Evapotranspiration and Its Energy Exchange in Alpine Meadow Ecosystem on the Qinghai-Tibetan Plateau

Evapotranspiration and Its Energy Exchange in Alpine Meadow Ecosystem on the Qinghai-Tibetan Plateau

Journal of Integrative Agriculture August 2013 2013, 12(8): 1396-1401 RESEARCH ARTICLE Evapotranspiration and Its Energy Exchange in Alpine Meadow...

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Journal of Integrative Agriculture

August 2013

2013, 12(8): 1396-1401

RESEARCH ARTICLE

Evapotranspiration and Its Energy Exchange in Alpine Meadow Ecosystem on the Qinghai-Tibetan Plateau LI Jie1*, JIANG Sha1*, WANG Bin1, JIANG Wei-wei1, TANG Yan-hong2, DU Ming-yuan3 and GU Song1 College of Life Science, Nankai University, Tianjin 300071, P.R.China National Institute for Environmental Studies, Tsukuba 305-8506, Japan 3 National Institute for Agro-Environmental Sciences, Tsukuba 305-8604, Japan 1 2

Abstract To understand the water and energy exchange on the Qinghai-Tibetan Plateau, we explored the characteristics of evapotranspiration (ET) and energy fluxes from 2002 to 2005 over a Kobresia meadow ecosystem using the eddy covariance method. The ratio of annual ET to precipitation (P) of meadow ecosystem was about 60%, but varied greatly with the change of season from summer to winter. The annual ET/P in meadow was lower than that in shrub, steppe and wetland ecosystems of this plateau. The incident solar radiation (Rs) received by the meadow was obviously higher than that of lowland in the same latitude; however the ratio of net radiation (Rn) to Rs with average annual value of 0.44 was significantly lower than that in the same latitude. The average annual ET was about 390 mm for 2002-2005, of which more than 80% occurred in growing season from May to September. The energy consumed on the ET was about 44% of net radiation in growing season, which was lower than that of shrub, steppe and wetland on this plateau. This study demonstrates that the Kobresia meadow may prevent the excessive water loss through evapotranspiration from the ecosystem into the atmosphere in comparison to the shrub, steppe and wetland ecosystems of the Qinghai-Tibetan Plateau. Key words: eddy covariance, evapotranspiration, net radiation, precipitation, Qinghai-Tibetan Plateau

INTRODUCTION Evapotranspiration (ET) is a significant water loss from the terrestrial ecosystem into the atmosphere, and the process of ET is one of the main consumers of solar energy at the Earth’s surface (Mitchell et al. 2009). Assessing ecosystem ET may provide insights into not only the water cycle, but also the energy exchange. The Qinghai-Tibetan Plateau with a mean altitude of more than 4 000 m is well known as the “water tower of Asia”, and plays an important role in global climate change. More than 60% of the plateau area is covered by extensive grasslands, in which most of the grassReceived 17 October, 2012

lands are dominated by alpine meadow (Cui and Graf 2009). Moreover, the plateau ecosystem is very fragile and sensitive to global climate changes because of its high elevation (Gu et al. 2008). Thus, information regarding ET and its energy exchange in meadow ecosystem is necessary to further understand the responses of alpine ecosystem to climate change. ET is controlled by several hydrometeorological factors as well as soil and vegetation conditions. The Qinghai-Tibetan Plateau is characterized by strong solar radiation, relatively high precipitation and low temperature, which provides an opportunity to study water budget and energy exchange of alpine meadow in the unique environment. Recently, several studies in the plateau

Accepted 10 January, 2013

Correspondence GU Song, Tel: +86-22-23508245, E-mail: [email protected] * These authors contributed equally to this study. © 2013, CAAS. All rights reserved. Published by Elsevier Ltd. doi:10.1016/S2095-3119(13)60546-8

