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

Abstract

Using the results from a long-term grazing trial in the Dry Highland Sourveld of the KwaZulu-Natal province, we prepared a water use efficiency value (the ratio of the increment in annual biomass to total annual evapotranspiration) for this rangeland type. Using seasonal biomass measurements recorded between March 2000 and March 2007, we developed a model for predicting the increment in annual biomass using a regression relationship between fPAR (fraction of photosynthetically active radiation intercepted by the vegetation canopy) and measured standing biomass. This regression model was used to estimate the growth rates from fPAR for the period 2001-2006. During the period 2004-2007, when complete meteorological records were available, we calculated reference or potential evapotranspiration (ETₒ) using the Penman-Monteith equation. To approximate actual evapotranspiration (ET), we used the MODIS leaf area index to approximate canopy conductance component (G) of the Penman-Monteith equation. Using the adjusted fPAR curves as a surrogate for plant growth, and ET, we calculated a water use efficiency value for these grasslands of 7.5 ± 0.48 kg DM mm⁻¹ ha⁻¹ y⁻¹. This value relates well to other published water use efficiencies for natural rangeland.