Mystery of the 3 USGS Gages in Mānoa

The runoff ratio is how much runoff responses to the rainfall. Here, we use the NCDC HMNL rainfall data (the diamond symbol in Waihi subwatershed on the Annual Rainfall Map), and 3 USGS gages and their information of the drainage areas.

Why do we need drainage area? Because both of runoff and rainfall represents as depth over the watershed area. (Notice the tricky part here: we assume both of runoff and rainfall depths are well-distributed/homogeneous, but they are usually not…)

Waihi Runoff Ratio

Waihi runoff ratio shows an extreme runoff response to the rainfall, which may indicate:

  • Waihi watershed has a very permeable surface with a threshold for overland flow (Wikipedia: overland flow).
  • Either runoff or rainfall is not well-spatially (homogeneously) distributed

Besides, we can also notice that the runoff ratio excessed 100% sometimes, which means the runoff we got is more than the rainfall we observed. It whispers to us: either rainfall or runoff is not well-spatially distributed  OR there is some extra water pouring into the stream.


Waiakeakua Runoff Ratio

With a similar drainage area, Waiakeakua’s runoff response more to the rainfall than Waihi’s runoff! Moreover, there were more runoff ratios larger than 100%… It looks not like only because of inhomogeneous of runoff or rainfall, but more likely some additional water input to the Waiakeakua stream.


Mānoa-Woodlawn Runoff Ratio

Surprisingly, Mānoa-Woodlawn runoff ratio has a similar pattern with Waihi but not Waiakeakua. I’d like to guess that there were more groundwater discharge to the Waiakeakua streams, but not Waihi and Mānoa streams. I wonder, which stream has more groundwater discharge since USGS Waiakeakua gage is actually the confluence of three streams.




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