Revisiting reference evapotranspiration calculation under regional advection and its effect on single and dual crop coefficients: An empirical approach for quinoa crop

Advection is a prevailing meteorological phenomenon in the arid and semi-arid environments that directly affects the crop and soil hydrology. It could have a great impact on the rate of standard crop evapotranspiration (ET_c) in the irrigated areas. Therefore, it is required to take it into consideration while computing crop water requirement through the physically based meteorological procedures. The objectives of this study are (1) simple modification of the Penman–Monteith (PM) equation in calculation of the grass reference evapotranspiration (ET_o) to implement the local advection, (2) comparing the single (K_c) and dual crop coefficient (K_cb) of two quinoa cultivars (Titicaca, and Q5) with and without advection correction and (3) presenting a simple model to calculate the advection factor using the maximum air temperature and mean relative humidity for the future crop growth modelling studies. Both Q5 and Titicaca showed unexpectedly very high ET_c and potential transpiration (T_p) as 1568 mm and 1003 mm, and 1156 mm and 829 mm, respectively, due to occurrence of regional advection, whereas very high unrealistic K_c and K_cb values for Q5 revealed the impact of strong local advection and external energy. Therefore, we modified ET_o to implement the advection effect through introducing the advection factor, ET_c/R_n (R_n is the net radiation), as a function of maximum air temperature and mean relative air humidity during the growing season [ET_c/R_n = exp (0.025T_max – 0.015RH_avg)] which resulted in higher ET_o values, and consequently lower and more realistic K_c and K_cb. As a result, modified maximum K_c values of 1.14 and 1.55 and the modified maximum K_cb of 0.94 and 1.0 were obtained for Titicaca and Q5 cultivars, respectively. This procedure leads to a more accurate site-specific K_c estimation and indirectly reliable ET_c estimation as a function of advection factor and its multiplication by the non-modified ET_o. Furthermore, for direct estimation of ET_c through the PM equation, the coefficients of aerodynamic and canopy resistance components of PM equation were modified for estimation of ET_c by the non-modified R_n, which resulted in accurate estimation of ET_c.