This paper reports a comparative analysis of nebkha morphology in central Tunisia and northern Burkina Faso based on a total of 473 measured nebkhas. Nebkhas are mounds composed of wind-borne sediment that accumulated around shrubs. The size of the mounds was measured by height (H) and a horizontal component (L), the latter being the mean of the length and width of the mound. In the Tunisian data set, three trends of development of the mounds were distinguished for Ziziphus lotus nebkhas. Initially, the height increases linearly with the increase in the horizontal component until it reaches about 17 m. When the horizontal component exceeds 17 m, the height stabilizes before a decreasing trend of the height occurs. These results provided the basis for mathematical modelling of nebkha development. A ion-linear and a linear fit were compared and a second-order polynomial function was found to best fit the data on nebkha size from Tunisia. The results indicate that nebkhas have three stages of development: a growing, a stabilizing, and a degrading stage. The model was applied to data on Acacia sp. nebkhas and Balanites aegyptiaca nebkhas from Burkina Faso with good results. The non-linear fit was slightly better than the linear fit. Nebkhas exist at all three stages of development in Tunisia, whereas Burkina Faso has nebkhas that are mainly at the growing stage. These differences are a function of time lag in land degradation between the two study areas, because the wind transport of sediment and the sediment supply are highly related to the density of the vegetation cover in semiarid areas. This conclusion is further supported by dating of nebkhas and of historical and recent vegetation changes in the study areas reported in the literature. The results show that differences in plant ecologies, however, are also very important for the rate of development of nebkhas, which is demonstrated by the much faster growth of Balanites nebkhas as compared to Acacia nebkhas, even though they occur in the same type of environment. (C) 1998 Elsevier Science B.V.
