Interannual Variation Of Evapotranspiration Research Articles

Environmental and biological controls on the interannual variations of evapotranspiration in a natural oak forest

Investigating the interannual variation (IAV) of evapotranspiration (ET) is important for understanding the water cycle in forest ecosystems. However, there has been little research on interannual variability of ET in warm-temperate forests under a changing environment. In this study, 6-year eddy covariance data (2017–2022) in a warm-temperate natural oak forest in central China were used to examine the IAV of ET and its driving factors. The mean and standard deviation of annual ET from 2017 to 2022 were 610 and 122.8 mm (with a coefficient of variation (CV) of 20 %), respectively. The annual ET did not exhibit an obvious trend over the six years with a larger IAV. Environmental factors had a greater impact on the IAV of annual ET compared to the biological factors, with reference crop evapotranspiration (ET0) exerting the most significant influence on ET. The IAV of monthly ET was primarily controlled by precipitation during the early growing season, but it changed to be controlled by radiation and temperature (i.e., atmospheric evaporation demand) during the mid-growing season. We concluded that atmospheric evaporation demand rather than water supply dominated the IAV of annual ET in the warm-temperate natural oak forest in central China, and occasional seasonal drought only affected the ET in a short time-scale (no more than three years). There is a need to measure three components of ET (transpiration, canopy interception evaporation and soil evaporation) in order to better understand the IAV of ET and the underlying biophysical control mechanisms.

Interannual variation in evapotranspiration in an urban forest reserve with respect to drought

IntroductionA warming global climate is expected to perturb the hydrological cycle, resulting in deviations in both frequency and duration of drought and thus being hypothesized to lead to interannual variation in evapotranspiration (ET). Interannual variation in ET in urban forest ecosystems in response to drought remains poorly understood.MethodsHere, ET in an urban forest reserve in the megalopolis of Beijing was investigated using eddy-covariance measurements collected over six consecutive years (2012–2017).ResultsThe mean annual cumulative ET was 462 ± 83mm (±first standard deviation), with a coefficient of variation of 18%. Interannual variation in both annual and monthly ET was shown to be largely controlled by canopy conductance (gs), affected by environmental factors. The main factors affecting interannual variation in monthly ET varied seasonally, namely, soil volumetric water content (VWC) and normalized difference vegetation index (NDVI) in spring, precipitation and soil temperature in summer, and VWC and net radiation (Rn) in autumn. Interannual variation in annual ET was driven largely by spring and mid-summer droughts induced by insufficient precipitation during the non-growing and mid-growing seasons, respectively. Spring drought reduced annual ET by restricting leafing out, shortening growing season length (GSL), and reducing the normalized difference vegetation index (NDVI). The summer drought reduced annual ET by reducing stomatal conductance.Discussion and conclusionResults from this study point to the importance of precipitation timing and volume and the soil moisture carry-over effect in controlling interannual variation in ecosystem ET. Irrigation during the early spring and mid-summer is viewed as a practical management measure for sustaining growth and better ecosystem services in urban forests in Northern China.

Towards effective drought monitoring in the Middle East and North Africa (MENA) region: implications from assimilating leaf area index and soil moisture into the Noah-MP land surface model for Morocco

Abstract. The Middle East and North Africa (MENA) region has experienced more frequent and severe drought events in recent decades, leading to increasingly pressing concerns over already strained food and water security. An effective drought monitoring and early warning system is thus critical to support risk mitigation and management by countries in the region. Here we investigate the potential for assimilation of leaf area index (LAI) and soil moisture observations to improve the representation of the overall hydrological and carbon cycles and drought by an advanced land surface model. The results reveal that assimilating soil moisture does not meaningfully improve model representation of the hydrological and biospheric processes for this region, but instead it degrades the simulation of the interannual variation in evapotranspiration (ET) and carbon fluxes, mainly due to model weaknesses in representing prognostic phenology. However, assimilating LAI leads to greater improvement, especially for transpiration and carbon fluxes, by constraining the timing of simulated vegetation growth response to evolving climate conditions. LAI assimilation also helps to correct for the erroneous interaction between the prognostic phenology and irrigation during summertime, effectively reducing a large positive bias in ET and carbon fluxes. Independently assimilating LAI or soil moisture alters the categorization of drought, with the differences being greater for more severe drought categories. We highlight the vegetation representation in response to changing land use and hydroclimate as one of the key processes to be captured for building a successful drought early warning system for the MENA region.

