Abstract

Inland valleys are being used under the Sawah technology for rice production to reduce rice imports to Ghana. Sawah technology is assumed to benefit from geological fertilization. However, there is no quantitative information on runoff and sediment flows in the agricultural watershed of Ghana. This study was carried out at Biemso in the southern part of the country. The aim was to estimate runoff and sediment transport using the water erosion prediction project (WEPP) model (version 2006.500), from hillslope to the valley bottom where rice is cultivated using the Sawah technology. A digital elevation model (DEM) was created from ground survey and used to select the various plots (hillslopes) and to select slope input parameters. Four plots (hillslopes) were selected for the model simulation. Data on local daily values of rainfall and on minimum and maximum temperatures were used to set a CLIGEN model station file to determine climate input parameters for the model. Rainfall characteristics (erosivity and distribution) were analysed. Soil erodibility was also determined. Soil and crop management input parameters required by the model were identified and or estimated from field measurements and secondary sources. The model was run for two management scenarios: Fallow and continuous maize systems. The results of the simulation showed that 2.9 to 3.9 and 6.8 to 10.2 t/ha/year of sediments were eroded from upper catchment to valley bottom under fallow system and maize, respectively. The range of values for runoff produced under fallow was 17.4 to 40 mm whereas that under maize system is 158.7 to 233.62 mm. The study has shown that land use system in the study area has a great influence on geological fertilization. In addition, the valley bottom where rice is produced under the Sawah system is enriched with organic matter from upslope.

Key words: Geological fertilization, sediment, sawah, inland valleys, runoff, water erosion prediction project (WEPP) (2006.500) model.