Observational Study of Hydrological Land-surface Processes on Semi-arid
Grassland Underlain by Warm Permafrost in Mongolia
Yinsheng Zhang1, M. Ishikawa1, T. Kadota1, E. Munkhtsetseg2, D. Oyunbaatar2, and T. Ohata1
1 Institute of Observational Research for Global Change (IORGC), Yokohama 236-0001, Japan
2 Institute of Meteorology and Hydrology, Ulan Bator, Mongolia

Abstract
Since July 2002 to June 2004, intensive observation, including micrometeorology, phenology, heat budget and soil
heat/water condition, has been conducted at site on sparse grassland in Mongolia, locating in southern periphery regionof Eurasia cryosphere. Hydrological land surface processes at study site, including the role of snow cover and grassplaying in water fluxes at ground surface, has been investigated in this work.
Key word: Mongolia, Grassland, evapotranspiration, soil moisture

1.

Introduction

The climate of Mongolian territory, which is distributed
in southern fringe of the Siberia permafrost occurrence,is characterized by a long and cold winter and a short but
warm summer. Due to far from oceanic influences,characteristic feature for Mongolian climate is the
combination of low relative humidity values with rather
low precipitation. The feature of land-surfacehydrological processes characterized by a few
precipitation, high potential-evaporation, developing ofsemi-arid vegetation and presence of permafrost.
To improve our understanding of water cycle
processes at the periphery of the Eurasian snow
cover/frozen ground region, a series of observation sites
were established on a semi-arid grassland, as part of the
project “Observational study of the impact of the water
cycle and thermal conditions at the periphery of theEurasian snow cover/frozen ground region in relation toclimate formation and climate change”, funded by the
Frontier Observational Research System for Global
Change of Japan. The observation terms include
radiation and heat fluxes, meteorology, phenology ofgrassland and water condition of surface ground layer.
The data has been gained continually by a full annual
cycle, and analyzed preliminarily.

2.

Observation site and terms

An observation site was established on sparse grassland,of Nalaikh in north-eastern Mongolia at 47o45'N,
107o20'E, 40 km southeast of Ulan Bator, in a field in a
mountain plain in the Tuul River basin (Fig. 1). The site
located on a sediment plain in the vast valley, nearest

mountains is far than 10 km, the topography of andaround this site is very smooth.

Fig. 1: Location of the study site
The soil in the study region is sandy, with a barren
organic. From ground surface to 60 cm depth, soil
porosity is decrease by range of 58-34%, and bulk density
r increase from 1.17 gcm3 to 1.71 gcm3. From surface to
50 cm depth, soil water capacity range 17.2 to 6.5 %, andwilting point range from 4.1 to 2.8%.
Observation site is located in the periphery region of
sub-arctic permafrost (Sharkhuu, 2001). The permafrost
is characteristic by thick active layer and higher groundtemperature. From the borehole observation of 30 m
depth at study site, the ground has been clarified to beunderlain by warm permafrost (Ishikawa et al, 2003).
The maximum of downward thaw front reach about 5 m,
and relative moist zone with gravity soil moisture about
10% was found at same depth.
In the two years from July 2002 to June 2004,
precipitation was observed to be 272.5 mm, 79% of the
precipitation was occurred in the warm season (May toSeptember). Air temperature, humidity and wind speed
were averaged to be –4.1oC, 61% and 2.6 m/s
respectively.
The annual mean global radiation flux was observed
to be 168.5 Wm-2. The all-wave radiation albedo shows
clear seasonal variation. That was low and stable insnow-free period (0.12) but high and variable in snow
cover period (0.71). Accordingly average net radiation
was 60.0 Wm-2 and –6.0 Wm-2 respectively.
Since July 2002, meteorological and heat budget
data were obtained using an automatic climate
observation system (ACOS), in which air temperature,humidity and wind speed profile were measured at heightof 0.5, 1.0, 2.0 and 4.0 m respectively; short-wave
radiation, long-wave radiation and photosynthesis active
radiation (PAR) were measured both in up- anddownward respectively; the sensors of air pressure,inferred radiative thermometer for surface temperatureand net radiation were installed at 1.5 m above groundsurface as well.
Soil moisture was observed both automatically andmanually. 7 TDR probes and 7 Pt thermometers, installed
parity at depth of 0, 0.2, 0.4, 0.8, 12, 2.4 and 3.0 m
respectively; moreover, 2 sets of heat flux meters wereset at 0.02 and 0.2 m. In addition, the soil moistureprofile was observed with manual samples from a
7-m-deep borehole every month. The soil moisture in thesurface layer (0-60 cm) was measured using manual
sampling.
Phenology observations, including the coverage andbiomass of grass, were made from July to September at
10-day intervals. Plant coverage is sparse, generally itreaches about 38-42%. Also, plant type and species
over pastureland are poor. About 60 percent of themoccupied Artemisia frigita. Others are occupied by
Arenaria and Leymus chinensis. The maximum grassheight in mid July was less than 20 cm.
3. Results and Analysis
3.1. Atmosphere and ground surface control on

