Abstract
Remote sensing can provide good alternatives for traditional in situ water status measurements in orchard crops, such as stem water potential (Ψstem). However, the heterogeneity of these cropping systems causes significant differences with regards to remote sensing products within one orchard and between orchards. In this study, robust spectral indicators of Ψstem were sought after, independent of sensor viewing geometry, orchard architecture and management. To this end, Ψstem was monitored throughout three consecutive growing seasons in (deficit) irrigated and rainfed pear orchards and related to spectral observations of leaves, canopies and WorldView-2 imagery. On a leaf and canopy level, high correlations were observed between the shortwave infrared reflectance and in situ measured Ψstem. Additionally, for canopy measurements, visible and near-infrared wavelengths (R530/R600, R530/R700 and R720/R800) showed significant correlations. Therefore, the Red-edge Normalized Difference Vegetation Index (ReNDVI) was applied on fully sunlit satellite imagery and found strongly related with Ψstem (R2 = 0.47; RMSE = 0.36 MPa), undoubtedly showing the potential of WorldView-2 to monitor water stress in pear orchards. The relationship between ReNDVI and Ψstem was independent of management, irrigation setup, phenology and environmental conditions. In addition, results showed that this relation was also independent of off-nadir viewing angle and almost independent of viewing geometry, as the correlation decreased after the inclusion of fully shaded scenes. With further research focusing on issues related to viewing geometry and shadows, high spatial water status monitoring with space borne remote sensing is achievable.
Highlights
In capital intensive cropping systems, such as pear orchards, prolonged periods of water deficiencies can lead to significant yield loss [1], making water status monitoring crucial to optimize irrigation scheduling [2]
Leaf water content is normally measured on tissue samples [8], which are subject to some variability, while Ψleaf provide an integrated measurement of environmental conditions on water availability within the leaf
This study aims at investigating the potential of WorldView-2 satellite imagery for the monitoring of
Summary
In capital intensive cropping systems, such as pear orchards, prolonged periods of water deficiencies can lead to significant yield loss [1], making water status monitoring crucial to optimize irrigation scheduling [2]. Technological advances in remote sensing provide non-destructive, time efficient and cost beneficial alternatives that visualize the spatial variability in water status at a wide range of temporal scales [4]. Leaf water potential (Ψleaf) has been found a more robust indicator of plant water status compared to plant water content [6,7]. Most researchers prefer stem water potential (Ψstem) as an indicator of plant water status in fruit orchards [7,11,12]. As opposed to plant water status and Ψleaf, the potential of remote monitoring of Ψstem is not yet fully explored nor widely adopted (but see examples for olives [13] and citrus [7,14])
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