Heat and Water Balance Estimatesover the Tibetan Plateau in 1997-1998*Jianqing XU1 , Shigenori HAGINOYA2,Kooiti MASUDA1, Rikie SUZUKI1(1: FRCGC/JAMSTEC, 2: MRI)
* Jianqing XU, Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science andTechnology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama City Kanagawa 236-0001, Japan. E-mail:jxu@jamstec.go.jp
Abstract
Xu and Haginoya (2001) used a multi-layer soil model with input data derived from routine meteorological data toestimate heat and water balances over Tibet in 1997. The present paper uses the same method to estimate heat and waterbalances for 1998. GAME-Tibet intensive observation period (IOP) data are used to verify calculation results. Observed
longwave radiation flux agrees well with calculations. Surface air temperatures were higher in 1998 than in 1997. Sensible
heat flux dominated the heat balance in the pre-monsoon season, and latent heat flux dominated it in the monsoon season.
Peaks of net radiation Rn appear in July of the monsoon season. The rainfall pattern influenced the amount of the drainage.
Comparing with 1997, the rainfall events were more concentrated in the monsoon season of 1998. The amount of rainfall
available for runoff was larger, and ground soil layers were wetter in 1998.Western Tibet was drier, and eastern Tibet waswetter, in 1998 than in 1997. Evaporation E was small (less than 100 mm/year) in western Tibet and large (about 400mm/year) in eastern Tibet. The wetness index(WI) distribution agrees well with the distribution of the normalized difference
vegetation index (NDVI). Results were also compared to precipitable water (from the NASA Water Vapor Project, NVAP)
in the atmosphere. Calculations were carried out for 14 stations in 1997 and 17 stations in 1998. Datasets of daily andseasonal variations based on 1997-1998 have been established; heat and water balances can be judged from such datasets.
Keyword: sensible heat flux, latent heat flux, heat and water balance, the Tibetan Plateau, IOP dataset.
1.

Introduction

Xu and Haginoya (2001) estimated the water andheat balances over the plateau using a multi-layer model
initialized with routine meteorological data from 1997.
Daily and seasonal variations were shown for 14 sites overTibet. Subsequently, an intensive observation period (IOP)of the GAME-Tibet Global Energy and Water CycleExperiment (GEWEX) Asia Monsoon Experiment overTibet took place in 1998. The present paper will determine
the heat and water balances for the Tibetan Plateau during
1997-1998; the results are compared to observations. The
relationship between the normalized difference vegetationindex (NDVI), as reported by the Center for Environmental
Remote Sensing (CEReS), of Chiba University, Japan, andthe wetness index (WI=Pr/Ep) is examined. Therelationship between calculated results and precipitablewater from the NASA Water Vapor Project (NVAP) dataset
will be shown.

2. Method
2.1. Model
The model used in this study was developed by
Kondo and Xu(1997a). Sub-surface soil water transport isbased on the multi-layer soil model proposed by Kondo
and Saigusa, 1994. The soil model has been put to practicaluse using a simplified calculation scheme (Kondo et al.
1995; Kondo and Xu 1996a, 1996b). Although thevegetation such as grass grows in eastern Tibet seasonally,
the surface is assumed to be bare soil surface in the model.
We should notice that the assumption might lead tounderestimate the amount of evaporation in the monsoon
season of eastern Tibet. Soil properties differ greatly fromplace to place on the earth's surface and also from the top
to the bottom of the horizontal layers that constitute a soil

profile. The present calculations assume that soil type isconstant from the surface to the lowest layer (0.7 m). Watertransport within the soil is partitioned into vapor and liquid
phases. Xu and Haginoya (2001) applied the model to theTibetan Plateau to analyze heat and water balances in 1997.
Soil parameters in the Tibetan Plateau have been studied
and used in the model.
Potential evaporation is also an important
climatological index. Kondo and Xu (1997b) and Xu(2001) defined potential evaporation Ep as the evaporation
expected from a hypothetical surface with saturated soil
and a roughness of 0.005 m; albedo ref is 0.06 (watersurface), emissivity 0.98, and evaporation efficiency andrelative humidity are 1.0. Typical surfaces that satisfy theseconditions are fields with a wet, rough, and black surface,or a newly planted paddy field, with dripping wet leaves(Kondo and Xu, 1997b, Xu and Haginoya, 2001).
Solar radiation flux was estimated in the present
study from sunshine duration; long-wave radiation flux is afunction of solar radiation flux, surface air temperature,and humidity (Kondo and Xu, 1997a).

