Applicability and limitations of using the crop water stress index as an indicator of water deficits in citrus orchards

Accurate measurements of the crop water status are becoming essential in irrigated agriculture, as water resources are limited and its use must be optimized, especially in semi-arid conditions. Indicators derived from thermal information have shown to be closely related to water status in several fruit tree species, and have shown promise for assessing the spatial variations among and within whole orchards. Here, a methodology is proposed for assessing the Crop Water Stress Index (CWSI) of mandarin (Citrus reticulata Blanco cv. Clemenvilla) and navel orange (Citrus sinensis L cv. Powell) located in Southern Spain, taking into account the short-term fluctuations in canopy temperature observed in both species. Infrared thermal sensors were installed above trees to record canopy temperature (Tc) continuously for three seasons from 2009 to 2011. The Non Water Stress Baseline (NWSB) was calculated using Tc of well irrigated trees on cloudless days during summertime. The NWSB was affected by flushes of growth that occurred periodically, depending on the crop load of a given year. Nevertheless, the close relationship observed between CWSI and stem water potential (R2 ranging between 0.59 and 0.66; p < 0.001) demonstrated that it is a suitable indicator of water status in citrus. The results showed that care must be taken when using CWSI in citrus to account for the presence of new growth at the top of the canopy, and for the short-term fluctuations in canopy temperature. The canopy temperature information acquired from point sensors was used in conjunction with high resolution airborne thermal imagery to derive CWSI maps. The approach presented here demonstrates that CWSI is a valuable method to assess the water status, and to quantify the spatial variability in water stress among and within citrus orchards using high-resolution airborne thermal imagery.