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

[摘要] The role of precipitation ( P ) variability with respect to 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 ecohydrological applications. Our study reports on 6 years of measured ET and estimated  T and  E from a grazed savanna grassland at Welgegund, South Africa. Annual  P varied significantly with respect to 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 CO 2 exchange rates. The computed annual  T for the 4 wet years with frequent early wet-season rainfall was nearly constant, 326±19  mm yr −1 ( T / ET = 0.51 ), but was lower and morevariable between the 2 dry years (255 and 154 mm yr −1 , respectively). Annual  T and T / ET were 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 C 4  grasses as well as trees' ability to use water from deeper layers. During extreme drought, grasses respond to water availability with a dieback–regrowth pattern, reducing leaf area and transpiration and, thus, increasing the proportion of transpiration contributed by trees. The works suggest that the early-season P  distribution explains the interannual variability in  T , which should be considered when managing grazing and fodder production in these grasslands.

[学科分类] 妇产科学