Evapotranspiration and Its Energy Exchange in Alpine Meadow Ecosystem on the Qinghai-Tibetan Plateau

showed the variations of ET in shrubs, wetland and meadow steppe alpine ecosystems (Gu et al. 2008; Liu et al. 2009, 2010). However, there is a lack of detailed information on the efficient use of water and energy in alpine meadow ecosystems. In this study, the main objectives are to: 1) describe the characteristics of ET and its energy partitioning in Kobresia meadow using the lone-term measured data; 2) compare ET and energy exchange between the Kobresia meadow and other ecosystems on the QinghaiTibetan Plateau, and analyze the impact of environmental factors and vegetation condition on the ecosystem ET.

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Fig. 1 Seasonal variations in evapotranspiration (ET) and precipitation (P) from 2002 to 2005 in Kobresia meadow.

RESULTS AND DISCUSSION Water budget in Kobresia meadow Precipitation is the most important component input to the ecosystem water budget, and mainly balanced by ET, infiltration, storage and runoff. There was an obvious variation in annual precipitation (P) with the average of 637 mm for 2002-2005 (Fig. 1), of which about 86% falls in the growing season from May to September. ET, as the most important consumer of water in ecosystems, was found having a significant seasonal variation in Kobresia meadow with the maximum in July and minimum in winter for 2002-2005 (Fig. 1). The mean annual ET was 390 mm for the same period, of which more than 80% occurred in growing season with the maximum value of 4.1 mm d-1. The ratio of evapotranspiration to precipitation (ET/P) is an important parameter describing water budget. ET is the largest component of water loss from terrestrial ecosystems, about 70% of precipitation returns to the atmosphere through evaporation and transpiration processes (Rosenberg et al. 1983). In this alpine meadow, the average annual ET accounted for about 60% of the precipitation for 2002-2005 (Fig. 2). Although the actual ET is the largest consumer of the precipitation in Kobresia meadow, the ET/P was significantly lower than those reported for tussock grassland (Hunt et al. 2002) and northern temperate grassland (Wever et al. 2002), as well as lower than those of shrub, steppe and wetland ecosystems on the plateau (Table).

Fig. 2 Changes in monthly precipitation (P) and evapotranspiration (ET) for 2002-2005 in Kobresia meadow.

In this meadow, the daily precipitation ranged from 0.1 to 44 mm d-1, however, more than 30% precipitation was less than 1 mm d-1, and over 70% precipitation was less than 5 mm d -1 (Fig. 3-D). Thus, it is reasonable to neglect the runoff because of the flat study site and/or small amount of precipitation for each rain event. There was a significant difference in soil water content at the different depths (Fig. 3-C). The soil water content at 50-cm depth was almost above the field capacity (0.40 cm3 cm-3 reported by Cao et al. (1998)) in growing season, indicating that there was a higher infiltration in this layer. However, a relatively large variation in soil water content at 5-cm depth was observed, and slightly decreased in the peak growing season. The results indicate that the soil water content at 5-cm depth was strongly affected by ET, and soil water at 50-cm was almost not influenced by ET. The water storage in the upper soil layer of 0-50 cm was estimated by soil

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Table Comparison of the parameters relating to evapotranspiration and energy exchange among alpine Kobresia meadow and other grasslands during the growing season β

Site

Rn/Rs

ET/P

LE/R n

gc

ET/ETeq

LAImax

Alpine Kobresia meadow Alpine shrubs meadow

0.55 0.60

0.60 0.99

0.47 0.56

0.39 0.64

11.5 9.22

0.69 0.63

3.8 2.8

This study Liu et al. (2009, 2010)

References

Alpine steppe Alpine wetland meadow

0.44 0.55

1.04 1.27

0.63 0.84

0.53 -

6.89 -

0.51 -

1.0 3.9

Liu et al. (2009, 2010) Hu et al. (2009)

Typical steppe Wet temperate grassland

0.68

0.98 0.83

0.41 0.80

1.03 0.04

3.12 15.2

0.46 1.10

1.5 5.5

Hao et al. (2007) Li et al. (2005)

Rn, net radiation; Rs, incident solar radiation; ET, evapotranspiration; P, precipitation; LE, latent heat; β, bowen ratio; gc, canopy conductance; ETeq, equilibrium evapotranspiration; LAImax, maximum leaf area index.

