Factors affecting dominant peak‐flow runoff‐generation mechanisms among five neighbouring granitic headwater catchments

Abstract

We examined how and why dominant peak-flow runoff-generation mechanisms differ among neighbouring headwater catchments. We monitored runoff and groundwater levels and performed terrain analyses in a granitic second-order catchment and its four neighbouring subcatchments in the Kiryu Experimental Watershed in Japan. Our analysis of lag times from peak rainfall to peak runoff suggests differences in the dominant peakflow runoff-generation mechanisms among the five catchments. For two of the three zero-order catchments, with few perennial groundwater bodies, subsurface flow from hillslopes was the dominant mechanism at some events. However, the dominant mechanisms were channel precipitation and riparian runoff at almost all events in first-and second-order catchments and in the third zero-order catchment, which has a large perennial groundwater body over a bedrock depression in the riparian zone. In this zeroorder catchment, the quick-flow ratio was the smallest of the five catchments because subsurface flow from the hillslope was buffered at the riparian zone. These facts suggest that the channel length, riparian buffering, and hillslope connectivity were the factors governing the different dominant peak-flow runoff-generation mechanisms among the catchments. Riparian buffering was affected, not only by surface topography, but also by bedrock topography and bedrock groundwater (BGW) dynamics. Our findings indicate that both of BGW dynamics and topography are important for catchment classification, and the relative importance of topography increases with the change from baseflow to stormflow. Furthermore, mismatching between a geographic source and a flow path resulted in different catchment classifications depending on the approach. Therefore, multiple approaches during both baseflow and stormflow periods are necessary for catchment classification to apply information obtained from one headwater catchment to other headwater catchments within the same region. K E Y W O R D S bedrock depression, bedrock groundwater, catchment classification, hillslope connectivity, lag time, riparian buffering

Geological Effects on the Scaling Relationships of Groundwater Contributions in Forested Watersheds

“…Since the total area of the individual study sites in the current study (Table 1) was smaller than this, the spatial variation in precipitation would be too small to cause the increase in baseflow-specific discharge with area by an order of one for the volcanic sites (Figure 3). As for the participation of storm flow, Iwasaki et al (2020) also reported that a subcatchment with the smallest baseflow-specific discharge had a relatively small direct runoff ratio by comparing five subcatchments in a granitic headwater catchment. Takahashi et al (1982) found that direct runoff ratio was not correlated to catchment area by examining 58 catchments with various bedrock geology between 1 and 1,000 km 2 .…”

Regional characteristics’ impact on the performances of the gated recurrent unit on streamflow forecasting

“…Similar to the pattern found in training losses, this phenomenon became more significant when the lead time of the forecasting increased, although it was not evident in the Tþ1 forecasting in the WRB. This can be partially explained by the time lag effect of the hydrologic responses (Ross et al 2019;Iwasaki et al 2020). The time lag of the rainfall-runoff events makes the rainfall data meaningful to the target streamflow data.…”

Flood patterns in a catchment with mixed bedrock geology: causes for flashy runoff contributions during storm events

“…Previous studies have shown that the organization and distribution of landscape units can control the differences in runoff responses between nested catchments (Sidle et al 2000;McGlynn, McDonnell, Seibert, and Kendall 2004;Iwasaki et al, 2020). Iwasaki et al (2020) studied 5 catchments with similar geology, climate and vegetation, but different geomorphological layout, and concluded on the key role of the riparian area in buffering fast hillslope flow mechanisms.…”

“…The contrasted hydrological response during summer in the KOE catchment -exhibiting no change in dominant hydrological processes -could be caused by:  the larger riparian zone and the gentle slopes in the downstream part of the catchment, buffering the inflow of quick runoff (Iwasaki et al, 2015, Iwasaki et al, 2020;…”

“…Several studies, focusing on subsurface flow celerity on hillslopes, assessed the quicker flows during dry conditions (Scaini et al, 2018;Anderson et al 2009;Asano et al, 2020), although they could not identify correlations between hillslope flow celerity and antecedent wetness conditions (Scaini, et al 2018;Iwasaki et al, 2020). The dry conditions carry a large variability of the hillslope responses in terms of volumes and timing, which decrease during wetter conditions (Scaini et al, 2018;Bergstrom et al, 2016;Teschemacher et al, 2019).…”