2013 Annual Report
1a.Objectives (from AD-416):
Test and validate three targeting tools proposed by NRCS to identify and prioritize land most in need of conservation treatment. These three tools are the Soil Vulnerability Index (SVI), the Conservation CEAP Benefits Index (CCBI), and field office APEX. We will verify whether the information provided by the tools is consistent with our observations of land degradation in GCEW, edge-of-field monitoring, and our own APEX and SWAT modeling results.
1b.Approach (from AD-416):
The project will first assess the correctness of the information that went into the derivation of the proposed indices and the parameterization of field office APEX given our knowledge of the area. We will then compare land classification using the proposed indices to that provided by our own indices on one hand and SWAT results at the hydrological response unit (HRU) level on the other hand. Finally, we will evaluate the parameterization of field office APEX for 3 fields and 30 1-acre plots that represent various tillage strategies for row crop systems in GCEW, and compare it with our own parameterization that resulted from full calibration/validation using measured flow and water quality data at one field. This comparison of parameterization may result in improvements in both the field office APEX and our own parameterizations.
The research described below directly supports Objective 3 (Improve watershed models for targeting conservation practices on the landscape and to better assess the aggregate impact of field- and watershed-scale management practices on surface water quality) of the parent 211 project.
The Soil Vulnerability Index (SVI) layer was obtained for Goodwater Creek Experimental Watershed (GCEW). The models Interactive_Agricultural Policy/Environmental eXtender (APEX) and the latest version of APEX0806 were installed and run for the points relevant to GCEW. Information about the Soil Vulnerability Index proposed by NRCS was obtained through direct contact with NRCS personnel. Initial comparison of APEX0806and APEX0604 showed considerable differences in the output for identical input files. Further examination of the input files, showed that there were more differences in the inputs than initially thought because of additional parameters. Detailed reading of the different user’s manuals and codes resulted in a set of documents on how to parameterize APEX0806 to make it equivalent to the Conservation Effects Assessment Project (CEAP) APEX parameterization.
Simultaneously to these efforts, the SVI was compared to the Conductivity-Claypan Index (CCI), an index developed locally based on APEX results for a GCEW field. A methodology was developed to compare spatial maps of index values, which is described in a proceeding paper presented at the July 2013 American Society of Agricultural and Biological Engineers (ASABE) meeting in Kansas City. The methodology includes contingency tables, evaluating the identified vulnerable areas using Google Earth images and our knowledge of the land, and comparing the range of input parameters for areas classified with each index in each vulnerability class. In order to refine the targeting power of SVI, we recalculated SVI using a slope value calculated from a Digital Elevation Model (DEM) instead of the representative value found in Soil Survey Geographic Database (SSURGO). SVI and CCI were re-calculated and compared at increasing resolutions for GCEW: SSURGO scale, field scale, and DEM grid cell scale. Between these scales, the critical parameter that impacts both CCI and SVI is the slope: at SSURGO scale, we used the representative value of the soil type at SSURGO scale; at the cell scale, we calculated a value for each DEM cell; and at the field scale, we averaged these individual cell values over the field area. We found that:
• CCI identified 2 to 3 times more highly vulnerable areas than SVI.
• CCI identified areas with very low vulnerability but SVI did not.
• Some areas we know are highly vulnerable were classified as highly vulnerable by CCI, but only as moderately high by SVI.
• The percentage of the watershed classified as highly vulnerable by both indices decreased by half when resolution increased from the SSURGO to field scale, but then tripled when resolution increased from field to DEM cell scale.
Work is continuing for larger watersheds than GCEW to confirm these results.
Chan, R., C. Baffaut, A.L. Thompson, E.J. Sadler. 2013. Evaluating the Soil Vulnerability Index and Conductivity Claypan Index for claypan soils in Missouri. Paper # 1620723 for presentation at the 2013 ASABE Annual International Meeting, Kansas City, Missouri, July 2013.