1.3. Results and discussion
1.3.2. River-aquifer exchange fluxes under monsoonal climate conditions
The focus of this study was on investigating how monsoonal precipitation events affect the dynamics of river-aquifer exchange and the corresponding flux rates as well as its impact on the local water quality.
The two-dimensional HGS model, for determining the river-aquifer exchange fluxes, was successfully calibrated to the measured temperatures by inversely estimating the hydraulic conductivities using the parameter estimation code PEST (Doherty, 2005). Even though, the simulated temperature at 10 cm and 30 cm depth were slightly underestimated (Fig. 3.4, Fig.
3.5) likely because solar radiation was not considered in the modeling approach (similar to Vogt et al., 2010), the statistical measures indicate that the model performs well in predicting both, the hydraulic heads and temperatures. Nash-Sutcliffe efficiencies and correlation coefficients ranged from 0.60 to 0.98 and 0.85 to 0.99, respectively. In the period directly after the scouring event (07/05/2010), observed temperatures at 10 and 30 cm depths show a stronger cooling than in the simulations. This was most likely caused by preferential upwelling of groundwater along the outside of the piezometer forced by a small gap between the pipe and the sediment which was created during the scour event.
As presented in Figure 1.6, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river-aquifer exchange fluxes, with frequently appearing riverbed flow reversals. Based on the spatio-temporal variability of the exchange fluxes at the river-aquifer interface, the monitoring period can be subdivided into the three main time periods. The first time period (i) refers to the pre-monsoon season (04/03/2010 - 07/05/2010). During this time period, river water was infiltrating into the aquifer with a flow mainly vertical down to about 0.50 to 0.80 m depth. Below this depth, river water was increasingly transported laterally with the regional groundwater flow direction. Already, small precipitation events resulted in a short but strong infiltration of river water into deep groundwater zones (depth > 2 m). The second time period (ii) starts after the first intensive monsoonal precipitation event at the 5th of July, which resulted in a significant riverbed elevation change correlated with a hydraulic gradient reversal (07/05/2010 - 08/01/2010). For the following 25 days, the studied river reach was always gaining. In these 25 days the first long-lasting precipitation event of the monsoon season 2010 (16-18 July 2010, total rainfall amount = 64 mm) took place. This event resulted in the highest positive volumetric fluxes within the entire monitoring period, due to a faster decrease of the river stage compared to the groundwater level recession after the event.
Spatially, the highest positive volumetric fluxes were measured close to the riverbed surface.
The third time period (iii), starts at the beginning of August, when monsoonal precipitation events occur at higher frequency (08/01/2010 - 09/20/2010), which caused the highest variability in river-aquifer exchange fluxes. During this time period, groundwater levels and river stage are steadily rising and start to slowly equilibrate. Consequently, the vertical hydraulic gradient approximated zero and almost every precipitation event resulted in a reversal of the vertical hydraulic gradient, correlated with the change of flow direction from groundwater gaining to short-time river water losing conditions as well as with high fluxes.
Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river
exchange fluxes
groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation events on river
completed.
Figure 1.
location of piezometer W8 (B) and the corresponding for the entire simulation period.
During the first time period (i), under losing conditions and tem
showed typically higher DOC concentrations in the river water relative to the groundwater.
this first
strong infiltration of river water into
concentrations were observed in groundwater enriched with
Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river
exchange fluxes and
groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation events on river-aquifer e
completed.
Figure 1.6: Simulated spatio
location of piezometer W8 (B) and the corresponding for the entire simulation period.
During the first time period (i), under losing conditions and temporal variability in DOC concentrations
showed typically higher DOC concentrations in the river water relative to the groundwater.
this first time period
strong infiltration of river water into
concentrations were observed in groundwater enriched with DOC,
Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river
and it has been
groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation
aquifer exchange fluxes and on the local water quality has
Simulated spatio-temporal pattern of vertical volume fluxes along a depths profile at the location of piezometer W8 (B) and the corresponding
for the entire simulation period.
Therefore, in this study we also
aquifer exchange fluxes on the local water quality.
and the local water quality the monitoring period can be
DOC and nitrate concentrations iii) as for the investigated exchange fluxes investigated exchange fluxes.
During the first time period (i), under losing conditions poral variability in DOC concentrations
showed typically higher DOC concentrations in the river water relative to the groundwater.
time period, already comparably small precipitation events result strong infiltration of river water into
concentrations were observed in groundwater
DOC, was pushed into the groundwater.
Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river
it has been argued that the dynamics of exchange between rivers and groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation
xchange fluxes and on the local water quality has
temporal pattern of vertical volume fluxes along a depths profile at the location of piezometer W8 (B) and the corresponding
also focused on examining potential implications of the river xes on the local water quality.
During the first time period (i), under losing conditions poral variability in DOC concentrations
showed typically higher DOC concentrations in the river water relative to the groundwater.
, already comparably small precipitation events result strong infiltration of river water into groundwater
concentrations were observed in groundwater
pushed into the groundwater.
