STUDY AREA

Playas are hydrologically and geologically unique because both groundwater discharge and surface-water runoff accumulate in their dry lakebeds. Predicting and estimating the extent of flood hazards are difficult for several reasons. Playas are large and very flat, which allow even small amounts of water to spread easily and quickly engulf a large area. In addition, playas are located at the lowest point within a watershed; therefore, precipitation occurring anywhere within the watershed may result in flooding on the dry lakebed. Finally, watersheds feeding these areas are usually large, covering several hundred square kilometers. Thus, unexpected and severe playa flooding can produce an expansive accumulation of water resulting in operational shutdowns and interruptions at airfield facilities.

Rosamond Lake, a playa lying within the western boundaries of EAFB and covering approximately 20sq mi, w as selected as the study site. Precipitation within the Rosamond Lake Watershed, which is located to the west and south of EAFB, supplies runoff to the Rosamond playa.

Industrial areas, taxiways, runways, and roadways associated with the EAFB complex have been located on or near these dry lakebeds. In addition to operational ramifications, EAFB also must address regulations pertaining to flood prevention and control at federal facilities. Scientific, engineering, and economic interests have driven the development of a methodology where frequency, duration, and volume of flooding events can be estimated.

Rosamond playa receives an annual average precipitation of 5.2 in. measured at the EAFB precipitation gaging station located on nearby Rogers playa. Summer (May–September) precipitation does not produce significant amounts of water on the Rosamond playa for several reasons. Summer precipitation events in the Mojave Desert usually result from convective storms producing intense rainfall over small areas, whereas winter precipitation events (October–April) likely consist of frontal systems producing less intense but widespread regional precipitation that lasts longer. In addition, birds are not migrating during summer months; therefore, habitat value or operational hazards associated with water on the lakebed is generally unimportant. Finally, evaporation rates are high in the summer, and water remains in the lakebed only for short periods. Consequently, summer precipitation events were not considered in the study. Since winter precipitation events produce the largest amounts of rainfall making the Rosamond playa more likely to become flooded, winter became the focus period for the study.