Robert Lichvar

PONDING DURATION, PONDING FREQUENCY, AND FIELD INDICATORS: A CASE STUDY ON THREE CALIFORNIA, USA, PLAYAS

Playas are temporarily ponded, shallow, intermittent lakes found in the arid southwest United States formed by rainfall and runoff in the wet season. Because playas are considered “Waters of the United States” (WoUS) under Federal Regulations 33 CFR 328.3 [a], their jurisdictional extent is delineated by either observation of ponded water or physical characteristics that represent ordinary high water marks that remain after the water has receded. To date, no surface hydrologic studies have described the frequency and duration of ponded water within desert playa systems to facilitate the understanding of “ordinary” for delineation purposes. To establish a baseline to support further the concept of “ordinary,” we used 20 years of historical satellite imagery of playas in the western Mojave Desert, California, in combination with 59 years of precipitation records, to provide a case study of frequency and duration of ponding in playas in the arid southwestern United States. Ponding was found to occur for at least 16 days, with a frequency of 51% or approximately every other year. We estimated the average antecedent precipitation needed to initiate ponding to be 8.29 cm. In years when rainfall exceeded 8.29 cm between October and March, the playas also ponded for 16 days into the growing season (March–November). The total length of the ponding period through the wet season ranged between 1 and 32 weeks, with a predictable relationship between length of inundation and total rainfall during the wet season. This range of occurrence of ponded water on these arid playas expresses the climatically unevenly distributed precipitation pattern, both spatial and temporally. Analysis of the ponding duration and frequency from this study site acts as a baseline for further refinement of the concept of ordinary high water and provides a basis for further development of field indicators for delineation purposes of arid southwestern playas.

Surficial features associated with ponded water on playas of the arid southwestern United States: Indicators for delineating regulated areas under the Clean Water Act

Desert playas can be unambiguously identified in a geological context. However, identifying those portions of desert playas that are defined as either three-parameter wetlands or Waters of the United States (WoUS) in the Clean Water Act (CWA), and thus under the jurisdiction of Federal agencies charged with enforcing the CWA, is sometimes problematic. Although the WoUS definition specifically includes playas, the guidance for playa delineation is not as highly developed as that for wetlands. Delineating WoUS on desert playas involves determining the Ordinary High Water Mark. Field experience has demonstrated that the indicators for Ordinary High Water on desert playas have not been fully identified nor have they been associated with ponding that represents the limits of Ordinary High Water. This report discusses the distribution of indicators above, below, and at the Ordinary High Water Mark. Fifteen playa features are identified for possible delineation use and are rated for reliability and their relationship to the Ordinary High Water position.

Relationships among wetland indicators in Hawaiian rain forest

We applied established methods for wetland identification in lowland and montane wet forests (rain forests) on the island of Hawaii to determine whether rain forests exhibited wetland indicators specified in delineation manuals and to examine relationships among indicators of hydrophytic vegetation, hydric soils, and wetland hydrology. Morphological characteristics and ferrous iron tests indicated pockets of hydric organic soils within areas mapped as Folists. Hydrophytic vegetation decisions based on prevalence values agreed with hydric soil determinations more often than did decisions based on dominant plant species. None of the rain forest types we studied exhibited wetland indicators throughout, but some sites contained scattered small wetlands occupying microtopographic lows created by cracks, folds, and undulating flow patterns in the lava bedrock. Further work is needed to identify reliable wetland indicators that can be used during drier portions of the year and to distinguish hydric from nonhydric organic rain forest soils.

Disagreements between plot-based prevalence indices and dominance ratios in evaluations of wetland vegetation

Methods for wetland identification and delineation require the investigator to determine whether vegetation is hydrophytic. Two widely used techniques for making hydrophytic vegetation decisions involve dominance ratios (i.e., the percentage of dominant species that are rated obligate (OBL), facultative wetland (FACW), and facultative (FAC) and prevalence indices (i.e., the weighfed-average wetland indicator status of all plants present). We sampled 338 vegetation plots on sites throughout the United States and calculated the dominance ratio and a plot-based prevalence index for each plot. We found that hydrophytic vegetation decisions based on the two methods disagreed on 16% of field plots. Analysis of simulated plot data (n=80,000) indicated that frequencies of disagreement increase as vegetation complexity (i.e., number of strata and number of species per stratum) increases. We conclude that the two methods for hydrophytic vegetation decisions disagree too often to be considered equivalent. Additional studies are needed in differnent biogeographic regions and plant community types to determine the conditions under which prevalence indices, dominance ratios, or some other treatment of vegetation data provide more reliable indicators of wetland vegetation.

