The most important environmental factors controlling successful plant germination and subsequent establishment in arid climates are rainfall amount, spatial and temporal rainfall distribution and soil moisture availability. In general, drier areas have a greater variability in rainfall. Agriculturists and ecologists commonly perceive total annual rainfall as a reliable indicator of vegetation growth and production during the rainy season. However, because of the intrinsic high spatial and temporal variability of annual rainfall in drier areas, it is in fact a poor indicator of actual vegetation growth.This paper aims at presenting simple methods modeling effective use of rain by grass for germination and establishment (effective rainfall) in the Etosha National Park (Etosha). Keywords: 10-day interval rainfall, annual rainfall, daily rainfall, effective rainfall, Etosha National Park, grass greenness, grass green cover, grass growth, grass production, modeling.