Nutrients in agricultural runoff may cause offsite environmental impacts. The objective of this investigation was to examine nutrient transport by overland sheet flow on sites containing swine slurry. Data examined in this study was collected during field rainfall simulation tests conducted on cropland sites in southeast Nebraska, USA. Inflow was added to the top of experimental plots in four successive increments to simulate runoff rates occurring at greater downslope distances. Runoff rates on the experimental sites ranged from 2.3 to 21.2 L min−1 and maximum equivalent downslope distances varied from 5 to 108 m. Phosphorus (P) and nitrogen (N) transport rates were found to increase in a linear fashion with runoff rate. Hypothesis testing using the student’s t-test affirmed the prediction that a linear equation, calibrated for site specific conditions, can be used to relate nutrient transport rates to runoff rates. P and N transport rates were thought to be influenced by (a) the quantity of nutrients released by swine slurry at a particular runoff rate and (b) the amount of overland sheet flow available to transport the released nutrients. If nutrient transport rates can be linked to runoff rates, it may be possible to extrapolate experimental results obtained from small plots to greater downslope distances. Existing process-based models used to route overland sheet flow along hillslopes on upland areas could also be modified to include nutrient constituents.
Nutrients contained in manure can be effectively used for crop production and thus reduce reliance on inorganic fertilizers. The application of manure has also been shown to improve crop yield and soil properties (Du et al., 2020). However, nutrients may accumulate within soils if manure is applied at excessive rates resulting in greater nutrient transport by overland sheet flow (Chen et al., 2018).
Nutrient best management procedures can be used to reduce the potential for offsite environmental impacts following land application (Kamrath & Yuan, 2023). The effectiveness of best management practices can be estimated using phosphorus indices or process-based models (Sharpley et al., 2017; Holloway et al., 2018). The parameters used to calibrate and test phosphorus indices and process-based models are often obtained from rainfall simulation tests conducted on small field plots (McGehee et al., 2024).
Nutrient losses were found to be significantly greater under high intensity rainfall due to larger runoff volumes (Kleinman et al., 2006). It has also been demonstrated that the forms and amounts of phosphorus (P) loss are greatly influenced by flow path length (McDowell & Sharpley, 2002). Increased runoff rates resulting from longer flow path lengths can be simulated by adding inflow to the top of experimental plots (Laflen et al., 1991; Misra et al., 1996; Monke et al., 1977). The results obtained with the addition of inflow are applicable to a much larger range of hydrologic conditions. The hypothesis established in this study is that P and nitrogen (N) transport rates can be related to runoff rates on sites containing swine slurry.
If the interaction between nutrient transport rate and runoff rate can be quantified, it may be possible to estimate the contributions to nutrient transport occurring at discrete locations along a hillslope. Existing analytical procedures used for routing overland sheet flow on upland areas could then be expanded to include nutrient constituents, or the routing of overland sheet flow along hillslopes could at least be considered as a viable alternative to existing approaches. The effectiveness of selected best management practices in reducing nutrient transport by overland sheet flow could also be more accurately quantified. The objective of this investigation was to examine nutrient transport by overland sheet flow on sites containing swine slurry.
2. Materials and methods
2.1. Study site characteristics
Data used in this study were obtained from three rainfall simulation investigations conducted at the University of Nebraska Rogers Memorial Farm, located 18 km east of Lincoln, Nebraska, USA, in Lancaster County following the land application of swine slurry (Table 1). The Aksarben silty clay loam soil on the farm developed in loess deposits under prairie vegetation and is considered a benchmark soil for this region. Manure had not been added to the study areas since at least 1966.
Table 1. Studies where the effects of varying runoff rates on dissolved P (DP), total P (TP), NO3–N, and total N (TN) transport rates were measured on sites containing swine slurry.
