Title : The Role of Physical , Numerical and Data Coupling in a Mesoscale Watershed

Full coupling of physical processes, natural numerical coupling, and parsimonious but accurate data coupling are three key steps in efficient and accurate simulation of distributed hydrologic states in watersheds. Here we present a physically-based, spatially distributed hydrologic model (called PIHM) that utilizes all the three coupling strategies. Interception, snow melt, transpiration, evaporation, overland flow, subsurface flow, river flow, macropore based infiltration and lateral stormflow, as well as flow through and over hydraulic structures such as weirs and dams are some of the physical processes handled in the model. A semi-discrete, Finite-Volume approach is used to define the distributed process equations on discretized unit elements, in terms of a fully-coupled system of ordinary differential equations. An implicit Newton-Krylov based solver that utilizes adaptive time stepping provides a robust and stable solution. Data-coupling is aided by the use of constrained unstructured meshes, and a flexible data model incorporated within an open-source GIS tool (PIHMgis). The spatial adaptivity of the mesh elements and temporal adaptivity of the numerical solver facilitates capture of multiple spatio-temporal scales, allowing important insight into hydrologic process interactions. The implementation of the model has been performed for a mesoscale watershed in central PA (Little-Juniata Watershed, 845 km). Model results are validated by comparison of observed and predicted streamflow and groundwater levels at multiple locations. The fully-coupled model unfolds a range of multiscale/multiprocess interactions including: 1) an apparent inverse relationship between fraction of total evapotranspiration rate due to transpiration and interception loss, 2) the role of forcing (precipitation, temperature and radiation), soil moisture and overland flow on evaporation-transpiration partitioning, 3) the importance of water table depth on evaporation-transpiration, 4) the influence of local upland topography and stream morphology on spatially distributed, asymmetric right-left bank river-aquifer interactions, and, 5) the role of macropore and topography on ground water recharge magnitude, time scale and spatial distribution.