A climatological study of soil moisture and irrigation water requirement in Paraiba State (NE Brazil)

Results of a study of soil moisture conditions in Paraiba state (NE Brazil) are presented in this paper. Paraiba is one of the several states in NE Brazil which is frequently affected by droughts. Considerable parts of the state have a semiarid climate and the highly irregular nature of rainfall makes the practice of agriculture a risky proposition. Information on soil moisture conditions is hence a matter of importance. The study is based on the estimation of daiiy soil moisture values for a minimum period of 25 years at twenty seven locations representing different parts of the state. A simple water balance model using daily precipitation and monthly mean temperature data is used for the estimation of soil moisture content. A first order Markov chain model is applied to the soil moisture data and the initial and conditional probabilities of days with dry and wet soil are computed for each decade of the year. Soil moisture averages and probabilities are used to evaluate the crop growing periods at the stations. The amounts of supplementary irrigation necessary to maintain the soil moisture content above selected levels during the crop growing periods are determined.