Estimation of Evapotranspiration in the Yellow River Basin from 2002 to 2020 Based on GRACE and GRACE Follow-On Observations

Evapotranspiration (ET) plays an important role in the hydrological cycle of river basins. Studying ET in the Yellow River Basin (YRB) is greatly significant for the scientific management of water resources. Here, we made full use of the advantages of the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) gravity satellites for monitoring large-scale hydrological changes to calculate the terrestrial water storage anomaly (TWSA) and terrestrial water flux in the YRB from May 2002 to June 2020. Furthermore, combined with terrestrial water flux, precipitation, and runoff data, ET in the YRB was calculated based on the water budget equation and then compared with other traditional ET products. The mutation of annual mean ET was identified by the Mann–Kendall trend test method, and the seasonal and interannual variations of ET were explored. ET was closely related to precipitation. Annual mean ET exhibited a sudden change in 2011, with an insignificant downward trend from 2003 to 2010, followed by an increasing trend from 2011 to 2019, particularly after 2016. Compared with the traditional ET monitoring methods and products, the ET estimated by GRACE/GRACE-FO observations provides a new way to effectively obtain continuous and reliable ET data in a wide range of river basins.

Interannual Variations of Evapotranspiration and Water Use Efficiency over an Oasis Cropland in Arid Regions of North-Western China

The efficient use of limited water resources and improving the water use efficiency (WUE) of arid agricultural systems is becoming one of the greatest challenges in agriculture production and global food security because of the shortage of water resources and increasing demand for food in the world. In this study, we attempted to investigate the interannual trends of evapotranspiration and WUE and the responses of biophysical factors and water utilization strategies over a main cropland ecosystem (i.e., seeded maize, Zea mays L.) in arid regions of North-Western China based on continuous eddy-covariance measurements. This paper showed that ecosystem WUE and canopy WUE of the maize ecosystem were 1.90 ± 0.17 g C kg−1 H2O and 2.44 ± 0.21 g C kg−1 H2O over the observation period, respectively, with a clear variation due to a change of irrigation practice. Traditional flood irrigation generally results in over-irrigation, providing more water than actual crop requirements. Unlike flood irrigation, which can infiltrate into deep soil layers, drip irrigation can only influence the shallow soil moisture, which can lead to decreases of soil moisture of approximately 27–32% and 36–42% compared with flood irrigation for shallow and deep layers, respectively. Additionally, drip irrigation decreases evapotranspiration by 13% and transpiration by 11–14%, leading to increases in ecosystem and canopy WUE of 9–14% and 11%, respectively, compared to the traditional irrigation practice. Therefore, the drip irrigation strategy is an effective method to reduce irrigation water use and increase crop WUE in arid regions. Our study provides guidance to water-saving cultivation systems and has implications for sustainable water resources management and agriculture development in water-limited regions.

CHANGES IN EVAPOTRANSPIRATION AND THE POTENTIAL DRIVERS IN ASIAN ARID REGIONS DURING 2003 TO 2017

Abstract. Evapotranspiration (ET) has changed greatly across the world in the past decades. There are various drivers that may affect the changes in ET. Understanding the changes in ET and the potential drivers is of great importance to water resources management in the arid regions. In this study, we examine the trends in ET using satellite-based data in the Asian arid regions from 2003 to 2017. Trends in different ET components, i.e., the evaporation from bare soil and transpiration from vegetation, are estimated to examine their respective contributions to the changes in ET. The results show that the evaporation from bare soil is the main component of ET and has contributed larger to the changes in ET from 2003 to 2017. Statistically significant increasing trend is mostly found in Southwest Pakistan, Northwest India, Southeast Iran, many areas in Northwest China, and some parts in South Kazakhstan. The relations between ET and the global sea surface temperature (SST) are also examined. It suggests that the interannual variation of ET is closely related to annual SST anomalies in most areas, especially in Kazakhstan, Turkmenistan, Northwest China, Southwest Pakistan, and some parts of eastern Iran. This analysis provides useful information for water resources management and planning with respect to forest and agricultural development in the Asian arid regions.

