Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
783 
A study with NOAA-7 AVHRR-imagery in monitoring ephemeral 
streams in the lower catchment area of the Tana River, Kenya 
J.W.van den Brink 
DHV Consulting Engineers, Amersfoort, Netherlands 
ABSTRACT: This article presents a case study in which the possibilities of the NOAA-7 AVHRR system are 
evaluated for the extraction of hydrological information through the monitoring of the vegetation growth 
in the lower catchment area of the Tana River,Kenya. Emphasis is placed on the assessment of the 
hydrologic influence of the ephemeral streams in the lower catchment area on the Tana River. Due to the 
presence of severe cloud cover over the lower catchment no imagery was obtained by the NOAA-7 AVHRR 
system with sufficient quality to derive the desired information in a period of five months and a near 
daily coverage of the satellite system. However the imagery shows that the NOAA-7 AVHRR system forms an 
additional source of information for observation and monitoring of vegetation of large or even moderately 
sized areas, such as the secundary catchment areas in this study, with sufficient radiometric and spatial 
discrimination. 
RESUME: Cet article présente unç étude dans laquelle les possibilités du système NOAA-7 AVHRR sont évaluées 
pour la déduction d'information hydrologique par 1'enrégistrement de la croissance de la végétation dans la 
zone de captage inferieur du Tana fleuve au Kénia. 
Beaucoup d'attention est donnée à l'obtention de l'influence causé par les cours d'eau secondaires (Laga's) 
qui, dans ce bassin ne produisent de l'eau que pendant une période restreinte de l'année. Parce que Les 
études étaient exécutées pendant les saisons des pluies, dans deux périodes de 5 mois totales. Il existaint 
une couverture de nuages persistante dans ces périodes et le système NOAA-7 AVHRR ne procurait, malgré la 
répétition des observations quotidiennes par la satellite, pas d'images de qualité suffisante pour en 
déduire l'information désirée. 
Cependant, les images montrent que le système NOAA-7 AVHRR constitue une source additionelle d'information 
pour l'observation et l'enregistrement de la végétation dans des zones par sa résolution radiométriques et 
spatiale même les zones de superficie surface modérée comme le cas des zones sécondaire de captage dans cette 
étude. 
1 INTRODUCTION. 
The monitoring study of ephemeral streams in the 
Lower Tana River catchment in Kenya with NOAA-7 
AVHRR imagery formed part of the larger Tana River 
Remote Sensing Study. The Tana River Remote 
Sensing Study was financed by the Dutch Ministery 
of Education and Science and the Netherlands 
Remote Sensing Board (BCRS) and executed by DHV 
Consulting Engineers, in cooperation with the 
Delft Hydraulics Laboratry (DHL) and the Research 
Institute for Nature Management (RIN), all from 
the Netherlands, together with Landevco from 
Kenya. The study was carried out as part of the 
Tana River Morphology Studies in which the same 
organizations participate. The Tana River Morpho 
logy has the objective to analyse the effect of 
current and future interventions, such as reser 
voirs and irrigation schemes and the morphology 
of the Tana River and the riverine lands. 
2 OBJECTIVES. 
The lower catchment area of the Tana River is a 
very large and inaccessible area only inhabited by 
a small number of roaming pastoralists. The area 
contains some 90 smaller and larger ephemeral 
streams, called Laga's, which flow into the Tana 
River. 
Only during the two rainy seasons, a period in 
may-june with long-rains when more than half of 
the annual precipitation falls and a period in 
october-december with short rains, it can be 
expected that water is carried by the Laga's. 
It was the objective of the case study to evaluate 
the hydrologic influence of these Laga's on the 
Tana River. Given the constraints described above 
any research in the area require a lot of manpower 
and logistics. Therefore the use of satellite 
imagery was investigated for the extraction of the 
desired hydrological information. 
Figure 1. Mosaic of Landsat images of the Lower 
Tana River area(near-infrared channel, June 1975)