4.1 Mechanism of flood wave affecting HE
The dynamic process of q , between the stream and aquifer, was not
only affected by variation of the stream water level, but also by the
rate of variation. This was due to the fact that groundwater moves
rather slowly compared to open water flow. When the stream water level
rose or fell to a certain height with different rates, the response time
left for groundwater would be different and the corresponding
groundwater level at such moment would never be the same, resulting in
variations of the surface water-groundwater hydraulic gradient and
consequently the q values. Hence, we concluded that q was
proportional to the polynomial of water level and its change rate:q \(\ \mathrm{\propto}\mathrm{\ }\)(ω \(\bullet\)h +v ),
where ω is the angular frequency of the flood wave. The rate of
water level change was less influential on q than that of the
water level itself due to the influence weight of water level (ω= 2pi/T) was generally greater than 1. This was also why many studies
(e.g. Chen & Chen, 2003; Gu et al., 2012; Shuai et al., 2017) have
concluded that q is positively correlated with the water level as
a whole.
Typical flood wave parameters (A , T , r ,t p) characterized the height, width, roundness
and skewness of a wave, respectively. Essentially, an increase inA signifies an increase in water level and its change rate at any
time; an increase in T mainly reduces the overall change rate of
the water level; an increase in r leads to an increase of change
rate near the wave peak and a decrease of change rate near the initial
value of the wave; increasing t p has the opposite
effect to increasing r . In summary, different parameters
reflected the water level and its rate of change with different
regularities throughout the duration of the flood wave, thus leading to
the very different influences on q . Similarly, under CUFV
condition the increase in T /A is equivalent to the
simultaneous increase in T and decrease in A . Therefore,
the influence mechanism of T /A on q was similar to
that of T , but its influence was greater. The effect on qof increasing N was equivalent to shortening the duration of each
pulse and increasing the overall change rate of water level accordingly.
The Q max induced by a flood wave was mainly
controlled by the wave duration and the average water level height,
showing positive correlations with both (e.g. Liu et al., 2018). When
the flood wave duration was constant, the higher (A ↗), rounder
(r ↘) and less skewed (t p↗) the flood
wave,the larger the integral of the flood wave over time, resulting in
a larger Q max. RT was generally positively
correlated with Q max (e.g. Siergieiev et al.,
2015), but this relationship was not valid for the flood wave under CUFV
condition. Fig. 7 showed that the change trends of RT andQ max were completely different under CUFV
condition. The main reason was that T and A were related
variables under CUFV condition. An increase in T would inevitably
lead to a decrease in A , and A was dominant toQ max.