Seasonal and Inter-annual Variation of Evapotranspiration in Amazonia Based on Precipitation, River Discharge and Gravity Anomaly Data

We analyzed seasonal and spatial variations of evapotranspiration (ET) for five Amazon sub-basins and their response to the 2015/16 El Nino episode using a recently developed water-budget approach. ET varied typically between ~7 and 10 cm/month with exception of the Xingu basin for which it varied between 10 and 15 cm/month. Outstanding features of ET seasonality are (i) generally weak seasonality, (ii) two ET peaks for the two very wet catchments Solimoes and Negro, with one occurring during the wet season and one during the drier season, and (iii) a steady increase of ET during the second half of the dry season for the three drier catchments (Madeira, Tapajos, Xingu). Peak ET occurs during the first half of the wet season consistent with leaf flush occurring before the onset of the wet season. With regards to inter-annual variation, we found firstly that for the Solimoes and Madeira catchments the period with large positive wet season anomalies (2012–2015) is associated with negative ET anomalies, and negative SIF (solar induced fluorescence) anomalies. Furthermore, we found negative ET of several cm/months and SIF (up to 50%) anomalies for most of the Amazon basin during the 2015/16 El Nino event suggesting down-regulation of productivity as a main factor of positive carbon flux anomalies during anomalously hot and dry conditions. These results are of interest in view of predicted warmer and more erratic future climate conditions.

Inter-comparison of water balance components of river basins draining into selected delta districts of Eastern India

Quantification of hydrological components in-terms of surface runoff, stream flow and evapotranspiration is important and useful in planning and management of water resources across the river basin, including downstream delta regions. River deltas water availability; management and related disaster risk are largely influenced by the hydrological state of upstream river basins. The paper presents the results of hydrological modelling (SWAT) based long-term water balance components in river basins draining into selected delta Districts of Eastern India. Mahanadi, Brahmani-Baitarani river basins and Hooghly river and adjacent small river basins are considered. The long-term water balance components of Mahanadi and Brahmani-Baitarani river basins are similar and significantly different in Hooghly river and adjacent small river basins. The runoff coefficient is significantly higher in Hooghly river and adjacent small river basins at 0.39 compared to other two river basins (0.247 & 0.256). The evapotranspiration component is relatively low in Hooghly river and adjacent small river basins with smaller range of long-term variation. The time-series model outputs brought out the basin-specific hydrological response variations in low and high rainfall years such as changes in fraction of evapotranspiration and surface runoff. Mahanadi and Brahmani-Baitarani river basins exhibit large inter-annual variation in evapotranspiration, surface runoff fractions. The developed hydrological modelling framework is capable of incorporating future climate data and to predict the basin-scale future water availability, demand, use and to bring out resulting water scenarios that would impact river deltas in-terms of their exposure towards water related adversities, such as drought and flood.

On the attribution of changing crop evapotranspiration in arid regions using four methods

Accurate evaluation of the relative contribution of climatic and vegetative drivers on land evapotranspiration (ET) is critical for interpreting the ET controlling mechanism, modeling and predicting. However, how to accurately separate and estimate the contribution of climatic and vegetative changes on ET variation remains uncertain. Our study attempted to interpret the interannual variation in ET during 2007–2013 over a maize field in northwest China, using four methods simultaneously, such as the Penman-Monteith (PM) model, the modified crop coefficient method, the Priestly-Taylor (PT) model and the regression linear model.Results indicate that compared to the ET in 2007, the ET decreased averagely by 12, 30, 29, and 73 W m−2 in 2008, 2011, 2012 and 2013, respectively. All models yielded similar results and showed that the vegetative controls played a more important role in regulating ET relative to climatic drives, and that more than half of decrease in ET was caused by vegetative factors, while the differences in net radiation, water vapor pressure deficit and air temperature among years were lesser source of variation in ET. Furthermore, the advantage and disadvantage of the four methods were discussed.Our study confirmed the great effect of vegetative drivers in regulating crop ET, highlighted the importance of estimating canopy conductance accurately in ET modeling and prediction, and provided new approaches for separating the climatic and vegetative contribution on ET changes.

