1. Soil Collection and Preparation
2. Packing Soil Boxes
3. Mounting Soil Boxes in the Rainfall Simulator
4. Selecting the Source of Irrigation Water
5. Selecting the Nozzle Size to Use
6. Rainfall Simulator Operation
7. Nozzle Calibration and Rainfall Uniformity
8. Conducting a Rainfall Simulation
One reason for conducting the current experiment was to explore factors that may have contributed to poor results from a previous experiment where urea loss in runoff was being compared across several forms of fertilizers and manures that contained urea. All treatments were applied to soils that had been saturated and allowed to drain to field capacity. Results for five replicates of the urea prill treatment ranged from concentrations of 1-12 mg/L urea-N in runoff. This order of magnitude variation among replicates was unacceptable under controlled conditions and confounded the results of the experiment. A strong positive relationship between total volume of runoff and urea-N concentration in runoff suggested that physical conditions, such as packing or variable antecedent moisture conditions due to different draining and drying conditions, were causative factors.
In order to investigate the cause for such extreme variation in urea concentrations in runoff, all boxes in the current experiment were carefully packed with equal weights of uniformly mixed silt loam soil as depicted in Figures 1 and 2 to minimize variation in physical conditions. To achieve 50, 60, 70, 80, 90, and 100% of approximate field capacity as determined by wetting, then oven drying a small quantity of sieved soil, the weight of water required to wet the soil to corresponding antecedent soil moistures of 14, 17, 19, 22, 25, and 27% was calculated, added to the boxes, and allowed to equilibrate O/N. The rainfall simulation followed the exact protocol described above and depicted in Figures 3-5. The 17 wsq Full Jet 3/8 HH nozzle (Table 1) was used to deliver a rainfall intensity of 3.2 cm/hr over a 40 min period which is equivalent to a natural precipitation event that commonly occurs on an annual basis on the Eastern Shore of the Chesapeake Bay in Maryland.
The resulting total runoff volumes, loads, and flow weighted concentrations are summarized in Table 2. There was a significant positive relationship between total runoff volume and antecedent moisture condition (Figure 6). Wetter soils had less capacity to store water and lower infiltration rates resulting in greater runoff volumes. There was a significant negative relationship between time to runoff and antecedent moisture condition (Figure 7). Water infiltrated into drier soils for a longer period of time before they became wet near the surface, causing runoff to occur. Not surprisingly, there was a positive relationship between total load urea-N in runoff and total runoff volume (Figure 8). It is well known in hydrologic studies that flow volume is usually a strong predictor of total load. How concentration will behave in response to a runoff event is less predictable. Flow weighted concentration was calculated by summing the loads for each 2 min runoff collection and dividing by total runoff volume. It is equivalent to the concentration in a single collection of runoff at the end of the 40 min rainfall period. In this study, there was a significant positive relationship between flow weighted concentration in runoff and antecedent moisture condition (Figure 9). Given the positive linear relationships between runoff volume and antecedent soil moisture and flow weighted concentration and antecedent moisture condition, a significant positive relationship between total load urea-N and antecedent moisture condition was expected. However, this significant relationship was best described by an exponential equation (Figure 10).
In order to visualize urea-N loss in runoff over time, individual 2 min concentrations and cumulative loads in one replicate of a soil box representing each antecedent moisture condition was plotted over the 40 min rainfall time interval (Figure 11). Although concentrations in runoff can vary somewhat erratically over time (e.g. in the case of the 90% moisture), concentrations generally start high and decrease over time. Cumulative loads over time are much smoother functions, and they illustrate the significant relationships previously discussed. Time to runoff is longer, urea-N concentrations in runoff are lower, and cumulative loads are less for drier soils. Although urea hydrolyzes rapidly in soils, when rainfall occurs within hours of surface application, much of the N is still present in urea form and is subject to loss in runoff. Urea is a neutral molecule and is not strongly sorbed to the surfaces of soil particles. As water infiltrates the drier soils during the early part of a rainfall event it carries dissolved urea down into the soil and away from the surficial runoff zone. When runoff does begin, there is less urea present and concentrations in runoff are lower. From a practical sense, urea would almost always be applied under drier conditions as farm equipment could not traverse soils that are at field capacity.
Figure 1. Schematic of packed soil runoff box. A metal box (100 cm x 20 cm x 7.5 cm) with a 5 cm lip on the forward end is packed with soil to a depth of 5 cm. Runoff that spills over the 5 cm lip is collected in an attached gutter that is shielded against rainfall falling directly into the gutter. Nine 5 mm diameter holes allow water that infiltrates the soil to drain from the boxes and prevent ponding. A nipple attached near the forward edge of the bottom of the gutter allows runoff water to drain into funnels and collection bottles positioned below the nipple.
