A simple model for estimating the Bowen ratio from climatic factors for determining latent and sensible heat flux

Abstract

The energy partitioning or Bowen ratio β at the surface, can be expressed as a function of a climatic factor C (which depends on the climatic characteristics through the climatological resistance r i) and of a surface factor S (which depends on the surface characteristics through the surface resistance r c). This paper explains an approach for estimating S and therefore β as a function of standard meteorological variables. The empirical approach is then validated by estimating the latent ( λE) and sensible ( H) heat flux. The experimental data were measured over grass at two semiarid locations in the Ebro river valley, NE Spain, with typical Mediterranean climates: at Mas Bove, using a Bowen ratio-energy balance method from 1991 to 1993, and at Zaragoza, using a weighing lysimeter from 1999 to 2000. Results show that the surface factor S depends on the square root of the absolute value of r i/ r a (where r a is the aerodynamic resistance), with two locally calibrated empirical parameters. This result is valid for the largest possible range of situations ( β values) throughout the day. Using three different hypotheses for computing r a, results for β (following Hypothesis 1) indicate a good level of performance by the model, although with a high degree of variability. The estimated latent heat flux λE est,1 tends to slightly underestimate the measured values. Under the semiarid conditions of the region, λE PM estimates obtained with the Penman–Monteith equation produce relative errors that are greater than those obtained with the proposed model once it has been calibrated. On average, sensible heat flux tends to be overestimated with the proposed method.