A feasibility study on mean areal rainfall estimation and hydrologic response in the Blue Nile region using METEOSAT images D. Tsintikidis a, * , K.P. Georgakakos a,b , G.A. Artan a , A.A. Tsonis c a Hydrologic Research Center, 12780 High Bluff Drive, Suite 250, San Diego, CA 92130-2069, USA b Scripps Institution of Oceanography, UCSD, La Jolla, CA 92023-0224, USA c Department of Geosciences, University of Wisconsin–Milwaukee, Lapham Hall, P.O.Box 413, Milwaukee, WI 53201, USA Received 12 December 1998; received in revised form 8 March 1999; accepted 20 May 1999 Abstract The feasibility of using a bi-spectral frequency analysis method to estimate daily mean areal precipitation (MAP) from 3- hourly METEOSAT visible (VIS) and infrared (IR) images over the Blue Nile river catchment (about 35–40° East Longitude and 8–12° North Latitude) in support of hydrologic studies is explored. The data record used spans the period 14 July 14 August 1995. At first, the study addresses the spatiotemporal variability of the satellite images, the determination of the relationship between topography and satellite data and the inference of MAP from satellite images using the bi-spectral method and a multi- variate regression. Validation of the estimated MAPs is carried out with data from a sparse raingauge network in the region. These estimates are next used in a sensitivity study to determine the dependence of the Blue Nile region hydrologic response on the type of precipitation forcing (raingauge-based vs. satellite-based estimates). A conceptual semi-distributed hydrologic model is used to simulate hydrologic processes pertaining to soil water and channel routing with a 1 × 1° resolution. Principal conclusions of this initial sensitivity study are: (a) use of the bi-spectral method, complemented with an appropriate multi- variate regression formulation, improves MAP estimates during the aforementioned time interval substantially, for daily rainfall rates greater than about 7 mm day -1 ; (b) aggregate hydrologic response of the Blue Nile region is very sensitive to the type of precipitation forcing used; and (c) substantial spatial variability of the sensitivity of hydrologic response to the type of precipitation forcing exists in the region. The use of satellite-derived MAP estimates is recommended together with recalibration of hydrologic models using spatially variable parameter values, and analysis of uncertainty propagation through model components and for various sub-catchments.  1999 Elsevier Science B.V. All rights reserved. Keywords: Daily mean areal precipitation; Spatio-temporal variability; Bi-spectral method 1. Introduction Reliable estimation of mean areal rainfall over hydrologic catchments is a crucial step for the simula- tion and prediction of catchment outflows. The Nile Forecast System (NFS) implemented at the Nile Fore- cast Center (NFC) in Cairo, Egypt, is an operational hydrologic prediction system, which uses mean areal rainfall and long-term-averaged potential evapotran- spiration over a number of sub-catchments of the Nile River catchment to predict daily inflows at the High Aswan Dam in Egypt (Barrett, 1993). The prediction methodology is based on a Monte Carlo simulation approach that utilizes historical mean areal rainfall estimates from previous years to provide future rain- fall forcing for the spatially-distributed hydrologic Journal of Hydrology 221 (1999) 97–116 0022-1694/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0022-1694(99)00071-2 * Corresponding author. Tel.: + 1-619-794-2726; fax: + 1-619- 792-2519. E-mail address: ditsinti@hrc.ucsd.edu (D. Tsintikidis)