Sap flow, leaf area, net radiation and the Priestley–Taylor formula for irrigated orchards and isolated trees

Abstract

This paper describes the goodness of fit for two simple methods to estimate the daily sap flow of irrigated, non-stressed apple and olive trees in an orchard, and a walnut tree in isolation. The required inputs for the calculation are the tree leaf area ( L A, in m 2 tree −1), the net (all-wave) radiation over grass ( R N, in MJ m −2 day −1) and the average air temperature. Data are presented for mid-summer when daily R N ranged between 2 and 20 MJ m −2 day −1. Tree leaf area ranged between 8.65 m 2 for a dwarf apple and 35.5 m 2 for a large apple. With the first method, daily sap flow ( S, MJ tree −1 day −1) was empirically found to equal approximately 1/4 of R N times L A ( R 2 = 0.92, n = 72 days). The second method used the Priestley–Taylor equation with tree canopy net radiation term ( A, in MJ tree −2 day −1) empirically computed as A = 0.32 R N L A. Estimates of S based on the original α value of 1.26 did not differ significantly from a linear relationship ( R 2 = 0.91; n = 72; p < 0.05), for sap flows up to 56 L tree −1 day −1. However, there was a small leaf-area dependence for the ‘best-fit’ α value i.e., α = 1.41 − 0.0064 L A ( R 2 = 0.94; n = 4 trees). On average, the daily sap flow equated to about 2/3 of A. Both relationships appear robust and capable of providing a simple working alternative to the traditional crop-coefficient approach that relates crop water use to the potential evapotranspiration rate. The problem then shifts to that of obtaining a reliable estimate of tree leaf area either by destructive sampling or using a remote sensing method such as light transmission.