2.2. Data
The calculations were conducted for 14 sites in 1997
and 17 sites in 1998. Eight sites were common to both1997 and 1998. Input datasets originated from routine
meteorological data as follows:・ daily mean air temperature TAM; temperature range
TA,MAX-TA,MIN; sunshine duration N; precipitation amountPr; wind speed UOBS; vapor pressure e; surface pressure ps.
・ The following output data were estimated fromcalculations: downward solar radiation flux S↓; downward
long-wave radiation flux L↓; latent heat flux ιE; sensible
heat flux H; evaporation E; potential evaporation Ep; soil

water content θ; wetness index WI= Pr/Ep; calculated
surface temperature TSE.
・The comparison datasets include: observed long-waveradiation flux from the IOP of the GAME-Tibet automatedweather station (AWS) dataset; NDVI dataset fromCEReS; precipitable water (mm) from NVAP.

2.3. Verification of long wave radiation
Observed and calculated solar radiation, soil watercontents, ground surface temperatures, and sensible andlatent heat fluxes were compared in Section 4 of Xu andHaginoya 2001. Verification of the long-wave radiation
flux is provided in the present study. Figure 1 shows theseasonal variation of the daily mean downward
long-wave radiation flux L ↓ at Shiquanhe for 1998.
Open circles are observations and the solid line iscalculated. Calculations and observations show goodagreement. Long wave radiation flux has an annual meanof about 237.2W/m2 and ranges from about 160W/m2 inwinter to about 310W/m2 in summer. Shiquanhe has anannual precipitation of about 60mm. The long-wave
radiation there is relatively small in the Tibetan Plateau.

Fig. 1: Calculated and observed long wave radiation
over Shiquanhe in 1998. The solid line is the calculatedL↓, the circles are observed L↓.

was late in 1998 (Fig.2k, l). Changes in the latent heat flux
(Fig.2i, j) suggest that the differences between the dry andrainy seasons were greater in 1998. The latent heat flux issmall in the dry season because of a lack of water;
evaporation occurs mainly during the rainy season. Most of
the net radiation converts to sensible heat flux during thedry season. In contrast, latent heat flux dominates the heatbalance during the rainy season. Observed surface airtemperatures differed between 1997 (annual mean -2.4℃)
and 1998 (annual mean -0.1℃), which suggests that theheat flux was also different. Net radiation and sensible heat
fluxes were frequently negative in 1997, but rarely so in
1998 (Fig.2a-d). Volumetric soil-water content was high(wet) in 1997 and low (dry) in 1998 (Fig.2g, h) because ofwarmer, drier conditions in 1998. Soil-water content
increases with rainfall.

Fig. 2: Comparisons of the calculated seasonalvariations at Nagqu for 1997 and 1998. Daily mean net
radiation (W/m2) in 1997 (a) and 1998 (b); Sensible heat
flux (W/m2) in 1997 (c) and 1998 (d); Surface airtemperatures (℃) in 1997 (e) and 1998 (f); Volumetricsoil-water content in the surface layer (0.01 m) in 1997(g) and 1998 (h); Latent heat flux (W/m2) in 1997 (i) and1998 (j); Precipitation (mm) in 1997 (k) and 1998 (l).

3.2. Heat balance
3. Heat and water balance
Model calculations were carried out for 14 sites in1997 and for 17 sites in 1998. Datasets of daily andseasonal variations for these two years have been
established. Heat and water balances can be inferred fromthe datasets.

