Heterogeneity Preserving Inversion Methods for Vadose Zone Flow Modeling

Schaap, Marcel

Heterogeneity Preserving Inversion Methods for Vadose Zone Flow Modeling

Friday, November 8, 2013: 3:45 p.m.

Marcel Schaap , University of Arizona, Tucsun, AZ

Yonggen Zhang , University of Arizona, Tucson, AZ

Natural recharge to groundwater in semi-arid regions relies on unsaturated flow through an often deep vadose zone. Understanding and modeling such flow requires intimate knowledge of surface-atmosphere interactions (which includes complex patterns caused by seasonally active vegetation) but also the subsurface heterogeneity of hydraulic properties. It is expensive and labor intensive to measure such properties, especially for the extensive and deep vadose zones which are prevalent in the western United States and other densely populated semi-arid regions in the world. Pedotransfer functions (PTFs) offer a cheap means to estimate hydraulic properties from soil or sediment texture, but suffer from inaccuracies that would likely bias modeled deep vadose zone flow.

In pursuit of the question “How much subsurface heterogeneity must be accounted for in vadose zone flow modeling?” we quantify the effectiveness of several methods for generating “fields” of subsurface hydraulic properties as needed for 1D to 3D numerical simulations. The main line of the presentation will deal with a detailed study carried out at Maricopa, Arizona. The study site is a 50×50 meter and 15 meter deep vadose zone at which a 28-day constant-rate infiltration experiment was conducted in 2001. Moisture content at this site was measured with neutron thermalization at 400 locations daily during the infiltration period, and at irregular intervals 100 and 200 days prior to and after infiltration, respectively. Our research shows that direct simulations based on PTF-estimated heterogeneous fields of hydraulic properties poorly represent the measured infiltration plume. However, several types of model inversions using different generalizations of heterogeneity present in the PTF estimates yield acceptable results and offer a potential means to limit the collection of site-specific data. In addition to the Maricopa site, we will also briefly outline surface soil-climate-deep vadose zone interactions for managed systems (a hypothetical golf course) and rangelands for long (10-25 year) time series. Much of this work is still ongoing, but presently these simulations indicate that the type of surface soil and the amount of winter rain strongly affects the amount of deep vadose zone infiltration. Conversely, surface soils as well as the amount of summer rain (the North American monsoon) have very limited effect on long-term nonriparian infiltration.

Marcel Schaap, University of Arizona, Tucsun, AZ

Marcel G. Schaap is currently an assistant professor in environmental physics at the University of Arizona’s Department of Soil, Water and Environmental Science. Previously he has worked as an assistant/associate professional research scientist at the University of California, Riverside; a cooperative employee at the George E. Brown Jr. Salinity Laboratory (USDA-ARS) in Riverside: and as a postdoctoral researcher at the University of California, Riverside. Schaap holds a B.A. in chemistry, an M.S. in physical geography, and a Ph.D. in environmental sciences from the University of Amsterdam in the Netherlands. .

Yonggen Zhang, University of Arizona, Tucson, AZ

Yonggen Zhang is a Ph.D student in China University of Geosciences since 2008, and a visiting student in Department of Hydrology and Water Resources at the University of Arizona since February 2012, where he is working with Professor Shlomo P. Neuman and Professor Marcel G. Schaap. His main research interests include inverse modeling of vadose zone and groundwater, soil pedotransfer functions to estimate soil hydraulic properties, geostatistical methods for spatial analyses of hydrologic data, and modeling of organic pollution in water, soil and sediment. He is determined to become a professional soil scientist and hydrogeologist who helps tackle some most challenging geosystem problems in the world.