Surface soils host unparalleled number of microbial species even in small volumes, serving as the richest storage of microbial genus in the biosphere. Spatial and temporal heterogeneity and multitude of niches and microhabitats in the vadose zone are believed to play key roles in fostering such diversity. In this study, a simple network model is constructed to simulate heterogeneous water distribution on a rough surface providing diffusional pathways supporting microbial activity. Because liquid may redistribute according to temporal variations in water potential (soil wetness), the resulting diffusion pathways and connectivity also vary with time. Local capillary properties on the surface and water contents may response differently to the overall water potential variations. Simulations are carried out using the network model as well as random walkers representing microbial activity, and reaction-diffusion model for nutrient transport and consumption. Results show that long term microbial growth is strongly determined by local conditions (location, microtopography) rather than its physiology. This is critical for fostering coexistence of different species and enhancing diversity.