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 (LA, in m2 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 m2 for a dwarf apple and 35.5 m2 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 (R2 = 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 = 0.32RNLA. Estimates of S based on the original a value of 1.26 did not differ significantly from a linear relationship (R2 = 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� a value i.e., a = 1.41 - 0.0064LA (R2 = 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