On the potential for high-resolution lidar to improve rainfall interception estimates in forest ecosystems

© The Ecological Society of America www.frontiersinecology.org T is a crucial need to understand ecological water balances, both in the present and in the future. This balance is currently shifting, as economies rapidly develop and urbanization accelerates (Hubacek and Sun 2005). In the future, global climate change and the resulting uncertainties will only add to the urgency of finding accurate and reliable prediction tools that can be used to understand regional water supply, ecosystem function, and landscape biogeochemistry. For nearly a century, researchers have developed hydrologic models to this end, but their effectiveness has been hampered by a shortage of detailed data (Singh and Woolhiser 2002). Canopy interception of rainfall is a key component in the hydrologic cycle (Figure 1). In closed-canopy ecosystems, the canopy can intercept as much as 10–50% of incoming total precipitation, depending on a variety of factors. This intercepted rainfall may be evaporated to the atmosphere (interception loss), absorbed by the canopy (storage), channeled downward along branches and stems (stemflow), or dripped to the ground (throughfall). Recent investigations have indicated that canopy structure, in combination with rainfall regime (ie short, intense versus long, sustained rainfall events), influences rainfall interception and evaporation to a greater extent than previously recognized (Liu 2001). Canopy structure is extremely difficult to quantify due to its high three-dimensional (3D) spatial variability over relatively short distances (Pypker et al. 2005). In extreme cases, researchers have resorted to using construction cranes to obtain detailed canopy structural measurements (Nadkarni and Sumera 2004). Emerging remote sensing technologies such as lidar (light detection and ranging) offer the ability to collect 3D canopy structural information at a resolution that has not previously been attainable (Lefsky et al. 1999). A better understanding of how rainfall is intercepted by the canopy of trees will broadly impact ongoing ecological research. The interacting processes that influence the amount and distribution of rainfall interception across a landscape remain somewhat poorly understood, and are difficult to model. For example, rainfall interception by forested canopies depends on individual rainstorm intensity, duration (Keim et al. 2004), and nominal droplet size (Murakami 2006). Thus, interception varies dramatically with the characteristics of rainfall events and the heterogeneity of gaps in the canopy (Liu 2001). Interception loss via evaporation from branch and leaf surfaces is depenREVIEWS REVIEWS REVIEWS