Sensitivity of terrestrial water and carbon fluxes to climate variability in semi-humid basins of Haihe River, China

Inter-annual climate variability dominates the evolution of eco-hydrological processes over the water limited basins. Haihe Basin in semi-humid northern China is under significant climate change and serious human disturbance during the recent decades. Integrated with NASA Terra-Modis remote sensing LAI data products, the VIP eco-hydrological model is used to predict the spatiotemporal variations of evapotranspiration (ET) and vegetation gross primary production (GPP) over two catchments of Haihe River branches from 2000 to 2013. Along with precipitation increasing, average annual ET and GPP are significantly increasing. The trends are more distinct over the mountainous upstream areas. The inter-annual variations of ET and GPP are highly correlated with the 12-month SPI (Standardized Precipitation Index), while the monthly GPP is related clearly with 3-month SPI. Sensitivity analysis shows that there are remarkable differences in the elasticity coefficients of both ET and GPP to the climatic factors. It is found that precipitation, solar radiation and leaf area index are the most sensitive factors to ET and GPP in the study basin. The results are helpful to understand the catchment eco-hydrological mechanisms and their responses to climate change.

Subsurface storage capacity influences climate–evapotranspiration interactions in three western United States catchments

Abstract. In the winter-wet, summer-dry forests of the western United States, total annual evapotranspiration (ET) varies with precipitation and temperature. Geologically mediated drainage and storage properties, however, may strongly influence these relationships between climate and ET. We use a physically based process model to evaluate how plant accessible water storage capacity (AWC) and rates of drainage influence model estimates of ET–climate relationships for three snow-dominated, mountainous catchments with differing precipitation regimes. Model estimates show that total annual precipitation is a primary control on inter-annual variation in ET across all catchments and that the timing of recharge is a second-order control. Low AWC, however, increases the sensitivity of annual ET to these climate drivers by 3 to 5 times in our two study basins with drier summers. ET–climate relationships in our Colorado basin receiving summer precipitation are more stable across subsurface drainage and storage characteristics. Climate driver–ET relationships are most sensitive to subsurface storage (AWC) and drainage parameters related to lateral redistribution in the relatively dry Sierra site that receives little summer precipitation. Our results demonstrate that uncertainty in geophysically mediated storage and drainage properties can strongly influence model estimates of watershed-scale ET responses to climate variation and climate change. This sensitivity to uncertainty in geophysical properties is particularly true for sites receiving little summer precipitation. A parallel interpretation of this parameter sensitivity is that spatial variation in storage and drainage properties are likely to lead to substantial within-watershed plot-scale differences in forest water use and drought stress.

Evapotranspiration of tropical peat swamp forests.

In Southeast Asia, peatland is widely distributed and has accumulated a massive amount of soil carbon, coexisting with peat swamp forest (PSF). The peatland, however, has been rapidly degraded by deforestation, fires, and drainage for the last two decades. Such disturbances change hydrological conditions, typically groundwater level (GWL), and accelerate oxidative peat decomposition. Evapotranspiration (ET) is a major determinant of GWL, whereas information on the ET of PSF is limited. Therefore, we measured ET using the eddy covariance technique for 4-6 years between 2002 and 2009, including El Niño and La Niña events, at three sites in Central Kalimantan, Indonesia. The sites were different in disturbance degree: a PSF with little drainage (UF), a heavily drained PSF (DF), and a drained burnt ex-PSF (DB); GWL was significantly lowered at DF, especially in the dry season. The ET showed a clear seasonal variation with a peak in the mid-dry season and a large decrease in the late dry season, mainly following seasonal variation in net radiation (Rn ). The Rn drastically decreased with dense smoke from peat fires in the late dry season. Annual ET forced to close energy balance for 4 years was 1636 ± 53, 1553 ± 117, and 1374 ± 75 mm yr(-1) (mean ± 1 standard deviation), respectively, at UF, DF, and DB. The undrained PSF (UF) had high and rather stable annual ET, independently of El Niño and La Niña events, in comparison with other tropical rainforests. The minimum monthly-mean GWL explained 80% of interannual variation in ET for the forest sites (UF and DF); the positive relationship between ET and GWL indicates that drainage by a canal decreased ET at DF through lowering GWL. In addition, ET was decreased by 16% at DB in comparison with UF chiefly because of vegetation loss through fires.

