A process-based distributed hydrologic model and its application to a Michigan watershed

The PAWS (Process-based Adaptive Watershed Simulator) model is a novel distributed hydrologic model that is based on solving partial differential equations (PDE) for physical conservation laws of the hydrologic cycle. The objective is to create an efficient physically-based modeling framework to describe the linkages between processes at different scales and to improve the applicability of physically-based models. The model simulates evapotranspiration, overland flow, channel flow, unsaturated soil moisture, groundwater flow, depression storage, vegetation growth and snowpack. PAWS focuses on the dynamic surface- subsurface interactions and integrated responses by efficiently coupling runoff and groundwater flow to the vadose zone processes governed by the Richards equation. This novel approach solves a long-standing bottleneck in PDE-based subsurface flow modeling by removing the computational limitations while maintaining physically consistent solutions. Surface flow is solved by ail efficient Runge-Kutta Finite Volume (RKFV) scheme.;We follow the Freeze and Harlan (1969) blueprint in that we believe each component of the model should be verifiable by itself. All flow components have been independently verified using analytical solutions and experimental data where applicable. PAWS utilizes readily available data from national databases. The model is applied to a medium-sized watershed in Michigan achieving high performance metrics in terms of streamflow prediction at two gages during the calibration period and the verification period. The baseflow flow periods are described particularly well. Starting from a rough initial estimate of the groundwater heads, the model describes the observed groundwater heads well (R2=0.98). The annual hydrologic fluxes are close to those estimated by a calibrated SWAT model. The model is considerably less expensive than previous physically-based models of similar complexity.;The model is able to elucidate the complex interactions of processes in space and time. Such detailed, quantitative and mechanistic descriptions cannot be produced by conceptual models. The watershed is found to be a subsurface-dominated system with saturation excess being the main runoff generation mechanism. Infiltration, recharge and ET are also found to be strongly related to topography and groundwater flow. The large seasonal variation of energy input drives the strong annual cycle and markedly different responses in streamflow.