The Penn State Integrated Hydrologic Model (PIHM) is a multiprocess, multi-scale hydrologic model where the major hydrological processes are fully coupled using the semi-discrete finite volume method. The model itself is "tightly-coupled" with PIHMgis, an open-source Geographical Information System designed for PIHM. The PIHMgis provides the interface to PIHM, access to the digital data sets (terrain, forcing and parameters) and tools necessary to drive the model, as well as a collection of GIS-based pre- and post-processing tools. Collectively the system is referred to as the Penn State Integrated Hydrologic Modeling System. The modeling system has been written in C/C++, while the GIS interface is supported by Qt. The Penn State Hydrologic Modeling System is open source software, freely available for download at this site along with installation and user guides.

It is our intention to begin a debate on the role of "Community Models" in the hydrologic sciences. Our research is a response to recent trends in US funding for "Observatory Science" that have emerged at NSF over the last few years, namely, the NSF-funded CUAHSI program (Consortium of Universities for Advanicing Hydrologic Sciences).

PIHM represents our strategy for the synthesis of multi-state, multiscale distributed hydrologic models using the integral representation of the underlying physical process equations and state variables. Our interest is in devising a concise representation of watershed and/or river basin hydrodynamics, which allows interactions among major physical processes operating simultaneously, but with the flexibility to add or eliminate states/processes/constitutive relations depending on the objective of the numerical experiment or purpose of the scientific or operational application.

The approach is based on the semi-discrete finite-volume method (FVM) which represents a system of coupled partial differential equations (e.g. groundwater-surface water, overland flow-infiltration, etc.) in integral form, as a spatially-discrete system of ordinary differential equations. Domain discretization is fundamental to the approach and an unstructured triangular irregular network (e.g. Delaunay triangles) is generated with constraints (geometric, and parametric) using TRIANGLE. A local prismatic control volume is formed by vertical projection of the Delauney triangles forming each layer of the model. Given a set of constraints (e.g. river network support, watershed boundary, altitude zones, ecological regions, hydraulic properties, climate zones, etc), an “optimal” mesh is generated. River volume elements are also prismatic, with trapezoidal or rectangular cross-section, and are generated along edges of river triangles. The local control volume contains all equations to be solved and is referred to as the model kernel. The global ODE system is assembled by combining all local ODE systems throughout the domain and then solved by a state-of-the-art parallel ODE solver known as CVODE developed at the Lawrence- Livermore National Laboratory.

The PIHM Modeling System was initially developed under research grants to The Pennsylvania State University (Penn State) from NSF (EAR 9876800, 1999-2007; EAR 03-10122, 2003-2007), NOAA (NA040AR4310085, 2003-2007), NASA (NAG5-12611, 2002-2005), with continuing grants from NSF and EPA for community model development.

An important partnership and motivation for this work was the Project Leaders participation in two community-science research activites over the last few years: The University of Arizona-led Science and Technology Center (SAHRA: Sustainability of Water Resources in Semi- Arid Regions), and the Chesapeake Community Modeling Project (CCMP). Each of these research programs has been essential to supporting the concept of "community models" for environmental prediction and helping to make it happen.

Penn State University makes no proprietary claims, either statutory or otherwise, to this version and release of PIHM and considers PIHM to be in the public domain for use by any person or entity for any purpose without any fee or charge. We request that any PIHM user include a credit to Penn State in any publications that result from the use of PIHM. The names Penn State shall not be used or referenced in any advertising or publicity which endorses or promotes any products or commercial entity associated with or using PIHM, or any derivative works thereof, without the written authorization o Penn State.

PIHM is provided on an "AS IS" basis and any warranties, either express or implied, including but not limited to implied warranties of noninfringement, originality, merchantability and fitness for a particular purpose, are disclaimed. Penn State will not be obligated to provide the user with any support, consulting, training or assistance of any kind with regard to the use, operation and performance of PIHM nor to provide the user with any updates, revisions, new versions, error corrections or "bug" fixes. In no event will Penn State be liable for any damages, whatsoever, whether direct, indirect, consequential or special, which may result from an action in contract, negligence or other claim that arises out of or in connection with the access, use or performance of PIHM, including infringement actions.