Reference
Condition (observation)
Nutrient Content of Slurry (mg kg−1)
Nutrient Transport Rates (g ha−1 min−1)
Runoff Rates (L min−1)
Equivalent Downslope Distance (m)
Transport Equation, Nutrient (g ha−1 min−1) Runoff Rate (L min−1)
Land application of swine slurry at annual corn N requirement (observations 3a and 3b)
Total P – 589
Total N −5490
DP – 10.1–29.8
2.3–12.6
5–22
DP = 2.55 Runoff Rate
0.983
TP – 12.9–35.5
TP = 3.06 Runoff Rate
0.974
NO3–N – 314–1340
NO3–N = 109 Runoff Rate
0.998
TN – 346 – 1460
TN = 119 Runoff Rate
0.998
2.2. Rainfall simulation procedures
The rainfall simulation equipment and experimental protocols adopted by the National Phosphorus Research Project were employed (Sharpley & Kleinman, 2003) (Fig. 1). Rainfall was applied to paired plots at an intensity of approximately 70 mm h−1 for 30 min using a rainfall simulator based on the design by Humphry et al. (2002). Two additional rainfall simulation tests were conducted on the same plots at approximately 24 h intervals. Two rain gauges were placed along the outside perimeter of the plots, and one was placed in the center between the plots. Water used in the study was obtained from an on-site irrigation well. Plot borders channeled runoff into a sheet metal lip that emptied into a collection trough that extended along the bottom of the plots.
Fig. 1. Rainfall simulation equipment and experimental protocols established by the U.S. National Phosphorus Research Project (Sharpley & Kleinman, 2003) were employed. The paired experimental plots shown above are each 0.75 wide x 2.0 m long.
2.3. Addition of inflow
Since the upslope contributing area under field conditions is much larger than that provided by the experimental plots, additional inflow was introduced at the top of the plots to simulate longer slope lengths (Fig. 2). Increased runoff rates resulting from longer slope lengths were simulated using previously established procedures (Laflen et al., 1991; Misra et al., 1996; Monke et al., 1977). The experimental results obtained with the addition of inflow are applicable to a much larger range of rainfall and runoff conditions. When simulated overland flow was introduced during the experimental tests, it was not practical to capture and store all the runoff that was produced. Therefore, nutrient samples were collected under steady-state runoff conditions, and nutrient load values per unit area and unit time were reported (g ha−1 min−1). Water from the same well used for the rainfall simulation experiments was applied at the upgradient end of each plot after the first 30 min of the third rainfall simulation event to examine the influence of varying runoff rates on nutrient transport.
Fig. 2. Paired plots were monitored during the rainfall simulation tests. Inflow was introduced onto the plot surfaces using PVC pipes. Small holes were drilled in the PVC pipes and inflow rates were adjusted using a pressure gauge and gate valve. A carpet was placed below the PVC pipes to prevent scouring and to distribute flow more uniformly across the plot surfaces.
Rainfall continued during the overland flow tests. A mat made of a green synthetic material used as an outdoor carpet was placed beneath the inflow device to prevent scouring and create more uniform flow distribution over the plot. The remainder of the plot surface remained undisturbed. Runoff generated during the inflow tests was transferred to a flume where a stage recorder was mounted to measure flow rate. The quantity of runoff occurring at a particular overland flow increment was only increased once the existing flow rate had achieved a steady runoff value (determined with the flume and stage recorder) and samples had been collected for nutrient analyses. Each inflow increment had a duration of approximately 8 min, which was the period required for steady-state flow conditions to become established and runoff samples for nutrient analyses to be collected.
2.4. Observations
The effects of slurry application method, swine growth stage, and runoff rate on runoff nutrient transport were examined by Gilley, Bartelt-Hunt, Lamb, et al. (2013). Swine slurry was obtained from production units containing grower pigs, finisher pigs, or sows and gilts. The swine slurry was applied using broadcast, disk, or injection methods at an N application rate of 151 kg ha−1, the amount required to meet annual N requirements for corn. The dissolved phosphorus (DP) yield of 0.20 kg ha−1 obtained on the broadcast treatment was significantly greater than the 0.11 and 0.08 kg ha−1 measured on the disk and injected treatments, respectively. The DP yield of 0.17 kg ha−1 measured for the sows and gilts treatment was significantly greater than the 0.11 kg ha−1 obtained for the finisher treatment.
Transport rates for DP, total phosphorus (TP), NO3–N, and total nitrogen (TN) increased from 8.4 to 40.1, 101 to 659, 420 to 2470, and 470–2850 g ha−1 min−1, respectively, as runoff rate increased from 3.2 to 21.2 L min−1. A mean overland flow rate of 0.82 L min−1 was measured without the addition of simulated overland flow on these 2 m long plots. The largest mean runoff rate was 21.2 L min−1 or approximately 26 times the value without the addition of inflow which corresponds with an equivalent downslope distance of approximately 52 m. Both the P (Fig. 3(a)) (Observation 1a) and N (Fig. 3(b)) (Observation 1b) transport rates increased in a linear fashion with runoff rate.
