Surface runoff dominates stream source and hydrochemical composition in
a semi-arid, thick, loess-covered catchment based on high resolution
intra-rainfall sampling
Abstract
The source and hydrochemical makeup of a stream reflects the
connectivity between rainfall, groundwater, the stream, and is reflected
to water quantity and quality of the catchment. However, in a semi-arid,
thick, loess covered catchment, temporal variation of stream source and
event associated behaviors are lesser known. Thus, the isotopic and
chemical hydrograph in a widely distributed, deep loess, semi-arid
catchment of the northern Chinese Loess Plateau were characterized to
determine the source and hydrochemical behaviors of the stream during
intra-rainfall events. Rainfall and streamflow were sampled during six
hydrologic events coupled with measurements of stream baseflow and
groundwater. The deuterium isotope (2H), major ions (Cl-, SO42-, NO3-,
Ca2+, K+, Mg2+, and Na+) were evaluated in water samples obtained during
rainfall events. Temporal variation of 2H and Cl- measured in the
groundwater and stream baseflow prior to rainfall was similar; however,
the isotope compositions of the streamflow fluctuated significantly and
responded quickly to rainfall events, likely due to an infiltration
excess, overland dominated surface runoff during torrential rainfall
events. Time source separation using 2H demonstrated greater than 72%
on average, the stream composition was event water during torrential
rainfall events, with the proportion increasing with rainfall intensity.
Solute concentrations in the stream had loglinear relationships with
stream discharge, with an outling anomaly during an intra-rainfall event
on Oct. 24, 2015. Stream Cl- behaved nonconservative during rainfall
events, temporal variation of Cl- indicated a flush and washout at the
onset of small rainfall events, a dilution but still high concentration
pattern in high discharge and old water dominated in regression flow
period. This study indicated that streamflow responded to rainfall
events quickly and composition was dominated by overland flow. Stream
isotope and hydrochemistry controlled by infiltration excess, overland
flow indicated that stored water in the thick, loess covered areas were
less connected with stream runoff. Solute transport may threaten water
quality in the area, requiring further analysis of the performance of
the eco-restoration project.