Linking Bedrock Groundwater, Landscape Structure and Mean Transit Time
Abstract
Contributions from hydrologically active bedrock have been shown to influence rainfall-runoff relationships, biogeochemical fluxes and mean transit times in headwater catchments. Yet, we still lack a complete understanding of the role of bedrock groundwater in the catchment storage-release relationship—arguably a key descriptor of catchment function. Here we explore the role of bedrock groundwater in runoff generation in a small temperate forested catchment. We focus on three questions: What is the link between landscape structure and bedrock groundwater dynamics? What is the role of the larger bedrock groundwater aquifer in stream flow and hydrochemistry? and, How does bedrock groundwater control mean transit time and the storage-discharge relationship? Streamflow and precipitation were measured for a 1 year period, along with soil and bedrock groundwater dynamics from an array of 60 wells strategically located in hillslope, hollow and riparian zones across the 4.5 ha Maimai M8 experimental watershed in New Zealand. We used water table and stable isotope data, combined with extensive aquifer testing to characterize groundwater contributions to catchment runoff. Tritium-based groundwater ages were determined for 23 wells and regressed against groundwater and streamwater silica concentrations. Vertical head gradients showed groundwater flux remained downward throughout all wetness conditions in all hillslope and most hollow and riparian locations, indicating minimal surface water-groundwater interaction. Thin soils and low-permeability unfractured hillslope bedrock acted to shunt vertically infiltrating soil-water laterally downslope at the soil-bedrock interface. This resulted in minimal bedrock groundwater recharge, high runoff ratios and a runoff regime characterized by both extremely high and extremely low flows due to minimal soil-water storage capacity. Catchment mean transit times were generally young (0.3 - 2.5 yrs), despite much older (12-23 yrs) bedrock groundwater found in deeper sub-alluvial riparian zones. These results shed light on the combined control of bedrock permeability and landscape structure in setting the storage-discharge relationship and catchment mean transit times.
Contributions from hydrologically active bedrock have been shown to influence rainfall-runoff relationships, biogeochemical fluxes and mean transit times in headwater catchments. Yet, we still lack a complete understanding of the role of bedrock groundwater in the catchment storage-release relationship—arguably a key descriptor of catchment function. Here we explore the role of bedrock groundwater in runoff generation in a small temperate forested catchment. We focus on three questions: What is the link between landscape structure and bedrock groundwater dynamics? What is the role of the larger bedrock groundwater aquifer in stream flow and hydrochemistry? and, How does bedrock groundwater control mean transit time and the storage-discharge relationship? Streamflow and precipitation were measured for a 1 year period, along with soil and bedrock groundwater dynamics from an array of 60 wells strategically located in hillslope, hollow and riparian zones across the 4.5 ha Maimai M8 experimental watershed in New Zealand. We used water table and stable isotope data, combined with extensive aquifer testing to characterize groundwater contributions to catchment runoff. Tritium-based groundwater ages were determined for 23 wells and regressed against groundwater and streamwater silica concentrations. Vertical head gradients showed groundwater flux remained downward throughout all wetness conditions in all hillslope and most hollow and riparian locations, indicating minimal surface water-groundwater interaction. Thin soils and low-permeability unfractured hillslope bedrock acted to shunt vertically infiltrating soil-water laterally downslope at the soil-bedrock interface. This resulted in minimal bedrock groundwater recharge, high runoff ratios and a runoff regime characterized by both extremely high and extremely low flows due to minimal soil-water storage capacity. Catchment mean transit times were generally young (0.3 - 2.5 yrs), despite much older (12-23 yrs) bedrock groundwater found in deeper sub-alluvial riparian zones. These results shed light on the combined control of bedrock permeability and landscape structure in setting the storage-discharge relationship and catchment mean transit times.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.H33J1690G
- Keywords:
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- 1804 Catchment;
- HYDROLOGYDE: 1843 Land/atmosphere interactions;
- HYDROLOGYDE: 1848 Monitoring networks;
- HYDROLOGYDE: 1855 Remote sensing;
- HYDROLOGY