Flow and transport modeling of a tracer isotope experiment at B2 LEO using integrated and distributed multisensor observation data
We performed an analysis of the first tracer experiment at the Landscape Evolution Observatory (LEO) of the Biosphere 2 facility in Oracle, Arizona. The experiment involved multiple periods (four) of water/tracer injection and produced a hydrological response characterized by water and solute (deuterium isotope tracer) outflow along the lower lateral boundary (seepage face) of LEO. The collected data consist of spatially integrated and point-scale responses for both flow and transport, measured at fine temporal resolution. The simulations were performed with the CATHY (CATchment HYdrology) model, a finite element based model for the solution of the three-dimensional Richards equation for variably saturated flow and advection-dispersion equation for solute transport. The analysis looked at the impact of the unsaturated zone, initial water storage, soil heterogeneity, and other factors on the water and solute dynamics over the hillslope, and it examined some numerical issues connected to mass conservation, velocity field accuracy, advection-dominated processes, and solute exchange across the soil/atmosphere boundary. The overall aim is to address, based on a very detailed laboratory and modeling study, the many challenges of simulating multivariate hydrological responses. To give one example, it is found that water balance partitioning between seepage face and total storage and the average concentration at the seepage face are well reproduced with a parameterization that is not overly complex, while for the point-scale (internal state) response it is necessary to make the model more complex by augmenting the degree of heterogeneity of the soil hydraulic parameters.