The present invention utilizes a fiber optic rising assembly for the detection and measurement of water volumes. The fiber optic sensing element is placed adjacent to a body of water-soluble polymer. A rigid enclosure surrounds the fiber optic sensing element and the polymer to act as both a protective casing and an abutment limiting the motions of the polymer. A water permeable jacket is wrapped around the polymer to improve contact with the fiber optic sensor. In the presence of water, the polymer expands in volume and bends the fiber optic sensor. The strain caused by the bending is transduced into a measurable signal. In one embodiment of this invention, a fiber Bragg grating sensor is pressed against a pellet ofpolyacrylimide.