4.1 Recession characteristics and impact parameter
Spatiotemporal changes in parameters a and b represent
regional differences and changes in catchment drainage behaviors after
environmental changes. According to a previous study that suggested thatb represents the main recession regime (Santos, Fernandes, Moura,
Pereira, & Pacheco, 2014; Sánchez-Murillo, Brooks, Elliot, Gazel, &
Boll, 2015), here we analyzed b with all data as a constant for
each period. Therefore, changes in a can show differences in
hydrological behavior caused by environmental changes. The results are
presented in Table 2. The apre ranged from 0.04
to 0.23, apost ranged from 0.03 to 0.16, andb ranged from 0.53 to 1.44. There are 10 catchments showing
decreasing a after 2000. In addition, the recession constantsc and d were estimated from the reformed parameter. In
most catchments, the d value was close to 1~1.5,
while the c values had a greater variability. The results showed
that a lower a value corresponding to higher c values
indicates the discharge contributed from a relatively large groundwater
storage when the recession constants d were fixed.
Impact parameter x represents the groundwater flux caused by
external factor (e.g. evapotranspiration, land cover change, etc.),
mainly affecting the recession curve in the portion of the lower
streamflow. An increase in x means that a decrease in groundwater
storage which contributes to streamflow and a higher flow variation.
Otherwise, a decreasing x indicates increasing groundwater
storage with lower flow variation. Therefore, the change in x(Δx ) can be explored to further understand its influence on
catchment dynamic storage properties. Table 3 and Figure 4a show the
results of x and its change. There is no obvious regional
difference in all catchments, indicating that the environmental impact
on each catchment varies greatly. The factors that affect Δx by
altering or interfering with the hydrological process could be
attributed to climate change, groundwater pumping, return flow of
agricultural irrigation, change in land use/cover and vegetation
coverage, topography, soil type, among others.