Seasonal and interannual variations of evapotranspiration, energy exchange, yield and water use efficiency of castor grown under rainfed conditions in northeastern Brazil

Brazil is the world's third largest producer of castor. The crop is mainly cultivated in the northeastern region of the country, but little information is available concerning the diurnal, seasonal and interannual variability of evapotranspiration (ET) and energy exchange of castor crops grown in the region, as well as the water use efficiency and yield. To address this gap in knowledge, the Bowen ratio method was used to measure energy balance and ET in 2004, 2005 and 2007. The experiments were performed in a 4-ha area at the Centro de Ciências Agrárias, Federal University of Paraíba in Areia in the state of Paraiba, Brazil (6°58′S, 35°41′W, elevation 620m). The study site had a micrometeorological tower with sensors for measuring air temperature and relative humidity located at two heights above castor canopy. Global and net radiation, rainfall, soil heat flux, and stored soil water at 0–1.0m depth were also measured. Measurements from all of the sensors were recorded by a data logger every 60s and mean/sum data were logged every 1800s. Over 3 years, the net radiation (Rn) varied from 53.2 to 461.7Wm−2 and soil heat flux (G) varied from −10.5 to 58.9Wm−2. Variation in energy partitioning into latent (LE) and sensible (H) heat fluxes was mainly associated with changes in stored soil water. H values were higher in 2004 and 2005 (35% and 37% of Rn, respectively) than in 2007 (25% of Rn). Daily ET was very similar in 2004 and 2005 (2.29 and 2.34mmday−1, respectively) but increased to 3.42mmday−1 in 2007, mainly due to increased volume and more even rainfall distribution throughout the growing season. Total ET was 299.5, 334.3 and 656.6mm in 2004, 2005 and 2007, respectively. Castor showed a low yield (60, 324 and 988.3kgha−1 in 2004, 2005 and 2007, respectively) and low water use efficiency (0.02, 0.10 and 0.15kgm−3 in 2004, 2005 and 2007, respectively), especially in drier years, indicating that under short water supply the water use efficiency of castor plants was very low. The seed oil content varied from 33.6% to 49.2% by weight. There was also a strong correlation between castor yield and daily ET (R2=0.9433, RMSE=91.1kgha−1), and between castor yield and rainfall (R2=0.9902, RMSE=30.6kgha−1).

Evaluating actual evapotranspiration and impacts of groundwater storage change in the North China Plain

AbstractAs a critical water discharge term in basin‐scale water balance, accurate estimation of evapotranspiration (ET) is therefore important for sustainable water resources management. The understanding of the relationship between ET and groundwater storage change can improve our knowledge on the hydrological cycle in such regions with intensive agricultural land usage. Since the 1960s, the North China Plain (NCP) has experienced groundwater depletion because of overexploitation of groundwater for agriculture and urban development. Using meteorological data from 23 stations, the complementary relationship areal evapotranspiration model is evaluated against estimates of ET derived from regional water balance in the NCP during the period 1993–2008. The discrepancies between calculated ET and that derived by basin water balance indicate seasonal and interannual variations in model parameters. The monthly actual ET variations during the period from 1960 to 2008 are investigated by the calibrated model and then are used to derive groundwater storage change. The estimated actual ET is positively correlated with precipitation, and the general higher ET than precipitation indicates the contributions of groundwater irrigation to the total water supply. The long term decreasing trend in the actual ET can be explained by declining in precipitation, sunshine duration and wind speed. Over the past ~50 years, the calculated average annual water storage change, represented by the difference between actual ET and precipitation, was approximately 36 mm, or 4.8 km3; and the cumulative groundwater storage depletion was approximately 1700 mm, or 220 km3 in the NCP. The significantly groundwater storage depletion conversely affects the seasonal and interannual variations of ET. Irrigation especially during spring cause a marked increase in seasonal ET, whereas the rapid increasing of agricultural coverage over the NCP reduces the annual ET and is the primary control factor of the strong linear relationship between actual and potential ET. Copyright © 2013 John Wiley & Sons, Ltd.

