A hydrological tank model assessing historical runoff variation in the Hieu River Basin

The Hieu River is the largest tributary on the left bank of the Ca River, which is one of the large basins in north-central Vietnam. Here, we use cumulative anomaly tests and Pettitt tests to ascertain the turning points in annual rainfall and discharge during the time period 1962-2014. The results of our statistical analysis reveal a breaking point in 1982 for the rainfall time series and in the late 1970s and late 1990s for the discharge time series. A storage-type hydrological model is used to determine runoff processes for different periods corresponding to detecting points of rainfall and discharge. The results of our model simulation confirm that a two-tank model with monthly input data is the most appropriate tank model for the Hieu River. The difference between the hydrographs improved when we used a rain factor function. A comparison between the observed and calculated runoff revealed a drastic decrease between 1999 and 2014. The rate of discharge loss in the Lower Basin was approximately six times higher than that in the Upper Basin, a finding potentially due to reservoir construction and intensive water use for agricultural and residential purposes.