Date of Award
Master of Science in Natural Resources Science
Natural Resources Science
Riparian zones are lands that occur along stream boundaries. These zones vary widely in their potential to protect stream and estuarine waters from groundwater nitrate contamination. Past research has found that riparian sites with hydric soils (i.e., soils associated with wetlands) possess high groundwater nitrate removal potential, while nonhydric soils appear to have minimal removal rates. Although the presence of hydric riparian corridors is a critical clue to watershed functions, they often occur as narrow bands that are challenging to map. The potential for identifying these high value riparian zones through existing spatial data bases coupled with geographic information systems (GIS) has not been rigorously explored. The objectives of my study were to 1) characterize the landscape attributes and occurrence of hydric soils along riparian corridors of lower (first and second) order streams and 2) investigate the accuracy of SSURGO digital soil maps (1: 15,840) to depict the patterns of soil drainage classes and occurrence of hydric soils along riparian corridors of lower order streams. My study compared SSURGO map data classification to the results of detailed field reconnaissance at 100 randomly selected riparian locations in Rhode Island. Soils in the study region were developed from stratified glacial outwash, unstratified glacial till, organic deposits and recent alluvium.
Each site consisted of three transects extending 30 m orthogonally from the stream into the riparian forest. At each site I documented selected field attributes, including the slope of the riparian zone, hydric soils width and presence of seeps. I also located each site on the SSURGO coverage and recorded the soil map unit classification.
Median hydric soils width of riparian sites for the study area was 11.9 m; however, the width of hydric soils in the study area was highly variable (interquartile range: 28.l m). Stratifying the data by slope class created a marked change in variability. Mean hydric soils width varied significantly at the p ≤ 0.5 level, with slope class. Within the 0-3 % slope class 83 % of sites had hydric soils widths > 15 m while most of the sites in the other slope classes had hydric soils widths < 7 m.
I examined the capacity of SSURGO hydric and geomorphic map categories to explain the variability and measures of central tendency in the field measurements of hydric soils width. The SSURGO soil survey data base appears to recognize the importance of hydric riparian soils, in spite of their narrow nature, and provides map users with valuable information beyond what might be expected based on map scale.
Over 85% of riparian sites classified as organic/alluvium and outwash-hydric had groundtruthed hydric soils widths > 10 m. These sites did not contain groundwater seeps suggesting a high potential for groundwater nitrate removal. In contrast, over 85% of sites classified by SSURGO as nonhydric contained < 5 m of hydric soils, minimizing the capacity of these riparian zones for ground water nitrate removal. Sites classified by SSURGO as till-hydric showed high variability in hydric soils width and over two-thirds of these sites had groundwater seeps. Only 14% of the till-hydric sites were found to contain both an absence of seeps and > 10 m of hydric soils, suggesting a low potential for groundwater nitrate removal.
Adding high resolution field slope data to the SSURGO hydric soils designations increased the overall map accuracy to characterize the occurrence or absence of hydric soils. By combining SSURGO with improved topographic spatial data, we may be able to improve the mapping and identification of riparian zones with high groundwater nitrate removal potential. Even without improved topographic data, SSURGO holds promise for the identification and management of high value riparian zones.
Rosenblatt, Adam E., "Hydric Soil Patterns in Riparian Corridors of the Glaciated Northeast: Groundtruthing the Soil Geographic Data Base (SSURGO)" (2000). Open Access Master's Theses. Paper 2065.