private double advectWaterParticle(double timeStep, Particle particle, int i, int j) {
Vector2d vel = interpolator.findVelocity(particle);
// scale the velocity by the cell size so we can assume the cells have unit dims
vel.scale(CellDimensions.INVERSE_CELL_SIZE);
double xChange = timeStep * vel.x;
double yChange = timeStep * vel.y;
particle.set( particle.x + xChange, particle.y + yChange );
particle.incAge( timeStep );