Abstract

The role of precipitation (P) variability on evapotranspiration (ET) and its two components, transpiration (T) and evaporation (E) from savannas, continues to draw significant research interest given its relevance to a number of eco-hydrological applications. Our study reports on six years of measured ET and estimated T and E from a grazed savanna grassland in Welgegund, South Africa. Annual P varied significantly in amount (508 to 672 mm yr−1), with dry years characterized by infrequent early-season rainfall. T was determined using annual water-use efficiency and gross primary production estimates derived from eddy covariance measurements of latent heat flux and net ecosystem CO2 exchange rates. The computed annual T was nearly constant, 331 ± 11 mm yr−1 (T/ET = 0.52), for the four wet years with frequent early wet-season rainfall, whereas annual T was 268 and 175 mm yr−1 during the dry years. Annual T/ET was linearly related to the early wet-season storm frequency. The constancy of annual T during wet years is explained by the moderate water stress of C4 grass and constant annual tree transpiration covering 15 % of the landscape. However, grass transpiration declines during dry spells. Moreover, grasses respond to water availability with a dieback-regrowth pattern, reducing leaf area and transpiration during drought. These changes lead to an anomalous monthly T/ET relation to leaf-area index (LAI). The results highlight the role of the C4 grass layer in the hydrological balance and suggest that the grass response to dry spells and drought is reasonably described by precipitation timing.

Highlights

  • Annual P varied significantly in amount (508 to 672 mm yr-1), with dry years characterized by infrequent early-season rainfall

  • The results presented here on the partitioning of ET must be viewed as necessary but insufficient for developing best practices for the management of grazing or fodder production

  • 15 Before addressing the study objectives, we first present the variability in precipitation and leaf-area index (LAI)

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Summary

Introduction

Similar to other semi-arid areas, wooded grasslands in central South Africa deliver essential ecosystem services such as grazing land and fodder (Bengtsson et al, 2019). The focus is restricted to processes operating over timescales ranging 15 between daily and seasonal, commensurate with controls over the annual partition of P into T, and the resulting carbon uptake in gross primary production (GPP). These longer timescales are of interest in the valuation of ecosystem productivity and their services when assessing climatic shifts (Godde et al, 2020). The results presented here on the partitioning of ET must be viewed as necessary but insufficient for developing best practices for the management of grazing or fodder production

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