Methods and Materials
This study was conducted in Dai Tu district of Thai Nguyen province, Vietnam during November 2001 to January 2003. Dai Tu district, about 100 km north of Hanoi, is a mountainous area, covering 57,618 ha (Fig. 1). The primary data sources were social-economic survey, field measurement and three SPOT multi-spectral band satellite images for land use change detection. Land use types, roads, and hydrological systems were taken from topographic maps and updated with SPOT satellite images. Field survey, digital topographic, land use map and related information were used to identify suitable sites for watershed ponds to be used in aquaculture.

F 1.Location map of the study area.

GIS Software and Systems used
The GIS software used in this study was ArcView GIS for windows (version 3.2) developed by Environmental Systems Research Institute, Inc. The remote sensing image analysis used in this study was ENVI (version 3.4) developed by Research Systems and MLR XP24 GPS for field sample collection and verification.

Surveys and field measurements
The socio-economic and environment data were collected using pre-test questionnaires and field measurement from randomly selected sixty farmers and sixty watershed ponds respectively. Soil and water samples were collected from these ponds for the determination of soil pH, soil texture, water pH (Boyd and Tucker, 1992. The locations of the selected watershed ponds were recorded by a GPS navigator (MLR SP24).

Remotely sensed data and thematic maps
The primary data sources were three SPOT multi-spectral bands satellite images acquired for the dates of 22 October 1994, 10 November 1998, and 9 November 2002. Cloud-free images of the study area were taken from these three scenes and geo-registered with topographical maps (1:25,000) using reference points.

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