Spatial-temporal variation patterns of groundwater tables in the middle section of the Hexi Corridor

It is of great significance to research spatial-temporal variations of groundwater tables from the perspective of the whole groundwater variable field in a basin. In this study, the original observation matrix was constructed using 38 monitoring wells data from 1984 to 2019 in the Heihe River Basin in the middle section of the Hexi Corridor and processed to be centered and standardized, respectively. The empirical orthogonal function (EOF) method was used to decompose the transformed matrices to obtain spatial and temporal variation patterns of groundwater tables which were represented by the spatial weights of typical uncorrelated modes with the corresponding temporal functions (TF). The resulting main variation pattern showed general drops in regional groundwater tables for both centered and standardized cases, especially in the southeast regions when using centered EOF modes. The second to fifth modes presented reverse distribution characteristics from the northwest to southeast directions. The downward trend in the first TF series indicated that the groundwater depth of the whole basin varied to be deeper. The local rise occurring at some sites demonstrated the positive effects of continuous ecological water delivery since 2000, but was still not recovered to the previous level due to long-term serious groundwater overdrawing.