3.2. Seasonal variations (Nagqu)
Nagqu (31 ° 29'E, 92 ° 04'N) is the nearest
meteorological observatory to Amdo, a planetary boundary
layer (PBL) observation site for IOP GAME-Tibet. Figure
2 compares the calculated seasonal variations for 1997 and1998 at Nagqu. Horizontal axes represent the day of theyear (DOY). Daily mean net radiation, sensible heat andlatent heat fluxes, surface air temperature, volumetricsoil-water content and precipitation are shown. Annual
total precipitation was 558mm in 1997, and 526mm in
1998, but the onset of the rainy season (i.e., the monsoon)

Figure 3 shows the seasonal variation in heat fluxes
over the Tibetan Plateau. A block with curves and two axes
represents the heat balance for each station. The figurelegend is in the lower left. Annual mean net radiation
fluxes range from about 50 to 80W/m2 over the plateau,changing from about 25W/m2 in the winter to 110W/m2 inJune or July. Peaks in net radiation flux were greater in1997 than in 1998 over western Tibet (Shiquanhe, Gerzeand Baingoin), but smaller in 1997 than in 1998 overeastern Tibet (Nagqu, Lhasa, Xigaze, Qamdo), except at
Nyingchi. In arid areas and during the dry season, sensible
heat flux is larger than latent heat flux and dominates theenergy balance. At the most arid station, Shiquanhe, morethan 90% of the net radiation becomes sensible heat flux
because so little water is available to evaporate. Peaks in H
appear in May just before the monsoon season. The
contribution of latent heat flux to total energy flux

gradually increases when, or where, precipitation isrelatively large. As rain wets the surface, latent heat flux
increases to a value about the same as H. The curved shapeof the latent heat flux line resembles the curve in theseasonal change in precipitation (Fig.4). The latent heat
flux due to precipitation in 1997 was larger in the east andsmaller in the west than in 1998.

and ground soil layers were wetter in 1998.

Fig. 4: Water balance over the Tibetan Plateau during
1997-1998. As in Fig. 3 except for the monthly total
precipitation Pr (histogram), the potential evaporation Ep(dotted lines), and the evaporation E (broken lines).

Fig. 3: Heat balance over the Tibetan Plateau during
1997-1998. The net radiation flux Rn(solid lines), thesensible heat flux H(dotted lines), the latent heat fluxιE(broken lines), and the heat flux into the ground
G(dashed chains) are shown in one block for eachobservatory. Figure legend is at lower left.

3.3. Water balance
Xu and Haginoya (2001) have shown that theclimate over the Tibetan Plateau is arid in the west andsemi-arid or semi-humid in the east (Fig.6). Precipitation
ranges from about 56 mm over western Tibet to almost
1000 mm over eastern Tibet. Figure 4 shows the seasonalvariation in the water balance of the Tibetan Plateau during
1997-1998. The differences between potential evaporationand evaporation (Ep-E) are large over western Tibet andsmall over eastern Tibet. Over southwestern Tibet,
precipitation was relatively heavy in September 1998,
resulting in a larger WI in 1998. At stations in westernTibet, such as Shiquanhe and Gerze, evaporation changes
with the precipitation because there is almost no drainage.
From January to May, rainfall and evaporation totals areboth small. Most precipitation and evaporation occurs
during the monsoon season (June to September), and thevalue of Pr is near, or greater than, the potential
evaporation Ep. Eastern Tibet is humid or semi-humid
during this season.
Figure 5 shows the monthly mean values of thecommon 8 sites of the seasonal variations of heat and waterbalance over the whole plateau during 1997-1998. Sensible
heat flux dominated the heat balance in the pre-monsoon
season, and latent heat flux dominated it in the monsoon
season. Peaks of net radiation Rn appear in July of themonsoon season. The rainfall pattern influenced theamount of the drainage. Comparing with 1997, the rainfall
events were more concentrated in the monsoon season of
1998. The amount of rainfall available for runoff was larger,

Fig. 5: Seasonal variations of heat and water balance over theTibetan Plateau in 1997 and 1998. (a) the monthly mean net
radiation Rn(solid line), the sensible heat flux H(dotted chaine),
the latent heat flux ιE(broken line), and the heat flux into theground G(dashed chain); (b) the monthly total precipitation Pr(histogram), the potential evaporation Ep (solid line), and theevaporation E(broken line); (c) the monthly mean values of thevolumetric soil-water content at the depths of 0-2cm, 2-6cm,6-14cm and 62-70cm.

