This paper discusses the influence of the moisture content on the seasonal behavior of both soil resistivity and ground resistance typical of power distribution grounding systems. Although field surveys indicate a wide variation in the moisture content only for the topmost soil layer, laboratory tests show that the resistivities of several types of soil are greatly affected. As a consequence, the ground resistance of systems with small dimensions may vary significantly and compromise the lightning performance of protection systems. The strong correlation, identified from measurements in this work, between soil moisture and, thus, soil resistivity, and monthly rainfall allows extrapolating resistivity measurements for the most critical conditions providing data that can be used to improve the design and performance of grounding systems.