Fig. 3 Annual variations in solar radiation (Rs), net radiation (Rn), air temperature (Ta), vapor pressure deficit (VPD), soil water content at 5- and 50-cm depths (θ-5 and θ-50) and the precipitation intensity (mm d-1) and its frequency for 2002-2005 in Kobresia meadow.

water content, and the annual average value was about zero. Therefore, the infiltration was about 40% of precipitation in this meadow ecosystem. The lower ET/P may due to the high water infiltration in the upper layer of soil (Gu et al. 2008).

ET and its energy exchange in the alpine meadow ET has a great influence not only on the water balance

but also on energy partitioning. Net radiation (Rn) is the dominant part of available energy to drive ET, and is largely dependent on short- and long-wave radiation, as well as related to the vegetation type (Malhi et al. 2002). Bowen ratio (β), as a key parameter in explaining the energy partitioning, is widely used to describe the contribution of latent heat and sensible heat to the surface energy balance. In growing season, the Bowen ratio of this meadow with 0.39 was much lower than

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Evapotranspiration and Its Energy Exchange in Alpine Meadow Ecosystem on the Qinghai-Tibetan Plateau

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In this study site, although the alpine meadow receives high incident solar radiation (Rs) (Fig. 3-A), the annual radiation efficiency (Rn/Rs) with an average of 0.44 was much lower than that for lowland grasslands (Li et al. 2005; Hammerle et al. 2008). The main reason for lower Rn/Rs on this plateau is due to the fact that the net long-wave radiation is much higher than that for the global surface or for lowland grasslands (Zhang et al. 2010). The result indicates that the low energy available for ET was one possible reason resulting in low ET/P despite of the high Rs on the alpine meadow. Vapor pressure deficit (VPD) is another important

factor affecting ET, because VPD is the meteorological variable used to quantify the drying power of the atmosphere and affects canopy conductance (gc). In this study, the average daytime VPD was ranging from 0.2 to 0.9 at the annual scale with high value in growing season (Fig. 3-B). Gu et al. (2008) indicated that VPD in this alpine meadow is much lower than many other grassland values. A low VPD will result in low ET/P in this meadow because the decrease in VPD tends to decrease ET. Temperature is an influential factor to ET based on the Penman-Monteith equation, since it reflects the amount of solar energy. Many studies reported that ET increases with the increase in temperature (Rosenberg et al. 1983; Liu et al. 2009). In the alpine meadow, temperature was lower than other lowland ecosystems due to the high elevation, low temperature is considered as one of the factors limiting ecosystem ET. Equilibrium ET (ETeq), defined as the evapotranspiration rate of a freely evaporating wet surface, is understood as a function of temperature and available energy. The field capacity for the alpine meadow soil is 0.37 cm3 cm-3 at depth of 0-20 cm (Cao et al. 1998). It was found that most data of the soil water content at 5-cm depth was above the field capacity during the growing season (Fig. 3-C). ET/ETeq is used as a measure of drought during the growing season. The mean ET/ETeq was 0.69 with the maximum in growing season in this meadow (Fig. 5), it was lower than wet temperate grassland, but much higher than those in alpine and typical steppes (Table), indicating that the water supply for evaporating is relatively sufficient in this

Fig. 4 Annual variation of LE/R n for 2002-2005 in Kobresia meadow.

