, artesian seepage without rain hydraulic gradient • Sp + R, artesian seepage with rain hydraulic gradient

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS CONCLUSIONS REFERENCES

 
PHOSPHORUS AND NITROGEN in runoff from agricultural lands are nonpoint sources of pollution and can accelerate eutrophication of surface waters (Daniel et al., 1994, 1998; Foy and Withers, 1995). Long-term applications of P and N in chemical fertilizers and animal wastes have resulted in elevated levels of soil P and N in many locations in the United States (Lovejoy et al., 1997). Soils high in P and N have aggravated water pollution problems in many areas. Damage to surface water quality, due to sedimentation and excessive nutrients from agricultural lands in the United States, was estimated to range from $2.2 to $7 billion dollars annually (Lovejoy et al., 1997). Extensive research efforts have identified and quantified factors contributing to chemical losses in runoff, such as soil properties, crop residue cover, slope, tillage, method and timing of fertilizer application, and rainfall pattern (Alberts and Spomer, 1985; Hubbard and Sheridan, 1983; Hubbard et al., 1991; Lowrance, 1992; Pote et al., 1996, 1999). These research findings enhance the understanding of how P and N are moved from soil to water bodies and help the development of management practices capable of minimizing the excessive nutrient problem.

Due to its high solubility, NO₃–N tends to be transported in drainage and subsurface flow. In southern Georgia, Hubbard and Sheridan (1983) reported that 20% of the applied N over a 10-yr period was lost in surface runoff and subsurface flow, and 99% of this loss occurred in subsurface flow. Other studies by Hubbard et al. (1991) and Lowrance (1992) at the same watershed and elsewhere by Alberts and Spomer (1985) with different crops showed the same trend with significantly greater NO₃–N movement in subsurface flow than in surface runoff.

Because P is strongly bound in soil and much less mobile than N, many efforts have been focused on relating different soil test P values to P in surface runoff (Daniel et al., 1993; Pote et al., 1996, 1999; Sharpley et al., 1996; Cox and Hendricks, 2000). Despite a general positive correlation between soil test P to runoff P, the slope of the trend line varied with methods of P extraction, soil type, organic matter, and soil management. Recent studies had been initiated to couple watershed hydrology and chemical transport using the variable source area (VSA) concept (Gburek and Sharpley, 1998; Gburek et al., 2000). Gburek and Sharpley (1998) studied P loss in east-central Pennsylvania and showed that zones of runoff production, and consequently, the areas that ultimately controlled most P transport, were the near-steam saturated areas of the watershed. Further analysis showed that most in-stream P came from soils within 60 m of the stream, rather than from the entire area of the watershed (Gburek et al., 2000).

In this study, we hypothesized that because most agricultural chemicals are in the surface layer, the near-surface hydraulic gradients could have a large influence on chemical transport. This study was initiated based on recent laboratory findings of increased sediment delivery and rilling from soil subject to saturation and artesian seepage or exfiltrating through flow (Huang and Laflen, 1996; Gabbard et al., 1998; Huang et al., 1999; Zheng et al., 2000). Artesian seepage is a common occurrence during the wet season such as early spring after a wet winter, when the shallow ground water emerges at the soil surface as return flow in the middle or lower portions of the hillslope (Fig. 1) . Soils with clayey subsoil or tight plow pan are prone to produce the return flow or seep (Whipkey and Kirkby, 1978; Dunne, 1978).

View larger version (14K): [in this window] [in a new window]   Fig. 1. Hillslope position and hydrologic condition.

The objective of this study was to quantify the effect of near-surface hydraulic gradients on NO₃–N and PO₄–P transport in surface runoff water. A laboratory rainfall simulation study was conducted using a silt loam soil under four different near-surface hydraulic conditions: free drainage (FD), saturation (Sa), artesian seepage without rain (Sp), and artesian seepage with rain (Sp + R).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS CONCLUSIONS REFERENCES

 
Soil Sample Collection and Soil Properties
The soil used in this study was a Waupecan silt loam (fine-silty, mixed, mesic Typic Argiudolls) collected from the surface to a 0.3-m depth near Dayton in Tippecanoe County, Indiana. The collected soil was air-dried and sieved through a 10-mm-opening sieve and stored in covered containers until used in the experiment.

