Elsevier

Journal of Hydrology

Volume 333, Issue 1, 30 January 2007, Pages 133-143
Journal of Hydrology

Evapotranspiration from a Mongolian steppe under grazing and its environmental constraints

https://doi.org/10.1016/j.jhydrol.2006.07.021Get rights and content

Summary

The magnitude and seasonal dynamics of evapotranspiration (ET) for a steppe in central Mongolia was estimated over a full year period (from 25 March 2003 to 24 March 2004, 366 days) using the eddy covariance (EC) technique. The steppe, typical of central Mongolia, is dominated by temperate C3 plants and experiences moderate grazing. The environmental constrains over ET for the steppe were evaluated by examining the responses of ET to biotic (leaf area index) and abiotic (atmospheric evaporative demand and soil moisture condition) factors. Seasonal variations in ET followed closely the variation in leaf area index. Change in soil moisture was the most important environmental factor controlling the dynamics of ET in this grassland ecosystem, indicated by the strong susceptibility of ET and the Priestley–Taylor parameter (α, calculated as the ratio of the measured ET to the equilibrium ET) to soil water content. The α also showed a distinct seasonal variability, but its value on average was lower than 0.5 during most of the measurement period, suggesting that the steppe was limited in water supply. The maximum daily ET rate was 2.8 mm d−1. Cumulative ET during the study period estimated directly by the EC method was 163 mm, which was 66% of the precipitation received at the site during the same time period (248 mm).

Introduction

Apart from precipitation, water vapor exchange (also called evapotranspiration, ET or latent heat flux, LE) between the atmosphere and terrestrial ecosystems is the major component of the terrestrial hydrological budget (Brutsaert, 1982). ET, by definition, is the process by which ecosystems return soil water, initially charged from precipitation (PPT), to the atmosphere to balance regional and global water cycling through two pathways: soil evaporation and plant transpiration (Brutsaert, 1982). ET at a given site depends largely on the availability of water and energy, and thus is site-specific. For example, in arid and semiarid environments across the world, limitations due to soil moisture are believed to be the most important factor affecting ET (Noy-Meir, 1973, Sala et al., 1992). ET is also controlled by canopy architecture and development, soil characteristics, and a variety of in situ environmental variables (Kelliher et al., 1995, Wilson et al., 2002a).

In Mongolia, steppe covers 83% of the territory (∼1.3 × 106 km2) (World Resources Institute, 2003). It lies mainly in the central part of the country, the transitional zone bordering the Gobi deserts to the south and mountain taiga forests to the north (Hilbig, 1995, Batjargal and Enkhbat, 1998). The steppe ecosystems are associated with the semiarid and arid continental temperate climates of the region, and are ecologically fragile and sensitive to climate change and anthropogenic disturbances. Recent studies have shown that in central and southeastern Mongolia, the annual mean temperature has shown an increasing trend (Yatagai and Yasunari, 1994, Dagvadorj and Mijiddorj, 1996) while summer precipitation has shown a decreasing trend over the past half century (Yatagai and Yasunari, 1995). The Mongolian steppe ecosystems play an important role in mitigating regional and even global climate through their interaction with the atmosphere (Yatagai and Yasunari, 1994, Yatagai and Yasunari, 1995). In turn, climate change may give rise to a modification in surface boundary conditions with respect to water vapor and energy exchange (Houghton et al., 2001). However, our knowledge of the ecosystem functioning of the Mongolian steppe is still lacking (Sugita et al., 2006). Therefore, it is crucial that information about water vapor and energy exchange is obtained in order to improve understanding of the coupling among water and carbon cycling, steppe management, and climate change in Mongolia. Only a few studies have assessed the hydrological processes of the steppe ecosystems in Mongolia (e.g. Ma et al., 2003, Miyazaki et al., 2004, Zhang et al., 2005). By using multiyear growing season measurements of hydrometeorological elements and surface energy fluxes from the Automatic Weather Station at a steppe in central Mongolia, Miyazaki et al. (2004) have found that ET and grass growth are highly associated with precipitation and soil water storage before July. By using observations obtained with microlysimeters, Zhang et al. (2005) have evaluated the growing season eco-hydrological characteristics of a sparse steppe at the edge of the Eurasian cryosphere in Mongolia, and have found that for grass growth, atmospheric heat stress is weaker than soil water stress, and soil evaporation is the prime contributor to ET. These works provide valuable information about the functioning of Mongolian steppe ecosystems. As part of the RAISE project, this study is designed to quantify water vapor exchange for a full year period and to establish its relationship to biotic and abiotic variables above a steppe in central Mongolia by means of the eddy covariance technique, which is now widely used for continuous flux measurements (Baldocchi, 2003).

Section snippets

Site information

The study site is located at Kherlenbayan-Ulaan (KBU), Hentiy province, Mongolia (lat. 47°12.838′N, long. 108°44.240′E). The elevation is 1235 m above sea level. The soil is Kastanozem. For the surface horizon (top 30 cm), the bulk density is 1.45 g cm−3 on average, the hydraulic conductivity at 15 °C averages 0.01–0.08 mm s−1, and the overall porosity is about 45% (Asano, 2004). There are scattered stones on the ground surface and in the top 2-m of soil profiles.The climate is continental in the

Daily patterns of energy partitioning in relation to canopy development and precipitation

The daily pattern for energy partitioning was clearly affected by precipitation (PPT) at the site. Fig. 2 illustrates the three day ensemble mean diurnal cycles of the energy components (LE, H and G) and the Bowen ratio (β = H/LE) prior to and immediately after the PPT at two canopy development stages: the early growth stage (Stage I; day of year, DOY 129–138) and the peak growth stage (Stage II, DOY 221–231). Both stages experienced a shift from dry to wet due to PPT (12.2 mm from DOY 132 to 135

Conclusions

We quantified evapotranspiration (ET) over a year long period for a Mongolian steppe using the eddy covariance (EC) technique. ET occurred dominantly during the growing season, fluctuated in a close relationship with precipitation or variation in soil moisture, and followed a linear relationship with leaf area index. Therefore, canopy development and soil moisture conditions acted as major constraints on ET. This was also depicted by the Priestley–Taylor parameter (α), which also showed a

Acknowledgments

This study has been supported in part, by the Japan Science and Technology Agency through a grant under the Core Research for Evolutional Science and Technology (CREST) program funded for the RAISE project. Partial support is from the Global Environment Research Fund of the Ministry of Environment of Japan. The first author is now also supported by the “Hundred Talents” Program of the Chinese Academy of Sciences. The authors acknowledge the help of W. Eugster, Institute of Plant Sciences, Swiss

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