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Groundwater level declines at the Oasis of Mara, Joshua Tree National Park, 1940-2015
NPS/MOJN/NRR—2016/1242
National Park Service
By:
Goeff J. M. Moret (National Park Service), Sarah Wright (National Park Service), Jennifer L. Bailard (National Park Service), Luke Sabala (National Park Service)


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Abstract:
The Oasis of Mara in Joshua Tree National Park (JOTR) is one of approximately 160 fan palm oases in North America. The oasis supports a diverse ecosystem, and is of great cultural significance. A paved interpretive trail rings the oasis, making it one of the most highly-visited sites in the park. The Oasis of Mara is located in the Eastern Subbasin of the Joshua Tree Groundwater Basin, immediately to the south of Pinto Mountain Fault Zone. The fault impedes flow, forcing water to the surface. Historically, the oasis was a flowing spring with pools of water. However, the pools dried up in the 1940s due to declining groundwater levels. These declines reduced the extent of riparian vegetation in and around the oasis, and made the recruitment of new fan palms impossible. A 1974 USGS study (Swain 1974) found that “the principal cause of the decline probably is pumpage that exceeded natural recharge to the aquifer”. There was very little groundwater pumping in the Eastern Subbasin between 1960 and 1990. Water levels stabilized during these three decades, but they did not recover to their previous levels. The Twentynine Palms Water District (TPWD) greatly increased its groundwater withdrawals from the Eastern Subbasin beginning in 1991. This increased pumping corresponded to a steep decline in groundwater levels in the subbasin. Currently, the groundwater level at the Oasis of Mara is more than 30 feet below ground surface in a monitoring well located 100 feet to the south of the oasis, with approximately half of that decline occurring since 1990. Fan palm seeds require wet soil at the ground surface to germinate, and the maximum depth that fan palm roots can reach to access groundwater is approximately 13 feet. Therefore, declining groundwater levels threaten the survival of the fan palm oasis. JOTR has responded to the groundwater declines by irrigating the plants in the oasis. Given the magnitude of the groundwater declines the site, and the fact that groundwater levels did not rebound when pumping was greatly reduced for 30 years, it is likely that continued irrigation of the oasis is the best option for the survival of the palms and other riparian vegetation.