2019 journal article

Determining Normal Precipitation Ranges for Hydric Soil Assessments


co-author countries: United States of America 🇺🇸
Source: Web Of Science
Added: June 17, 2019

Core Ideas Normal rainfall ranges are best defined by the 30th and 70th percentiles of historic data. Mean ± SD produces a normal rainfall range twice as large as that of percentiles. Mean ± SD normal rainfall will cause some upland soils to be classified as hydric soils. Water table data collected for hydric soil and wetland identification studies require supporting analysis of rainfall normality. Water table measurements made after periods when precipitation is within a normal range are believed to represent long‐term trends, whereas data collected following periods of abnormally high precipitation represent rare events, potentially resulting in erroneous hydric soil determinations. The USDA‐NRCS currently uses two different methods to assess normal precipitation ranges; both have been used to assess hydric soils. This study compared methodologies that identify normal precipitation periods by using: (i) the range defined by the 30th and 70th percentiles observed within a 30‐yr period [i.e., the Climate Analysis for Wetlands Tables (WETS) method] and (ii) long‐term monthly mean precipitation ± one SD (i.e., the U.S. Soil Taxonomy method). Comparisons were made for 30 geographically diverse locations and soil moisture regimes. The results demonstrated that the U.S. Soil Taxonomy method yielded normal precipitation ranges approximately twice as large as those from the WETS method. As a result, the U.S. Soil Taxonomy method precluded the occurrence of drier than normal conditions in many instances and displayed increased sensitivity to infrequent high rainfall events. Three case studies evaluated the implications of method selection on hydric soil identification, demonstrating that the U.S. Soil Taxonomy method identified normal conditions more frequently than the WETS method. As a result, the adoption of the WETS method, which accounts for the non‐normal distribution of precipitation data, as the sole method to determine normal precipitation periods for hydric soil assessment is recommended.