Groundwater level in Wuwei Oasis of Northwest China

The Wuwei Oasis, situated in the middle reaches of the Shiyang River Basin in the arid inland of northwest China, is intensively cultivated using both irrigation water originating from Qilian Mountains and ground water. Overuse of irrigation water in the Wuwei oasis causes desertification in the lower reaches of the Shiyang River Basin. Temporal variation in ground water levels of the Wuwei Oasis allows us to determine irrigation water overuse. To accurately estimate the groundwater table surface, eight interpolation methods of observed well levels for 1983, 1988 and 1992 were compared by cross-validation and orthogonal-validation. Root mean squared error, the mean error and the correlation coefficient were calculated for each of the interpolation methods and years. The three kriging methods were superior over the other interpolations methods consisting of inverse distance weighting, global and local polynomial interpolations, regularized and tension splines. Ordinary kriging using the Yamamoto's method for estimating the error variance for determining ground water was again better than simple and universal kriging methods because there were a smaller number of points with large differences in estimated and predicted values. Based on the groundwater surfaces determined by the ordinary kriging as modified by Yamamoto, we found that the ground water decline from 1983 to 1992 was a modest 2.1 m on the average.