Walsh Index                       

Walsh index reflects the intensity and length of the dry season (Walsh 1996). The beginning of the dry season is defined when it rains less than 100 mm in a month for the first time in a year (Nelson, personal communication). Professor Bruce Nelson from National Institute for Amazon Research (INPA - Instituto Nacional de Pesquisas da Amazônia) used WorldClim monthly accumulated rainfall (Hijmans et al. 2005) as input data to calculate Walsh Index.