Evaluation of surface features for delineating the ordinary high water boundary on playas in the arid Western United States

Delineation of Ordinary High Water (OHW) under the Clean Water Act (CWA) is based on the use of physical features that represent “ordinary” levels of ponding or flowing waters. On arid western United States playas, where the climate is an unevenly distributed series of precipitation events that are spread over many years, the use of surface water monitoring can be limiting due to occasional years with almost no hydrological information. To substitute for a general lack of monitored surface water conditions, we used processed satellite images and precise topographic modeling to determine ponded water areas. To test the reliability of select field indicators for delineation purposes, we used a two-phase field test on a hard playa in the Mojave Desert, California. First, we verified that ponded water was associated with these playa surface features. Then, to test the statistical reliability of these surface features, we developed a decile ponding zone map by stacking processed satellite imagery, collected detailed laser altimetry (LiDAR) elevation data, sampled surface features occurring in the various decile zones, and developed reliability statements for these OHW delineation features. These field indicators represent surface features that have developed over a series of years representing the wetter portion of the El Niño climatic cycle.

Discrepancies in Hydrophytic Determinations Produced by Three Vegetation Formulas Used for Wetland Delineations

We examined disagreement among three methods used in the USA to make hydrophytic vegetation determinations during wetland delineations: the Dominance Ratio (DR), the Prevalence Index (PI), and the FAC-neutral Dominance Ratio (FN-DR). We had two objectives: to determine whether the number of dominant species in a plot affects the percentage of hydrophytic vegetation determinations made by each of the three methods and, if so, to explain the mathematical origin of disagreements among the methods. We compared the percentage of hydrophytic vegetation determinations produced when each method was applied to 200,000 simulations. The PI was the most consistent method for making hydrophytic vegetation determinations. We found that the DR is biased toward nonhydrophytic vegetation when there is an even number of dominant plant species in a plot; it is biased toward hydrophytic vegetation when there is an odd number of dominants. As the number of dominant species and strata increased, there were >20% more hydrophytic determinations made when we used DR than when we used PI. The FN-DR was also biased; it consistently produced fewer hydrophytic determinations than the other methods. When the DR disagrees with hydric soil and hydrology indicators, delineators should re-examine vegetation using the plot-based PI approach.

The Effect of a Low-Cover Stratum—Woody Vines—on Vegetation Determinations Made During Wetland Delineations

We examined the effect of a low-cover stratum—woody vines—on 1) the outcome of vegetation determinations made using the Prevalence Index (PI) and the Dominance Ratio (DR), and 2) agreement between vegetation and soils during wetland delineations in the United States. Different vine abundance measures—stem counts vs. percent cover—had no effect on the percentage of hydrophytic vegetation determinations made by either formula. Artificial increases and decreases to the woody vine stratum’s minimum cover threshold of 5.0% also had no effect. However, in plots that contained borderline hydrophytic/nonhydrophytic vegetation, the percentage of hydrophytic vegetation determinations made by the DR decreased significantly when vine indicator status was artificially increased (p = 0.048). The PI produced significantly fewer hydrophytic determinations in plots with nonhydric soils than in plots with hydric soils (p < 0.001). The DR produced large percentages (81.8–100%) of hydrophytic determinations, regardless of soil type. Plots in which the DR and the PI differed had many commonalities, including nonhydric soils, nonhydrophytic PI/hydrophytic DR values, borderline hydrophytic vegetation, and an odd number of dominant species. During wetland delineations, the PI should be used in plant communities with low-cover strata, high species richness, or a high frequency of hydrophytes.