Final answer:
Filling pore space with water does not alter soil porosity because it is a measure of void space which remains constant; therefore, there is no change in porosity.
Step-by-step explanation:
When water fills the pore space in soil, leading to saturated conditions, the water does not cause the soil grains to push apart in a way that would increase or decrease the porosity. Instead, the water simply occupies the existing space between the grains. Porosity is a measure of the void space in the material and is a physical property related to the size, shape, and packing of particles.
Saturating the soil with water does not alter these physical characteristics; therefore, the porosity remains the same. When soil is saturated, all pore spaces are filled with water, and when it reaches field capacity, it holds all the water it can against gravity's pull. In the context of the question, when pore space is filled with water and the grains are pushed apart, there would be no change in porosity.