Remote sensing temporal and spatial patterns of evapotranspiration and the responses to water management in a large irrigation district of North China

To deal with water resources crisis caused by climate change and intensified human activities, the Hetao Irrigation District of Inner Mongolia in North China has been experiencing rehabilitation for water-saving since 1999. This has significantly changed its regional hydrological cycle and thus water use patterns over different land use types. In this paper, Surface Energy Balance Algorithm for Land (SEBAL) model fed with MODIS data was applied in Hetao area from 2000 to 2010 to examine the spatial and temporal patterns of evapotranspiration (ET). The SEBAL estimated ET agreed well with that from other methods in the study area. The results indicate that inter-annual variability in ET over agricultural land, water body, woodland and irrigated grassland are primarily explained by the variation of reference ET. So it is with the seasonal variability in agricultural land ET on monthly basis. However, the inter-annual variations of ET over sandy land, Gobi desert and mountain areas are mainly controlled by precipitation. Over the study period, a reduction in river water diversion for irrigation has not reduced the agricultural land ET, indicating no significant impact on agricultural production. But ET over the non-irrigated grassland tends to decrease, which was likely caused by declining groundwater table in recent years.

Interannual variation of evapotranspiration in an eastern Siberian larch forest

AbstractIn a deciduous larch forest in eastern Siberia, the mean and standard deviation of the total evapotranspiration (E) during May to September (day of year (DOY) = 121–274) for 2003–2006 were 181.5 and 26.4 mm, respectively. The interannual variation (IAV) in the total E was caused by the IAV in E for the canopy‐foliated period (DOY = 164–253), not by the IAV in the dates of leaf expansion and leaf fall. For the years with higher total E, E in the canopy‐foliated period was consistently higher, which corresponded to the higher soil water content in these years. Copyright © 2012 John Wiley & Sons, Ltd.

Seasonal and Interannual Variations of Evapotranspiration and Energy Exchange over Different Land Surfaces in a Semiarid Area of China

AbstractSeasonal and interannual variations of evapotranspiration (ET) and energy exchange were estimated over degraded grassland and cropland land surfaces in a semiarid region of northeastern China using the eddy covariance technique from 2003 to 2008. The peak daily ET, which occurred in August, was 1.5–4.5 mm day−1for the degraded grassland and 1.5–5.5 mm day−1for the cropland land surface. Annual cumulative ET was roughly equal to annual precipitation at both sites. However, the annual cumulative ET at the cropland site was slightly larger (about 10–30 mm) than it was at the grassland at the end of each year. More water might come from irrigation at seedtime and from the soil. With the factor analysis technique, the results revealed that the atmospheric water demand was the most important factor in the ET process on a half-hour time scale in this semiarid area. On a seasonal time scale, ET was greatly constrained by surface conductance and precipitation; on an annual time scale, ET was greatly constrained by the total amount of precipitation at both sites. The accuracy of ET estimation using the Penman–Monteith formula in this semiarid area was also discussed.

The role of permafrost in water exchange of a black spruce forest in Interior Alaska

