Saturated hydraulic conductivity (K...) is an essential parameter for understanding soil hydrology. This study evaluated the K... of in situ monoliths and intact cores and compared the results with other studies for Missouri claypan soils. These K... values were used as runoff. model inputs to assess the impact of K... variation on simulated runoff. Lateral in situ K... of the topsoil was determined on 250 by 500 by 230 mm deep monoliths. These values were compared with the K.., of 76 by 76 mm diam. intact cores with and without bentonite to seal macropores. Mean

The influence of dead roots on soil erodibility, splash detachment, and aggregate stability was studied in the laboratory using a rainfall simulator on a Mexico silt loam (fine, montmorillnitic, mesic, Udollic Ochraqualf). Soil was collected from four cropping treatments including alfalfa, Canada bluegrass, corn, and soybeans. Rainfall of 64 mm h-l intensity was applied for 1 h during the first day. On the second day, a 30-min run of constant intensity (64 mm h-l) was appliedwhichwasfollowedbyfour l5-min stormsat intensitiesof 25, 100,50, and 75mm h-l. Dead rootmassand dead root length in the 0-to 0.l5-m depth from the perennial crops (alfalfa and bluegrass) were much higher than those from annual row crops (corn and soybean). There was almost afive-fold difference in root mass and root length between alfalfa and soybeans. The study showed that dead roots did not affect runoff, but had significant effect (p <0.05) on soil loss and sediment concentrations. However, the differences in soil loss and sediment concentrations were small relative to the differences in dead root mass and dead root length. lnterrill erodibility (K) decreased as dead root mass and dead root length increased. There were exponential relationships between Ki and dead root mass, and Ki and dead root length. Dead roots had significant effects (p <0.05) on soil shear strength, aggregate index, and dispersion ratio. Soil shear strength and aggregate index from alfalfa and Canada bluegrass were approximately 20 and 50%, respectively, higher than those from corn and soybean. Dispersion ratios from alfalfa and bluegrass were about 30% lower than those from corn and soybean. There was no significant difference (p <0.05) in soil splash among the crops. Splash detachment was highest during the initiall 0 min of the simulation and then decreased exponentially.

Starting in 1971, stream flow and climatologic data have been collected in the Goodwater Creek Experimental Watershed, which is part of the Central Mississippi River Basin (CMRB) Long-Term Agroecosystem Research (LTAR) site. Since 1992, water quality and socio-economic data have complemented these data sets. Previous modeling efforts highlighted the challenges created by the presence of a claypan. Specific changes were introduced in the Soil and Water Assessment Tool (SWAT) (i) to better simulate percolation through and saturation above the claypan and (ii) to simulate the spatial and temporal distributions of the timing of field operations throughout the watershed. Our objectives were to document the changes introduced into the code, demonstrate that these changes improved simulation results, describe the model's parameterization, calibration, and validation, and assess atrazine [6-chloro-N-ethyl-N¢-(1-methylethyl)-1,3,5-triazine-2,4-diamine] management practices in the hydrologic context of claypan soils. Model calibration was achieved for 1993 to 2010 at a daily time step for flow and at a monthly time step for water quality constituents. The new percolation routines ensured correct balance between surface runoff and groundwater. The temporal heterogeneity of atrazine application ensured the correct frequency of daily atrazine loads. Atrazine incorporation by field cultivation resulted in a 17% simulated reduction in atrazine load without a significant increase in sediment yields. Reduced atrazine rates produced proportional reductions in simulated atrazine transport. The model can be used to estimate the impact of other drivers, e.g., changing aspects of climate, land use, cropping systems, tillage, or management practices, in this context.

No abstract

No abstract