The repellency of a water droplet by a leaf surface (i.e., leaf hydrophobicity) is a common adaptation among plant species in habitats exposed to daily precipitation. Leaf hydrophobicity may be an important variable that influences canopy storage capacity during a rainfall event. Canopy storage capacity is the amount of water held by the canopy during a rainfall event before water starts to drip as indirect throughfall. Species with highly repellent leaf surfaces may increase the quantities of throughfall at a site and result in greater hydrological inputs beneath the canopy.

Professor Curtis Holder in the Department of Geography and Environmental Studies at the University of Colorado at Colorado Springs will examine the extent to which leaf hydrophobicity influences canopy storage capacity in common species of the semi-arid Western United States. Specifically, the three objectives of this project are to determine if leaf hydrophobicity and canopy storage capacity differs between species with contrasting leaf habits and growth forms, to compare methodologies that calculate leaf hydrophobicity and canopy storage capacity, and to determine if leaf hydrophobicity influences canopy storage capacity on a leaf area basis. To address these three objectives, the investigator will measure canopy storage capacity by a mass-balance approach with rainfall simulation and an immersion method. Additionally, the investigator will measure leaf hydrophobicity by calculating the contact angle between a water droplet and the leaf surface using a digital camera and goniometer. Canopy characteristics (leaf and woody area, leaf orientation, and leaf surface properties) of each species will also be determined. If differences in leaf hydrophobicity and canopy storage capacity between species are significant, then hydrological inputs to a watershed may be significantly influenced by vegetation changes. Within this context, this project will increase our understanding of hydrological processes and inform the planning and management of vegetation cover in watersheds contributing as raw water source regions for municipalities.

This project will enhance our understanding of hydrologic fluxes within watersheds by examining the significance of leaf hydrophobicity as a mechanism that influences canopy storage capacity. By measuring leaf hydrophobicity and canopy storage capacity of common species within a watershed, this project will provide information on the significance of leaf hydrophobicity in forest hydrology models that could enhance our understanding of the delivery of water resources from municipal watersheds. Several undergraduate and graduate students will receive valuable educational opportunities by participating directly in field data collection during this project.

Project Report

This study examined the relationships between leaf hydrophobicity, water droplet retention, and canopy storage capacity in 24 common species in semi-arid regions of the western United States. Canopy storage capacity is the amount of water held on the canopy at the end of a rainfall event, after drip has ceased and in the absence of wind. Leaf hydrophobicity is a measure of how repellent a water droplet is on a leaf surface, and is commonly measured by calculating the contact angle between a sessile water droplet and the leaf surface. Water droplet retention is a measure of how easily a water droplet drains off a leaf surface, and is commonly measured as the angle at which a sessile water droplet will begin to drain off a leaf surface as the leaf surface is incrementally tilted. Eleven students were active participants in this research project. The research objectives were: (1) to determine if leaf hydrophobicity, water droplet retention, and canopy storage capacity differ between the species, (2) to compare methodologies of calculating leaf hydrophobicity, water droplet retention, and canopy storage capacity, and (3) to determine if leaf hydrophobicity and water droplet retention influence canopy storage capacity on a leaf area basis. This study found that the canopy storage capacity of species in Colorado was influenced by leaf characteristics such as leaf hydrophobicity and water droplet retention. Leaf surface storage was lower in species with greater leaf hydrophobicity and greater water droplet retention. Small differences in leaf hydrophobicity and water droplet retention between species influence leaf surface storage. Although branches and juvenile plants of selected species were used in the experimental design of this study, the influence by leaf hydrophobicity and water droplet retention between species would be magnified at the scale of large trees with all branches and leaves intact or at the scale of a watershed with several trees of various species. Changes in canopy surface properties such as leaf area index have been found to significantly influence stream discharge from watersheds. Because increases in leaf hydrophobicity and increases in water droplet retention lower leaf surface storage, vegetation changes may also impact discharge of watersheds. Humans have consistently altered vegetation in watersheds by replacing existing species with new species. Changes in vegetation type in watersheds (e.g., agriculture, urban landscaping, etc.), may influence the discharge from watersheds because of the influence that leaf hydrophobicity and water droplet retention has on interception processes, including leaf surface storage and canopy surface storage. The impact of changes in leaf hydrophobicity and water droplet retention on ecohydrological processes is an important avenue for further investigations.

Agency
National Science Foundation (NSF)
Institute
Division of Behavioral and Cognitive Sciences (BCS)
Type
Standard Grant (Standard)
Application #
0961474
Program Officer
Antoinette WinklerPrins
Project Start
Project End
Budget Start
2010-05-15
Budget End
2011-10-31
Support Year
Fiscal Year
2009
Total Cost
$97,831
Indirect Cost
Name
University of Colorado at Colorado Springs
Department
Type
DUNS #
City
Colorado Springs
State
CO
Country
United States
Zip Code
80918