Regional precipitation anomalies under El Niqo or global warming scenarios manifest considerable spatial variability, especially in the vicinity of tropical deep convective zones. Using a simplified set of equations, we develop an analytic prototype of convective margins for tropical land regions subject to inflow from adjacent ocean regions. The approach yields an expression for the location of the convective margin from which it is straightforward to deduce how climate perturbations impact the margin. We compare the results of the prototype to full model simulations of the equatorial South American precipitation response to El Niqo forcing and find general agreement. Analysis of the observations supports the convective inflow margin prototype's applicability to appropriately simple basic state configurations.