There is a difference in US National Weather Service (NWS) normal precipitation year standard (past 30 years, 30 and 70 percentile)and USDA Soil Taxonomy (Taxonomy) normal precipitation year. The Taxonomy standard requires a normal precipitation year (NWS) and then 8 normal precipitation months in that year (Taxonomy 12th Edition, 2014 pg 29).
Over a period of 15 years (1989-2004) there were three formal USDA-NRCS and university sponsored long term wet soil studies in the Texas Gulf Coast Prairie (Victoria County USDA International Committee on Aquic Soils, 1989-1996; interagency Brazoria County Bottomland Hardwood Vertisols, 1996-2004; Galveston Island, Baylor University MS, 1995-1998). Two of the studies lasted for six to ten years .
During the course of the studies the NWS normal precipitation year was only met for about 2 or 3 years. The Taxonomy requirement of those normal years also having 8 normal months was never met. The typical range of normal precipitation months in a normal year was about 4 to 5 months. Later studies by Texas A&M researchers for peer reviewed papers from one of the studies (Victoria County Vertisol crack study) confirmed our initial field findings
We forwarded this information to the USDA-NRCS National Office in the late 1990's suggesting a change be made in Taxonomy to a standard similar to NWS, or a standard that could easily be determined and met at the field level during formal field studies and soil Taxonomy classification. We were told the Taxonomy standard was based on statistics, that no one else had reported a similar finding from field studies at the time, so nothing was done.
Has anyone else really compared actual soil field study and precipitation data to the Taxonomy normal precipitation year standard in the US, and was the Taxonomy standard ever met?
Wesley Miller
USDA-NRCS Soil Scientist (retired)
Victoria, TX
