MONITORING EVOLUTION OF LAVAKAS (GULLIES) IN THE ALAOTRA-MANGORO DISTRICT OF EASTERN MADAGASCAR USING 20TH CENTURY AIR PHOTOS AND 21ST CENTURY ORTHO-IMAGERY

Madagascar’s highlands are characterised by unusual gullies known as lavakas. Conventional wisdom links hillslope erosion and lavaka formation with slash-and-burn agriculture and overgrazing, but there are few data on rates of lavaka initiation and evolution, and how those relate to population dynamics is not known. The Alaotra-Mangoro district is particularly sensitive, both because of the ecological importance of Lac Alaotra and because it is Madagascar’s most important rice-growing region. The population has grown exponentially in recent decades, doubling from about 0.3 million in mid 20^th C to 0.6 million in 1993, and doubling again to >1 million in 2013 (density increase of 6 to 19 and then to 31 people/km²). The hills in the watershed have many lavakas; the question is whether increasing human activity has driven an increase in lavaka erosion.

We studied a 450 km² area in the steep treeless uplands of the Lac Alaotra watershed, just east of the district capital (Ambatondrazaka). We used low-altitude aerial photograph suites taken in 1949, 1957 and 1969, georectified and overlaid on 2006 Bing orthoimagery in ArcGIS. For each time-slice, all identifiable lavakas were outlined, and their areas measured.

Despite the dramatic increase in human activity, there has been almost no initiation of new lavakas since 1949. In the 438 km² covered by 1949 and 2006 imagery, 525 out of 527 lavakas were present in 1949. Lavakas range in size from 325 m² to 460,000 m². The median lavaka area increased by about 6% between 1949 and 2006, from 12,000 to 12,700 m². This does not necessarily mean that overall erosion increased, however, because as lavakas evolve through time from young and active to old and inactive they also change from narrow and deep to wide and shallow as their walls collapse and the gully fills in. Examination of the ratio of vegetation to bare earth in the lavakas suggests that there has been an overall lessening of lavaka activity since 1949.

There are many unvegetated, active lavakas in the study area, but the data suggest that lavaka activity is in fact less overall than it was in the mid-20^th C. These results are counter to expectations, given prevailing narratives linking human activities to increased lavaka formation in Madagascar, and demonstrate the importance of decadal-scale analysis of regional landscape histories.