Gilley, Bartelt-Hunt, Li, et al. (2013) determined the effectiveness of a narrow grass hedge in reducing runoff nutrient loads following swine slurry application. Manure was applied at N application rates of 0, 151, 302, and 453 kg ha−1. The grass hedge did not significantly reduce runoff nutrient transport rates following application of swine slurry. Increasing the N application rate from 151 to 453 kg ha−1 also did not result in a significant increase in N or P yields.
The rates of transport of DP, TP, NO3–N and TN increased from 7 to 25, 72 to 382, 273 to 1204, and 323–1490 g ha−1 min−1, respectively, as runoff rate increased from 3.6 to 19.7 L min−1. A mean overland flow rate of 0.70 L min−1 was measured without the addition of inflow on these 4 m long plots. The largest mean overland flow rate was 19.7 L min−1 or approximately 27 times the value without the addition of inflow which corresponds with an equivalent downslope distance of approximately 108 m. The P (Fig. 4(a)) (Observation 2a) and N (Fig. 4(b)) (Observation 2b) transport rates also increased in a linear fashion with runoff rate.
The effects of swine slurry application method, time following slurry application, and runoff rate on selected water quality characteristics were examined by Schuster et al. (2017). Slurry from a commercial swine operation was broadcast or injected on field plots at a N application rate of 151 kg ha−1. Rainfall simulation tests were conducted at five periods following slurry application. The DP and TP yields of 0.35 and 0.46 kg ha−1 measured for the broadcast treatment were significantly greater than the 0.13 and 0.19 kg ha−1 obtained for the injection treatment. As time following slurry application increased from 1 to 43 days, DP and TP loads decreased from 0.35 to 0.14 and from 0.52 to 0.18 kg ha−1, respectively.
Transport rates for DP and TP increased from 10.1 to 29.8 and from 12.9 to 35.5 g ha−1 min−1, respectively, as overland flow rate increased from 2.3 to 12.6 L min−1. A mean overland flow rate of 1.1 L min−1 was measured without the addition of simulated overland flow on these 2 m long plots. The largest mean overland flow rate was 12.6 L min−1 or approximately 11.2 times the value without the addition of inflow which corresponds with an equivalent downslope distance of approximately 22 m. The P (Fig. 5(a)) (Observation 3a) and N (Fig. 5(b)) (Observation 3b) transport rates again increased in a linear fashion with runoff rate.
Fig. 5. Transport rate for (a) DP and TP and (b) NO3–N and TN versus runoff rate from study performed by Schuster et al. (2017).
2.5. Statistical analysis
Linear regression analysis was first used to relate P and N transport rates (g ha−1 min−1) to runoff rates (L min−1) on selected sites (Gilley, Bartelt-Hunt, Lamb, et al., 2013, and 2013b; Schuster et al., 2017) (Fig. 3(a), 3(b), 4(a), 4(b), 5(a), and 5(b)). The coefficient of determination was then employed to identify the goodness-of-fit of the linear regression models (Table 1). Linear regression equations were also derived relating P and N transport rates to runoff rates on selected plots on which varying amounts of slurry had been applied (Gilley, Bartelt-Hunt, Li, et al., 2013) (Fig. 6, Fig. 8) (Eqs. (1), (2), (3), (4)). Linear regression analysis was also utilized to compare predicted nutrient transport rates obtained from the regression relationships with measured values (Fig. 7(a), 7(b), 9(a), and 9(b)). Finally, the students-t test was used to evaluate the hypothesis that the regression coefficient of the predicted versus measured nutrient transport values equals one at the 95% confidence level (Table 2).
Fig. 6. DP and TP transport rate versus runoff rate for plots 401, 501, 503, 504, 601, and 603 of study performed by Gilley, Bartelt-Hunt, Li, et al. (2013).
Fig. 7. Predicted versus measured (a) DP and (b) TP transport rates for plots 402, 403, 404, 502, 602, and 604 of study performed by Gilley, Bartelt-Hunt, Li, et al. (2013).
Fig. 8. NO3–N and TN transport rate versus runoff rate for plots 401, 403, 501, 504, 601, and 603 of study performed by Gilley, Bartelt-Hunt, Li, et al. (2013).
