Development of an automated procedure for estimation of the spatial variation of runoff in large river basins Narayanan Kannan a, * , Chinnasamy Santhi a , Jeffrey G. Arnold b a Blackland Research and Extension Center, Texas, A&M University System, 720, East Blackland Road, Temple, TX 76502, USA b Grassland Soil and Water Research Laboratory, Agricultural Research Service–United States Department of Agriculture, 808, East Blackland Road, Temple, TX 76502, USA Received 3 June 2007; received in revised form 23 May 2008; accepted 3 June 2008 KEYWORDS SWAT; Calibration; Regional modeling; Upper Mississippi; Parameterization; Runoff Summary The use of distributed parameter models to address water resource manage- ment problems has increased in recent years. Calibration is necessary to reduce the uncer- tainties associated with model input parameters. Manual calibration of a distributed parameter model is a very time consuming effort. Therefore, more attention is given to automated calibration procedures. This paper describes the development and demonstra- tion of such an automated procedure developed for a national/continental scale assess- ment study called Conservation Effects Assessment Project (CEAP). The automated procedure is developed to calibrate spatial variation of annual average runoff components for each USGS eight-digit watershed of the United States. It uses nine parameters to cal- ibrate water yield, surface runoff and sub-surface flow respectively. If necessary, the pro- cedure uses a linear interpolation method to arrive at a better value of a model parameter. When tested for the Upper Mississippi river basin of the United States, the automated calibration procedure gave satisfactory results. Other test results from the procedure are very encouraging and show potential for its use in very large-scale hydro- logic modeling studies. ª 2008 Elsevier B.V. All rights reserved. Introduction In recent years, distributed parameter models are widely used to address watershed and large-scale water quality management problems. These models use many different 0022-1694/$ - see front matter ª 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2008.06.001 * Corresponding author. Tel.: +1 254 774 6122; fax: +1 254 774 6001. E-mail address: nkannan@brc.tamus.edu (N. Kannan). Journal of Hydrology (2008) 359,1– 15 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jhydrol