Proxy detection of wheat water stress from photochemical reflectance index and land surface temperature data

In semi-arid and arid regions, crops face elevated atmospheric demands and endure prolonged periods of moderate to severe water scarcity. In this context, this study investigated the effectiveness of the photochemical reflectance index (PRI) and a normalized surface temperature index (T_norm) for proxy detection of the water stress of winter wheat crops. Furthermore, the potential of PRI for characterizing water, atmospheric or photo-inhibition stress, and wheat transpiration was assessed over experimental drip-irrigated crop fields in the Haouz plain, central Morocco. In practice, PRI observations were compared to agro-environmental variables such as Leaf Area Index (LAI), Available Water Content (AWC) at a root zone depth, net Radiation (R_n), Vapor Pressure Deficit (VPD) and the wheat transpiration derived from sap flows, lysimeters and a crop water balance model. Due to the strong relationship between PRI and LAI (R² = 0.91), another index named PRI_j was derived to correct for this effect. The PRI_j was found to be independent of structural effects related to LAI and significantly correlated with AWC (R² = 0.85). Using the PRI_j index, we can reflect the level of water stress experienced by the wheat field throughout the experiment with an R² of 0.69 for a FAO-56 water stress coefficient (K_s) of less than 1. Under dry conditions, for an AWC below 30%, the correlation between AWC and T_norm gives an R² of 0.29. However, comparison of PRI_j with the T_norm index showed that PRI_j is an early water stress index and provides information on the state of the vegetation cover at all stages of wheat development. The study's findings can have a significant impact on the use of the PRI as a water stress indicator, helping in the optimal irrigation of crops.