Journal of Animal & Plant Sciences, 2010. Vol. 7(2): 789- 800.Rainwater harvesting (RWH) is the process of interception and concentration of runoff and its subsequent storage either in soil for direct use by plants or in reservoirs for later application when needed to mitigate dry spells. RWH varies from macro to micro to in-situ systems based on the size of the catchments and runoff transfer distances. Macro RWH systems with or without storage has shown to be more applicable among communities as compared to micro catchments RWH systems. The study aimed at looking on the complexity of biophysical and social economic factors affecting potentiality of the use of runoff harvested from macro catchment. The results of the study identified two broad categories of constraints which are hydro-climatic and management of harvested runoff at the farm level scale. The hydro-climatic challenges are more related to climate while management looks on the transaction cost reflected on the maintenance of the systems, equitable access to runoff and related resources. Results indicated that during the short rain season, the seasonal rainfall amount received does not meet maize water requirements, hence requires supplementary irrigation water to mitigate dry spells. Other biophysical challenge is the change of the runoff conveyance channels due to erosion and deposition. The results showed that fields in close proximity to runoff sources can receive from 70 m3/ha to 300 m3/ha of runoff and the crop yields on these fields that received extra water from external catchments (macro RWH), increased by more than 120% as compared to fields that received rainfall only. The result also showed that the amount received in the field is not the only factor that can contribute to the water use efficiency but also depends on infield management. The study therefore, recommends that the modeling of macro catchments RWH models should not only deal with hydro-climatic challenges but also looks on the social economic for efficient and equitable distributions of resources runoff from macro-catchment.