Table 2. Statistical analyses of predicted versus measured dissolved P (DP), total P (TP), NO3–N, and total N (TN) transport rates (g ha−1 min−1) determined from data collected by Gilley, Bartelt-Hunt, Li, et al. (2013). (F, F ratio; β1, slope of the regression line; β0, intercept of the regression line).
Two predictions were made based on the hypothesis that nutrient transport rates can be related to runoff rates on sites containing swine slurry. Prediction 1 (Observations 1a, 2a, and 3a) is that DP and TP transport rates can be related to runoff rates on sites containing swine slurry. Prediction 2 (Observations 1b, 2b, and 3b) is that NO3–N and TN transport rates can also be related to runoff rates on sites containing swine slurry.
3.2. Testing of predictions
Prediction 1 was tested using data collected in the previously described study reported by Gilley, Bartelt-Hunt, Li, et al. (2013). Linear regression equations were first derived relating P transport rates to runoff rates on selected sites on which varying amounts of slurry had been applied (plots 401, 501, 503, 504, 601, & 603) (Fig. 6). These sites represent the range of slurry application rates applied to plots that did not contain a grass hedge. The regression equations relating DP and TP transport rates to runoff rates, which had R2 values of 0.990 and 0.992, respectively, were:(1)DP = 1.44 Runoff Rate(2)TP = 9.82 Runoff Rate
These regression equations were next used to relate predicted DP and TP transport rates obtained at varying runoff rates to measured DP and TP transport rates for the sites not used for calibration (plots 402, 403, 404, 502, 602, & 604) (Table 2) (Fig. 7 (a) and 7 (b)).
The student’s t-test test was then employed to evaluate the hypotheses that the regression coefficients equal one at the 95% confidence level. The regression coefficients were found to be not significantly different from one (Table 2). Therefore, analyses of the experimental data collected by Gilley, Bartelt-Hunt, Li, et al. (2013) suggest that DP and TP transport rates can be related to runoff rates on sites where slurry have been applied, and, thus, prediction 1 is affirmed.
Data obtained from the study conducted by Gilley, Bartelt-Hunt, Li, et al. (2013) was also used to test prediction 2. Linear regression equations were first derived relating N transport rates (g ha−1 min−1) to runoff rates (L min−1) on selected sites (plots 401, 403, 501, 504, 601, & 603). The regression equations relating NO3–N and TN transport rates to runoff rates, which each had R2 values of 0.999 (Fig. 8), were:(3)NO3–N = 54.5 Runoff Rate(4)TN = 61.3 Runoff Rate
These regression equations were next used to compare predicted N transport rates to measured values for the sites not used in the calibration (plots 402, 404, 502, 503, 603, and 604) (Fig. 9(a) and 9(b)).
Fig. 9. Predicted versus measured (a) NO3–N and (b) TN transport rates for plots 402, 404, 502, 503, 603, and 604 of study performed by Gilley, Bartelt-Hunt, Li, et al. (2013).
The student’s t-test was again employed to evaluate the hypotheses that the regression coefficients equal one at the 95% confidence level. The regression coefficients were found to be not significantly different from one (Table 2). Analyses of the experimental data collected by Gilley, Bartelt-Hunt, Li, et al. (2013) also suggests that NO3–N and TN transport rates can be related to runoff rates on sites where slurry had been applied, and thus prediction 2 is affirmed.
4. Discussion
Limitations have been reported in the use of small-scale rainfall simulators to estimate DP losses from hillslopes (Hollaway et al., 2018; Nash et al., 2021). It can be difficult to find larger plots that have consistent longitudinal and cross-sectional grades that do not encourage concentrated flow along the borders (Verbree et al., 2010). The introduction of inflow to the top of small experimental plots to simulate flow rates occurring at greater downslope distances could minimize some of the limitations inherent in the used of small-scale rainfall simulators (Laflen et al., 1991; Misra et al., 1996; Monke et al., 1977).
McDowell & Sharpley (2002) investigated the effects of flow path length (1–10 m long) on P transport by overland flow with and without dairy manure application on 1 m wide plots within an agricultural watershed in Pennsylvania, USA. Simulated rainfall was applied for a 1-h duration at an intensity of 7 cm h−1. The loads of dissolved reactive P and TP on site 1 increased in a linear fashion from 0.001 to 0.046 g and 0.056–0.677 g, respectively. P transport rates were also found to increase in a linear fashion with runoff rate (plot length) in the present investigation.
