Examining Discrepancies Among Three Methods Used to Make Hydrophytic Vegetation Determinations for Wetland Delineation Purposes

Three methods for making vegetation determinations the Prevalence In-dex PI, the Dominance Ratio DR, and the Hydrophytic Cover Index HCI were compared using national wetland delineation data. The PI and the DR produced significantly fewer hydrophytic determinations 69, p 0.01, and 76, p 0.04 than the HCI 80. The three methods disagreed 16 of the time. The HCI produced hydrophytic determinations only in plots where hydrophyte Facultative, Facultative Wetland, and Obligate species cover was greater than 50 of the total cover and never produced hydrophytic determinations in plots where hydrophyte cover was 50 or less. The PI disagreed with the HCI 12 of the time, producing nonhydrophytic determinations in plots where hydrophyte cover was greater than 50. The DR disagreed with the HCI 9 of the time. Disagreements included nonhydrophytic determinations in plots where hydrophyte cover was greater than 50, hydrophytic determinations in plots where hydrophyte cover was 50 or less, and a nonhydrophytic bias in plots dominated by even numbers of plant species. These results demonstrate that HCI determinations are more accurate and consistent than those of the PI and the DR. The HCI method is recommended for making vegetation determinations during wetland delineations in future revisions of the Corps delineation manual and its supplements.