(a) The monthly precipitation anomaly time series in the upper stream of the Heihe River (upper panel) and its local wavelet power spectrum (lower panel) using the Morlet wavelet, normalized by 1/σ2 (σ2 = 126.45 mm2). The dashed curve depicts the cone of influence beyond which the edge effects become important. The shaded contours are at normalized variances of 0.5, 1, 2, 4, and 8. The contour closes regions of greater than 95% confidence for a red-noise process with a lag-1 coefficient α of 0.10. (b) Same as (a), but for the middle reach, with σ2 = 110.09 mm2 and α = 0.09. (c) Same as (a), but for the lower reach, with σ2 = 95.96 mm2 and α = 0.09.