A hydrograph is a time-series graph showing the rate of streamflow (discharge) a specific point in a river, a stream, or another channel. We can understand the streamflow rate changes with time at the point in the stream by a hydrograph.
The streamflow is related to rainfall, thus we like to put them together and see. However, we notice that the stream gage and rain gage usually are not at the same point. For example, in Mānoa watershed, we have three stream gages but only one rain gage (see the Mānoa Annual Rainfall Map in Map Page). It is such a pain for hydrologist…(now we should learn that nature is not a separated system, we better communicate with different field first, and then set up our observation stations…) Moreover, a stream gage is at a point of a stream, but it collects all the runoff (the water flow) from the entire watershed. Besides, point rainfall cannot represent the rainfall in entire watershed, neither. Thus we have to be very aware while we’re interpreting the rainfall and streamflow at each stream gage.
Waihī USGS Stream Gage
Waihī stream gage is the one closest to the rain gage, so the streamflow responses to the rainfall should be the most accurate within three stream gages. The minimum discharge (flow) at Waihi gage is about 0.002 cms on 2/11/2012, which is almost no flow…, and the maximum discharge is 6.8 cms on 12/13/2011 during the data period. The mean discharge of Waihi gage is about 0.09, which is more than three times ‘Aihualama stream. It’s very obvious that the streamflow didn’t response to the rainfall the same way (sometimes larger discharge with larger rainfall, but sometimes more rainfall has okay discharge)
Waiakeakua USGS Stream Gage
Waiakeakua stream gage has mean discharge 0.13 cms, including minimum discharge, 0.037, on 1/17/2014 to 1/20/2017 and maximum discharge, 1.71 cms on 9/15/2015. We noticed that the discharge from Waiakeakua is not as extreme as Waihī. There are some possibilities:
- There is more groundwater discharge into Waiakeakua stream (since the minimum discharge at Waiakeakua is much higher than the minimum discharge at Waihī)
- The rainfall amount is not the same at Waihī and Waiakeakua, especially heavy rainfall (because the maximum discharge at Waiakeakua is much lower than the maximum discharge at Waihī)
- The flow paths are different in each watershed. The flow paths influence the time the water stays on or in the ground until it goes into the stream.
- The slopes of each watershed are different. I think that the discharge will increase immediately if the slope is steeply toward to the stream.
- Geology and soil distributions lead the distribution of infiltration rate. The infiltration rate plays an important role in the flow paths.
- The shapes of each watershed are different. The water paths may also related to the shape of a watershed.
Mānoa-Woodlawn USGS Stream Gage
Mānoa-Woodlawn stream gage has much higher discharge than Waihī and Waiakeakua because of its larger drainage area that allows the stream gain more water. The maximum discharge, 8.27 cms, occurred on 7/24/2016, and minimum discharge, 0.057, occurred on 12/8/2012.