A feasibility study on mean areal rainfall estimation and hydrologic response in the Blue Nile region using METEOSAT images

The feasibility of using a bi-spectral frequency analysis method to estimat e daily mean areal precipitation (MAP) from 3-hourly METEOSAT visible (VIS) and infrared (IR) images over the Blue Nile river catchment (about 35-40 d egrees East Longitude and 8-12 degrees North Latitude) in support of hydrol ogic studies is explored. The data record used spans the period 14 July 14 August 1995. At first, the study addresses the spatiotemporal variability o f the satellite images, the determination of the relationship between topog raphy and satellite data and the inference of MAP from satellite images usi ng the bi-spectral method and a multivariate 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 determi ne the dependence of the Blue Nile region hydrologic response on the type o f 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 X 1 degree s resolution. Principal conclusions of this initial sensitivity study are: (a) use of the bi-spectral method, complemented with an appropriate multiva riate regression formulation, improves MAP estimates during the aforementio ned time interval substantially, for daily rainfall rates greater than abou t 7 mm day(-1); (b) aggregate hydrologic response of the Blue Nile region i s very sensitive to the type of precipitation forcing used; and (c) substan tial spatial variability of the sensitivity of hydrologic response to the t ype of precipitation forcing exists in the region. The use of satellite-der ived MAP estimates is recommended together with recalibration of hydrologic models using spatially variable parameter values, and analysis of uncertai nty propagation through model components and for Various sub-catchments. (C ) 1999 Elsevier Science B.V. All rights reserved.