Experimental Setup
The study was conducted on soil pans that were 45 cm long, 32 cm wide, and 35 cm deep. Each soil pan had six drainage holes at the bottom. A water supply system was designed to supply water to the soil pan from the bottom to control the near-surface hydraulic gradient (Fig. 2) . A saturation condition was created when the supply water level was set at the soil surface and an artesian seepage condition was created when the supply water level was set higher than the soil surface, forcing water to flow out of the soil. In this study, the artesian seepage condition was created with the supply water level set 20 cm above the soil surface. For the free drainage treatment, the water supply tubes were not connected to let the soil pan drain freely under gravity.

View larger version (24K): [in this window] [in a new window]   Fig. 2. Experimental setup.

Experimental treatments in this study included three rates of fertilizer input: control (no fertilizer input), low (40 kg P ha^–1, 100 kg N ha^–1), and high (80 kg P ha^–1, 200 kg N ha^–1), hereafter designated as C, L, and H, respectively. These fertilizer treatments were subjected to four near-surface hydraulic gradient treatments: free drainage (FD), saturation (Sa), artesian seepage without rain (Sp), and artesian seepage with rain (Sp + R). The detailed experimental treatments appear in Table 1. For each fertilizer level, three replicates were made for rainfall simulation for the FD and Sa treatments, and three additional soil pans were prepared and used to collect the before-run soil profile samples three days after prewetting under free drainage; four replicates were made for the Sp and Sp + R treatments, and four additional soil pans were prepared and used to collect the before-run soil profile samples three days after prewetting under free drainage.

After soil pan preparation, a prewetting rain (15 mm of rain at 9.5 mm h^–1 intensity) was applied at 0% slope. The 9.5 mm h^–1 rain intensity was selected to avoid the initiation of runoff during the prewetting event. To further avoid raindrop-induced splash and surface sealing, two layers of wire mesh were placed over the soil surface. This initial rain was applied to allow fertilizer leaching into the soil profile, create a uniform surface soil moisture condition before the experiment, and reduce surface variability from preparation. After the prewetting rain, the soil pan was covered with a plastic sheet and allowed to equilibrate under free-drained conditions for three days.

Rainfall Experiments
Three days after the prewetting rain, soil pans were set to 5% slope and subjected to the experimental hydraulic gradients. A simulated rainstorm of 50 mm h^–1 for 90 min was applied to the FD, Sa, and Sp + R treatments. For the Sa and Sp + R treatments, the rainfall was applied after the surface was either saturated or when seepage flow started. During rainfall simulation runs, runoff samples were collected at 10-min intervals. For the Sp treatment, due to the small quantity of seepage flow, runoff samples were collected at 15-min intervals for 90 min immediately after the seepage flow started. During each run, the rainfall amount was measured at least twice with a hyetometer on the right and left sides of the soil pan.

Immediately after each run, runoff samples were centrifuged (4000 rpm, 15 min) and filtered through 2.5-µm paper (Whatman [Maidstone, UK] no. 5). The filtered solution was stored in a refrigerator and analyzed 24 h later. Sediment samples at the bottom of the centrifuge bottle were washed into cans, and oven-dried at 55°C. This dry sediment was weighted to calculate sediment delivery.

Collection of Soil Samples from the Soil Pans
Soil samples taken before and after each run were analyzed for moisture and nutrient contents. The before-run soil profile samples were taken three days after prewetting under the free drainage. The after-run samples were taken one day after the runs were completed. Samples were taken at five depth intervals: 0 to 2, 2 to 5, 5 to 10, 10 to 15, and 15 to 20 cm. To ensure a proper representation of the profile distribution, soil moisture samples were collected from five different locations and nutrient samples from 12 different locations in each soil pan. Soil moisture samples from a given depth increment in a soil pan were combined in an aluminum can and oven dried at 105°C. Soil samples for determining soil nutrient concentration from a given depth increment in a soil pan were combined, air-dried, sieved (2 mm), and stored for chemical analysis.

