An Increase in Specific Discharge With Catchment Area Implies That Bedrock Infiltration Feeds Large Rather Than Small Mountain Headwater Streams

Abstract

Mountains are a source of water for downstream areas; thus, it is important to understand the storage and discharge characteristics of steep mountain catchments. Nested catchment studies have indicated that the relation between catchment area and specific discharge during baseflow can represent mesoscale storage and discharge characteristics, but this is poorly understood. We found that baseflow-specific discharge increased with catchment size in the headwater of the Arakawa River and identified the processes responsible for this spatial pattern. Synoptic discharge measurements obtained in catchment areas of 0.05 to 93.58 km 2 showed that specific discharge increased more than threefold with increasing drainage area. Analyses of the spatial variation in precipitation, hydrographs from three continuous gauging stations, and isotopic tracers implied that in this catchment, considerable amounts of water infiltrated in bedrock on hillslopes and did not discharge into small streams, but instead fed surface flow into a larger downstream catchment. A review of previous nested studies demonstrated three spatial patterns for specific discharge: Specific discharge may increase or decrease with catchment area, or it may be independent of area. An increase in specific discharge with area was observed only in catchments with permeable bedrock, which implies that such an increase is a useful indicator of the importance of the bedrock flow path to mountain watershed storage. The pattern of relationships between catchment area and specific discharge can be used to assess the storage and discharge properties of mesoscale catchments when the processes driving each pattern have been clarified.

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

“…Shaman et al (2004), investigating low-flow specific discharge in the Neversink River watershed, USA, which is underlain by sedimentary bedrock, suggested that deep groundwater contributions increase with increasing drainage area in subcatchments smaller than a representative elementary area (REA) of 8-21 km 2 but become constant in subcatchments larger than the REA. Asano et al (2020) found an increase in baseflow-specific discharge with drainage area in a 93.58-km 2 sedimentary watershed of the Arakawa River, Japan. In addition to baseflow-specific discharge, stream and spring baseflow chemistry have been commonly used to understand groundwater dynamics across spatial scales by separating stream water into shallow and deep groundwater.…”

“…An increase in baseflow-specific discharge with area can be caused by three factors: downstream discharge of water that infiltrates into bedrock at upstream hillslopes, increase in precipitation with area, and decrease in specific discharge during storm flow with area. Asano et al (2020) examined these factors in a 93.58-km 2 Mesozoic sedimentary watershed and showed that the downstream discharge of bedrock groundwater was the main factor causing the increase in baseflow-specific discharge with area. They reported that the spatial variation in the precipitation was small (<13%), and the specific discharge during storm flow was also smallest in the subcatchment with the smallest baseflow-specific discharge.…”

“…All sampling was considered to have occurred during baseflow periods because no increase in runoff through rainfall or snowmelt was observed in rivers downstream of the study sites (for the details of the downstream river discharges, see supporting information Text S1 and Figures S1 and S2). In accordance with Asano et al (2020), for the analysis of the scaling relationship, we used sampling points with drainage areas >0.05 km 2 (called stream water). We also sampled spring water, defined as water from a drainage area ≤0.05 km 2 , at various elevations for use as end members (EMs).…”

“…Asano et al. (2020) found an increase in baseflow‐specific discharge with drainage area in a 93.58‐km 2 sedimentary watershed of the Arakawa River, Japan. In addition to baseflow‐specific discharge, stream and spring baseflow chemistry have been commonly used to understand groundwater dynamics across spatial scales by separating stream water into shallow and deep groundwater.…”

Thermal video recording while walking: A simple method for mapping groundwater discharge points along forested headwater streams

“…The variability of groundwater discharge along a river network is a critical scientific question that needs to be addressed to fill the scale gap between the headwater stream, downstream river, and deep groundwater flow (Wagener et al, 2021). Non-negligible amounts of water and solutes are lost through subsurface flow paths from the headwater catchment, and the deeply infiltrating groundwater usually discharges to downstream channels in basins with permeable geology (Asano et al, 2020;Egusa et al, 2016;Fan, 2019;Iwasaki et al, 2015). Thus, mapping the spatial variability of the groundwater discharge is key to understanding the changes in the stream runoff and chemistry from the headwater to downstream as well as the connectivity between local and regional groundwater systems.…”