evapotranspiration
Evapotranspiration (E) has been calculated using heatbudget method and displaying in second panel of Fig. 2.
And precipitation (Pr) and surface soil moisture (θ) areshowing by same period in top and third panel of the
figure. The lines show declining processes ofcomponents, black lines show processes relating toprecipitation water, and dash lines shows processes
relating to snow melting water.
Pr was variable even in annual basis. In the twoyears from July 2002 to June 2004, 65% of precipitation
occurred in 2003. The summer of 2003 was moister thanothers.

Fig. 2: Seasonal variation of precipitation (Pr),calculated evapotranspiration (E), surface soil
moisture (θ at surface) and evaporation coefficient(E/Ep) in the pre-growth and grass growing period(early-growth to senescence) at study site since July
2002 to June 2004. The lines show declining processesof E, θ and E/Ep, black lines show processes relating
to precipitation, and dash lines shows processes
relating to snow melting water.
Most notable of the processes shown in Fig. 2 is thesensitive response of E to precipitation events or snow
melting. It can be seen clearly that E tentatively
correlated well to coupling to Pr varying through θ atsurface. Every declining process of E and θ at surface
can be initially corresponded to the precipitation events(beside of declining relating to snow melting water).

Firstly, the declining of E can be seen in the period
just after snow disappearing. The snow disappeared at
end of the March or beginning of April. By coupling byvery shallow melting ground, water contented in groundsurface layer was penetrated from snow melting but
rarely relate to precipitation. From the Figure 2, it can beseen also that declining process of surface soil moisturecausing by snow melting. Within ten days, θ at surface
dropped intensively from 40 to 20%, leading notably
decrease in E and E/Ep.
General seasonality of evapotranspiration istheoretically limited by available energy, expressed in
potential evapotranspiration (Ep) as well, which is a
useful index to investigate the controlling of groundsurface condition to evapotranspiration (E) processes. If
Ep completely determined
mined by atmospheric condition, E
normalized by Ep should depend on only surface(Soil/vegetation). However, most concepts of Ep
involving net radiation, which relating to surface
condition as well. In this study, Ep has been calculated,
and it’s normalized index of E/Ep, so called evaporation
coefficient, showing in lowest panel of Figure 2.
All of the declining processes in E/Ep can be
dressed tentatively in temporal variation of θ at surface,
including that just after snow melted. The results that
E/Ep indicate the controlling of ground surface condition
to evapotranspiration (E) efficiently in study region.
Surely, sustained period of every declining process wasdifferent relating to precipitation and soil water capacity,which partly determined by grass growing situation.
Therefore, on such semi-arid sparse grassland, all peaksof evapotranspiration variation can be elucidated by
precipitation events or snow melting.
It is necessary to noted that all the declining line
were not exactly linear, that imply the evapotranspiration
at the study site not determined by precipitation
completely, but anticipated to affect by vegetation as
well, for instance, even if with lower biomass, grass still
uptake water through its root from soil to supply
transpiration.
3.2. Role of grass playing in water cycle and its

response to atmospheric forcing
Using the parameterization soil surface moisture,
evapotranspiration (E) has been partitioned to be soil
evaporation (Esoil) and transpiration (Etrans) at studysite (Table 1).
Phenological observation at study site show biomassof grass in summer of 2003 was higher than others of
50%. However, it didn’t lead a anticipated increase in

Etrans. Etrans has been totaled to be 23.2 mm insummer of 2003 proportioned just of 15% in E, which
was a moister summer with lower AT. Etrans waspartitioned of 34-39% in drier and warmer summer.
The transpiration determined by two parameters: leaf
conductance and leaf-to-air specific deficit. leaf
conductance has been concept clarified to be a functionof temperature. The analysis shows Trans tentatively
increase as temperature rising. When temperature is high,higher leaf conductance lead grass uptake water even if
soil moisture is lower but higher than wilting point,
which has been measured to 2.8 to 4.1% in root zone.
Once precipitation event occurring, lower temperatureplus lower leaf-to-air specific deficit, causing by higher
air humidity.
Tab. 1: Integrated ground surface water budget overgrowth periods at study site: Precipitation (Pr), Air
temperature (AT), Evapotranspiration (E), Soilevaporation (Esoil) and Transpiration (Etrans).
PeriodSurface(mm/dd/yy)
7/1-10/2/02

condition
Sparsegrass

10/3/02-Snow/

3/31/03frozen4/1-4/30/03
5/1-10/7/03

BaregroundSparsegrass

10/8/03-Snow/

2/26/04

frozen

3/27-4/30/04
5/1-6/30/04

Bare
groundSparsegrass

Total/mean

Pr

AT

E

(mm)

