Performance of spatio-temporal scale in the streamflow trend - Evidence in jialing river basin, China

The scale effect plays an important role in the hydrological responses to environmental change, which has been testified in many studies. However, the effect of temporal and spatial scale on hydrological change is not sufficiently studied. In this paper, a case study of such effect on the streamflow trend in Jialing River Basin is carried out. It is assumed that if the effect of spatial scale on the results of hydrological change does exist, it will escalate from the upper reach of the river to the lower reach. In that case, the confluence of large tributaries could be treated as spatial scale jump, and it may weaken, or reinforce the hydrological change in the mainstream evidently. So it is convenient to carry out such a study in Jialing River Basin, as two of its largest tributaries, the Fu River and the Qu River, flow into the mainstream of the Jialing River through two nearby infalls. The trend of the runoff and maximum flood peak at annual and monthly scale is tested by the Mann-Kendall method. The effect of temporal scale on the hydrological trend is identified by comparisons between the trend test results of the runoff and maximum flood peak at annual and monthly scale, while the effect of the spatial scale is identified through comparing the trend tests at the mainstream after the confluence and before. The annual runoff of Fu River shows a downward trend in the past 50 years while the runoff in January, February and March shows an upward trend and there is a larger downtrend in July. The annual maximum flood peak shows a downward trend before the confluence, but the trend disappears after the confluence. The results show that the annual scale eliminate the complex hydrological change occurred at the monthly scale, and the basin scale may also hide the complex hydrological change occurred in its tributaries.