Estimating transpiration dynamics of a low-density litchi orchard using crop coefficients derived from a variable leaf conductance model, canopy cover, and tree height in Northeastern South Africa

Abstract

Commercial production of litchi (Litchi chinensis Sonn.) in arid and semi-arid areas is almost exclusively cultivated with irrigation. In this study we have improved the calculation of the basal crop coefficient ( Kcb) of litchi for estimating orchard transpiration following the Allen and Pereira (Irrig Sci 28(1):17–34, 2009) (A&P) approach. The original A&P approach calculates Kcb from fixed leaf resistance ( rl) values for specific growth stages, but in reality rl varies depend¬ing on a number of factors such as genotype, environmental conditions, crop growth stages, management, etc. We show that significantly improved Kcb, and therefore transpiration estimation, can be obtained using variable values instead of fixed values of rl in the A&P approach. The original A&P approach uses a typical leaf resistance (rtyp) of 100 s/m, a value derived for annual crops. This study derived a specific rtyp of 55 s/m for litchi trees using measured data, whereas rl was modelled as a variable by Jarvis leaf resistance model. Orchard transpiration was subsequently calculated as the product of the Kcb and the reference evapotranspiration (ETo). A comparison of calculated and measured transpiration rates resulted in a coefficient of determination of 0.82, a normalized root mean square error of 0.12, a normalized mean absolute error of 0.10, and a Nash-Sutcliffe Efficiency of 0.64. We conclude that the use of the variable rl and rtyp which are specific to litchi trees gives a more accurate estimation of the transpiration of litchi trees.