MEASUREMENT AND MODELING OF RAINFALL INTERCEPTION BY 3 SEMIARID CANOPIES

The main aims of this study were, firstly, to adapt the rainfall inter ception model of Rutter et al. (Agric. Meterology, 1971, 9, 367-384) t o individual plants of two semiarid shrubs (Anthyllis cytisoides L. an d Retama sphaerocarpa (L.) Boiss.) and a tussock grass (Stipa tenaciss ima L,) and secondly, to understand how the different canopy structure s influence rainfall partitioning by individual plants. The selected s pecies represent contrasting canopy types typical of vegetation of sem iarid areas. Free throughfall coefficients were estimated from field m easurements of low volume rainfall events and vertical photographs tak en beneath the plant canopy. Canopy drainage curves were measured by c ontinuous weighing of wetted plants. Canopy boundary layer conductance s were calculated by measuring the evaporation of water from wet canop ies, Field measurements of gross rainfall, throughfall and stemflow we re taken for each rainfall event for A. cytisoides and R. sphaerocarpa . The Rutter type model of rainfall interception was adapted for indiv idual shrubs and tested with measured rainfall events showing a good a greement between observed and predicted values for R. sphaerocarpa and for A. cytisoides. The interception model was then run to simulate in terception loss during actual rainfall events, using atmospheric condi tions measured every 5 s. The results from this simulation showed sign ificant differences in interception loss between species, which can be explained by differences in canopy drainage and boundary layer conduc tance, 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. cytisoidesh ad an intermediate value. The low interception loss by R. sphaerocarpa can be explained by its low total area index, thus, high free through fall and high canopy drainage rate per unit projected canopy area. On the other hand, S. tenacissima and A. cytisoides, show a low free thro ughfall and drainage rate per unit projected canopy area because of th eir higher aerial biomass density. The ecological implications of thes e adaptations are discussed. (C) 1998 Elsevier Science B,V, All rights reserved.