Seasonal variability of surface run-off, recharge and soil moisture dynamics in lowland catchments
Authors: Miri Mahdi , Galys Helena , Hübner Christof , Sauter Martin , Becker Felix , Engelhardt Irina 
Published: March 2024

For an effective water resources management in regions faced with scarcity understanding infiltration dynamics in the unsaturated zone at a high temporal-spatial resolution is essential for quantifying stored water volumes in the vadose zone.Our pilot-site (area 790 km&#178;) is the Lower Spree catchment, located E of Berlin, Germany.This area is characterized by a continental climate and a low water supply compared to other German states. Increasing industrial water-use (e.g.Tesla factory) and increased irrigation demand causes conflicts over water availability for the different water user.Three aquifers(GWL 1 to 3)in the Lower Spree consist of sands and gravel-sands. Since 1980, the groundwater level in all aquifers has collectively dropped by 5m. The planned phase-out of coal mining up-stream of Berlin will reduce the discharge of the river Spree by 50-75%. With an average precipitation of 549 mm/y,average recharge decreases to 114 mm/y. Landuse varies between forests(46%) and grasslands(20%).The soil types range from Histosols and Fluvisol and are followed by an unsaturated zone's thickness varying from 5 to over 50m. &#160;We installed in pilot site 8 pressure-sensors in lakes, 12 pressure-sensors in streams, 8 pressure-sensors in groundwater observation wells and 21 Time Domain Reflectometry (TDR)sensors at various depths (25, 50 and 75 cm)in the unsaturated zone within different soil types and landuse.Hydrological regimes,in ground and surface water, are affected by a high seasonal variability. Approximately 35% of the river discharge results from baseflow, which feed lakes and ecosystems. Comparison between coniferous forest-dominated and grassland-dominated areas shows that coniferous- forest plays a crucial role in attenuating streamflow variability.Lag-times between precipitation and discharge response are similar for both landuse(2-4 days).However, coniferous-forests result in decreasing river discharge during and immediately after precipitation.Due to the shallow thickness of unsaturated-zone(15m)and the lag-time increases to 96-153 days.The groundwater flow system provides a highly relevant water resource for rivers and lakes and due reduced baseflow(35% of the discharge)and the short lag-time of a few days summer periods and droughts with limited precipitation results in a drying of streams and enormous lake level drop.5 streams dried from (May-August),also 7 streams and 5 lakes exhibited declining water levels from(winter-summer).Soil Moisture Active Passive(SMAP)satellite estimates near-real time surface soil-moisture(5 cm-depth) and root zone soil-moisture(1 m-depth)with 9 km resolution.We compared SMAP with our measured soil moisture obtaining correlation-coefficients of(0.31-0.63).Higher soil-moisture values are observed in grassland and peat-soil.The soil-moisture curves indicate that the soils below coniferous-forests have a larger capacity to store and release water than those below in grassland.Based on these measurements we will be able to design a sophisticated water management concept:using the surplus of discharge during autumn,winter and store it our lakes.For the later infiltration into the unsaturated zone and groundwater we can identify regions with i)optimal storage capability of the vadose zone,ii)best protection of the artificially enriched groundwater from evapotranspiration loss.iii) maximum storage volumes,and iv) minimum discharge loss into lakes and streams.