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

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,,, in m(2) tree(-1)), the net (all-wave) radiation over grass (RN, in Mj m(-2) day(-1)) and the average air temperature. Data are presented for mid-summer when daily RN 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 RN times LA (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 = 0.32R(N) L-A. Estimates of S based on the original a 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'bestfit' a value i.e., alpha = 1.41 - 0.0064L(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. (c) 2007 Elsevier B.V. All rights reserved.