Preprints
https://doi.org/10.5194/hess-2021-292
https://doi.org/10.5194/hess-2021-292

  09 Aug 2021

09 Aug 2021

Review status: this preprint is currently under review for the journal HESS.

The effect of rainfall amount and timing on annual transpiration in a grazed savanna grassland

Matti Räsänen1, Mika Aurela2, Ville Vakkari2,3, Johan P. Beukes3, Juha-Pekka Tuovinen2, Pieter G. Van Zyl3, Miroslav Josipovic3, Stefan J. Siebert4, Tuomas Laurila2, Markku Kulmala1, Lauri Laakso2,3, Janne Rinne5, Ram Oren6,7,8, and Gabriel Katul6,8 Matti Räsänen et al.
  • 1Institute for Atmospheric and Earth System Research, University of Helsinki, Finland
  • 2Finnish Meteorological Institute, Helsinki, Finland
  • 3Atmospheric Chemistry Research Group, Chemical Resource Beneficiation, North‐West University, Potchefstroom, South Africa
  • 4Unit for Environmental Sciences and Management, North‐West University, Potchefstroom, South Africa
  • 5Department of Physical Geography and Ecosystem Science, Lund University, Sweden
  • 6Nicholas School of the Environment, Duke University, Durham, North Carolina, USA
  • 7Department of Forest Science, University of Helsinki, Finland
  • 8Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, USA

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.

Matti Räsänen et al.

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  • RC1: 'Comment on hess-2021-292', Russell Scott, 17 Sep 2021 reply

Matti Räsänen et al.

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Dataset for "The effect of rainfall amount and timing on annual transpiration in grazed savanna grassland" M. Räsänen, M. Aurela, V. Vakkari, P. Beukes, J.-P. Tuovinen, S. Siebert, P. Van Zyl, M. Josipovic, T. Laurila, M. Kulmala, L. Laakso, J. Rinne, R. Oren, G. G. Katul https://doi.org/10.6084/m9.figshare.11322464

Matti Räsänen et al.

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Short summary
The productivity of semi-arid grazed grasslands is linked to the variation in rainfall and transpiration. By combining carbon dioxide and water flux measurements, we show that the annual transpiration is nearly constant during wet years while grasses react quickly to dry spells and drought, which reduce transpiration. The planning of annual grazing strategies could take into account the early-season rainfall frequency that was linked to the portion of annual transpiration.