TRACKING 20TH CENTURY LAVAKA DEVELOPMENT IN CENTRAL MADAGASCAR USING AIR PHOTO AND SATELLITE IMAGE ANALYSIS

Lavakas, the unusual gullies found throughout Madagascar’s central highlands, are poorly understood and their contribution to the erosion budget is controversial. We compared 2006 satellite imagery (DigitalGlobe, through Google Earth) and 1966 air photos, with ground-truth from field measurements. From these data we calculated a rate of addition of new lavakas, and also examined evolution of individual lavakas over a 50-year period.

The study focused on a small area (57 km²) in a region where lavakas are extensively developed and where increasing agricultural use in recent decades has spurred concerns about erosion (Amparafaravola, west of Lac Alaotra in north-central Madagascar). In 2006 this area had 157 active lavakas that we digitised from satellite images (≈2.8 lavakas km^-2). Of these, 125 were measured and photographed in the field. The main source of error in our analysis is imprecision of area measurements from the 1966 air photos. Comparison of lavaka areas measured from overlapping 1966 air photos gives mean precision (in 1966 area measurement) of 9±2%. Thus we take 10% as a ballpark error bar on our 1966 measurements, and consider 1966-2066 changes in lavakas size greater than 10% to be real.

The data show some increase in lavaka activity. Thirty-five lavakas (25% of the 2006 total, representing 0.03 km²) appeared after 1966, an addition rate of 0.7 lavakas km^-2 yr^-1. Many of the 1966 lavakas grew larger over time. The median increase in area was +19% (range -50% to +793%). The largest individual lavaka in 1966 had area 17,597 m², and the median was 1306 m². By 2006 these values had increased slightly, to 18,301 m² and 1405 m², respectively. The total area covered by lavakas increased from 0.27 km² to 0.34 km².

Over time, lavakas evolve from incising and active (deep relative to their width) to collapsed and vegetated (shallow and wide), and this study provides preliminary information about the rate at which that happens. Of the lavakas present in 1966 for which image quality was sufficient to permit evaluation, 86 (72%) appeared as active in 2006; 16 (13%) evolved from deeply incised to partially collapsed; and a further 17 (14%) were invaded by natural vegetation and became largely inactive.