Hydrologic Modeling of an Eastern Pennsylvania Watershed with NEXRAD and Rain Gauge Data Latif Kalin 1 and Mohamed M. Hantush 2 Abstract: This paper applies the Soil Water Assessment Tool SWAT to model the hydrology in the Pocono Creek watershed located in Monroe County, Pa. The calibrated model will be used in a subsequent study to examine the impact of population growth and rapid urbanization in the watershed on the base flow and peak runoff. Of particular interest in this paper is the exploration of potential use of Next Generation Weather Radar NEXRAD technology as an alternative source of precipitation data to the conventional surface rain gauges. NEXRAD estimated areal average precipitations are shown to compare well with the gauge measured ones at two climate stations in the study area. Investigation of the spatially distributed NEXRAD precipitation estimates revealed that average annual precipitation can vary spatially as much as 12% in the Pocono Creek watershed. The SWAT model is calibrated and validated for monthly stream flow, base flow, and surface runoff. Hydrographs generated from both gauge and NEXRAD driven model simulations compared well with observed flow hydrographs. Although little effort is spent on daily calibration, model simulations and observed flows were in good agreement at the daily scale as well. Almost similar model efficiency statistics, i.e., mass balance error MBE, coefficient of determination R 2 , and Nash-Sutcliffe efficiency E NS , were obtained during the calibration period in the gauge and NEXRAD driven simulations. In the validation period, NEXRAD simulations generated higher model efficiencies at the monthly scale. On the other hand, simulations with gauge precipitations resulted in slightly better model efficiencies at the daily time scale. The spatial representation of precipitation did not contribute much to model performance when stream flow at the watershed outlet was the required output. However, the use of NEXRAD technology appears to offer a promising source of precipitation data in addition to currently existing surface gauge measurements. Discussions on new directions in radar-rainfall technology are provided. DOI: 10.1061/ASCE1084-0699200611:6555 CE Database subject headings: Watershed management; Radar; Rain gages; Hydrologic models; Calibration; Spatial distribution; Pennsylvania. Introduction Distributed hydrologic models are effective tools for environmen- tal decision making and water resources planning and manage- ment. They are helpful not only in making predictions of future flow conditions, but also in assessment of hydrologic impacts of changes in management scenarios, land cover, and climate. This study calibrates and validates a watershed-scale distributed hydrologic model to the Pocono Creek, which drains 120 km 2 of an area in Monroe County, Pa. Pocono Creek has very good water and biological quality and has been designated as Special Protec- tion Waters by the State and the Delaware River Basin Commis- sion DRBC. The Creek has a wild brown trout population, sig- nificant to outdoor recreation, which is the largest economic generator of the region. Population growth and projected urban- ization threaten to stress the natural resources, including the brown trout habitats in the watershed. The anticipated increase in the demand for groundwater coupled with the expected decrease in groundwater recharge caused by increased imperviousness are expected to reduce base flow and increase peak runoff in the Pocono Creek. This paper focuses on a comparative assessment of the use of Next Generation Weather Radar NEXRAD estimated precipita- tion as an alternative to gauge-measured precipitation data. The impact on the hydrology of projected future land use changes due to urbanization in the Pocono Creek watershed will be addressed in a future effort. The distributed hydrologic Soil Water Assessment Tool SWATNeitsch et al. 2002a,b, was chosen to fulfill the pro- ject objectives. The SWAT model was originally developed to quantify the impact of land management practices in large, com- plex watersheds with varying soils, land use, and management conditions over a long period of time, on the order of years. It has been developed and maintained by U.S. Department of Agri- culture USDA scientists and is freely available from http:// www.brc.tamus.edu/swat. It is widely accepted and used, and numerous applications can be found in the peer-reviewed litera- ture. For instance, as of March 2005, the SWAT model Web site cited 156 peer-reviewed publications in the form of journal papers or book chapters http://www.brc.tamus.edu/swat/swat-peer- 1 Assistant Professor of Forest Hydrology, School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849; formerly, ORISE Postdoctoral Researcher, U.S. Environmental Protection Agency, National Risk Management Research Laboratory, 26 W. Martin Luther King Dr., Cincinnati, OH 45268. E-mail: latif@auburn.edu 2 Research Hydrologist, U.S. Environmental Protection Agency, National Risk Management Research Laboratory, 26 W. Martin Luther King Dr., Cincinnati, OH 45268 corresponding author. E-mail: hantush.mohamed@epa.gov Note. Discussion open until April 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on July 28, 2005; approved on April 4, 2006. This paper is part of the Journal of Hydrologic Engineering, Vol. 11, No. 6, November 1, 2006. ©ASCE, ISSN 1084-0699/2006/6-555–569/$25.00. JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / NOVEMBER/DECEMBER 2006 / 555