Estimation of soil boundary-layer resistance in sparse semiarid stands for evapotranspiration modelling

Abstract

Summary The aim of this work was first, to compare two methods for obtaining soil boundary-layer resistance, one based on the turbulent diffusion theory (TD) and another based on the energy balance of heated and unheated sensors (EB), and secondly, to analyse how the results obtained from both methods are affected by canopy structure parameters in typical sparse vegetation. The study was carried out in stands of three different species characteristic of the semiarid South East of Spain ( Retama sphaerocarpa , Anthyllis cytisoides and Stipa tenacissima ). Comparison of the two methods showed that EB boundary-layer resistances, unlike TD, implicitly considers the peculiarities of the plant architecture and structure, as well as its distribution in the field and the presence or absence of substrate under the canopies. In contrast, for the TD method, quantitative attributes of the vegetation (average vegetation height, h , leaf area index, L , fractional vegetative cover, f , drag coefficient, c d and the eddy diffusivity decay constant, n ) are necessary, which are not only sometimes difficult to obtain in the field, but also seem to be insufficient to explain the development of the soil boundary layer in sparse semiarid vegetation. The advantages of using the EB method are both practical and theoretical. Positioning of the sensors and data storage with automatic data loggers is easy, and does not require knowledge of the aerodynamic attributes of the vegetated area. Furthermore, it allows qualitative interpretation of the effect of non-transpirable vegetative elements, as well as the architecture of the vegetation and its distribution in the field, neither of which are considered in the turbulent diffusion theory.