Measurement and modelling of rainfall interception by three semi-arid canopies

The main aims of this study were, firstly, to adapt the rainfall interception model of Rutter et al. (Agric. Meterology, 1971, 9, 367-384) to individual plants of two semiarid shrubs (Anthyllis cytisoides L. and Retama sphaerocarpa (L.) Boiss.) and a tussock grass (Stipa tenacissima L,) and secondly, to understand how the different canopy structures influence rainfall partitioning by individual plants. The selected species represent contrasting canopy types typical of vegetation of semiarid areas. Free throughfall coefficients were estimated from field measurements of low volume rainfall events and vertical photographs taken beneath the plant canopy. Canopy drainage curves were measured by continuous weighing of wetted plants. Canopy boundary layer conductances were calculated by measuring the evaporation of water from wet canopies, Field measurements of gross rainfall, throughfall and stemflow were taken for each rainfall event for A. cytisoides and R. sphaerocarpa. The Rutter type model of rainfall interception was adapted for individual shrubs and tested with measured rainfall events showing a good agreement between observed and predicted values for R. sphaerocarpa and for A. cytisoides. The interception model was then run to simulate interception loss during actual rainfall events, using atmospheric conditions measured every 5 s. The results from this simulation showed significant differences in interception loss between species, which can be explained by differences in canopy drainage and boundary layer conductance, and are caused primarily by the structural differences in their canopies. R. sphaerocarpa gave lower interception than the other two species, S. tenacissima gave higher interception, while A. cytisoideshad an intermediate value. The low interception loss by R. sphaerocarpa can be explained by its low total area index, thus, high free throughfall and high canopy drainage rate per unit projected canopy area. On the other hand, S. tenacissima and A. cytisoides, show a low free throughfall and drainage rate per unit projected canopy area because of their higher aerial biomass density. The ecological implications of these adaptations are discussed. (C) 1998 Elsevier Science B,V, All rights reserved.