Fig. 5 Annual variation of ET/ETeq from 2002 to 2005 in Kobresia meadow.

that of other ecosystems (Table), and comparable to that of wet temperate grassland in Japan (Li et al. 2005), implying that the most available energy was consumed on ET during this period. There is a significant seasonal change in ratio of latent heat (LE) to net radiation (LE/Rn), with the maximum and minimum values in growing season and winter, respectively (Fig. 4). The average annual LE/Rn was 0.30, and was 0.47 during the growing season. Although ET is the important consumer of the incoming energy, LE/Rn in this meadow was lower than that in wet temperate grassland reported by Li et al. (2005), and as well as lower than those in shrub, steppe and wetland on the Qinghai-Tibetan Plateau (Table). The results show that the available energy consumed on ET was relatively low in comparison to other ecosystems.

Effects of environment factors on ET

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meadow. Therefore, the vegetation condition is considered to play an important role in limiting water loss from this ecosystem.

Effects of vegetation on ET Vegetation has a significant impact on ET, and the ET is well correlated with the distribution of vegetation types (Wever et al. 2002). In this alpine meadow, the aboveground biomass reached its maximum in midAugust for 2002-2005 (Fig. 6), however, the ET reached its maximum in roughly mid-July. The peak biomass lagged approximately one month behind the maximum ET. Leaf area index (LAI) was estimated by biomass because it is greatly related to biomass (Shi et al. 2001). The peak ET appeared at about LAI of 1.5 m2 m-2, indicating that ET generally increased with LAI, and decreased when LAI is more than 1.5 m 2 m -2 in this meadow. The decrease in ET might be primarily due to the decrease in available energy (Li et al. 2005) because Rn started to decrease after July (Fig. 4-A). But the LAI of 1.5 was lower than other reported value with approximately 3-5 (Rosenberg et al. 1983). The results indicate that LE was not strongly linked with LAI of more than 1.5 m2 m-2. Canopy conductance (gc) is an important parameter for ET, and affected by vegetation conditions. gc reached the peak value in mid-July, and has the same tendency as ET (Fig. 6). Although the LAI kept at higher level after July, g c declined at the same time. Thus, decrease of gc after July might limit the transfer of water

LI Jie et al.

from the meadow ecosystem to the atmosphere.

CONCLUSION We explored the characteristics of ET and its energy exchange over a Kobresia meadow ecosystem on the Qinghai-Tibetan Plateau for 2002-2005. Despite of the high incident solar radiation input into this meadow ecosystem, the relatively low ET/P and LE/Rn are mainly due to the lower Rn/Rs and VPD in comparison to other grassland ecosystems. The results imply that the Kobresia meadow may reduce water loss through the ET from the ecosystem to the atmosphere. In addition, ET measured by eddy covariance system may be underestimated because of the lack of energy balance closure, and further study of ET will be made in future research.

MATERIALS AND METHODS Our study site (37°36´N, 101°18´E; alt. 3 250 m) is a Kobresia meadow located in the northeast of Qinghai-Tibetan Plateau. The annual solar radiation was 6 242 MJ m-2 during the period from 1980 to 2005. Mean air temperatures for January and July are -15°C and 10°C, respectively, and mean annual precipitation is 567 mm (over 80% falls during the period for May-September). The maximum aboveground biomass is about 300-350 g m-2 in late August. The openpath eddy covariance system and a meteorological observation system installed in a flat Kobresia meadow were used to measure water vapor flux and environmental factors continuously. More detailed information can be found in references (Gu et al. 2003, 2005; Kato et al. 2004).

Acknowledgements This study was supported by the National Natural Science Foundation of China (31070433) and Japan-China Research Cooperative Program (2010DFA31290). This study was also partly supported by the project of Early Detection and Prediction of Climate Warming Based on the LongTerm Monitoring of Alpine Ecosystems on the Tibetan Plateau funded by the Ministry of Environment, Japan.

References

Fig. 6 The seasonal variations in ET, canopy conductance (gc) and leaf area index (LAI) in Kobresia meadow.

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Evapotranspiration and Its Energy Exchange in Alpine Meadow Ecosystem on the Qinghai-Tibetan Plateau

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