Vertical-gradient sign conventions

For absolute representative elevations, vertical separation is upper minus lower elevation. The upward-driving gradient is (lower_head-upper_head) / vertical_separation. ps_vertical_gradient() can return upward- or downward-positive values and always records the convention. Reversed, zero, or unidentified separations fail. A hydraulic gradient indicates driving potential; it is not groundwater flux.

ps_vertical_gradient(upper_head=10, lower_head=12,
                     upper_elevation=100, lower_elevation=80,
                     positive="upward")