Surface Detention on Cropland, Rangeland, and Conservation Reserve Program Areas

One of the factors contributing to overland flow on upland areas is water stored temporarily in a thin sheet on the soil surface as surface detention. This study was conducted to quantify surface detention on selected cropland, rangeland, and Conservation Reserve Program (CRP) sites. Surface detention was determined from the recession portion of runoff hydrographs corresponding with the period when rainfall had ceased but runoff continued. The hydrographs were generated from six previously reported rainfall simulation studies conducted on paired 3.7 m wide 10.7 m long plots on which approximately 128 mm of rainfall was applied. Surface detention values were found to increase as crop residue or vegetative cover increased. Eleven fallow cropland sites in the eastern U.S. had surface detention values that varied from 1.7 to 4.6 mm. Surface detention on plots in southwestern Oklahoma containing Old World bluestem, no-till wheat, and conservation-till wheat was 9.4, 7.3, and 5.2 mm, respectively. No-till sorghum, tilled sorghum, no-till wheat, and tilled wheat plots in southeast Nebraska had surface detention values of 6.7, 4.5, 6.7, and 4.6 mm, respectively. Mean surface detention on no-till and tilled cropland sites in southwest Iowa containing corn residue was 7.2 and 5.9 mm, respectively. CRP study sites in southwestern Iowa had mean surface detention of 10.8 mm. When data from the six field studies were combined, mean surface detention values for fallow cropland, tilled cropland, no-till cropland, rangeland, and CRP areas were 3.1, 5.0, 6.9, 9.6, and 10.8 mm, respectively

Artificial Neural Network estimation of soil erosion and nutrient concentrations in runoff from land application areas

The transport of sediment and nutrients from land application areas is an environmental concern. New methods are needed for estimating soil and nutrient concentrations of runoff from cropland areas on which manure is applied. Artificial Neural Networks (ANNs) trained with a backpropagation (BP) algorithm were used to estimate soil erosion, dissolved P (DP) and NH4–N concentrations of runoff from a land application site near Lincoln, Nebraska, USA. Simulation results from ANN-derived models showed that the amount of soil eroded is positively correlated with rainfall and runoff. In addition, concentrations of DP and NH4–N in overland flow were related to measurements of runoff, EC and pH. Coefficient of determination values (R2) relating predicted versus measured estimates of soil erosion, DP, and NH4–N were 0.62, 0.72 and 0.92, respectively. The ANN models derived from measurements of runoff, electrical conductivity (EC) and pH provided reliable estimates of DP and NH4–N concentrations in runoff

Darcy-Weisbach Roughness Coefficients for Selected Crops

Total hydraulic resistance on an upland agricultural site may be influenced by several factors including standing vegetation. In this laboratory study, Darcy-Weisbach roughness coefficients were measured for corn, cotton, sorghum, soybeans, sunflower, and wheat vegetation. Experimental variables used in this investigation in addition to crop type included plant population, row spacing, row orientation, and flow rate. For some of the experimental tests, a single row of vegetation was oriented within a flume parallel to the principal flow direction. For the remainder of the tests, rows of vegetation were placed perpendicular to the flow using row spacings and plant populations recommended by crop management specialists. Measurements of discharge rate and flow velocity were used to calculate roughness coefficients for Reynolds number values ranging from approximately 530 to 22,000. Regression equations which relate roughness coefficients to plant population, row spacing, and Reynolds number were developed from the laboratory data. With the exception of wheat placed perpendicular to flow, roughness coefficients produced by standing vegetation were negligible. On upland agricultural areas, total hydraulic roughness will be influenced primarily by frictional drag over the soil surface, and residue and ground cover

Chemical Tracing Techniques for Evaluating Rill Hydraulics

Development of water erοsiοn and surface water quality control practices requires information concerning the hydraulic characteristics of upland areas. The relatively small flow rates normally found within rills make measurement of hydraulic parameters difficult. Chemical tracing procedures, originally developed for stream and river systems, have been successfully used to measure rill flow properties. A chemical tracer of known concentration is added to the rill and by knowing the degree of dilutiοn at a downstream sampling point, flow rate can be calculated. Rill flow velocity can be measured by determining the time required for a slug of tracer material to travel a designated distance. Measurements of flow rate and velοcity can be used tο calculate οther hydraulic variables. The ability to understand and properly mοdel rill flοw will improve as additional information [is gathered]

Accumulation and Release of Nutrients by Immersed Stalks Collected on Selected Dates Following Harvest

