Agricultural and Forest Meteorology 184 (2014) 1–11 Contents lists available at ScienceDirect Agricultural and Forest Meteorology jou rn al hom ep age: www.elsevier.com/locate/agrformet Fraction of canopy intercepted radiation relates differently with crop coefficient depending on the season and the fruit tree species Jordi Marsal a,∗ , Scott Johnson b , Jaume Casadesus a , Gerardo Lopez a , Joan Girona a , Claudio Stöckle c a Irrigation Technology, Institut de Recerca i Tecnologia Agroalimentàries (IRTA), Centre UdL-IRTA, Avda. Rovira Roure, 191, 25198 Lleida, Spain b Plant Sciences Department, University of California Davis Kearney Ag Center, Parlier, CA 93648, USA c Biological Systems Engineering Department, Washington State University, Pullman, WA 99164-6120, USA a r t i c l e i n f o Article history: Received 3 April 2013 Received in revised form 21 August 2013 Accepted 26 August 2013 Keywords: Irrigation CropSyst Lysimeter Transpiration Stem water potential a b s t r a c t It is commonly assumed that the fraction of canopy intercepted radiation (f IR ) should be well correlated with irrigation crop coefficients (Kc) throughout the season. However, in fruit trees there is some evidence that such a correlation is different between pre-harvest and postharvest periods. Over two different years, basal Kc (K cb ) data from three different weighing lysimeters (one in California growing peach trees, and the other two in Catalonia growing apple and pear trees) were analyzed using two parameters of the CropSyst growth model: full canopy Kc (Kc fc ) and maximum plant hydraulic conductance (C max ). In CropSyst, K cb is approximated as f IR times Kc fc . The latter is usually seasonally fixed, but for this study it was made variable so that it could be adjusted every fortnight throughout the season. Variable Kc fc implies the possibility that the K cb relationship with f IR is not constant. The objective of this study was to evaluate possible seasonal patterns in the Kc fc . The results indicated that Kc fc was variable in all species and it followed a distinctive pattern in three different time periods: (i) initial rise (spring), (ii) plateau or slight decline (mid-summer), and (iii) decline (autumn). However, the magnitude of Kc fc fluctuation was different among the three species. It fluctuated the most in the slowest growing species (pear), and the least in the fastest growing species (peach). Apple had an intermediate response. In conclusion, Kc is not a fixed function of f IR . Assumption of a fixed function will introduce errors in plant water use estimation, which could be especially large in pears and apples. This will be by 50% in pears during postharvest. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Modern fruit production is facing the challenge of limited water resources. In order to optimize irrigation it is necessary to improve the accuracy of irrigation scheduling programmes. Water requirements can be calculated using the following equa- tion: ETc = (Kcb + Ke) × ETo (Allen et al., 1998), where ETc is crop evapotranspiration, ETo is the reference evapotranspiration, Kcb is basal crop-specific coefficient that primarily represents plant transpiration, and Ke accounts for soil evaporation. Accurate deter- mination of Kc (Kcb + Ke) is a prerequisite for sound irrigation scheduling. It is widely acknowledged that the fraction of crop intercepted radiation (f IR ) is a major determinant of Kc (Suay et al., 2003). It represents the energy that can be absorbed by the canopy and therefore be used for transpiration, and it has been assumed that the relationship between absorbed energy and transpiration does not change throughout the season (Pereira et al., 2007). This is ∗ Corresponding author. Tel.: +34 973 702670; fax: +34 973 70 2420. E-mail address: jordi.marsal@irta.es (J. Marsal). supported by the literature published on peach growing in lysimeters in California reporting that noon intercepted radiation produced a significant linear relationship with Kc (Ayars et al., 2003, Johnson et al., 2005). However, experiments done in apple and pear lysimeters in Catalonia indicated that Kc showed moderate declines after harvest without changes in canopy foliage (Girona et al., 2011). Auzmendi et al. (2011) explained such declines after apple harvest by a reduction in the ratio of transpiration to intercepted radia- tion. This seems to emphasize that there are also other factors to consider such as canopy conductance. Therefore, there seems to be some basis for challenging the assumption of constancy in the relation between f IR and Kc. For instance it has been found that fruit sinks are related to leaf conductance which decreases when fruit are thinned or harvested in peach (Marsal and Girona, 1997). In terms of tree transpiration, this has also been shown in apple (Reyes et al., 2006). Such a principle of constancy has been successfully used in modelling to calculate evapotranspiration for annual crops in Crop- Syst (CS) (Stöckle et al., 2003). In CS the f IR is used as a multiplier coefficient of maximum evapotranspiration to separate crop tran- spiration from soil evaporation. This maximum ET is calculated as 0168-1923/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agrformet.2013.08.008