G-WADI, Beijing, 26 OCT 2016 Hydrologic change in the Upper Heihe Basin of the Northeast Tibetan Plateau Dawen YANG Professor of hydrology & water resources Department of Hydraulic Engineering Tsinghua University, Beijing, China
OUTLINE: I. Background II. Development of a distributed eco-hydrological model III. Ecohydrological changes in the upper Heihe Basin IV. Summary
I. Background Eco-hydrological Change on the Tibet Plateau due to global warming Effect of glacier/snow melting on the river discharge Effect of soil freezing thawing on the ecohydrological processes and runoff Tibet Plateau Degradation of permafrost Glacier melting 3
Ecohydrological characteristics of the high altitude mountain Heihe River 黑河 Elevation: 790 5547m Precipitation: 50 500mm/a Population: 1,300,000 The upper mountain region has 20% of the basin area, but it provides 70% of river discharge.
Vegetation pattern in the upper Heihe basin Aspect and altitude are two key factors for determining the vegetation pattern.
The major objectives of Integrated Research on Ecohydrological Modeling in the Upper Heihe River project funded by NSFC are: Develop a distributed ecohydrological model. Understand the ecohydrological change in the past and predict change in the future.
1-km Grid II. A distributed ecohydrological model GBEHM (geomorphology-based ecohydrological model) River network: using Horton-Strahler ordering system to determine the flow routing sequence in the river network. Connectivity of hillslope and river: using the flow distance to determine the connection between hillslope and river in a sub-catchment. Sub-catchment area: 20km 2 7
(1) Cryosphere Hydrology Glacier melting: simple energy balance model Snowpack: snowpack metamorphism and energy balance Soil frozen and throwing: multi-phase water movement and energy balance Energy and mass exchange at snow surface Snow surface SNOWPACK Melt runoff Underlying conditions(soil, VEGETATION, etc.)
(2) Vegetation Dynamics Basic balance: energy-water-carbon Photosynthesis (Farquhar et al., 1980) CO 2 Solar flux Atmosphere Temper. CO VPD 2 Stoma Transpiration Canopy Rainfall Evaporation Respiration (LPJ; Biome-BGC) Total Respiration Photosynthesis Interception Leaf Respiration Allocation Leaf Phenology Carbon pool decomposition Interfering process: grazing (Yak and sheep) Stem Respiration Soil Respiration Litter Stem Root Soil moisture decomposition Soil layer -9-
(3) Model Validation Plot Scale Comparing with observed fluxes and soil moisture and temperature Heat fluxes: Simulation shows a good agreement with the observations, RMSE < 25 W m -2. Snow melting and soil thawing: The simulated snow melting and soil thawing occur earlier than the insitu observation.
Plot Scale
Catchment Scale: Comparing with the observed soil moisture
Catchment Scale: Comparing with the observed soil moisture Relative error of monthly average soil moisture: -13% ~ 141% Relative error from April to October: -13% ~ 14%
Catchment Scale: Comparing with the remote sensing-based ET The annual mean relative error is 6% (-8% ~ 21%)
Catchment Scale: Comparing with the remote sensing-based ET Annual Mean Actual Evapotranspiration during 2001-2010 Annual Mean Actual Evapotranspiration simulated by the ecohydrological Model Annual Mean Actual Evapotranspiration estimated from remote sensing data
Catchment Scale: Comparing with the observed discharge
III. Ecohydrological change in upper Heihe River (1) Annual water balance in the upper Heihe basin (1981~2010) (in growing season)
(2) Ecohydrological pattern in the upper Heihe basin Precipitation increasing with elevation Evapotranspiration similar pattern with vegetation Runoff distribution increasing with elevation
(3) Influence of vegetation on the water balance In the same elevation interval, precipitation and air temperature are close, thus vegetation type is the main factor affecting the water balance (ET and R). Elevation interval (area) 3,000 3,199m (123 km 2 ) Vegetation type Area ratio P (mm/a) ET (mm/a) R (mm/a) R/P Shrub 24% 436.5 381.1 53.2 0.12 Steppe 12% 439.6 398.6 37.7 0.09 Forest 13% 450.8 421.9 34.1 0.08 Meadow 51% 418.5 383.8 36.0 0.09 Shrub 18% 447.5 384.4 68.8 0.15 3,400 3,599m (590 km 2 ) Steppe 2% 466.6 401.5 68.3 0.15 Forest 1% 458.1 402.5 56.4 0.12 Meadow 78% 437.8 384.4 65.6 0.15 4,200 4,399m (634 km 2 ) Meadow 35% 458.4 337.3 129.7 0.28 Sparse Vegetation 60% 526.8 264.8 259.0 0.49
(4) Contributions of different land cover to the total runoff The area ratio of meadow, sparse vegetation and shrub is 45.5%, 20.1% and 16.5%. The contribution to the total runoff of meadow, sparse vegetation and shrub is 39.4%, 36.5% and 13.7%. Glacier area accounts for only 0.8%, but its contribution to the total runoff is 4%. Vegetation type, area (km 2 ) and area ratio (%) P (mm/a) ET (mm/a) R (mm/a) Runoff (10 8 m 3 /a) Contribution to the total runoff (%) Desert 91 0.9 253.1 238.0 15.1 0.01 0.1 Shrub 1,652 16.5 495.9 355.0 140.9 2.33 13.7 Steppe 1,063 10.6 396.7 331.5 65.2 0.69 4.0 Forest 561 5.6 402.1 331.6 70.5 0.40 2.3 Meadow 4,549 45.5 488.5 348.7 147.8 6.72 39.4 Sparse Vegetation 2,009 20.1 547.3 237.2 310.1 6.23 36.5 Glacier 80 0.8 586.7 82.7 846.2 0.68 4.0
(5) Change of the permafrost and seasonally frozen soil
Permafrost changed to seasonally frozen soil at elevation of 3800-4200 m
(6) Changes of the runoff and actual evapotranspiration
Summary It is desired to understand the permafrost change and its hydrological impacts on the Tibetan Plateau under global warming. A distributed permafrost hydrological model has been developed during the Heihe project, and it would be useful for this purpose. 25
Acknowledgement It was supported by an on-going research project funded by the Natural Science Foundation of China (NSFC). Drs. Gao Bing (CUG), Li Xin (CAS), Zheng Yuanrun (CAS), Wang Yanhui (CAF) and PhD students (Qin Yue, Wang Yuhan) contributed to this talk.
Thanks for your kind attention. Dawen YANG (yangdw@tsinghua.edu.cn) Department of Hydraulic Engineering Tsinghua University