Top-left panel: left-hand side Y^sep(θ) of Eq. (15) as a function of the right-hand side C^sepX^sep(θ) of Eq. (15) for a separable model. Bottom-left panel: left-hand side Y^obs(θ) of Eq. (15) as a function of the right-hand side C^sepX^obs(θ) of Eq. (15) for the observations. Top right panel: left-hand side Y^sep(θ) and right-hand side C^sepX^sep(θ) of Eq. (15) as a function of distance for a separable model. Bottom right panel: left-hand side Y^obs(θ) and right-hand side C^sepX^obs(θ) of Eq. (15) as a function of distance for the observations. We show angular distances up to 30 Mm where the supergranular velocities are largest. Further away from the center of the supergranule, both X(θ) and Y(θ) go to zero. In all panels the errors are 3σ. In all panels, blue error bars and curves correspond to the right-hand side of Eq. (15) and red error bars and curves correspond to the left-hand side of Eq. (15).