Starting from a stochastic model of the water balance at a point proposed by Rodriguez-Iturbe et al. <1999>, the unsteady dynamics of mean soil moisture under varying climatic conditions is analyzed. The exact differential equations that describe the evolution of the mean soil moisture are obtained, and the simplifications that are needed to solve these equations are detailed. Within this framework we are able to study the importance of the seasonal variations in precipitation and evapotranspiration and the role of the root depth and soil characteristics, as well as the possible occurrence and relevance of steady state conditions for the soil moisture process during the growing season.