Based on the Mezentsev-Choudhury-Yang equation (with n representing catchments characteristics): and water balance equation R = P ─ E, Yang et al. [2011] derived climate elasticity of runoff to precipitation and potential evaporation as 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 * 1. Introduction 3. The parameter n 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. 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: /2010WR 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: Climate elasticity and climate contribution 2. Data and Method Climatic variables were collected from 735 meteorological stations across China during 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. where R, P, E 0 are runoff, precipitation, and potential evaporation; f represents the function R=f(P, E 0, n); and ε 1 and ε 2 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. Fig catchments across China and two catchments for validating the climate elasticity method Fig.2 The parameter n of the 207 catchments Fig.4 Precipitation elasticity to runoff of the 207 catchments Fig.3 Relationship of the parameter n with (A) catchment slope (S) and (B) vegetation coverage (M) in the 207 catchments 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 ε 1 and ε 2. The change trends in P and E 0 were detected as the linear slope of annual series from 1961 to Potential evaporation was estimated according to Penman equation. In addition, two small catchments were chosen for method validation. 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 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.5 Contribution of (A ) precipitation and (B) potential evaporation to runoff during Fig.5 Climate contribution to runoff during 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.