A comparison of methods for determining forest evapotranspiration (original) (raw)
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Evapotranspiration (ET) represents the second largest flux in terrestrial ecosystem water budget. In recent years, much attention has been given to the coherent linkages among hydrological cycle, ecophysiological processes, disturbances, and ecosystem function. However, quantification of ET at various temporal and spatial scales remains challenging (e.g., continuous changes of ET with time of a forest). Large uncertainties and measurement errors exist in fully accounting the ET flux, a process that involves both the physical (atmospheric and soil water control) and biological processes (leaf stomata and stem conductance control). In 2004, we established three research sites to study the climatic and forest management effects on ecosystem carbon and water balances in three contrasting forests: an oak openings in NW Ohio, a recent plantation of loblolly pine in eastern North Carolina, and a 13 year-old loblolly pine stand in eastern NC. The oak-opening ecosystem in a dry, cold environ...
Forest Science, 2012
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.
Journal of Hydrology, 2015
Patchy planted forest Stand level flux Water vapor exchange s u m m a r y Evapotranspiration (ET) was estimated from a planted coniferous forest in southwestern Japan by applying three methods: the eddy covariance method; the measurement of rainfall (P) and runoff (Q) in a small watershed; and a combination of rainfall interception loss (I C), upper canopy transpiration based on a sap-flux density measurement in Japanese cedar (Cryptomeria Japonica D. Don) stands (E UC), and modeled sub-canopy ET (E SC). After inverse multiplication of the energy imbalance ratio, ET by the eddy covariance method (ET EC) was 839.9 mm in 2007 and 811.8 mm in 2008. The yearly values of P-Q were partially affected by P in the previous autumn. After continuous data collection for more than 5 years, P-Q became stable. The 9-year (2000-2008) average P-Q, which was considered most reliable in this study, was 897.5 mm y À1. The cumulative ET EC during the daylight hours from the right stream bank, covered mainly with large Japanese cedars, was 894.1 mm from April 2007 to March 2008. The value was almost the same as that calculated as the components sum (ET COMP = I C + E UC + E SC : 911.4 mm), and the comparison suggested that the annual totals of ET EC with an energy imbalance correction provide a reliable estimate of ET in a forest stand on a complex topography. Spatial variation in the watershed was likely caused by differences in soil water retention at each slope position. The slight difference in annual ET EC in 2007 compared with 2008 was attributed to differences in the radiative energy input. In the monthly-weekly analysis, ET COMP was frequently higher than ET EC after heavy rainfall, while ET EC was higher under dry conditions and during active ET. Even under dry canopy conditions, daily ET EC was often higher than E UC + E SC. The results suggested a time-lag in evaporation from the ecosystem and/or underestimated ET EC after rainfall.
Basic and Applied Ecology, 2002
Sapflow density was measured in six stands in a boreal forest in central Sweden, to assess its dependence on soil moisture and stand characteristics. The stands were mixed and pure Scots pine and Norway spruce stands, which were between 34 and 105 years old. Sapflow was measured in 12 trees per stand using the Granier method during two contrasting growing seasons; one warm and dry and one wet and cool. The canopy conductance of the stands was estimated by the inverse of the Penman-Monteith equation, using time-lag-adjusted sapflow as input. Maximum canopy conductance varied between 8 mm s-1 and 88 mm s-1 for the stand with the lowest and highest conductance, respectively. Transpiration was higher in the dry, warm season, mean values for the different stands ranging between 1.30 to 4.64 mm day-1 during July to September. The corresponding range in the wet, cool season was 0.95 to 2.65 mm day-1. Besides climatic factors, stand age, stem density and diameter explained most of the variation in sapflow density. By use of multiple regression analysis for 5-day periods it was possible to estimate sapflow density and transpiration for a larger area of the forest. This upscaled areal transpiration was compared with evaporation measured by an eddy-correlation system located centrally in the area. It was shown that areal transpiration constituted 78% of total evaporation in the warm, dry season and 52% in the wet, cool season. It was not possible to establish with confidence a critical limit for soil water at which transpiration began to be reduced, mainly because of wide scatter in the relationship between potential and actual transpiration. Die Saftflussdichte wurde in sechs Beständen des borealen Waldes Zentralschwedens gemessen, um ihre Abhängigkeit von der