Biomass production and water use efficiency of grassland in KwaZulu-Natal, South Africa

Introduction: Water use efficiency (WUE) is a useful concept to use when comparing rangeland condition. The MODIS fPAR and LAI products make it possible to calculate WUE for extensive rangeland systems. Materials and Methods: The study was conducted Kokstad Research Station (30° 31' S, 29° 25'E) in the Dry Highland Sourveld (annual rainfall=790mm). Growth was measured by clipping the sward within and adjacent to exclosure cages. Values were extracted from MODIS fPAR and LAI products for pixels that matched dates when the ground measurements occurred. Hourly reference evapotranspiration (ET0) was calculated using the Penman-Monteith equation (Allen et al. 1998). Following Leuning et al. (2009), MODIS LAI product was used to approximate the stomatal conductance component (Gs) of the P-M equation to model the daily actual ET (ETa). Using the measured biomass and the fPAR value from MODIS, we prepared a regression relationship which predicts dry matter from fPAR and used this to predict total standing dry biomass (kg ha⁻¹). The difference between the initial and the maximum standing biomass values accounts for most of the annual growing season increment in dry matter. Using ETa we calculate the water use efficiency (kg mm⁻¹) of this rangeland.