WP10 - Isotopes and hydrological fluxes

A suite of innovative hydrological and hydrogeophysical methods are used already in HOBE to monitor dynamic changes in soil moisture and temperature under different land coverage and from these to estimate groundwater recharge. Monitoring of stable isotope composition in precipitation and in the unsaturated zone at these three sites will provide valuable information and be a specific constraint on estimating recharge. The HOBE rain gauge stations have been equipped with rainfall collectors in order to monitor spatio-temporal changes (daily, weekly, monthly) in stable isotope composition of precipitation in the Skjern River Catchment. Techniques to obtain profiles of isotopes in groundwater are available and can be carried out, in principle, basin-wide.

Objectives:
1. to establish a data base on stable isotopes (δ¹⁸O and ²H) in different water compartments in the Skjern River Basin (SRB).

2. to quantify water fluxes across hydrological domains using stable isotopes, i.e. recharge/discharge to/from groundwater systems in different systems (recharge or discharge areas) with different land use (agriculture, forest, meadow).

3. to understand rainfall-runoff mechanisms at different catchment scales using stable isotope hydrographs.

4. to assimilate these results in a distributed hydrological model for the Skjern River Basin in order to improve our understanding of the origin of stream water and transit times in large catchments.

5. to initiate a long term monitoring of isotopes in precipitation in Denmark, as a contribution to the Global Network of Isotopes in Precipitation (GNIP) managed by the IAEA.

Work content:
A 2 year warm-up phase with instrumentation and data collection has been initiated. This will be followed by a 3 year PhD study.

Year 1-2 (2012-2013) (Data Collection Phase):
1. Equip rain gauge sites with precipitation collectors in coherence with the GNIP procedures
2. Collect rainfall samples and ensure high-quality analysis via cooperation with the IAEA
3. Continue monitoring stable isotope stream flow events (winter, summer)
4. Collect soil moisture samples for stable isotope analysis at the three land use sites
5. Collect groundwater samples near discharge and recharge zone of stream and lake in SRB

Year 3-5 (2014-2015) (Interpretation Phase):
1. Continue monitoring program for precipitation, soil moisture, groundwater, stream/lake.
2. Stream tracer hydrograph analysis
3. Perform rainfall-runoff modelling using numerical distributed hydrological model
4. Correlation of recharge-discharge patterns with land use and landscape characteristics and
geology/geomorphology.

2016 onwards: Long term monitoring:
1. Ensure continued sample collection for isotopes in precipitation under the Danish Network of Isotopes in Precipitation (DNIP).
2. Report data/submit samples to the Global Network of Isotopes in Precipitation (GNIP), providing public free data access.

Historical Danish δ¹⁸O and δD contributions to GNIP is shown in Figs. 1 to 4. The data primarily date back to 1965-1971 and ceased in 1984. In isotope hydrology, monitoring of the precipitation δ¹⁸O and δD values is fundamental, as this forms the input function to the hydrological system. Global warming effects on the isotopic composition of precipitation today increases the need for initiating a long term monitoring.

Fig. 1. Precipitation δ¹⁸O values from the Odense and Taastrup stations (GNIP).


Fig. 2. Seasonal δ¹⁸O variation. “1” indicates cumulated precipitation of January, etc. (Odense and Taastrup data (1962-1984), GNIP).


Fig. 3. Seasonal d-excess variation. “1” indicates cumulated precipitation of January, etc. (Odense and Taastrup data (1962-1984), GNIP).


Fig. 4. δD vs. δ¹⁸O. LMWL is for combined Odense and Taastrup data (GNIP) of 1962-1984.