Bodenfeuchte und den Bestandseigenschaften zu beurteilen. Die Bestände waren reine und gemischte Bestände von Waldkiefer und Fichte, zwischen 34 und 105 Jahre alt. Der Saftfluss wurde mit der Granier-Methode an 12 Bäumen pro Bestand in zwei unterschiedlichen Wachstums-Perioden gemessen: eine warme, trockene und eine nasse, kühle Saison. Die Kronendachleitfähigkeit wurde mit der inversen Penman-Monteith-Gleichung unter Verwendung eines verzögerungskorrigierten Saftflusses als Eingabe abgeschätzt. Die maximale Kronendachleitfähigkeit der Bestände variierte zwischen 8 mm s-1 bei geringster und 88 mm s-1 bei höchster Leitfähigkeit. Die Transpiration in der trockenen, warmen Saison im Zeitraum Juli bis September war mit Mittelwerten zwischen 1.30 und 4.64 mm Tag-1 höher. Die entsprechende Spannweite in der nassen, kalten Saison war 0.95 bis 2.65 mm Tag-1. Neben den klimatischen Faktoren erklärten Bestandsalter, Stammdichte und-durchmesser den größten Teil der Variation in der Saftflussdichte. Unter Verwendung einer multiplen Regressionsanalyse für 5-Tages-Zeiträume war es möglich, Saftflussdichte und Transpiration für ein größeres Areal des Waldes abzuschätzen. Diese hochskalierte Areal-Transpiration wurde mit der Verdunstung verglichen, die mit einem "eddy correlation"-Sys
Theoretical and Applied Climatology, 2005
Evapotranspiration of a mixed European beech -Norway spruce forest was measured by means of the eddy covariance technique during a 19 day fine weather period in August 2000. Site conditions were non-ideal as to conventional micrometeorological standards. Sloping terrain and heterogeneously composed, small fetch required a thorough evaluation of the results by (1) the degree of energy balance closure (EBC), (2) footprint analysis and (3) cross-checking against soil water depletion estimates of evapotranspiration. Spatially distributed soil moisture measurements (194 permanent measuring points across a 70 Â 70 m plot) guaranteed extended representativeness of the soil water budget method. Due to the convectively active atmosphere during the observation period and to the low measuring height, the source area of the latent heat flux remained small. Footprint analysis revealed that in 95% of the situations with positive net radiation the cumulative latent heat flux reached 50% of the total flux at a maximum upwind distance of 135 (AE27) m. Fifty five percent of the peak footprints occurred within the soil moisture measuring plot, providing good spatial comparability between the two methods for determining evapotranspiration. They differed only by 0.2 mm for the whole period, showing better agreement in the second half of the period, when water redistribution in the subsoil had ceased. EBC evaluated on a daily basis ranged between 73 and 92%. No clear relation between magnitude of the closure gap and meteorological parameters could be identified. Overall, 46% of the incident net radiation drove evapotranspiration and 28% were released as sensible heat, the Bowen ratio being sensitive to weather conditions and wind direction, though. The results of this study give confidence that the eddy covariance technique is a useful tool to measure forest evapotranspiration also in complex terrain.
Ecohydrology, 2014
Evapotranspiration (ET) comprises a major portion of the water budget in forests, yet few studies have measured or estimated ET in semi-arid, high-elevation ponderosa pine forests of the south-western USA or have investigated the capacity of models to predict ET in disturbed forests. We measured actual ET with the eddy covariance (eddy) method over 4 years in three ponderosa pine forests near Flagstaff, Arizona, that differ in disturbance history (undisturbed control, wildfire burned, and restoration thinning) and compared these measurements (415-510 mm year À1 on average) with actual ET estimated from five meteorological models [Penman-Monteith (P-M), P-M with dynamic control of stomatal resistance (P-M-d), Priestley-Taylor (P-T), McNaughton-Black (M-B), and Shuttleworth-Wallace (S-W)] and from the Moderate Resolution Imaging Spectroradiometer (MODIS) ET product. The meteorological models with constant stomatal resistance (P-M, M-B, and S-W) provided the most accurate estimates of annual eddy ET (average percent differences ranged between 11 and À14%), but their accuracy varied across sites. The P-M-d consistently underpredicted ET at all sites. The more simplistic P-T model performed well at the control site (18% overprediction) but strongly overpredicted annual eddy ET at the restoration sites (92%) and underpredicted at the fire site (À26%). The MODIS ET underpredicted annual eddy ET at all sites by at least 51% primarily because of underestimation of leaf area index. Overall, we conclude that with accurate parameterization, micrometeorological models can predict ET within 30% in forests of the south-western USA and that remote sensing-based ET estimates need to be improved through use of higher resolution products.