Chemical Analysis
Concentrations of NO₃–N in surface runoff water were determined by the cadmium reduction method, and PO₄–P in surface runoff water was measured by the ascorbic acid method (Sparks, 1996). The P in soil was extracted using the Bray–Kurtz P-1 method and determined using the ascorbic acid method. Soil NO₃–N was extracted with KCl solution and measured by the cadmium reduction method (Sparks, 1996).

Calculation of Dissolved Nitrogen and Phosphorus Loss in Surface Runoff Water
For each run, NO₃–N and PO₄–P loss in surface runoff water for the event, L_n (mg), was calculated by the following formula:

where C_i is NO₃–N or PO₄–P concentration (mg L^–1) in surface runoff water at time increment i; R_i is runoff volume (L) at time i; and n is the total number of collected samples.

For each run, the average NO₃–N and PO₄–P concentration in surface runoff water, C_n (mg L^–1), was calculated as:

where L_n is NO₃–N or PO₄–P loss (mg) and TR is total runoff volume (L).

For each hydraulic gradient treatment under the same fertilizer treatment, the average NO₃–N and PO₄–P concentration in surface runoff water, C_m (mg L^–1), was calculated as:

where p is the number of replications in each hydraulic gradient treatment (p = 3 or 4), (L_n)_j is NO₃–N or PO₄–P loss (mg), and (TR)_j is total runoff volume (L).

Determination of Statistical Significance
Within each fertilizer treatment, the LSD test was used to determine whether differences in runoff, sediment, concentrations, and losses of NO₃–N and PO₄–P among the four hydraulic gradients were statistically significant at the 95% confidence level. Similarly, within each hydraulic gradient treatment, the LSD test was performed to determine whether differences in the concentrations and losses of NO₃–N and PO₄–P among the three fertilizer treatments were statistically significant.

	RESULTS AND DISCUSSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS CONCLUSIONS REFERENCES

 
Soil Properties
Soil texture and antecedent soil nutrient content affect soil loss and chemical transport. The soil consists of 58% silt, 20% clay, and 22% sand. The soil contains 1.7% organic matter, 61.3 mg kg^–1 NO₃–N, and 0.8 mg kg^–1 Bray–Kurtz P-1. The pH in water was 7.0, measured with a 1:1 solid to water ratio on a weight basis.

Rainfall, Runoff, and Sediment Delivery
Rainfall ranged from 67.8 to 80.8 mm (Table 2). Variability of 10% from the target intensity is expected for this type of rainfall simulator. Runoff from the Sa treatment was on average 10 mm greater than the applied rainfall. This could have been due to the release of stored soil water by pressure wave advance (Torres, 2002). This physical process may influence the chemical transport process.

Concentrations of Nitrogen and Phosphorus in Surface Runoff Water
The average concentrations of NO₃–N and PO₄–P in surface runoff water from the FD treatments were less than 0.3 mg L^–1 regardless of the fertilizer treatments (Table 3). Except for PO₄–P concentrations under the control treatment, when the near-surface hydraulic gradients shifted from FD to Sa, then to Sp, concentrations of NO₃–N and PO₄–P increased greatly. For the low and high fertilizer treatments, NO₃–N concentrations in surface runoff water from the Sp and Sp + R treatments were statistically greater than those from the Sa and FD treatments. Under the low fertilizer treatment, NO₃–N concentrations in runoff water from LSp and LSp + R averaged 6618 and 897 times greater than those from LFD, and 241 and 33 times greater than those from LSa, respectively. Under the high fertilizer treatment, NO₃–N concentrations in surface runoff water from HSp and HSp + R averaged 11665 and 1509 times greater than those from HFD, and 127 and 16 times greater than those from HSa, respectively. The NO₃–N concentrations from Sp and Sp + R were also significantly different. Due to rain water dilution, NO₃–N concentration from the Sp + R treatment decreased by 87% compared with the Sp treatment under the low and high fertilizer treatments.