Gilley et al. (2007) examined temporal changes in nutrient transport following the application of swine slurry to a cropland site. Significant reductions in the transport of nutrients were measured during the year following slurry addition with the smallest concentrations usually occurring on the final sampling date. Temporal effects following slurry application must be accurately parametrized to properly estimate annual nutrient delivery from land application areas.
Slurry generated in swine production facilities typically contains relatively small amounts of solid material and large quantities of water. Relatively large quantities of dissolved nutrients and small easily transported solid materials are present within the slurry. Thus, a large quantity of readily available nutrient constituents is present following slurry application. Nutrient delivery was found to increase in a linear fashion with runoff rate in the present investigation. The amount of overland flow available to transport nutrients may have served as a constraint in this system. P and N transport rates were thought to be influenced by (a) the quantity of nutrients released by swine slurry at a particular runoff rate and (b) the amount of overland sheet flow available to transport the released nutrients.
It is possible that at flow rates much larger than those examined in the present investigation, the quantity of nutrients released to overland flow reaches an upper limit (point of inflection), and nutrient transport rates become constant as runoff rates continue to increase. The constraint in this system becomes the maximum rate of release of N and P constituents to overland flow. Nutrient transport rates would be expected to eventually decrease as the source of nutrients is depleted. Additional field tests are needed to identify and quantify nutrient transport mechanisms occurring over time at greater downslope distances and flow rates.
In contrast to swine slurry, manure generated in many animal production facilities including those used for beef cattle and poultry contains relatively small amounts of water and much larger quantities of solid materials. The quantity of nutrients readily available for transport by overland sheet flow would be expected to be much smaller following land application of primarily solid materials. The constraint in this system could be the quantity of nutrients released, not the amount of overland sheet flow available to transport the released nutrients. Again, additional field tests will be needed to identify and quantify the mechanisms influencing nutrient transport on land application sites containing primarily solid manures.
Hydraulics, hydrology, geomorphology, and land management exert interacting controls on P fluxes (Sharpley et al., 2013). Subsurface hydrologic pathways including interflow were not addressed in the present investigation. Therefore, the observations which were made, hypotheses that were developed, and predictions that were advanced are only applicable for overland sheet flow conditions.
Extraction coefficients have been used to relate soil P to DP in runoff (Vadas et al., 2005). The Surface Phosphorus and Runoff Model (SurPhos) addresses the direct transfer of P from manure to runoff during a rainfall event (Vadas et al., 2007, 2017). It is possible that algorithms used to quantify extraction coefficients, such as those incorporated into SurPhos, could be useful in estimating the amount of DP released into overland flow on upland areas.
If generalized nutrient transport rate – runoff rate relationships are shown to be accurate, it may be possible to extrapolate the experimental results obtained from previous rainfall simulation experiments conducted on small plots to larger slope lengths. The database generated from other rainfall simulation studies, like those performed as part of the U.S. National Phosphorus Research Project (Sharpley & Kleinman, 2003), could then be utilized to predict the effects of varying soil, cropping, and management conditions on nutrient transport by overland flow.
5. Conclusions
Data examined in this study was obtained from previous field rainfall simulation tests where inflow was added to the top of the experimental plots in successive increments to simulate runoff rates occurring at greater downslope distances. P and N transport rates were found to increase in a linear fashion with runoff rate in each of the previous studies. Linear regression equations, calibrated for site specific conditions, were first used to relate nutrient transport rates to runoff rates. Statistical analyses of predicted versus measured nutrient transport values indicated that P and N transport rates can be related to runoff rates on sites containing swine slurry.
Nutrient transport rates were thought to be influenced by (a) the quantity of N and P released by swine slurry at a particular runoff rate and (b) the amount of overland sheet flow available to transport the released nutrients. It may be possible to extrapolate experimental results obtained from small plots to greater downslope distances by linking nutrient transport rates to runoff rates. Existing physical process-based models used to route overland sheet flow along hillslopes could then be modified to include nutrient constituents.
CRediT authorship contribution statement
John E. Gilley: Writing – original draft, Formal analysis, Conceptualization.
Declaration of competing interest
John E. Gilley reports that financial support was provided by USDA – Agricultural Research who also happen to be their employer.