Interannual Invariability of Forest Evapotranspiration and Its Consequence to Water Flow Downstream
Ecosystems, 2010
Although drought in temperate deciduous forests decreases transpiration rates of many species, stand-level transpiration and total evapotranspiration is often reported to exhibit only minor interannual variability with precipitation. This apparent contradiction was investigated using four years of transpiration estimates from sap flux, interception-evaporation estimates from precipitation and throughfall gauges, modeled soil evaporation and drainage estimates, and eddy covariance data in a mature oak-hickory forest in North Carolina, USA. The study period included one severe drought year and one year of well above-average precipitation. Normalized for atmospheric conditions, transpiration rates of some species were lower in drought than in wet periods whereas others did not respond to drought. However, atmospheric conditions during drought periods are unlike conditions during typical growing season periods. The rainy days that are required to maintain drought-free periods are characterized by low atmospheric vapor pressure deficit, leading to very low transpiration. In contrast, days with low air vapor pressure deficit were practically absent during drought and moderate levels of transpiration were maintained throughout despite the drying soil. Thus, integrated over the growing season, canopy transpiration was not reduced by drought. In addition, high vapor pressure deficit during drought periods sustained appreciable soil evaporation rates. As a result, despite the large interannual variation in precipitation (ranging from 934 to 1346 mm), annual evapotranspiration varied little (610-668 mm), increasing only slightly with precipitation, due to increased canopy rainfall interception. Because forest evapotranspiration shows only modest changes with annual precipitation, lower precipitation translates to decreased replenishment of groundwater and outflow, and thus the supply of water to downstream ecosystems and water bodies.
Evapotranspiration components determined by stable isotope, sap flow and eddy covariance techniques
Agricultural and Forest Meteorology, 2004
Understanding and modeling water exchange in arid and semiarid ecosystems is complicated by the very heterogeneous distribution of vegetation and moisture inputs, and the difficulty of measuring and validating component fluxes at a common scale. We combined eddy covariance (EC), sap flow, and stable isotope techniques to investigate the responses of transpiration and soil evaporation to an irrigation event in an olive (Olea europaea L.) orchard in Marrakech, Morocco. The primary goal was to evaluate the usefulness of stable isotope measurements of water vapor in the turbulent boundary layer for partitioning evapotranspiration under such dynamic conditions. The concentration and deuterium isotope composition (δ 2 H) of water vapor was collected from different heights within the ecosystem boundary layer of the olive canopy before and over several days following a 100 mm surface irrigation. 'Keeling plots' (isotope turbulent mixing relationships) were generated from these data to estimate the fractions of evaporation and transpiration contributing to the total evapotranspiration (ET) flux. Transpiration accounted for 100% of total ET prior to irrigation, but only 69-86% of ET during peak midday fluxes over the 5-day period following irrigation. The rate of soil evaporation and plant transpiration at the stand level was calculated from eddy covariance measurements and the evaporation and transpiration fractions from isotope measurements. Soil evaporation rate was positively correlated with daily atmospheric vapor pressure deficit (D), but transpiration was not. Component fluxes estimated from the isotope technique were then compared to those obtained from scaled sap flow measurements. Sap flow in multiple-stemmed trees increased following the irrigation, but large single-stemmed trees did not. We matched the source area for eddy covariance estimates of total ET fluxes with scaled sap flow estimates developed for the different tree types. Soil evaporation was determined from the difference between total ET and the scaled sap flow. Ecosystem-level transpiration and soil evaporation estimated by the isotope approach were within 4 and 15% of those estimated by scaled sap flow, respectively, for periods of peak fluxes at midday. Our data illustrate the utility of the isotope 'Keeling plot' approach for partitioning ET at the ecosystem scale on short time steps and the importance of accurate spatial representation of scaled sap flow for comparison with eddy covariance measurements of ET.
Journal of Hydrology, 2007
Evapotranspiration over a Japanese cypress forest was estimated using both the eddy covariance and water budget methods. The long-term water budget revealed that there has been no obvious change in the annual amount of evapotranspiration in this watershed for 33 years despite tree growth, succession, occasional cutting and natural disturbance. Comparison of the eddy covariance method, with a correction for the energy budget, and long-term and short-term water budget methods strongly suggests the validity of the results of both methods and also demonstrates that both the amplitude and characteristics of the seasonal fluctuation of evapotranspiration show no significant interannual differences despite considerable fluctuation in precipitation and hence soil moisture. The three-year average amount of evapotranspiration from the eddy covariance method, with a correction for the energy budget (735 mm) were very close to the 33-year average amount of evapotranspiration from the water budget method (749 mm).