Figure 2. Box packing materials. Approximately 4 layers of cheesecloth in the bottom of the box prevent soil loss but allow water to drain freely. A leveling gauge consisting of acrylic glass sandwiched between two wooden boards is as wide as the box (20 cm) and as deep (2.5 cm) as the difference between the sides of the box (7.5 cm) and the top of the gutter (5 cm). By resting the board on the lip of the box the acrylic glass is used to grade soil to the depth of the gutter.
Figure 3. Positioning the platform. Position the platform so that when the packed soil boxes are in position, they all have the same slope. For this study, the desired slope was 3%. While holding a board level, position the platform so that the down slope, gutter end of the box is 3 cm below the upslope end. The platform should be level in the cross slope direction.
Figure 4. Rainfall simulator controls beginning from the water source and progressing through the plumbing system to the nozzle. (1) Single lever ball valve: This is a quick shutoff valve. Lever in line with pipe is on; lever at 90 degree angle across pipe is off. Use this valve to turn flow on and off without disturbing valves that control pressure and flow rate. Open fully and close fully. Do not try to use this valve to control flow rate. (2) Sediment filter: Check filter periodically and replace element as needed to prevent clogging with sediment. (3) Pressure regulator valve: This valve controls the pressure in the line from this point forward. Too much pressure may break pipes, hoses or connections. (4) In-line flow control valve (Gate valve): This valve is used to fine tune the flow to the nozzle in order to achieve the desired flow rate and nozzle pressure. (5) Flow meter: Measures approximate flow rate. (6) Pressure gauge: Measures approximate pressure at the nozzle.
Figure 5. Boxes positioned on the platform for rainfall simulation. Place 5 or 6 boxes in marked positions for each rainfall simulation event. Avoid positioning a box directly under the nozzle to prevent dripping directly onto a box surface.
Figure 6. Total runoff volume is positively correlated with antecedent soil moisture content (R2 = 0.64).
Figure 7. Time to runoff is negatively correlated with antecedent soil moisture content (R2 = 0.48). The surface of a wet soil saturates quickly. Rainfall that exceeds the hydraulic conductivity of the saturated soil generates runoff.
Figure 8. Total load urea-N is positively correlated with runoff volume (R2 = 0.81). Differences in runoff volume overwhelm differences in concentration of urea-N in runoff.
Figure 9. Flow weighted concentration of urea-N is positively correlated with antecedent soil moisture content (R2 = 0.66). Drier soils allow infiltration that leaches urea-N into the soil and away from the soil surface. When runoff does occur, less urea-N is available at the surface for movement in runoff.
Figure 10. Total load urea-N is positively correlated with antecedent soil moisture content (R2 = 0.74). The positive relationships between total runoff volume and antecedent soil moisture content and between flow weighted concentration of urea-N and antecedent moisture content combine to result in an exponential relationship (y = 0.2043 e0.0405x).
Figure 11. Urea-N concentration and cumulative load relationships over time for one replicate of each antecedent soil moisture content. Although urea-N concentration is not always a smooth function through time, the significant relationships previously discussed can be visualized.
Nozzle Size | Intensity | Optimum Pressure | Flow | 10 sec Flow |
cm/hr | psi | gpm | ml | |
17 wsq Full Jet 3/8 HH | 3.2 | 6.0 | 1.5 | 940 |
24 wsq Full Jet 3/8 HH | 3.3 | 6.0 | 1.8 | 1,140 |
30 w Full Jet 1/2 HH | 6.0 | 5.0 | 2.2 | 1,250 |
50 w Full Jet 1/2 HH | 7.0 | 4.1 | 3.7 | 2,300 |
Table 1. Nozzle size chart. Nozzle sizes that have been identified for use with this rainfall simulator and their associated rainfall intensity, pressure and flow parameters are presented. Selection of nozzle size depends on the desired rainfall intensity. Rainfall intensity and duration correspond to a precipitation event of a certain return period for a specified study location. Nozzle size 17 wsq was used for this study. Rainfall of 40 min duration at an intensity of 3.2 cm/hr is equivalent to a natural precipitation event that commonly occurs on an annual basis on the Eastern Shore of the Chesapeake Bay in Maryland.
