A NEW SOIL-MOISTURE BASED CLASSIFICATION OF RAINDAYS AND DRYDAYS AND ITS APPLICATION TO SIERRA-LEONE

The paper proposes a new classification of raindays and drydays incorp orating soil moisture status. This is of particular value for agricult ural purposes and also allows the adoption of a low rainfall threshold to distinguish between raindays and drydays. This is important since, as indicated in the paper, small falls may be important and yet in th e tropics, for agricultural purposes, a high threshold is often adopte d to take account of the high evaporative demand of the atmosphere. So il moisture is expressed as a percentage of available soil moisture st orage capacity (SMSC) and conditions are described as deficit, limitin g, adequate, and surplus, if soil moisture levels are 0-29%, 30-59%, 6 0-100% and > 100% of SMSC respectively. Combining this with rainday an d dryday occurrence, three types of drydays and four types of raindays are identified. This rain-soil moisture index classification more nea rly approaches a universal definition suitable for different tropical regions than previous ones. Application of the concept is illustrated with data from Sierra Leone, using a simple water balance model to est imate soil moisture.