Playas have an unpredictable and rapidly changing wet-dry cycle, which is essential to how they work — and provide benefits including groundwater recharge and wildlife habitat. Since playas depend on rainstorms to fill with water, it is not uncommon for a playa to be dry for long periods, years even, or to be wet several times during a growing season. No matter the amount of water — whether dry, moist, saturated, or flooded — playas are still working. In fact, without this natural cycle, they wouldn’t be as effective.
When dry, the clay soil in the playa basin contracts and forms large cracks. Then, when it rains, the first flush of water from surrounding uplands flows toward the playa. As water moves through the grasses surrounding the playa, sediment that may carry contaminants from farm fields is trapped and stopped from entering the playa. The water continues into the playa and into the cracks — beginning its journey to the underlying aquifer.
Once the clay becomes saturated, the cracks close, allowing the playa to hold water. Some of the water in the playa is used by plants or evaporates. Even when the clay basin is sealed, water continues its journey through the clay along roots and other small channels. When the playa is full, water also flows around the clay, through surrounding porous soil.
After the water is gone and the soil dries, prairie winds scour the playa basin, blowing out some of the sediment that has found its way into the playa. Over time, cracks reappear in the clay soil, and the playa awaits the next big storm when the cycle will repeat.
Plants and animals have adapted to this cycle and respond rapidly when playas become wet. Tiny seeds and eggs lay dormant in the soil, sometimes for several years, waiting for a heavy rainfall to germinate and hatch. The variable nature of playas enhances the plant diversity, which in turn leads to increased wildlife diversity. Birds and other prairie wildlife use wet playas for much-needed rest stops, water and food.
Why Playas Stop Working
Accumulated sediment and playa modifications dramatically affect how a playa functions by reducing the available water surface area, changing the composition of wetland plants, making the playa more susceptible to invasive species, and disrupting aquifer recharge.
Sediment accumulation has a significant effect on playa hydrology, with sediment removal being one of the most commonly needed restoration activities. When there is excess sediment on top of the clay soil, it can act like a sponge, soaking up and holding the water, so it takes larger amounts of rain and runoff to see water in the playa. Excess sediment also reduces the volume of water a playa will hold and the length of time a playa stays wet, which significantly affects the plant and wildlife community supported by the playa. Sediment accumulation in playas can lead to increased exotic plant species, decreased water availability, and diminished habitat for desired wildlife.
There are also a number of modifications that impact the hydrology of a playa and disrupt recharge: pits and ditches that concentrate the water; channels or gullies leading to the playa that deposit sediment in the basin; and diversions, terraces, bar ditches and roads that keep water from reaching the playa.