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 xx ) 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.