The concentrations of phosphorus (P) and nitrogen (N) in runoff from cropland areas may be influenced by accumulation and release of P and N by stalk residues. A laboratory study was conducted to measure the effects of time since harvest and immersion period on accumulation and release of P and N by corn, soybean, and wheat stalks. Experimental variables included type of stalk material (corn, soybean, and wheat), time since harvest (six residue collection dates over an approximate 1-year period), and stalk immersion period (25 s (0.42 min), 250 s (4.2 min), 2500 s (42 min), 25,000 s (6.9 h), and 86,400 s (24 h)). The initial concentration of each of the P and N constituents in a test solution was 6 μg mL−1. The soybean, wheat, and corn residue released PO4-P at mean rates of 40, 69, and 141 μg g−1 residue, respectively. The amount of PO4-P that was released consistently increased as immersion period became greater. Corn and wheat residue either accumulated or released NO3-N depending on residue collection date. Soybean residue accumulated an average of 20 μg NO3-N g−1 residue. Wheat residue obtained on five of the collection dates accumulated an average of 13 μgNO3-N g−1 residue. Residue collection date also influenced accumulation of NH4-N by soybean and wheat residue. Corn residue released an average of 77 μg NH4-N g−1 residue. The type of crop residue material, the amount of time the residue has remained in the field following harvest, and residue immersion period were found to influence nutrient concentrations of solution

Hydraulic Roughness Coefficients as Affected by Random Roughness

Random roughness parameters are used to characterize surface microrelief. In this study, random roughness was determined following six selected tillage operations. Random roughness measurements agreed closely with values reported in the literature. Surface runoff on upland areas is analyzed using hydraulic roughness coefficients. Darcy-Weisbach and Manning hydraulic roughness coefficients were identified in this investigation on each soil surface where random roughness values were determined. Hydraulic roughness coefficients were obtained from measurements of discharge rat巳and flow velocity. The experimental data were used to derive regression relationships which related Darcy-Weisbach and Manning hydraulic roughness coefficients to random roughness and Reynolds number. Random roughness values available in the literature can be substituted into the regression equations to estimate hydraulic roughness coefficients for a wide range of tillage implements. The accurate prediction of hydraulic roughness coefficients will improve our ability to understand and properly model upland flow hydraulics

Size Distribution of Sediment as Affected by Surface Residue and Slope Length

Runoff samples for determination of size distribution of sediment were collected under simulated rainfall conditions at selected downslope distances on plots covered with sorghum and soybean residue at rates ranging from 0.00 to 6.73 t/ha . The effects of surface residue and slope length on size distribution of sediment were evaluated. Substantial movement of sediment in the form of aggregates was found for each of the residue treatments. Significant differences in size distribution of sediment occurred between residue treatments. For a given residue rate, differences in sediment size distribution were found between sorghum and soybean residue. Size distribution of sediment was also determined to be significantly different at selected downslope distances

Runoff and Erosion as Affected by Sorghum and Soybean Residue

A rainfall simulator was used to measure the effects of varying rates of sorghum and soybean residue on runoff and erosion. In general, increased surface cover caused reduced runoff, sediment concentration and soil loss. Substantial reductions in erosion resulted from the use of small amounts of crop residue. Regression equations were obtained which related surface cover to residue mass. Equations describing relative runoff, sediment concentration and soil loss as a function of surface cover were also developed. Runoff, sediment concentration and soil loss were all found to be highly correlated to surface cover

Hydraulic Conditions Required to Move Unanchored Residue Materials

Hydraulic conditions required to initiate movement of unanchored residue materials are identified in the present study. Selected amounts of corn, cotton, pine needles, sorghum, soybean, sunflower, and wheat residue are placed in a flume on a sand surface, and flow is then introduced at the top of the flume in progressive increments. The discharge rate and flow velocity necessary to cause residue movement are determined. The ratio of critical flow depth to residue diameter, critical Reynolds number, critical shear stress, dimensionless shear stress, and boundary Reynolds number are calculated from hydraulic measurements. Regression equations are developed to relate dimensionless shear stress to boundary Reynolds number and residue diameter. Boundary Reynolds number, in turn, is related to residue diameter and cover. Close agreement is found between predicted and actual parameter values obtained from the regression relations. The regression equations can be used to estimate the beginning of motion for other residue materials if residue diameter and cover are known

Hydraulic Characteristics of Rills

Rill density and rill flow rates were determined during rainfall simulation tests conducted at 11 sites located throughout the eastern United States. A mean rill density of 1.0 rills/m was found for the study locations. From measurements of the relative distribution of flow rates, a procedure is identified for partitioning flow between individual rills. Regression equations were developed for relating rill width and hydraulic roughness coefficients to flow rate. Equations were also derived for predicting mean flow velocity from visually determined measurements of advance velocity. Information reported in this study can be used to estimate hydraulic characteristics of rills