Variations in annual stream hydrograph of montane watershed in northern Thailand: Mid-wet-season shift in runoff response observed in some years

Abstract:

In this study, we examined the rainfall-runoff relationship of the 74.2-km² Mae Sa watershed, a basin with mixed forest, agriculture, and peri-urban land covers in northern Thailand. Mae Sa has a distinct annual streamflow discharge (Q) hydrograph related to the strongly seasonal rainfall (RF) regime characteristic of the Asian monsoon region. Hourly RF observations during 2004-2012 from each of the 11 rain gauges in Mae Sa were adjusted for lag time with respect to Q, based on the time difference between the peak rainfall at the specific station and the peak discharge at the watershed outlet, and then integrated into a daily watershed RF time series. Quickflow (QF) and baseflow (BF) time series were obtained from hygrograph separation using the Sliding Average with Rain Record (SARR) method (Koskelo et al. 2012), with the integrated daily watershed RF and Q as input. The event quickflow runoff coefficient (RC) was calculated as total QF/total RF for each storm event identified by the SARR method. Results suggest a “switch-point” in the rainfall-runoff relationship annual cycle in some years, after which similar RF events generate higher Q than earlier, i.e., an abrupt upward shift in RC is observed. In general, the switch-point occurs in the second half of the wet season from July to August; however, the occurrence and timing of the switch-point varied from year to year during study period. This variability appears to be the result of interannual differences in total RF and the temporal RF patterns. For example, the switch-point in years with average or lower annual RF, such as 2007 and 2010, are more obvious compare to years with higher annual RF, such as 2006 and 2011, in which the RC in the early stage of wet season are already high.