The use of remote sensing to evaluate the effectiveness of soil and water conservation measures in the West Usambara Mountains, Tanzania

Abstract

The highlands in Tanzania are seriously affected by soil erosion, which leads to the formation of gullies, the collapse of soil and water conservation measures and the loss of fertile topsoil, which in turn leads to a reduction of crop yields, food deficiency, silting-up of waterways, damage to structures and loss of land value. Over the course of the past century considerable efforts have been undertaken to reduce the amount of erosion by means of soil and water conservation measures, but only minimal adoption of the promoted measures has been achieved, making soil erosion an on-going problem.

The aim of this study was to locate soil and water conservation measures in the Usambara Mountains, Tanzania and to model their effectiveness. This was done by (1) assessing erosion risk in the entire Lushoto district, (2) locating soil and water conservation measures in two smaller, 10x10 km blocks near the villages of Soni and Sunga, (3) modelling the erosion in those blocks, and (4) model the effectiveness of the soil and water conservation measures.

An erosion risk map of the entire district was created by using a Landsat Enhanced Thematic Mapper+ (ETM+) image and a digital elevation model created from a contour lines map with a vertical resolution of 50m. Land use maps of the 10x10 blocks were created using two standard pixel-based methods and an object-based, nearest neighbourhood method on two different scale levels. The pixel-based methods that were used were a minimum distance to means and a maximum likelihood classification method.

The locations of bench terraces and grass strips were located by using object-based image analysis and the land use map with the highest accuracy, which in this case was the map created with the maximum likelihood classification method. This land use map was used to mask out areas that were not of interest. The percentage of agricultural area that contained soil and water conservation measures ranged from 2.9% to 19.75% for the Soni and Sunga blocks, respectively.

The Universal Soil Loss Equation was used to quantitatively assess the erosion in the 10x10 km blocks. Areas containing soil and water conservation measures had an average modelled soil loss of 39.82-51.96 t ha-1 yr-1. The effectiveness of the soil and water conservation measures was modelled to be 42-58%.