The roles of channels and hillslopes in rainfall/run‐off lag times during intense storms in a steep catchment

Abstract

The response time (lag time) between rainfall input and run‐off output in headwater catchments is a key parameter for flood prediction. Lag times are expected to be controlled by run‐off processes, both on hillslopes and in channels. To demonstrate these effects on peak lag times within a 4.5‐km2 catchment, we measured stream water levels at up to 16 channel locations at 1‐min intervals and compared the lag times with topographic indices describing the length and gradient of the hillslope and channel flow path. We captured storm events with a total precipitation of 38–198 mm and maximum hourly precipitation intensity of 9–90 mm/hr. There were positive relationships between lag time and flow path length as well as the ratio of the flow path length and the square root of the gradient of channels for the most intense storms, demonstrating that channel flow paths generally defined the variation in lag times. Topographic analysis showed that hillslope flow path lengths were similar among locations, whereas channel flow path length increased almost one order of magnitude with a 100‐fold increase in catchment area. Thus, the relative importance of hillslope flow path decreased with increasing catchment area. Our results indicate that the variation in lag times is small when hillslopes are sufficiently wet; thus, catchment‐scale variation in lag times can be explained almost entirely by channel processes. Detailed topographic channel information can improve prediction of flood peak timing, whereas hillslopes can be treated as homogeneous during large flood events.

A Novel Method of Quantifying Catchment‐Wide Average Peak Propagation Speed in Hillslopes: Fast Hillslope Responses are Detected During Annual Floods in a Steep Humid Catchment

“…To analyze the roles that channels and hillslopes play in runoff lag times during intense storms, we measured water levels at up to 16 channel locations ( Figure 1; in catchment areas of 0.02-4.5 km 2 ) at 1-min intervals and compared the lag times with topographic indices that discretely describe the lengths of the hillslopes and channel flow paths. We found significant linear positive relationships between lag time and median channel flow path length for most of the intense storms analyzed (Asano & Uchida, 2018). Topographic analysis demonstrated that hillslope flow path lengths were similar among locations, whereas channel flow path lengths increased by almost one order of magnitude with a 100-fold increase in catchment area.…”

“…We previously showed that the lag time in hillslopes is short in our study catchment compared to the lag time in the channel when the catchment is wet, despite many previous theoretical studies noting the relatively large contribution of hillslope processes in smaller catchments (e.g., D'Odorico & Rigon, ). Moreover, in this catchment, the distributions of flow path lengths in hillslopes were similar among different measurement locations, whereas flow path lengths in channels increased by almost one order of magnitude with a 100‐fold increase in catchment area (Asano & Uchida, ). These observations suggest that the spatial pattern of T p,j is controlled mainly by the most frequent flow path length in the channel ( L cm,j ), as follows: where is the catchment‐wide average travel time in hillslopes and L cm,j is the modal flow path length from the channel to gauging station j .…”

“…Water Resources Research 100-fold increase in catchment area (Asano & Uchida, 2018). These observations suggest that the spatial pattern of T p,j is controlled mainly by the most frequent flow path length in the channel (L cm,j ), as follows:…”

“…Storm IDs correspond with Asano and Uchida (). Second number connected with “‐” shows that there are two distinct rainfall peaks for each storm.…”

“…In order to improve this new method, we must evaluate the following limitations and uncertainties associated with the method: The effect of spatial variation in precipitation on response time estimation. We used 1‐km mesh radar rainfall data in this study to evaluate the effect of spatial variation in rainfall, although it appears to have little effect on differences in peak lag times among locations in our study area (Asano & Uchida, ). Treatment of rainfall events that have multiple rainfall and runoff peaks. In previous research, we defined lag time simply as the difference between the recorded times of maximum rainfall and maximum water level for each storm, regardless of the rainfall pattern.…”