(oC)

(mm) (%)

(%)

39.4

12.7

48.9

66

34

17.9

-19.5

7.5

2.2

1.5

14.6

100

0

141.0

10.4

163.0

85

15

26.4

-19.5 13.1

9.9

0.3

15.3

100

0

35.7

11.8

39.1

61

39

272.5

-4.1

301.6

78

22

Esoi/E

Etras/E

Comparing to Etrans, Esoil is more sensitive tosurface soil moisture. The later transfer vapor toatmosphere by taken water from very thin soil layer, theformer transfer water from soil through mechanism ofroot-stalk-leaf, and could be taken water a thicker soil
layer to transfer into atmosphere. In the period just aftersnow disappeared of 10-15 days, Esoil was equal to E
perfectly. Therefore, Etrans is anticipated to be steadily
to soil moisture variation comparing to Esoil. To
investigation of dependence of soil evaporation
efficiency on the availability of stored water show that

Esoil/Ep increases near linearly with ground surface
moisture when the volumetric water content is less than
30%, but does not increase with moisture content beyond
that level. This stresses the importance of the criticalvalue of soil moisture of 30%. When soil moisture is lessthan 30%, the soil evaporation process is restrained by
the deficiency of available water. An similar criticalvalue has been noted on near bare soil ground surface inTibetan Plateau and on Siberia tundra.
3.3. Seasonality of water budget at groundsurface
Having considering the ground surface condition, water
budget at study site could be analyzed in accordance withperiod of ground surface frozen/snow covered, bareground surface and grass-growing.
The ground surface at study site was frozen/snow
covered for near half year. From snow survey
observation, Pr has been estimated to be 17.9 and 26.4
mm in winter of 2002/03 and 2003/04 respectively,
which was taken of 16% to annual precipitation.
Correspondingly, sublimation was 7.5 and 13.1 mm,
proportioned of 7% to annual E. More than half ofprecipitation accumulated on ground surface as snow
cover.
In the period just after snow disappeared but before
grass growth, ground surface melted but barred, snow
cover still affect evaporation process. Snow-melting
water penetrated to thin thawed layer leading soil nearsaturated. Soil evaporation partitioned of 10% to annual
coupling precipitation just of 4%. By accounting thepenetrated snow-melting water, Pr-E was near balancedin the period.
From July 1 to Oct. 2 of 2002, the period before
ground surface starting frozen, the E was totaled to be
48.9 mm coupling precipitation of 39.4 mm. E and Pr
was totaled to be 177.6 mm and 143.2 mm during April 1
to October 7 of 2003, and to be 54.5 mm 45.6 mm during
March 26 to June 30 of 2004. The ratio of E/Pr was 1.24,
1.24 and 1.20 respectively. Pr was nearly to be balancedby E in whole study period. Even if grass-growing period
is quite short in such periphery region of cryosphere, butwater fluxes between atmosphere and ground surface inthe period contribute significantly to annual water cycle.
88% of E occurred in grass growing period (early-growth
to senescence) coupling to 94% precipitation.
In the observation period from July 2002 to June
2004 consisting two cycle of cross pre-growth tosenescence period, evapotranspiration was totaled to be
301.6 mm coupling to precipitation of 272.5 mm.

Partition of transpiration was 22%.

4.

Discussion

By analysis based on data for two year on the sparse
grassland in fringe region of cryosphere, it has
demonstrated that evapotranspiration is sensitive toprecipitation forcing, which show a clear declining
process initial from significant rainfall events. On suchsemi-arid grassland, evapotranspiration shows a fast
response to precipitation (Scott et al., 1997).
Scott et al. (1997) has dress that soil evaporation andvegetation transpiration behaves differently in response
to atmosphere forcing. Increasing in evapotranspiration
for moister summer was dominant by increasing in soil
evaporation leading by more precipitation. By contrast,
transpiration was higher in drier summer determined byhigher temperature through enhancement in grass leaf
conductance. Miyazaki et al. (2001) dressed anincreasing in evapotranspiration of 20% coupling same
summer precipitation but twice LAI of grass and warmer
soil temperature. The results from this work imply thatsemi-arid grass affect land surface processes not only by
its changes in albedo and roughness (Li et al., 2000) but
also changing in transpiration. Transpiration is
anticipated to enhance drought in warm and driersummer.

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