Assessing plant water status in a hedgerow olive orchard from thermography at plant level

Abstract

a b s t r a c tWater scarcity is the most limiting factor in many irrigated areas of Mediterranean countries such as South Spain. Olive growing has been traditionally associated to rain-fed agriculture, although irrigation and practices related to intensive agriculture have been progressively introduced, requiring a more precise irrigation scheduling to save water. Thermal imaging is among the alternatives to assess the crop water status, especially when deficit irrigation (DI) strategies are applied. However, this technique requires of new advances to be more user friendly and robust for practical usage. The aims of this study were: i) to define threshold values of canopy temperature (T C ), Crop Water Stress Index (CWSI) and the temperature difference between canopy and the surrounding air ( T canopy-air ) for the assessment of the olive water status when a DI strategy is applied; ii) to define the best time of the day and the best area of the canopy to carry out thermal measurements, and iii) to obtain relationships between thermal indicators and main physiological parameters useful to estimate the crop water status from thermal data. The trial was conducted during 2015, in a hedgerow olive orchard (SW Spain) with 8-year-old trees (Olea europaea L., cv. Arbequina), under three irrigation regimes: a full-irrigation treatment (FI) and two regulated deficit irrigation treatments aimed to supplying 45% of the irrigation needs. In one of them, irrigation was scheduled from leaf turgor pressure related measurements (45RDI TP ). In the other, the crop coefficient approach was used to schedule irrigation (45RDI CC ). Significant correlations between Tc versus stem water potential (« st ) and leaf gas-exchange parameters (stomatal conductance to water vapour, g s ; net CO 2 assimilation, A N ; transpiration, E) were obtained (p ≤ 0.05), in particular from measurements taken at 10:30 GMT in the lower part of the sunlit side of the canopy. Moreover, the relationships between both T canopy-air and CWSI with the monitored physiological variables were very robust. We concluded that values of T canopy-air higher than 0 • C and values of CWSI up to 0.2 reliably reflect the plant water stress. Our results, therefore, suggest that both T canopy-air and CWSI measured at midday provide reliable information on the tree water status and are useful to schedule irrigation in hedgerow olive orchards, especially under DI conditions.

Determination of Crop Water Stress Index by Infrared Thermometry in Grapefruit Trees Irrigated with Saline Reclaimed Water Combined with Deficit Irrigation

“…The close behavior encountered between Tc-Ta and Tc with Ψ stem (p < 0.005) (Table 4) confirmed the previous findings on the use of canopy temperature as the crop water status indicator in most orchards [18,48,[51][52][53] including citrus [24] where the authors found higher correlations in Navelina orange trees (r 2 = 0.75). On the contrary, other authors such as those from Reference [13] reported on the difficulty of using Tc measurements obtained with fixed IRTs and normalized with air temperature (Tc-Ta) as a water stress indicator in citrus because they were poorly related with Ψ.…”

Simplified Evaluation of Cotton Water Stress Using High Resolution Unmanned Aerial Vehicle Thermal Imagery

“…Canopy temperature can be used to monitor crop water status. CWSI is also a good indicator of crop water status [44,45], and some previous studies have suggested that good relationships exist between CWSI and the stomatal conductance, transpiration rate as well as soil water moisture respectively [23,46]. Here the Canny edge detection algorithm was used to eliminate soil background pixels from UAV thermal infrared orthomosaic images, and CWSIsi parameters (T l , T wet , and T dry ) were calculated from a canopy temperature histogram.…”

“…In particular, Unmanned Aerial Vehicle (UAV) thermal infrared remote sensing has been widely used in agriculture for crop water stress monitoring and irrigation management purposes [19][20][21]. In addition, some studies have pointed out that for crops such as olive, potato, and cotton, there is a robust relationship between CWSI calculated using parameters obtained from UAV thermal infrared images and physiological measurements of stomatal conductance (gs), transpiration rate (tr), and soil water content [22][23][24][25].…”

A New Low-Cost Device Based on Thermal Infrared Sensors for Olive Tree Canopy Temperature Measurement and Water Status Monitoring

“…With this approach, we discarded 2.2% of the data situated at the upper and lower thresholds of the normal distribution of temperature. These data were considered outliers, which could represent, for example, surfaces of the tree without transpiration capacity (branches or stems) [39].…”