This study investigates options for assessing suitability for water harvesting. A conceptual model has been developed to explain the results of a rainfall simulation campaign involving the use of two instantaneous kinetic energies, a range of rainfall intensities and various degrees of crust disturbance in order to test the hypothesis of a surface (crust) control on runoff reported for other semi-arid environments. The resultant conceptual model purports to explain runoff as a function of both the initial state of the surface and of rainfall instantaneous kinetic energy and rainfall intensity. The advantages and disadvantages of rainfall simulation as opposed to alternative possible techniques for assessing the relative runoff potential of various surface types in the study area are discussed, as well as the prospects of upscaling from 'point' hydrological measurements to surfaces of spectral data (satellite imagery), with which an entire area of interest can be surveyed in terms of runoff potential for water harvesting suitability assessment.