Water exchange in a black spruce forest on ice-rich permafrost in Interior Alaska was examined over seven years to clarify its seasonal and interannual variations. Partitioning of available energy to latent heat flux was strongly constrained by leaf area and thaw depth of the soil. Water balance on a monthly timescale showed that evapotranspiration was greater than precipitation in the early growing season. It is likely that evapotranspiration was supported by soil thawing and snowmelt water stored in microtopographic hollows during this period. Precipitation increased in the latter half of the growing season, whereas evapotranspiration decreased along with a decrease in net radiation and the leaf maturation and the senescence of understory deciduous vegetation. This resulted in an increase of stored water in the soil, which can be utilized by vegetation in the following growing season. The presence of ice-rich permafrost helps the accumulation of water in the soil by blocking deep percolation of water. Cumulative evapotranspiration during the snow-free period did not show noticeable interannual variability, in contrast to a significant variation in precipitation. Cumulative evapotranspiration was primarily controlled by the available energy. Water resources for this black spruce forest remained sustainable under the current climate due to net water gain in the late growing season, combined with the presence of ice-rich permafrost. From a comparison to other black spruce forests with non-permafrost soil, we found that whether ice-rich permafrost exits or not is an important factor for determining the magnitude in the interannual variation of evapotranspiration in boreal forests.

Interannual variation of evapotranspiration from forest and grassland ecosystems in western canada in relation to drought

Climate models predict rising temperatures and more frequent and prolonged droughts, particularly in the northern hemisphere and in the Canadian Prairies. However, few studies have examined the interannual variation in evapotranspiration (E) of northern terrestrial ecosystems in relation to drought. This study analyses multi-year (1998-2006), eddy-covariance measurements to quantify the seasonal and interannual variability in E from grassland and mature aspen, black spruce, and jack pine forest ecosystems in the dry interior plains of western Canada. It also investigates the response of E to the historic 2001-2003 drought in this region. Leaf area index (LAI) was a primary factor controlling the difference in E among different ecosystems in the same ecozone. Annual E was higher and more variable for the aspen forest (405 ± 84 mm, mean ± s.d.) and grassland (395 ± 90 mm) than the black spruce (374 ± 34 mm) and jack pine (300 ± 20 mm) forests. Interannual variation of E was controlled by early spring soil temperature at all four sites, with warm springs enhancing annual E. Only the aspen forest and grassland showed a significant suppression of E by summer drought, related to reduced surface conductance at the aspen site, and to reduced surface conductance and early leaf senescence at the grassland. We conclude that the potential for drought impacts on annual E of northern ecosystems is greatest for grasslands, moderate for deciduous broadleaf aspen forests, and smallest for coniferous black spruce and jack pine forests. The forests of the Boreal Plains, adjacent to the prairie region, may ameliorate the onset of drought through the recycling of moisture to the atmosphere, whereas the prairie grasslands have only limited capacity to counteract drought through moisture recycling.

Seasonal contribution and interannual variation of evapotranspiration over a reed marsh (Phragmites australis) in Northeast China from 3‐year eddy covariance data

AbstractAt present, one of unresolved critical issues relating to climate change is whether evapotranspiration (ET) is increasing or decreasing with global warming. There has been great interest, therefore, in studying ET in a variety of ecosystems to better understand the nature of the controlling interactions and the links between ET and other ecosystem processes. Especially, relatively few studies on ET have been conducted in wetland environments. In this paper, we present the results from 3 years (2005–2007) of direct ET measurements, using eddy covariance method, over a reed (Phragmites australis) marsh in Liaohe Delta, Northeast China. The seasonal and interannual variation of daily ET and its main controls were analysed. Annual ET for the reed marsh was 432, 480, and 445 mm in 2005, 2006 and 2007, respectively. The contribution of ET during non‐growing season was not negligible; it ranged from 13% to 16% of the annual ET for the investigated years. On annual course, ET increased associating with increasing temperature and reed plant growth in late‐April and early‐May, and peaked in the rapid growing season with weekly mean rates of 3·7 mm day−1 (2005), 3·7 mm day−1 (2006), and 3·4 mm day−1 (2007). Decoupling factor (Ω) also varied seasonally, with its minimum values occurred in the non‐growing season and the maximum values in the rapid growing season, especially in July. During the rapid growing season, the values of decoupling factor (Ω) were generally between 0·5 and 0·8, indicating that solar radiation was main factor affecting ET other than vapour pressure deficit (VPD). Moreover, biological factor and canopy conduction also controlled the interannual ET variation in the reed marsh. The results would be helpful to better understand ET for this particular reed marsh. Copyright © 2009 John Wiley & Sons, Ltd.