Abstract
Surface Canopy Water (SCW) is the intercepted rain water that resides within the tree canopy and plays a significant role in the hydrological cycle. Challenges arise in measuring SCW in remote areas using traditional ground-based techniques. Remote sensing in the radio spectrum has the potential to overcome the challenges where traditional modelling approaches face difficulties. In this study, we aim at estimating the SCW by fusing information extracted from the radar imagery acquired with the Sentinel-1 constellation, aerial laser scanning, and meteorological data. To describe the change of radar backscatter with moisture, we focused on six forest stands in the H.J. Andrews experimental forest in central Oregon, as well as four clear cut areas and one golf course, over the summers of 2015–2017. We found significant relationships when we executed the analysis on radar images in which individual tree crowns were delineated from lidar, as opposed to SCW estimated from individual pixel backscatter. Significant differences occur in the mean backscatter between radar images taken during rain vs. dry periods (no rain for >1 h), but these effects only last for roughly 30 min after the end of a rain event. We developed a predictive model for SCW using the radar images acquired at dawn, and proved the capability of space-based radar systems to provide information for estimation of the canopy moisture under conditions of fresh rainfall during the dry season.
Highlights
Surface canopy water (SCW) is the water resting on the outer surfaces within the canopy layer, and plays a significant role on the hydrological cycle and on the characteristics of the boundary layer of the troposphere [1]
The stands were comprised of different sizes of trees, they behaved in a similar way with regard to their mean backscatter (Figures 6–8 and Table 2), which decreased in the overall brightness with temperature (Figure 7) and during rain events (Figure 9)
Our study provides evidence that C-band radar is sensitive to summertime tree canopy moisture in the hilly Pacific Northwest forests
Summary
Surface canopy water (SCW) is the water resting on the outer surfaces within the canopy layer, and plays a significant role on the hydrological cycle and on the characteristics of the boundary layer of the troposphere [1]. SCW can refer to canopies of crops, grasses, and forests, in this paper, we refer to the structurally complex forest canopies of the Pacific Northwest region of North America. The extensive networks of weather stations make the prediction of SCW relatively easy and accurate in flat areas [2,3], but significant challenges are faced in mountainous or remote regions, as more complex extrapolations are required or crucial parameters may be unknown [4]. Remotely sensed data can enhance prediction of SCW in areas where the traditional modelling approaches are challenged. The critical aspect of using remote sensing data in any investigation is the selection of the appropriate wavelengths. Radar (RAdio Direction And Ranging) is preferred over passive optical sensors, as they exhibit a series of sought-after properties, such as: (1) ability to penetrate cloud cover and vegetation ( the penetration power depends highly on the wavelength), (2) independence of local solar intensity (which allows operation irrespective of day or night), and (3)
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