دانلود رایگان مقاله انگلیسی کاربرد جدید سنجش از دور برای مدل سازی اثرات سایه درخت بر کیفیت آب - الزویر 2019

ABSTRACT

Uncertainty in capturing the effects of riparian tree shade for assessment of algal growth rates and water temperature hinders the predictive capability of models applied for river basin management. Using photogrammetry-derived tree canopy data, we quantified hourly shade along the River Thames (UK) and used it to estimate the reduction in the amount of direct radiation reaching the water surface. In addition we tested the suitability of freely-available LIDAR data to map ground elevation. Following removal of buildings and objects other than trees from the LIDAR dataset, results revealed considerable differences between photogrammetryand LIDAR-derived methods in variables including mean canopy height (10.5 m and 4.0 m respectively), percentage occupancy of riparian zones by trees (45% and 16% respectively) and mid-summer fractional penetration of direct radiation (65% and 76% respectively). The generated data on daily direct radiation for 2010 were used as input to a river network water quality model (QUESTOR). Impacts of tree shading were assessed in terms of upper quartile levels, revealing substantial differences in indicators such as biochemical oxygen demand (BOD) (1.58–2.19 mg L−1 respectively) and water temperature (20.1 and 21.2 °C respectively) between ‘shaded’ and ‘non-shaded’ radiation inputs. Whilst the differences in canopy height and extent derived by the two methods are appreciable they only make small differences to water quality in the Thames. However such differences may prove more critical in smaller rivers. We highlight the importance of accurate estimation of shading in water quality modelling and recommend use of high resolution remotely sensed spatial data to characterise riparian canopies. Our paper illustrates how it is now possible to make better reach scale estimates of shade and make aggregations of these for use at river basin scale. This will allow provision of more effective guidance for riparian management programmes than currently possible. This is important to support adaptation to future warming and maintenance of water quality standards.

 Conclusions

We have presented a methodology to estimate tree height and canopy extent, thereby allowing the calculation of daily shade, the fractional penetration of radiation and its effect on global incoming radiation for water quality modelling purposes. This methodology includes the use of high resolution spatial data capable of capturing riparian canopy structure and a model that simulates the position of the sun across the sky for hourly or sub-hourly intervals to model the daily shade over the river surface. It also uses measurements of hourly radiation and daily sunshine duration, which are corrected to account for the shade effect in order to be input to the water quality model. The results demonstrate:

• Consideration of riparian shading is important for water quality simulation, as is demonstrated by the big differences arising when considering or not considering shade (Table 1).

• An increased level of confidence about riparian shade condition in terms of percent occupancy (proportion of channel length with trees) and fractional penetration (fraction of direct radiation reaching the river through the tree canopy) along the River Thames.

• Water quality impacts are sensitive to the level of shade as estimated using the two datasets. Calculations using EA LIDAR DSM are different to those using NTM (partly due to the seasonal coverage of the EA LIDAR DSM data).