Fertilizer application rate greatly influenced NO₃–N and PO₄–P concentrations in surface runoff water. Except for the FD treatment, concentrations of NO₃–N and PO₄–P in surface runoff water from Sa, Sp, and Sp + R under the high fertilizer treatment were statistically greater than those under the low fertilizer treatment (Table 4). Doubling the fertilizer rate increased NO₃–N and PO₄–P concentrations in surface runoff water by 13 and 145%, respectively, under the FD treatment. Under Sa, Sp, and Sp + R treatments, NO₃–N concentrations with the high fertilizer treatment increased 275, 98, and 89% from the low fertilizer treatment. Compared with the low fertilizer treatment, PO₄–P concentration in surface runoff under the high fertilizer treatment was 126, 415, and 181% greater under the Sa, Sp, and Sp + R treatments, respectively.

Average PO₄–P loss from CSp + R was 4.7 times greater than loss from CFD. Under the low fertilizer treatment, average PO₄–P losses from Sp + R and FD were statistically different. The PO₄–P loss from LSp + R was 9.6 times greater than that from LFD. However, PO₄–P losses from LSp and LFD were not statistical different. Under the high fertilizer treatment, average PO₄–P losses from HSa, HSp, and HSp + R were statistically higher than those from HFD. Average PO₄–P losses from HSa, HSp, and HSp + R were 5.6, 4.0, and 10.3 times greater, respectively, compared with HFD. The PO₄–P loss from Sp + R was significantly higher than that from Sp under both low and high fertilizer treatments, with a magnitude of 8.0 and 2.9 times greater, respectively (Table 3).

Under the FD treatment, NO₃–N loss was approximately 0.01% of the applied N. Under the Sp and Sp + R treatments, NO₃–N loss accounted for 11 to 16% of the applied N (Table 3). Seepage flow alone (9.0–13.0 mm) comprised only 9 to 13.5% of the total runoff from the Sp + R treatment, but it appeared to produce most of the NO₃–N loss in surface runoff water. These results show that seepage flow could make an important contribution to total chemical transport.

Temporal Trends of Nitrogen and Phosphorus Concentrations
Under the FD treatment, the NO₃–N concentrations in runoff were greatest in the initial runoff and decreased gradually during the run (Fig. 3) . With seepage flow, a gradual increasing trend of NO₃–N in runoff water was observed as the run progressed. These responses were somewhat expected, because under the free drainage treatment, the downward movement of rain water leached NO₃–N deeper into the soil profile, while under the seepage treatments, NO₃–N in the profile was brought to the surface by the seepage flow. The temporal trend under the saturation treatment was between those from the drainage and seepage treatments. Without the fertilizer application, the control condition showed little change in NO₃–N loss as time progressed for all hydraulic gradients tested. The temporal variation of the PO₄–P concentration during the 90-min run was less pronounced than the NO₃–N variation (Fig. 4) .

View larger version (31K): [in this window] [in a new window]   Fig. 3. The NO3–N concentration in runoff during a 90-min run under different hydraulic gradients. Error bars indicate standard derivation.

View larger version (30K): [in this window] [in a new window]   Fig. 4. The PO4–P concentration in runoff during a 90-min run under different hydraulic gradients. Error bars indicate standard derivation.

Distribution of Soil Nitrogen and Bray–Kurtz Phosphorus in the Soil Profile
Concentrations of soil NO₃–N and Bray–Kurtz P-1 in the soil profile before and after the run provided additional information for explaining different patterns of NO₃–N and PO₄–P losses in runoff water (Table 5). Three days after the 15-mm pre-rain, the highest NO₃–N concentration was found at the 5- to 10-cm layer under the low and high fertilizer treatments, and at the 10- to 15-cm layer under the control treatment.

View this table: [in this window] [in a new window]   Table 5. Means and standard deviations (in parentheses) of soil NO3–N and Bray–Kurtz P-1 distribution in soil profile.

The P movement in the soil profiles under all hydraulic gradients was less pronounced than NO₃–N. Soil Bray–Kurtz P-1 content in the 0- to 2-cm layer under all hydraulic gradients was very high, which could be attributed to fertilizer being applied in the top 2-cm soil layer and little vertical movement of the applied P with rain water. Soil Bray–Kurtz P-1 in the top 2-cm layer after the 90-min run significantly decreased under the LSa, LSp, LSp + R, HSa, HSp, and HSp + R treatments. Under the low fertilizer treatment, soil Bray–Kurtz P-1 content in the top 2-cm layer from LSa, LSp, and LSp + R decreased by 35, 53, and 50%, respectively, compared with the soil Bray–Kurtz P-1 content in the top 2-cm layer before the run. A similar degree of decrease of the soil Bray–Kurtz P-1 content in the surface layer after the 90-min run was observed for HSa, HSp, and HSp + R under the high fertilizer treatment. This decline appears to be a result of greater PO₄–P loss rates in runoff from these treatments except the LSp treatment.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS CONCLUSIONS REFERENCES

