Bridget Scanlon of the University of Texas at Austin, has been selected as the 2007 Birdsall-Dreiss Distinguished Lecturer, sponsored by the GSA Hydrogeology Division. At the request of institutions, she will present one of two lectures for audiences interested in broad aspects of water resources.

About Bridget Scanlon

Bridget Scanlon received a B.S. in Geology at Trinity College, Dublin (Ireland), an M.S. at the University of Alabama, and a Ph.D. from the University of Kentucky (Lexington). She is currently a Senior Research Scientist at the Bureau of Economic Geology, the Jackson School of Geosciences. The primary objective of her research group is to assess sustainability issues with respect to water resources, within the context of climate variability and land-use change. Studies integrate physical, chemical, and isotopic analyses and numerical modeling. Much of her research focuses on groundwater recharge in semiarid regions in natural and cultivated ecosystems. Bridget Scanlon has taught Vadose Zone Hydrology at the Dept. of Geological Sciences and Civil Engineering at UT. She participated in focus groups on global recharge issues within the IAEA. She served on NAS committees on radioactive waste disposal and is currently serving on the Integrated Observations on Hydrologic Sciences committee.

Talk Topics

Implications of Climate Variability for Groundwater Resources and Waste Disposal in Semiarid Regions – A Look at Ecological Controls from Annual to Millennial Timescales

Understanding impacts of climate variability on groundwater recharge is essential for management of water resources and waste disposal. Water scarcity is a critical issue in semiarid regions and potential contaminant transport by recharge to groundwater is also important because of waste disposal. A key question is how do climate variability and related vegetation dynamics impact groundwater recharge.

This talk will explore the role of vegetation dynamics in regulating the impact of climate variability on groundwater recharge. Results from a unique field data set from weighing lysimeters (large, soil-filled concrete containers) beneath nonvegetated and vegetated systems in the Mojave Desert, Nevada unequivocally show that vegetation dynamics controls the impact of elevated winter precipitation related to El Nino Southern Oscillation (ENSO) on groundwater recharge. The lysimeter data indicate that rapid increases in vegetation productivity in response to 2.5 times normal winter precipitation reduced soil water storage to half of that in the nonvegetated lysimeter; thereby precluding deep drainage below the root zone that would otherwise result in groundwater recharge. Satellite vegetation data provided regionalization of the “point scale” lysimeter results. Unsaturated zone chloride and pressure data at sites across the southwestern U.S. indicate that similar feedbacks have minimized inter-stream basin-floor recharge since the last glacial period, 10,000–15,000 years ago. Strong correlations between satellite vegetation productivity and interannual precipitation variability related to ENSO in deserts in Australia, South America, and Africa indicate that the processes described in the southwestern U.S. may apply to deserts globally. The two-way coupling between the water cycle and vegetation dynamics is critical in controlling how climate variability influences water resources, with important implications for waste disposal in semi-arid regions.

References
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 Academy of Sciences of the United States of America 102:6033-6038. [PDF]

Scanlon, B. R., K. Keese, R. C. Reedy, J. Simunek, and B. J. Andraski. 2003. Variations in flow and transport in thick desert vadose zones in response to paleoclimatic forcing (0-90 kyr): field measurements, modeling, and uncertainties. Water Resources Research 39:1179; doi:1110.1029/2002WR001604. [PDF]

Impacts of Changing Land Use on Subsurface Water Resources in Semiarid Regions

The most widespread changes in land use have occurred because of agricultural expansion. In the past 300 years, cultivated cropland and pastureland have increased globally by 560% and 660%, respectively. Irrigated agriculture has expanded by 580% since 1900 and is projected to increase by 20% by 2030 in developing countries Agricultural food production accounts for ~85% of global fresh water consumption, led by irrigated agriculture. What impacts have these land-use changes had on water resources?

Measurements of pressure head, soil pore water chemistry, groundwater levels, and groundwater quality provide an archive of system response to past land-use changes. The presentation will focus on the Texas Southern High Plains, which is one of the largest agricultural areas in the United States. Cultivation of natural grasslands has changed the system from discharging through evapotranspiration since Pleistocene times (~10,000 to 15,000 yr) to recharging during the past 50 to 100 yr. Recharge under rain-fed agriculture is shown by large groundwater-level rises (average 7 m over 3,400 km2 area of rain-fed agriculture) during the last few decades, resulting in a median recharge rate of 21 mm/yr (5% of precipitation). Changes from discharge to recharge conditions reflect long fallow periods (~7 months/yr) associated with cultivation. Recharge under irrigated agriculture is shown by downward hydraulic head gradients. Large groundwater-level declines (as much as 75 m) under irrigated areas indicate that irrigated agriculture is not sustainable. Results from land-use changes in this region will be compared with those from other regions globally. Although past land-use changes had unintended impacts on the water cycle, a comprehensive understanding of these impacts could be used to alter land-use practices for better management of water resources.

References
Scanlon, B. R., R. C. Reedy, and J. A. Tachovsky. in press. Semiarid unsaturated zone chloride profiles: archives of past land-use change impacts on water resources. Water Resources Research. [PDF]

Scanlon, B. R., I. M. Jolly, M. Sophocleous, and L. Zhang. 2007. Global impacts of conversions from natural to agricultural ecosystems on water resources: Quantity versus quality. Water Resour. Res., 43, W03437, doi:10.1029/2006WR005486. [PDF]

Scanlon, B. R., R. C. Reedy, D. A. Stonestrom, D. E. Prudic, and K. F. Dennehy. 2005. Impact of land use and land cover change on groundwater recharge and quality in the southwestern USA. Global Change Biology 11:1577-1593. [PDF]

Dr. Bridget Scanlon, Senior Research Scientist
Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin
University Station, Box X, Austin, Texas 78713-8924
E-mail: bridget.scanlon@beg.utexas.edu; Telephone: (512) 471-8241