The abilities of the Richards and convection-dispersion equations approach (LEACHNR) and the capacity model approach (LEACHNA) of the nitrogen version (LEACHN) of the LEACHM model to simulate nitrate leaching were evaluated using field data from a 5-year nitrate leaching experiment conducted in central Pennsylvania on Hagerstown silt loam soil (fine, mixed, mesic, Typic Hapludalf). Nitrate leaching losses from N-fertilized and manured corn below the 1.2-m depth were measured with zero-tension pan lysimeters. Three N-fertilized and manured treatments for 1988-1989, 1989-1990, and 1990-1991 and two N-fertilized treatments for 1991-1992 and 1992-1993 were used from the leaching experiment to evaluate both approaches of LEACHN. The individual monthly simulations of nitrate leaching were compared with the mean of pan efficiency corrected-measured data fro these 5 years. Both approaches of the model were calibrated to the site conditions using the data of 1989-1990 and were then evaluated using 1988-1989, 1990-1991, 1991-1992 and 1992-1993 nitrate leaching data. Simulated results for the calibration year for both models were reasonably accurate (31 of 36 months simulated within the experimental 95% confidence limits). The statistical analysis used in this study indicted that both LEACHNA and LEACHNR adequately (91 to 120 months within the 95% confidence limits) predicted nitrate leaching below the 1.2-m depth for treatments in the refinement years. Much of the simulation error in some treatments in the refinement years seemed to be related to the sub-routine controlling soil nitrogen transformation processes and their rate constants in the model. The large deviations in NO3-N leached in some winter months may be related, in part, to problems with simulated water flow associated with the frozen soil conditions and snow accumulation. The addition of a dual-pore water flow option (LEACHNA) to the nitrogen version of LEACHM did not improve prediction of nitrate leaching beyond the rooting zone of corn under Pennsylvania conditions.
I reviewed the literature on soil penetration measurement to resolve the question, “How does one compare measurements using various types of soil penetrometers?” Topics covered in the review included the theory of penetraton of a rigid probe into soil and the associated experimental validation studies; experimental studies on the effect of cone angle, rate of penetrometer movement, penetrometer size, overburden pressure, and shaft friction; the effect of structured soil; interpretations for root growth; and other considerations. The theory of penetration of a rigid probe into soil involves the calculation of the pressure required to expand cylindrical and spherical cavities in the soil. Although this theory is extremely helpful, it does not completely answer the question posed. Instead, a combination of experimental studies was used to develop a standard interpretation procedure that will answer the question posed. The standard corrects an actual penetrometer measurement for shaft friction, measurement depth, sample size and confinement, penetrometer diameter, penetration rate, and cone angle. Error inherent in the various steps of the standard procedure ranged from 0 to 92%, which emphasizes the urgency of adopting a standard penetrometer design. Cone angle and the avoidance of confinement, nonconfinement, and depth effects were most important. Penetration rate and penetrometer diameter effects were of lesser importance. It is recommended that the standard be used with suitable caution to compare existing penetrometer data taken with various types of penetrometers.
Nearly one-third of the soils in Pennsylvania contain fragipans. On-site sewage disposal systems on these soils historically have been failure prone. This leads to a public health hazard due to contamination of surface and groundwaters with unrenovated effluent. In order to gain a better understanding of water flow in a soil that contains a fragipan, a field study was conducted to determine in situ, two-dimensional, steady-state water flow patterns from a surface line source. Tensiometers were used to determine the soil water pressure head distribution for five application rates ranging from 125 to 625 mm d-1 in 125 mm d-1 increments. Field results indicated that steady-state groundwater mounds (mound-shaped regions of saturation) developed for all application rates because of the reduced permeability of the fragipan and the presence of small lateral hydraulic gradients on either side of the saturated region. Field results were simulated with a finite difference solution of the steady-state Richards' equation. Simulated results agreed well with field measurements within the region of saturation for the three highest application rates. However, the model underpredicted the height of the mounds for the two lowest application rates. The application rate both experimentally and theoretically seemed to have little influence on the hydraulic head beyond 2 m laterally in either direction from the source.
The borehole permeameter method for measuring field-saturated hydraulic conductivity, Kfs, is prone to variability due to soil smearing during borehole excavation, and swelling that results from differences in initial soil-water content. The effects of borehole preparation techniques and initial soil-water pressure in Kfs were analyzed in this study. Eighty boreholes were studied in a strongly structured clayey soil (a fine, mixed, mesic Typic Hapludalf). A version of the borehole permeameter method was used to measure Kfs in soil that was initially at soil-water pressures of 0, -0.050 and -0.080 MPa. Three different borehole excavation techniques were also studied to determine the effect on Kfs of soil smearing during borehole preparation. The treatments consisted of removing a uniform thickness of soil from the sides of the borehole with an ice pick, scarifying the sides of the borehole with a wire brush, and no treatment. Field-saturated hydraulic conductivity for the brushed treatments was similar to Kfs values obtained when no treatment was used. The ice pick method resulted in significantly higher Kfs. When Kfs values measured using the ice pick method were analyzed, no significant effect of initial soil-water pressure was evident. This could possibly be due to soil variability.
This document was last modified on January 2, 2008.