A Statistical Method for Recovering the Depth to Shallow Groundwater Table in China

1. A Statistical Method for Recovering the Depth to Shallow Groundwater Table in China 袁 星 谢正辉，梁妙玲 中国科学院大气物理研究所 xyuan@mail.iap.ac.cn 2006.08.10

2. Background • Data • Methodology • Validation & Application • Summary

3. Groundwater 在全球总水量中，海洋占97%以上，偏远而难以利用的两极冰帽及冰川约占2%，其余不到1%才是人类可取用的水资源，而其中地下水的储存总量居冠。 地下水的过量开采会造成地下水位的大幅下降，引起地面沉降。地下水位过高会对农作物生长不利，造成渍害。因此，研究地下水位的动态对国计民生具有重大意义。

4. 气候条件、植被地形和人类活动的变化能引起地下水埋深时空分布的变化；反之，大尺度地下水埋深的变化，导致土壤含水量、地表径流和基流的改变，进一步影响下垫面的蒸散发和低层大气感热和潜热的分配，从而对气候产生影响。气候条件、植被地形和人类活动的变化能引起地下水埋深时空分布的变化；反之，大尺度地下水埋深的变化，导致土壤含水量、地表径流和基流的改变，进一步影响下垫面的蒸散发和低层大气感热和潜热的分配，从而对气候产生影响。 估计浅层地下水埋深变化对水资源研究、陆面过程模拟、陆地生态系统及陆气相互作用的研究具有重要意义。

5. Purpose To recover monthly data for the depth to shallow groundwater table since 1961 in continental China. Scheme Transfer function-noise (TFN) models & parameter transfer method.

6. Data Meteorological Data (1961-2000) Daily time-series of precipitation, maximum temperature, and minimum temperature are obtained by interpolating station values from 740 meteorological stations in China. Soil Data The soil texture information is derived from Food and Agriculture Organization dataset (FAO). Groundwater Phreatic data from monitor wells.

7. Locations of the meteorological stations

8. Locations of wells interpolated into 60×60 km2 grids

9. MethodologyⅠ: Calibration

10. TFN model

11. Input: precipitation surplus (precipitation minus potential evapotranspiration). The instantaneous evaporative demand (mm/s) is calculated following Jarvis and McNaughton (1986):

12. TFN model State-space representation

13. State equation Measurement equation A linear discrete stochastic system

14. If there is an observation at time t If no observation taken at time t Recursive application of the Kalman filter

15. Runningthe Kalman filter for the calibration period with a parameter set resulting in the following objective function: Using SCE-UA (shuffled complex evolution method developed at The University of Arizona) method to minimize the objective function.

16. Identification of TFN model Transform data to improve normality and stationarity, and determine parameters which will be calibrated. Output calibrated parameters Yes Representation of TFN model in vector notations(state space form) convergence criteria satisfied? No Generate sample Sample s points randomly in the feasible parameter space. Shuffle complexes Combine the points into a single sample population. Running Kalman filter for Optimal prediction (calculate the criterion values) Rank points Sort the s points in order of increasing criterion value. Partition and evolve Partition the s points into p complexes,evolve each complex Flow chart: the calibration method of TFN model

17. Methodology Ⅱ: Parameter Transfer

18. 1 Tropical climate 2 Dry, cold climate 3 Rainy, midlatitude climate 4 Continental climate with hot summer 5 Continental climate with cool summer 6 Continental climate with short cool summer

19. 聚类 (clustering) • 基于平方误差的聚类 • K均值(K-Mean) • 基于概率密度估计的聚类 • 高斯混合模型：GMM • 核密度估计：mean-shift • 层次聚类 • 基于图的聚类 • 模糊聚类 • 基于神经元网络的聚类

20. 高斯混合模型(GMM)(Mixture of Gaussians Model) • 基本思想：将聚类视为一个概率密度估计问题 • 给定一堆多峰分布的数据，估计其概率密度

22. Expectation-Maximum likelihood (EM) Algorithm

23. EM Algorithm

26. Validation of TFN models Mean Absolute Error: 0.18m 0.15m 0.19m 0.15m

27. Estimation and 95% confidence intervals of the depth to groundwater table for the four grids

28. Moving average parameter of transfer model Autoregressive parameter of transfer model

29. Autoregressive parameter of noise model Variance of noise series

30. Cross Validation

31. Time series of errors for cross validation. (a) ME(t); (b) RMSE(t); (c) MAE(t)

34. r=0.003 r=0.357 r=0.293

35. Reconstruction Scheme Transfer function-noise(TFN) models are calibrated by SCE-UA method coupled with Kalman filter in each observed grids. Parameters for gauged grids are transferred to ungauged grids by GMM clustering method based on soil property data and 40-years meteorologic data such as precipitation and temperature. The depth to groundwater table for continental China are estimated by the TFN models with parameters calibrated or transferred.

36. Conclusions (1) Validated by phreatic data, TFN models not only provide results with high accuracy, but also can quantify the prediction uncertainty reasonably well. (2) Cross validation shows that the parameter transfer scheme is an effective way for the recovery. (3) The seasonal variations of recharge and discharge for groundwater in China are obtained by our scheme. (4) The second EOF match the pattern of mean depth of the groundwater reasonably well.

37. Future work Further validation and modification of our data by satellite data such as GRACE. Assessing the improvement of Land surface model or climate model, running with initial conditions provided by the recovered data.

38. 谢谢指导！ Thank You!