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Title: Validation of the ANOCOVA model for regional scale ECa-ECe calibration

Author
item Corwin, Dennis
item LESCH, SCOTT - City Of Riverside

Submitted to: Soil Use and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2016
Publication Date: 5/3/2016
Citation: Corwin, D.L., Lesch, S.M. 2016. Validation of the ANOCOVA model for regional scale ECa-ECe calibration. Soil Use and Management. doi: 10.1111/sum.12262.

Interpretive Summary: Soil salinity is a major agricultural concern in arid and semi-arid agricultural areas throughout the world. The complex spatial variability of soil salinity has stood as a barrier to its measurement and mapping at field scales and larger spatial extents. Even though progress has been made in the area of mapping salinity at regional scales as a result of the combined use of satellite imagery and geophysical techniques, it remains as a challenge since current approaches are complex and technology intensive. Scientists at the USDA-ARS U.S. Salinity Laboratory have developed a simplified methodology for mapping soil salinity at regional scale using a statistical regression technique referred to as an analysis of covariance (ANOCOVA) model to calibrate easily obtained apparent soil electrical conductivity measurements to soil salinity over areas ranging from thousands to hundreds of thousands of acres. An extensive validation of the approach was made using 77 fields within a 30 by 30 km area of California’s Coachella Valley. The ANOCOVA model outperformed other approaches and provided estimates for depth predictions of soil salinity that were well within acceptable limits for mapping. The implication of this evaluation is that once ANOCOVA models for each depth are established for a representative set of fields within a regional, then the slope coefficients can be used at all future fields, thereby significantly reducing the need for ground truth soil samples at future fields, which substantially reduces labor and cost by up to 66% over previous approaches. The methodology has the practical simplicity to allow broad application by government agencies, such as NRCS, with minimal cost and has international implications, particularly for use in countries with limited resources. Land resource managers, producers, agriculture consultants, extension specialists, and Natural Resource Conservation Service field staff are the beneficiaries of regional scale maps of soil salinity.

Technical Abstract: Over the past decade two approaches have emerged as the preferred means for assessing salinity at regional scale: (1) vegetative indices from satellite imagery (e.g., MODIS enhanced vegetative index, NDVI, etc.) and (2) analysis of covariance (ANOCOVA) calibration of apparent soil electrical conductivity (ECa), as measured with electromagnetic induction, to salinity. The later approach is most recent and least extensively verified. It is the objective of this study is to provide extensive verification of the ANOCOVA approach as a means for regional-scale salinity assessment. The verification comprised 77 fields in California’s Coachella Valley, ranging from 1.25 to 30.0 ha in size with an average size of 12.8 ha. The fields were surveyed with mobile electromagnetic induction (EMI) equipment to obtain geospatial measurements of ECa. Using ECa-directed soil sampling protocols, sample sites were selected that characterized the range and spatial variation in ECa across the field. From the data a regional ANOCOVA model was developed. The regional ANOCOVA model successfully reduced cross-validated, average log salinity prediction error (variance) estimate by more than 30% across the 77 fields and improved the depth-averaged prediction accuracy in 58 out of the 77 fields. The results show that the ANOCOVA modeling approach improves soil salinity predictions from EMI signal data in most of the surveys conducted, particularly fields where only a limited number of calibration sampling locations were available. Once ANOCOVA models are established for each depth for a representative set of fields within a regional-scale study area, the slope coefficients can be used at all future fields, thereby significantly reducing the need for ground-truth soil samples at future fields, which substantially reduces labor and cost. Land resource managers, producers, agriculture consultants, extension specialists, and Natural Resource Conservation Service field staff are the beneficiaries of regional-scale maps of soil salinity.