 
This paper presents a laboratory study of near-surface hydraulic gradient effects on losses of NO₃–N and PO₄–P in surface runoff water under a simulated rainstorm. The results showed a significant increase of NO₃–N and PO₄–P transport in surface runoff when the downward drainage gradient was removed under the saturation or reversed under the artesian seepage conditions. These results demonstrate the importance of understanding watershed hydrology and its spatial and temporal patterns in predicting areas of high chemical loading potential.

The results of this study also challenge current rainfall simulation methods used in the field to quantify soil erodibility and chemical transport. Field studies are often performed when the fields are dry enough to make the runs, hence under a drainage gradient. Results from prior studies on near-surface hydraulic gradient effects on soil erosion and the current study on NO₃–N and PO₄–P transport show that the saturation and artesian seepage conditions can cause greater soil loss and chemical transport than the drainage condition. An examination of soil loss and chemical transport data at various soil moisture conditions allows us to identify critical conditions that would cause water quality problems at field scale.

Although extensive efforts have been made to determine factors contributing to P and N losses, the specific effects of near-surface hydraulic gradient have not been previously quantified. Incorporating the basic understanding of hydraulic factors may contribute to more effective control measures that can minimize the chemical loading to surface runoff at both hillslope and watershed scales.

	ACKNOWLEDGMENTS

 
The authors would thank the Chinese Academy of Science and National Natural Science Foundation for providing financial support (KZCX3-SW-422, 40335050, 90302001) for research cooperation between the United States and China. The authors would also thank the associate editor, Dr. Gregory McIsaac, and reviewers, whose comments significantly improved the manuscript.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSIONS CONCLUSIONS REFERENCES

 