20
Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river
argued that the dynamics of exchange between rivers and groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation
xchange fluxes and on the local water quality has
temporal pattern of vertical volume fluxes along a depths profile at the location of piezometer W8 (B) and the corresponding river stage and total heads measured in W8 (A)
focused on examining potential implications of the river xes on the local water quality.
and the local water quality are strongly linked to each other. Already the subdivided
DOC and nitrate concentrations into the same t iii) as for the investigated exchange fluxes, point out
During the first time period (i), under losing conditions
poral variability in DOC concentrations (groundwater and river water)
showed typically higher DOC concentrations in the river water relative to the groundwater.
, already comparably small precipitation events result groundwater. During
concentrations were observed in groundwater samples pushed into the groundwater.
Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river
argued that the dynamics of exchange between rivers and groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation
xchange fluxes and on the local water quality has
temporal pattern of vertical volume fluxes along a depths profile at the river stage and total heads measured in W8 (A)
focused on examining potential implications of the river xes on the local water quality. Our results indicate that the riv
are strongly linked to each other. Already the subdivided via the measured
into the same t
point out that local water quality is During the first time period (i), under losing conditions we
(groundwater and river water)
showed typically higher DOC concentrations in the river water relative to the groundwater.
, already comparably small precipitation events result During this time period
samples, which emphasizes that pushed into the groundwater. In the
Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river
argued that the dynamics of exchange between rivers and groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation
xchange fluxes and on the local water quality has
temporal pattern of vertical volume fluxes along a depths profile at the river stage and total heads measured in W8 (A)
focused on examining potential implications of the river results indicate that the riv
showed typically higher DOC concentrations in the river water relative to the groundwater.
, already comparably small precipitation events result time period
, which emphasizes that
second time period (ii) when Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river
argued that the dynamics of exchange between rivers and groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation
xchange fluxes and on the local water quality has never
temporal pattern of vertical volume fluxes along a depths profile at the river stage and total heads measured in W8 (A)
focused on examining potential implications of the river results indicate that the river showed typically higher DOC concentrations in the river water relative to the groundwater.
, already comparably small precipitation events resulted in a short but (i) increasing DOC , which emphasizes that river water time period (ii) when Although, our results demonstrate highly variable hydrologic conditions within the monsoon season, which are characterized by a high temporal and spatial variability in river-aquifer argued that the dynamics of exchange between rivers and groundwater may strongly influence the quality of water resources (i.e. Stonestrom and Constantz, 2003), to our knowledge research on the influence of monsoonal precipitation never been
temporal pattern of vertical volume fluxes along a depths profile at the river stage and total heads measured in W8 (A)
focused on examining potential implications of the river-er-aquifer showed typically higher DOC concentrations in the river water relative to the groundwater. In in a short but ncreasing DOC river water time period (ii) when
21
groundwater was predominantly discharging into the river, we observed a lower temporal and spatial variability in DOC and nitrate concentrations. Finally, within the third time period (iii) when storm events occurred more frequently and the simulated water fluxes were found to be strongest, also the spatial variability of DOC concentrations in the groundwater increased again as frequently DOC enriched river water was pushed into the aquifer.
In contrast, nitrate concentrations in the river water and groundwater (all piezometers) were found to decrease during this time period (iii). Basically two effects might have been responsible for the decreasing nitrate values. The river nitrate concentrations might have decreased during monsoonal precipitation events due to dilution effects as suggested by Rusjan and Miko (2008). But dilution effects fail to explain the even lower nitrate concentrations observed in the groundwater sample at piezometer (W5), relative to river nitrate concentrations.
Further measurements of groundwater, which were extracted out of additional piezometers throughout the Haean Catchment, indicate nitrate concentrations up to four times higher in the groundwater than in the river water. Generally, highest nitrate concentrations measured at the piezometer transect were observed in piezometer W9, which is upstream of the river channel with respect to the groundwater flow direction and hence less influenced by river water. In contrast, at piezometers W5 and W8, which are regularly influenced by the river water, either by strong river water infiltration or in terms of groundwater flow direction, showed lower nitrate concentrations relative to the observed river water concentrations.
During the storm events in August, river water high in DOC was frequently pushed into the anoxic, nitrate enriched groundwater, which created favorable conditions for denitrification. Hence, denitrification might have been the main biogeochemical process responsible for the nitrate attenuation observed in W5. Although, we cannot prove this hypothesis on the basis of the limited chemical data set, we postulate that under prevailing hydrologic conditions the potential for denitrification in the streambed below the river is likely high and therewith also the self-cleaning capacity (in terms of nitrate) of systems like the Haean Catchment.
In summary, this study demonstrates that during the monsoon season a high temporal and spatial variability in river-aquifer exchange fluxes is evident with frequent riverbed flow reversals and that the local water quality is controlled by these river-aquifer-exchange fluxes.
We further showed that using hydraulic gradient monitoring along a piezometer transect coupled with heat as a tracer and numerical modeling is a useful combination of methods in order to investigate river-aquifer exchange fluxes under monsoonal climate conditions.
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1.3.3. Evaluation of fertilizer best management practices for reducing nitrate leaching