Step 11. Repeat Steps 5 through 10 for two 

 other transects. After completing the three tran- 

 sects, proceed to Step 12. 



Step 12. Calculate a mean prevalence index for 

 the three transects. To be considered wetland, a 

 hydric soil area usually must have a mean preva- 

 lence index (PIm) of less than 3.0. A minimum of 

 three transects are required in each delineated area 

 of hydric soil, but enough transects are required so 

 that the standard error for PIj^ does not exceed 



0.20 percent. 



Compute the mean prevalence index for the three 

 transects by using the following formula: 



N 



where 



PIjvI = mean prevalence index for transects; 



PI7 = sum of prevalence index values for all 



transects; 

 N = total number of transects. 



After computing the mean prevalence index for the 

 three transects, proceed to Step 13. 



Step 13. Calculate the standard deviation (s)for 

 the prevalence index using the following formula: 



(PIi-PIm)2 + (Pl2-PlM)^ + (Pl3-PlM)^ 



s = 



N-1 



{Note: See formulas in Steps 8 and 10 for symbol 

 definitions.) 



After performing this calculation, proceed to Step 

 14. 



Step 14. Calculate the standard error (sx) of the 

 mean prevalence index using the following 

 formula: 



sx = 



where 



s = standard deviation for the Prevalence Index 

 N = total number of transects 



(Note: The sx cannot exceed 0.20. If sx exceeds 

 0.20, one or more additional transects are required. 

 Repeat Steps 6 through 14, as necessary, for each 

 additional transect.) When sx for all transects does 

 not exceed 0.20, proceed to Step 15. 



Step 15. Record final mean prevalence index 

 value for each hydric soil map unit and make a wet- 

 land determination. All areas having a mean preva- 

 lence index of less than 3.0 meet the hydrophytic 

 vegetation criterion (see p. 5). One should also 

 look for evidence or field indicators-of wetland 

 hydrology, especially if there is some question as 

 to whether the wetland hydrology criterion is met. 

 If such evidence or indicators are present or the 

 area's hydrology has not been disturbed, then the 

 area is considered a wetland. If the area has been 

 hydrologically disturbed, one must determine 

 whether the area is effectively drained before mak- 

 ing a wedand determination (see disturbed area dis- 

 cussion, p. 50). If the area is effectively drained, it 

 is considered nonwetiand; if it is not, the wetland 

 hydrology criterion is met and the area is consid- 

 ered a wedand. 



Areas where the prevalence index value is greater 

 than or equal to 3.0 (especially greater than 3.5) are 

 usually not wetiands, but can, on occasion, be wet- 

 lands. These exceptions are disturbed or problem 

 area wetiands (see discussion on pp. 50-59) and 

 funher evaluation of wetiand hydrology must be 

 undenaken. When the prevalence index falls 

 between 3.0 and 3.5 (inclusive) in the absence of 

 significant hydrologic modification, the area is pre- 

 sumed to meet the wetland hydrology criterion and 

 is, therefore, wetiand; the plant community is con- 

 sidered hydrophytic vegetation since the plants are 

 growing in an undrained hydric soil. If the preva- 

 lence index of the plant community is greater than 

 3.5, stronger evidence of wetland hydrology is 

 required to make a wetiand determination. Walk 

 through the area of concern and look for field indi- 

 cators of wetland hydrology. If field observations, 

 aerial photographs or other reliable sources provide 

 direct evidence of inundation or soil saturation 

 within 6, 12, or 18 inches depending on soil 

 permeability and drainage class for one week or 

 more during the growing season, or if oxidized 



49 



