Impact of ENSO on Groundwater Recharge in the Southwestern U.S.

Bridget Scanlon, principal investigator

The impact of climate variability on the water cycle in desert ecosystems is controlled by biospheric feedback at interannual to millennial timescales. This paper describes a unique field dataset from weighing lysimeters beneath nonvegetated and vegetated systems that unequivocally demonstrates the role of vegetation dynamics in controlling water cycle response to interannual climate variability related to El Nino southern oscillation in the Mojave Desert. Extreme El Nino winter precipitation (2.3–2.5 times normal) typical of the southwestern U.S. would be expected to increase groundwater recharge, which is critical for water resources in semiarid and arid regions. However, lysimeter data indicate that rapid increases in vegetation productivity in response to elevated winter precipitation reduced soil water storage to half of that in a nonvegetated lysimeter, thereby precluding deep drainage below the root zone that would otherwise result in groundwater recharge.

Vegetation dynamics have been controlling the water cycle in interdrainage desert areas throughout the southwestern U.S., maintaining dry soil conditions and upward soil water flow since the last glacial period (10,000–15,000 yr ago), as shown by soil water chloride accumulations. Although measurements are specific to the southwestern U.S., correlations between satellite-based vegetation productivity and elevated precipitation related to El Nino southern oscillation indicate this model may be applicable to desert basins globally. Understanding the two-way coupling between vegetation dynamics and the water cycle is critical for predicting how climate variability influences hydrology and water resources in water-limited landscapes.

Measured soil water storage to a depth of 2m in both vegetated and nonvegetated lysimeters and daily precipitation depths at the NTS location. Above-ground biomass productivity (g/m2) is shown for measurements in May 1995 and May 1996 for annual and perennial vegetation at the nearby Yucca Mountain site and May 1998 and May 1999 for annual vegetation at the nearby free air CO2 experiment site. (Inset) Relationship between the multivariate ENSO index (blue shading; El Nino, red shading; La Nina) and percent of normal 1971–2000 winter precipitation (normal, 46 mm, December–February; columns) at the NTS location.

Scanlon, B. R., D. G. Levitt, K. E. Keese, R. C. Reedy, and M. J. Sully. 2005. Ecological controls on water-cycle response to climate variability in deserts. Proceedings of the National Acadamy of Sciences of the United States of America 102:6033–6038.

March 2006