Delineation of small reservoirs using radar imagery in a semi-arid environment: A case study in the upper east region of Ghana

Small reservoirs serve many people living in semi-arid environments. Water stored in these reservoirs is used to supplement rainfed agriculture, allow for dry season irrigated agriculture and ensure the availability of water for domestic purposes. In order to manage the water effectively for competing uses, the actual storage of these reservoirs needs to be known. Recent attempts to delineate these reservoirs using remote sensing with Landsat imagery have been successful, especially in the upper east region of Ghana, West Africa.This paper shows that radar images (ENVISAT ASAR) can be used to provide similar information all year-round. Radar images have as an important advantage that they are not impaired by cloud cover and thus can be used during the rainy season. Another advantage of radar images is that images taken during night time are usable. The paper compares satellite derived data with field measurements of 21 small reservoirs. Whereas ENVISAT images on the average tend to overestimate the surface areas of small reservoirs, in certain reservoirs these areas are systematically under-estimated due to the shallow tail-ends of reservoirs that tend to have reed vegetation. These cannot be readily distinguished from the surrounding vegetation outside the reservoirs. This paper therefore provides a proof of concept of the monitoring of small reservoir volumes by radar imagery.