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.
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Vol. 24 • No. 2