Soil moisture | Total runoff | Flow weighted | Total load |
% | volume (L) | concentration | (mg urea-N) |
(mg L-1 urea-N) | |||
27† | 2.96 | 4.99 | 13.66 |
27 | 2.87 | 4.37 | 12.55 |
25 | 2.52 | 3.57 | 8.62 |
25 | 1.81 | 2.33 | 4.21 |
22 | 2.52 | 2.18 | 5.50 |
22 | 2.47 | 1.54 | 3.81 |
19 | 1.99 | 1.72 | 3.41 |
19 | 2.35 | 3.70 | 8.68 |
17 | 1.91 | 1.69 | 3.22 |
17 | 1.66 | 0.90 | 1.50 |
14 | 1.51 | 0.78 | 1.18 |
† Duplicate numbers represent two replications for each moisture level |
Table 2. Antecedent soil moisture content, total runoff volume, flow weighted urea-N concentration and total urea-N load after rainfall simulation. Duplicate numbers represent two replications for each moisture level
Rainfall Simulator | Joern's Inc. | TLALOC 3000 | Size 1.5m x 2.0m (size optional) |
Rainfall Simulator | Joern's Inc. | TLALOC 4000 | Size 2.0m x 2.0m (size optional) |
Rainfall Nozzle | Spraying Systems Inc. | 3/8HH-SS17WSQ | Size 17 nozzle |
Rainfall Nozzle | Spraying Systems Inc. | 3/8HH-SS24WSQ | Size 24 nozzle |
Rainfall Nozzle | Spraying Systems Inc. | 1/2HH-SS30WSQ | Size 30 nozzle |
Rainfall Nozzle | Spraying Systems Inc. | 3/8HH-SS50WSQ | Size 50 nozzle |
Rainfall is a driving force for the transport of environmental contaminants from agricultural soils to surficial water bodies via surface runoff. The objective of this study was to characterize the effects of antecedent soil moisture content on the fate and transport of surface applied commercial urea, a common form of nitrogen (N) fertilizer, following a rainfall event that occurs within 24 hr after fertilizer application. Although urea is assumed to be readily hydrolyzed to ammonium and therefore not often available for transport, recent studies suggest that urea can be transported from agricultural soils to coastal waters where it is implicated in harmful algal blooms. A rainfall simulator was used to apply a consistent rate of uniform rainfall across packed soil boxes that had been prewetted to different soil moisture contents. By controlling rainfall and soil physical characteristics, the effects of antecedent soil moisture on urea loss were isolated. Wetter soils exhibited shorter time from rainfall initiation to runoff initiation, greater total volume of runoff, higher urea concentrations in runoff, and greater mass loadings of urea in runoff. These results also demonstrate the importance of controlling for antecedent soil moisture content in studies designed to isolate other variables, such as soil physical or chemical characteristics, slope, soil cover, management, or rainfall characteristics. Because rainfall simulators are designed to deliver raindrops of similar size and velocity as natural rainfall, studies conducted under a standardized protocol can yield valuable data that, in turn, can be used to develop models for predicting the fate and transport of pollutants in runoff.
Rainfall is a driving force for the transport of environmental contaminants from agricultural soils to surficial water bodies via surface runoff. The objective of this study was to characterize the effects of antecedent soil moisture content on the fate and transport of surface applied commercial urea, a common form of nitrogen (N) fertilizer, following a rainfall event that occurs within 24 hr after fertilizer application. Although urea is assumed to be readily hydrolyzed to ammonium and therefore not often available for transport, recent studies suggest that urea can be transported from agricultural soils to coastal waters where it is implicated in harmful algal blooms. A rainfall simulator was used to apply a consistent rate of uniform rainfall across packed soil boxes that had been prewetted to different soil moisture contents. By controlling rainfall and soil physical characteristics, the effects of antecedent soil moisture on urea loss were isolated. Wetter soils exhibited shorter time from rainfall initiation to runoff initiation, greater total volume of runoff, higher urea concentrations in runoff, and greater mass loadings of urea in runoff. These results also demonstrate the importance of controlling for antecedent soil moisture content in studies designed to isolate other variables, such as soil physical or chemical characteristics, slope, soil cover, management, or rainfall characteristics. Because rainfall simulators are designed to deliver raindrops of similar size and velocity as natural rainfall, studies conducted under a standardized protocol can yield valuable data that, in turn, can be used to develop models for predicting the fate and transport of pollutants in runoff.
Rainfall is a driving force for the transport of environmental contaminants from agricultural soils to surficial water bodies via surface runoff. The objective of this study was to characterize the effects of antecedent soil moisture content on the fate and transport of surface applied commercial urea, a common form of nitrogen (N) fertilizer, following a rainfall event that occurs within 24 hr after fertilizer application. Although urea is assumed to be readily hydrolyzed to ammonium and therefore not often available for transport, recent studies suggest that urea can be transported from agricultural soils to coastal waters where it is implicated in harmful algal blooms. A rainfall simulator was used to apply a consistent rate of uniform rainfall across packed soil boxes that had been prewetted to different soil moisture contents. By controlling rainfall and soil physical characteristics, the effects of antecedent soil moisture on urea loss were isolated. Wetter soils exhibited shorter time from rainfall initiation to runoff initiation, greater total volume of runoff, higher urea concentrations in runoff, and greater mass loadings of urea in runoff. These results also demonstrate the importance of controlling for antecedent soil moisture content in studies designed to isolate other variables, such as soil physical or chemical characteristics, slope, soil cover, management, or rainfall characteristics. Because rainfall simulators are designed to deliver raindrops of similar size and velocity as natural rainfall, studies conducted under a standardized protocol can yield valuable data that, in turn, can be used to develop models for predicting the fate and transport of pollutants in runoff.