4. The relationship between WI and NDVIClimate affects, and is affected by, the biosphere.
The use and management of water resources strongly
depend upon the climate. The Tibetan Plateau features avery complex topography, and the vegetation on theplateau is therefore highly variable. The WI constitutes acomprehensive value that represents atmosphericconditions, including temperature, radiation, and wind. Xuand Haginoya (2001) showed how WI is calculated. A WI
dataset for the plateau was prepared for 1997-1998.
The NDVI measures vegetation. An NDVI dataset
was acquired from the ``Twenty-year global 4-minute
AVHRR NDVI dataset" produced by CEReS, using atransformed version of the Pathfinder global 10-day
composite 8-km NDVI data from the Advanced Very High
Resolution Radiometer (AVHRR). The data weretransformed from an Interrupted Goode Homolosine map
projection to a Plate Carree map projection to simplify datausage. In addition, NDVI temporal changes were smoothedusing the Temporal Window Operation (TWO). The annual
mean and May-October mean NDVI in 1998 werecalculated for each 1°×1°grid box over the TibetanPlateau. Figure 6 shows the distributions of annually
averaged WI and NDVI in 1998. The NDVI distribution isin color. Lines representing the WI are overlain on theNDVI map. The two fields match very well. The climate isvery arid where the WI < 0.3 and NDVI values are small.
Wetness index (WI) and vegetation (NDVI) increases
gradually from the west to east over Tibet. Figure 7 shows
the regression relationship between WI and NDVI over theTibetan Plateau in 1998. The solid line is the regression
line for the annual mean and the broken line is for
May-October. WI and NDVI are correlated with each other.

Fig. 7: Regression analysis of WI and NDVI over theTibetan Plateau, 1998. The solid line is the regression
for the annual mean; the broken line is for
was less in western Tibet than in eastern Tibet in 1998, ascompared to 1997. Western Tibet was relatively dry andeastern Tibet was relatively wet in 1998. In 1998, monsoon
onset was delayed relative to 1997.
3) The WI distribution was similar to the NDVI distribution.
NDVI values were small where WI was small (aridclimates), and large where WI was large (humid areas).
4) Sensible heat flux dominated the heat balance in thepre-monsoon season, and latent heat flux dominated it in
the monsoon season. Peaks of net radiation Rn appear in
July of the monsoon season. The rainfall pattern influenced
the amount of the drainage. Comparing with 1997, therainfall events were more concentrated in the monsoon
season of 1998. The amount of rainfall available for runoff
was larger, and ground soil layers were wetter in 1998.

References

Fig. 6: The distribution of annual averaged WI (lines) versus
NDVI (color) in 1998. Color bar shows the value of NDVI.

5. Summary and conclusion
A dataset including heat and water balances has beendeveloped for the Tibetan Plateau for the period 1997-1998.
A multi-layer soil model was used to facilitate thecalculations. NDVI and NVAP data were compared to thepresent dataset.
1) Calculated long wave radiation flux agreed withobservations at Shiquanhe in 1998. The annual mean longwave radiation flux was 237.2W/m2; it ranged from160W/m2 in winter to 310W/m2 in summer. Shiquanhe
receives annual precipitation of about 60 mm. Long wave
radiation is relatively low over the Tibetan Plateau.
2) Surface air temperatures were higher and precipitation

Kondo, J., and J. Xu, An estimation of heat balance for arid
and semi-arid regions in China (1): climatologicalconditions, soil parameters and calculation method.
J. Japanese Soc. Hydro. and Water Resour.}, 9,
162—174, 1996a.
—, and —, An estimation of heat balance for arid andsemi-arid regions in China (2): results. J. JapaneseSoc.
Hydro.
andWater
Resour.,9,175—187,1996b.
—, and —, Seasonal Variations in the Heat and Water
Balances for Nonvegetated Surfaces. J. Appl.
Meteor., 36, 1676—1695,1997a.
—, and —, Potential evaporation and climatologicalwetness index. Tenki. J. Meteoro. Soc. of Japan},44, 875—883,1997b.
—, —,and M. Fukumoto, Evaporation from a bare field of
Ando-soil. J. Japanese Soc. Hydro. and Water
Resour., 8, 174—183,1995.
Xu, J., An nanlysis of the climatic changes in eastern Asia
using the potential evaporation. J. Japanese Soc.
Hydro. and Water Resour., 14, 151—170, 2001.
Xu, J., and S. Haginoya, An estimation of heat and waterbalances in the Tibetan Plateau. J. Meteor. Soc.
Japan, 79(1B), 485—504, 2001.