• Alberts, E.E., and R.G. Spomer. 1985. Dissolved nitrogen and phosphorus in runoff from watersheds in conservation and conventional tillage. J. Soil Water Conserv. 40:153–157.
• Cox, F.R., and S.E. Hendricks. 2000. Soil test phosphorus and clay content effects on runoff water quality. J. Environ. Qual. 29:1582–1586.[Abstract/Free Full Text]
• Daniel, T.C., D.R. Edwards, and A.N. Sharpley. 1993. Effect of extractable soil surface phosphorus on runoff water quality. Trans. ASAE 36:1079–1085.
• Daniel, T.C., A.N. Sharpley, D.R. Edwards, R. Wedepohl, and J.L. Lemunyon. 1994. Minimizing surface water eutrophication from agriculture by phosphorus management. J. Soil Water Conserv. 49:30–38.
• Daniel, T.C., A.N. Sharpley, and J.L. Lemunyon. 1998. Agricultural phosphorus and eutrophication: A symposium overview. J. Environ. Qual. 27:251–257.
• Dunne, T. 1978. Field studies of hillslope flow processes. p. 227–293. In M.J. Kirkby (ed.) Hillslope hydrology. John Wiley & Sons, New York.
• Foster, G.R., F.P. Eppert, and L.D. Meyer. 1979. A programmable rainfall simulator for field plots. p. 45–49. In Agricultural reviews and manuals. ARM-W-10. USDA-ARS Western Region, Oakland, CA.
• Foy, R.H., and P.J.A. Withers. 1995. The contribution of agricultural phosphorus to eutrophication. Proc. Fert. Soc. 365:1–32.
• Gabbard, D.S., C. Huang, L.D. Norton, and G.C. Steinhardt. 1998. Landscape position, surface hydraulic gradients and erosion processes. Earth Surf. Processes Landforms 23:83–93.
• Gburek, W.J., and A.N. Sharpley. 1998. Hydraulic controls phosphorus loss from upland agricultural watersheds. J. Environ. Qual. 27:267–277.[Abstract/Free Full Text]
• Gburek, W.J., A.N. Sharpley, L. Heathwaite, and G.J. Folmar. 2000. Phosphorus management at the watershed scale: A modification of the phosphorus index. J. Environ. Qual. 29:130–144.
• Huang, C., and J.M. Laflen. 1996. Seepage and soil erosion for a clay loam soil. Soil Sci. Soc. Am. J. 60:408–416.[Abstract/Free Full Text]
• Huang, C., L.K. Well, and L.D. Norton. 1999. Sediment transport capacity and erosion processes: Concept and reality. Earth Surf. Processes Landforms 24:503–516.
• Hubbard, R.K., R.A. Leonard, and A.W. Johnson. 1991. Nitrate transport on a sand coastal plain soil underlain by plinthite. Trans. ASAE 34:802–808.
• Hubbard, R.K., and J.M. Sheridan. 1983. Water and nitrate losses from a small upland coastal plain watershed. J. Environ. Qual. 12:291–295.
• Lovejoy, S.B., J.G. Lee, T.O. Randhir, and B.A. Engel. 1997. Research needs for water quality management in the 21st century: A spatial decision support system. J. Soil Water Conserv. 52:19–23.
• Lowrance, R. 1992. Nitrogen outputs from a field-size agricultural watershed. J. Environ. Qual. 21:602–607.[Abstract/Free Full Text]
• Pote, D.H., T.C. Daniel, D.J. Nichols, A.N. Sharpley, P.A. Moore, Jr., D.M. Miller, and D.R. Edwards. 1999. Relationship between phosphorus levels in three Ultisols and phosphorus concentration in runoff. J. Environ. Qual. 28:170–175.[Abstract/Free Full Text]
• Pote, D.H., T.C. Daniel, A.N. Sharpley, P.A. Moore, Jr., D.R. Edwards, and D.J. Nichols. 1996. Relation extractable soil phosphorus to phosphorus losses in runoff. Soil Sci. Soc. Am. J. 60:855–859.[Abstract/Free Full Text]
• Sharpley, A.N., T.C. Daniel, J.L. Sims, and D.H. Pote. 1996. Determining environmentally sound soil phosphorus levels. J. Soil Water Conserv. 51:160–166.
• Sparks, D.L. (ed.) 1996. Methods of soil analysis. Part 3. SSSA Book Ser. 5. SSSA, Madison, WI.
• Torres, R. 2002. A threshold condition for soil–water transport. J. Hydrol. Processes 16:2703–2706.
• Whipkey, R.Z., and M.J. Kirkby. 1978. Flow within the soil. p. 121–144. In M.J. Kirkby (ed.) Hillslope hydrology. John Wiley & Sons, New York.
• Zheng, F., C. Huang, and L.D. Norton. 2000. Vertical hydraulic gradient and run-on water and sediment effects on erosion processes and sediment regimes. Soil Sci. Soc. Am. J. 64:4–11.[Abstract/Free Full Text]

Related articles in JEQ:

This Issue in Journal of Environmental Quality

JEQ 2004 33: 1947-1953. [Full Text]  

This article has been cited by other articles:

				P. J. A. Kleinman, M. S. Srinivasan, C. J. Dell, J. P. Schmidt, A. N. Sharpley, and R. B. Bryant Role of Rainfall Intensity and Hydrology in Nutrient Transport via Surface Runoff. J. Environ. Qual., July 1, 2006; 35(4): 1248 - 1259. [Abstract] [Full Text] [PDF]

				J. W. Cox, J. Varcoe, D. J. Chittleborough, and J. van Leeuwen Using Gypsum to Reduce Phosphorus in Runoff from Subcatchments in South Australia J. Environ. Qual., November 7, 2005; 34(6): 2118 - 2128. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in JEQ Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (9) Citing Articles via Google Scholar Google Scholar Articles by Zheng, F.-L. Articles by Norton, L. D. Search for Related Content PubMed PubMed Citation Articles by Zheng, F.-L. Articles by Norton, L. D. Agricola Articles by Zheng, F.-L. Articles by Norton, L. D. Related Collections Vadose Zone Processes and Chemical Transport Surface Water Quality Surface Hydrology Soil Hydrology Water Pollution