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Role of vegetation in modeling response of hydrological processes to climate change

5th International Symposium on IWRM & 3rd International Symposium on Methodology in Hydrology. Role of vegetation in modeling response of hydrological processes to climate change. Xi Chen State Key Lab. of Hydrology-Water Resources and Hydraulic Engineering, Hohai University. November 21, 2010.

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Role of vegetation in modeling response of hydrological processes to climate change

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  1. 5th International Symposium on IWRM & 3rd International Symposium on Methodology in Hydrology Role of vegetation in modeling response of hydrological processes to climate change Xi Chen State Key Lab. of Hydrology-Water Resources and Hydraulic Engineering, Hohai University November 21, 2010

  2. Outlines 1 Background 2 Role of plant on hydrological processes 3 Challenges in predicting response of hydrologic processes to climate change 4 Conclusions

  3. Background- Climate change

  4. Background- Climate change& hydrological cycle Carbon dioxide warms the Earth because it is a greenhouse gas in the atmosphere • Exchange of heat and moisture between atmosphere and Earth’s surface affect dynamics/thermodynamics of climate system1 • 50% of surface cooling results from evaporation • Water vapor acts as a greenhouse gas and nearly doubles effects of greenhouse warming CO2, methane, and all other gases “Although a very small fraction of water resides in the atmosphere, the rapid recycling of atmospheric water vapor makes it exert disproportionate control over the energetic of the climate system.”1 Carbon dioxide Water vapor 1. Chahine, Moustafa T., 1992: The hydrological cycle and its influence on climate. Nature,359, 373-381.

  5. Background- Climate change & Plant Map of globe shows percentage of predicted warming due to the direct effect of carbon dioxide on plants. cooling VOC Atmosphere CO2 (NPP) less cooling Vegetation Trees and other plants help keep the planet cool because of more evapotranspiration While the radiative effect of rising CO2 produces most of the warming, the physiological effect contributes additional warming by weakening the hydrologic cycle through reduced evapotranspiration. It also causes plants to provide less evaporative cooling.

  6. Role of plant on hydrological processes in the changing environment Long-term climate change • Climate and Soil Development Wet Dry Image Source: University of Wisconsin, 2002 • Changes of soil texture and network of macrospores • Changes of hydrological cycle and climate. • e.g. energy needed to convert soil moisture to water vapor • Dependent on surface albedo, which is dependent on vegetation/soil conditions, which alter “partition between evaporation & runoff”  changes soil conditions (endless cycle) Ye Weimin. 2008

  7. Role of plant on hydrological processes in the changing environment • Infiltration Rate Function of Vegetation Source: Texas Council of Governments, 2003. • Vegetation increases soil network of macrospores, which increases infiltration rate and hydraulic conductivity • This increase is more significant for forest Source: Gray, D., “Principles of Hydrology”, 1973.

  8. Role of plant on hydrological processes in the changing environment • Hydraulic conductivity •The soil Ks is significantly increased by vegetation roots in the near surface. •Mean Ks are 2.10×10-4 and 2.56×10-4m/s for the plots with bushwood and grass respectively. •They are much larger than 0.98×10-4m/s for the bare soil.

  9. Role of plant on hydrological processes in the changing environment • Plant adaptation mechanism for the climate change G-flux of green water to the atmosphere (plant transpiration), D-atmospheric demand, S-water supply from the soil-plant system Evaporation observations of forests in the Netherlands during periods of regular drought show that trees are able to evaporate almost at full potential, while surrounding crops and grasses show an evaporation reduction During a drought period, a tree will maximize its energy (i.e. carbon) gain by the optimal choice of the following trade-off strategies: (1) keeping photosynthesis going by investing in replacement of fine root mass and extracting water from deeper down the profile or (2) decreasing maintenance respiration by decreasing leave area. Daniëls, E.E, Geowetenschappen Scripties (2010)

  10. In the extreme water limited karst area, large trees survives because they take groundwater in the epikarst zone, which can be as much as 50% of the total water uptake by plant in the drought season 强度石漠化样地 中度石漠化样地 轻度石漠化样地 无石漠化样地 Role of plant on hydrological processes in the changing environment Water utilization efficient: WUE中度>WUE轻度>WUE无

  11. Role of plant on hydrological processes in the changing environment • Evapotranspiration: integrated effect of climate-plant, and Scale effect Murray C. Peel et al., Global investigation of vegetation impact on mean annual catchment Evapotranspiration, Geophysical Research, 12, 2010 • assessed through over 200 of paired catchment studies from around the world • Integrated effect of climate-plant • Tropical and temperate forested catchments had significantly higher median evapotranspiration (170mm and 130mm, respectively) than non-forested catchments. • Cold forested catchments unexpectedly had significantly lower median evapotranspiration (90mm) than non-forested catchments. • Scale effect: the impact of differing vegetation types on evapotranspiration diminishes as catchment area increases • The significant difference in median evapotranspiration between temperate forested and nonforested catchments persisted for catchments with area <1,000km2, but not for catchments with area 1,000km2,

  12. Future climate change GCMs (P, T…) Downscaling • Hydrological Model (parameters) Challenges in predicting response of hydrological processes to climate change • Hydrological Model • When coupled with GCM, can simulate soil moisture conditions, ET, and runoff changes • Cannot assess the relationship between biosphere-atmosphere Obs Data (P, ETpan, Q) • Hydrological Model calibration and validation Changes of Hydrological processes Hydrological processes

  13. Challenges in predicting response of hydrological processes to climate change • Model improvement- boundary of hydrologic model In traditional model In physical model • shape & physiology of plants in determining radiation and latent heat fluxes • better biosphere-atmosphere relational understanding The tree level water balance takes the tree itself and the soil around it as the control volume, with the top of the tree canopy as one boundary and the deepest soil moisture probe as the other. The water balance at the stand level is centered around a control volume with the eddy covariance system at the top boundary and the deepest soil moisture probe at the bottom boundary. Poor representation of vegetation in hydrological models Baldocchi et al., 2004

  14. Challenges in predicting response of hydrological processes to climate change • Hydro-ecological Model • When coupled with GCM, can simulate Water (soil moisture conditions, ET, and runoff changes) and Carbon and Nitrogen. • Plant adaptation mechanism as part of eco-hydrological models. • does not reflect influences of parameter changes along with changes of soil, vegetation, and topography changes

  15. Challenges in predicting response of hydrological processes to climate change • Coupling hydrologic model with ecosystem model & climate model • Mechanisms of surface interactions What are the mechanisms of interactions between the formation of terrain, soils, vegetation ecotones, and hydrologic response?

  16. Challenges in predicting response of hydrological processes to climate change • Coupling hydrologic model with climate and ecosystem models • SCALE AND EFFECT OF SPATIAL VARIATION • : Are there critical scales at which spatial variations in surface properties should be explicitly represented in models of land–atmosphere exchange? • Does lateral soil water redistribution significantly affect large-scale soil–vegetation–atmosphere exchange processes? • PARAMETERIZATION: • Can macroscale equations be formulated by upscaling (parameterization) process descriptions at the microscale and under what conditions can effective parameters be used to represent upscaled hydrologic processes?

  17. Conclusions • Plant ecosystem takes an important role in describing hydrological processes changing with climate • Long-term prediction of response of hydrologic processes to climate change needs to couple hydrologic model with ecosystem & climate model • Model parameters used for long-term prediction should be changed with Climate, vegetation and Soil Development • The hydrological cycle encompasses a variety of disciplines, there is a necessity for cross-disciplinary work in all Earth sciences

  18. Questions? Image from the University of Illinois WW2010 Project.

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