A Comparison of Three Methods to Estimate Evapotranspiration in Two Contrasting Loblolly Pine Plantations: Age-Related Changes in Water Use and Drought Sensitivity of Evapotranspiration Components

Abstract

Increasing variability of rainfall patterns requires detailed understanding of the pathways of water loss from ecosystems to optimize carbon uptake and management choices. In the current study we characterized the usability of three alternative methods of different rigor for quantifying stand-level evapotranspiration (ET), partitioned ET into tree transpiration (T), understory transpiration, interception, and soil evaporation (E S ) and determined their sensitivity to drought, and evaluated the reliability of soil moisture measurements by taking into account deep soil moisture dynamic. The analyses were conducted in an early-and in a mid-rotation stand of loblolly pine, the predominant species of southern US forest plantations. The three alternative methods for estimating ET were the eddy covariance measurements of water vapor fluxes (ET EC ), the water table fluctuation (ET WT ), and the soil moisture fluctuation (ET SM ). On annual and monthly scales, the three methods agreed to within 10 -20%, whereas on a daily scale, the values of ET SM and ET EC differed by up to 50% and ET SM and ET WT differed by up to 100%. The differences between the methods were attributed to root water extraction below measurement depth and to the sampling at different spatial scales. Regardless of the method used, ET at the early-rotation site was 15-30% lower than that at the mid-rotation site. The dry years did not affect ET at the mid-rotation site but reduced significantly ET at the early-rotation site. Soil moisture trends revealed the importance of measuring water content at several depths throughout the rooting zone because less than 20% of the water is stored in the top 30 cm of soil. Annually, E S represented approximately 9 and 14% of ET EC at the mid-rotation site and the early-rotation site, respectively. At the mid-rotation site, T accounted for approximately 70% of ET EC . Canopy interception was estimated to be 5-10% of annual precipitation and 6 -13% of total ET EC . At the early-rotation site, T accounted for only 35% of ET EC . At this site, transpiration from subdominant trees and shrubs represented 40 -45% of ET EC , indicating that understory was a significant part of the water budget. We concluded that the eddy covariance method is best for estimating ET at the fine temporal scale (i.e., daily), but other soil moisture and water table-based methods were equally reliable and cost-effective for quantifying seasonal ET dynamics. FOR. SCI. 58(5):497-512.Keywords: eddy covariance, loblolly pine, Pinus taeda, sapflow, soil moisture probes, water table L OBLOLLY PINE (PINUS TAEDA L.) REPRESENTS ONE-HALF OF THE STANDING PINE VOLUME in the southern United States (11.7 million ha) and is by far the single most commercially important forest tree species for the region, with more than 1 billion seedlings planted annually (McKeand et al. 2003). More than 1 million ha of intensively managed loblolly pine plantations are found along the lower coastal plain in eastern North Carolina. Large areas of the North Carolina...

Daily Landsat-scale evapotranspiration estimation over a forested landscape in North Carolina, USA, using multi-satellite data fusion

“…The study area is flat, about 3 m a.s.l. and has been ditched (fourth-order ditches at 100 m spacing) to manage the water table and improve tree productivity (Domec et al, 2012). The soil is Belhaven series Histosol, with a 50-85 cm organic layer over coarse glacial outwash sand .…”

“…While EC is a widely used observation method and provides an important data source to many research fields , it measures turbulent fluxes over a relatively small footprint area (10 2 -10 4 m 2 ), which is determined by the microclimate conditions around the flux tower and the instrument height. Catchment water balance is also a frequently used method, calculating ET from long-term precipitation and streamflow observations with the assumption that the soil water storage change is negligible (Domec et al, 2012;Wilson et al, 2001). All these observation methods have their inherent advantages and limitations, especially when considering both temporal and spatial resolution issues.…”

Hurricane impacts on a pair of coastal forested watersheds: implications of selective hurricane damage to forest structure and streamflow dynamics

“…Similarly, Sun et al (2010) showed that transpiration and soil evaporation accounted for about 83 % of total ET. In forested watersheds dominated by closed canopies and thick litter layers, soil evaporation is likely to be minimal (e.g., Ford et al, 2007), though Domec et al (2012) report a value of 9 % of ET attributed to soil evaporation from a mid-rotation managed pine plantation. Interception losses or canopy evaporation losses are closely linked to forest structure - Bryant et al (2005) reported interception losses equivalent to 17.7 and 18.6 % of total precipitation for a mixed forest and lowland hardwood forest, respectively.…”

“…Based on Ford et al (2007) and Wilson et al (2006), reported transpiration losses ranged between 53 and 57 % of ET, with the balance comprising canopy interception and direct soil evaporation. Using eddy flux tower measurements, Domec et al (2012) showed that transpiration accounted for 70 % of total ET in a mid-rotation managed pine forest. Similarly, Sun et al (2010) showed that transpiration and soil evaporation accounted for about 83 % of total ET.…”

Water extraction variability in the banana root zone affects the reliability of water balance

“…It is risky to assume that the moisture variations in the root zone of a crop can be represented by a few moisture samples because the water extraction amplitude is uneven in the different soil moisture monitoring positions. So, it is safer to monitor soil moisture at various positions of the root zone (Domec et al, 2012). …”