Earth Systems Modeling in the Brazos River Alluvium Aquifer: Improvement of Computational Methods and Development of Conceptual Model

Traditional hydrologic modeling has compartmentalized the water cycle into distinct components (e.g. rainfall-runoff, river routing, or groundwater flow models). In river valley alluvium aquifers, these processes are too interconnected to be represented accurately by separate models. An integrated modeling framework assesses two or more of these components simultaneously, reducing the error associated with approximated boundary conditions. One integrated model, ParFlow.CLM, offers the advantage of parallel computing, but it lacks any mechanism for incorporating time-varying streamflow as an upstream boundary condition. Previous studies have been limited to headwater catchments. Here, a generalized method is developed for applying transient streamflow at an upstream boundary in ParFlow.CLM.

The upstream inflow method was successfully tested on two domains – one idealized domain with a straight channel, and one small stream catchment in the Brazos River Basin. The stream in the second domain is gaged at the upstream and downstream boundaries. Both tests assumed a homogeneous subsurface, so that the efficacy of the transient streamflow method could be evaluated with minimal complications by groundwater interactions.

Additionally, an integrated conceptual model is presented for the Brazos River Alluvium Aquifer (BRAA), the Brazos River, and the overlying terrain. The BRAA is a floodplain aquifer in central to southeast Texas. This aquifer is highly connected to the Brazos River and experiences localized semi-confined conditions beneath thick surface clay layers. The conceptual model is designed to be implemented in an Earth system modeling framework and is limited to the central portion of the aquifer in Brazos and Burleson Counties, Texas. Unlike previous models in ParFlow.CLM, this is a high-order subbasin with large inflows from upstream. Additionally, the model incorporates noflow, transient head, and free drainage boundaries. Preliminary tests suggest the need for a long spin-up period. Long-term simulations will require calibration of surface and subsurface parameters before using the model to assess system behavior.