Hydrological effects of karst in the Mahajanga basin， Madagascar

It is of great significance to strengthen the overseas hydrological research of water resources for the strategy of China’s opening up policy. From this perspective, this work studies the hydrological effects of karst in the Mahajanga basin, Madagascar. It builds on previous research on geological evolution, hydrogeological data available, field surveys and monitoring data. Under the control of conditions such as lithology and geological structure, the eastern part of the Mahajanga basin has evolved into a karst valley with a length of nearly 300 km,and the southwestern part of the basin has formed a karst platform with an area of 12，000 km2. The development of the karst valley and karst platform is mainly in the horizontal direction, while extending downward to a relatively smaller depth, and the karst form exhibits an obvious characteristic of equal dissolution. The karst valley and platform play important roles in local water cycling, which can be described from 3 aspects: (1) The karst valley intercepts most of the rivers originated from the plateau, controls the directions of the rivers and forms a unique underground confluence system, which makes the valley the most abundant area of water resources and is the most important confluence area in Madagascar. (2) The karst valley, a complex and huge underground karst system, has created a large number of lakes and seasonal waterlogged depressions and broad loose sediments on both sides of the rivers, all of which produced a huge water cycle regulation system in the east margin of the basin, which will have a great retention effect on the hydrological process. (3) The karst platform in the southwest of the basin is a huge natural water storage reservoir with a vast area and large scale, which plays an important role in adjusting the surface runoff ,especially in dry months.