Regional variation in climate elasticity and climate contribution to runoff across China: according to the Budyko hypothesis Hanbo YANG* and Dawen Yang State key Laboratory of Hydro-Science and Engineering & Department of Hydraulic Engineering, Tsinghua University, Beijing, China *E-mail: yanghanbo@tsinghua.edu.cn 1. Introduction Climate change has an increasing impact on water resources. One basic question is how much runoff change occurs due to a 10% change precipitation. The climate elasticity was proposed to answer this question. Based on the Mezentsev-Choudhury-Yang equation (with n representing catchments characteristics): E0 P E n n 1/ n P E0 207 catchments were chose for this study, except 3 inland river catchments of the 210 catchments. Based on mean annual precipitation, potential evaporation and runoff, we calibrated n and then calculated ε1 and ε2. The change trends in P and E0 were detected as the linear slope of annual series from 1961 to 2010. Potential evaporation was estimated according to Penman equation. In addition, two small catchments were chosen for method validation. and water balance equation R = P ─ E, Yang et al. [2011] derived climate elasticity of runoff to precipitation and potential evaporation as dE0 dR dP 1 2 R P E0 1 f 1 P P PE 4. Climate elasticity and climate contribution The precipitation elasticity has a large regional variation. The largest values appear in the Hai River basin, the Liao River basin, the Huai River basin, and the Yellow River basin. Fig.4 Precipitation elasticity to runoff of the 207 catchments 3. The parameter n f E0 E0 2 PE where R, P, E0 are runoff, precipitation, and potential evaporation; f represents the function R=f(P, E0, n); and ε1 and ε2 are the climate elasticity of runoff to precipitation and potential evaporation. Both the climate and catchment characteristics have large spatial variations in China, which will lead to a spatial variation in climate elasticity. Therefore, we divide China into 210 catchments, calibrate n for each catchment, and estimate the climate elasticity to further understand its spatial variation and reveal the impacts of climate change on hydrology. The parameter has a large regional variation. It has a significant correlation with catchment slope and a relative weaker correlation with vegetation. The vegetation coverage was estimated as Fig.5 Contribution of (A ) precipitation and (B) potential evaporation to runoff during 1961-2010 The largest positive contributions of climate change to runoff occur in the Northwest, ranging from 1.1–3.1%/a, while largest negative contributions occur in the middle reach of the Yellow River basin, ranging from −1.3%/a to −1.0%/a. M NDVI NDVImin NDVImax NDVImin Fig.2 The parameter n of the 207 catchments 2. Data and Method Fig.1 210 catchments across China and two catchments for validating the climate elasticity method Climatic variables were collected from 735 meteorological stations across China during 19612010. Potential evaporation was estimated according to Penman equation. The catchment slope (S) was estimated according to DEM, and the vegetation coverage (M) was estimated from NDVI. Fig.5 Climate contribution to runoff during 1961-20100 Fig.3 Relationship of the parameter n with (A) catchment slope (S) and (B) vegetation coverage (M) in the 207 catchments Reference Yang, H. B., and D. W. Yang (2011), Derivation of climate elasticity of runoff to assess the effects of climate change on annual runoff, Water Resour. Res., 47, W07526, doi:10.1029/2010WR009287. Yang, H. B., J. Qi, X. Y. Xu, and D. W. Yang (2014), The regional variation in climate elasticity and climate contribution to runoff across China. J. Hydrol. 517: 607-615.