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Tuesday, January 28, 2020

When Technology and Nature Collide: LiDAR and the Importance of Tree Canopy in the Water Cycle

Water and Tree Canopy

Tree canopy collects rain serving as a vital component of the hydrologic cycle.
Part One in a Three-Part Series on HARC’s program of work and research utilizing LiDAR in the Houston-Galveston region.

From Data to Images
Light Detection and Ranging (LiDAR) is a remote sensing method that uses light in the form of a pulsed laser to measure distances of various surfaces on the Earth. These light pulses—combined with other data recorded by the airborne system— generate a precise, three-dimensional representation of objects and surface characteristics of the Earth[1].

Once captured, points are stored as data in the form of a three-dimensional point cloud which represents buildings, vegetation, and the ground surface, among other features. Due to airborne LiDAR’s ability to sample large scale areas at a fine resolution, typically collecting four points of information within a square meter, high precision datasets can be produced. The final products such as a digital elevation model, detailed elevation contours, and the classification of ground returns, have many applications for forestry and hydrology.

However, it is the area of research where forestry and hydrology overlap, termed eco-hydrology, that we stand to benefit from a higher-level understanding of interactions between tree canopy and rainfall.

Water and Tree Canopy
Tree canopy and rainfall are intertwined in a series of processes, known as the hydrologic cycle, which describes the exchange of water through the Earth’s systems. Tree canopy, whether located in an urban or natural environment, contains an intricate structure of leaves, twigs, and branches that create ample surface area to capture, store, and eventually dissipate rainfall back to the atmosphere through evaporation. Canopy capture offers the first substantial stormwater volume reduction before rainfall reaches the Earth’s surface.

A tree’s role in the exchange of water doesn’t end in the upper layers of canopy. A portion of rainfall eventually reaches the Earth’s surface through fall or stemflow. Two other processes, infiltration and transpiration, take over from here and further reduce the volume of stormwater left as runoff. Infiltration occurs as rainfall works its way through the soil where it is stored, reused by plants, replenishes groundwater, and feeds waterways. Trees further process rainwater by pulling it up from the soil eventually releasing large quantities to the atmosphere as transpired water vapor.

The three-fundamental processes of evaporation, infiltration, and transpiration work to naturally dissipate rainfall, reducing the remaining volume of stormwater runoff. These processes function whether the interactions occur within a single tree or a group of trees forming a forest canopy.

LiDAR: Providing the Accuracy Needed
LiDAR provides a detailed representation of the Earth’s surface and, once transformed, can be used to describe tree canopy characteristics such as height, density, and degree of closure. Analyzing relationships between tree canopy and rainfall is critical to better understand how different combinations of canopy structure relate to stormwater. LiDAR provides a unique, highly accurate, opportunity to study urban and natural tree canopy at multiple scales from a single residential lot to a forested habitat within a watershed.


Wrap Up
Maintaining and restoring forests or incorporating trees into development projects can contribute to a robust system of green infrastructure throughout the region. HARC scientists seek to explore questions such as: How do differing tree canopy structures impact stormwater retention? How can remaining forest canopy best be leveraged? Where are these areas located?

The answers can be used to inform tree plantings for restoration or green infrastructure projects, and to help guide conservation efforts by identifying areas of forest canopy ideal to capitalize on flood mitigation benefits. Sharing this research will continue to provide decision-makers with science-backed information to formulate effective policy while serving as a public resource.

Preliminary data processing was sponsored by the Texas Forest Service under the Forest and Floods Project.
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1https://oceanservice.noaa.gov/facts/lidar.html)