Effective rainfall defined using measurements of grass growth in the Etosha National Park, Namibia

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). These methods, which aim at providing a simple predictive tool for grass growth and production, are based on measurements of grass growth in relation to 10-day interval rainfall and daily measurements of rainfall. Studying the influence of soil moisture and other factors on grass growth is outside of the scope of this study. Effective rainfall calculated using daily measurements of rainfall from the rainy season of 1994/1995 indicate that between 35•5 to 57•6% of total annual rainfall is effective rainfall for Etosha.