Bret Harper
Hydro-Climate Project
I.
Introduction to watershed – the key water management issues
I selected USGS gauge number 16229000 on Kalihi Stream near Honolulu, Oahu, Hawaii
because it had observations from 1913 – 2004 for a total of 33086 observations. This gauge has
very little regulation because it is in an undisturbed watershed. This climate division is entirely
fed my rainwater. While the watershed is protected from development there is concern that
development on other parts of the island are creating a water deficit.
Figure 1: Honolulu County, Hawaii; Hydrologic Unit Code 20060000; Latitude 21o22’00”,
Longitude 157o50’49” NAD27; Drainage area 2.61 square miles; Gage datum 464.4 feet
above sea level NGVD29.
II.
Data sets used and how they were obtained
The following is a plot of the climatology of the flow, which is directly correlated to the
precipitation. I identified the peak season of interest as December – April when the flows are
highest.
Average Flows
10
9
8
7
ft3/s
6
5
4
3
2
1
0
0
2
4
6
8
10
12
14
Hydrologic month
I also looked at the long term trends over the entire observation period (91 years). There appears
to be a decrease in the stream flow over the last century which may indicate a climate trend.
Seasonal Streamflow
30
Seasonal streamflow (ft3/s)
25
20
15
10
5
0
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
Year
seasonal streamflow
10 per. Mov. Avg. (seasonal streamflow)
Linear (seasonal streamflow)
2000
2010
III.
Description of analysis techniques
Next I took the top and bottom 10% of flow years during the wet season, and compared the
average climate during that time. Looking at the upper air flow (Figure 2), we can see that the
subtropical jet tends to pass more directly over the state during high flow years. In low flow years
it tends to pass further to the northeast. The subtropical jet may bring in more precipitation from
storms that form off the cost of Japan that would otherwise pass to the north.
Figure 2: 200mb Vector Wind for high (left) and low (right) flow years
Looking at the geopotential height, I observed that the atmosphere over the equatorial Pacific
tends to be lower in high flow years. This could be an indication that the atmosphere in this
region tends to be cooler although I’m not sure how this would effect Hawaii’s precipitation.
Figure 3: 500mb Geopotential Height with high (left) and low (right) flow years.
Looking at the surface winds, I saw that winds over the state always come from the northeast
(trade winds), however, the equatorial winds were stronger in high flow years. This may be
indicative of La Nina conditions that accumulate warm water in the western Pacific. The El Nino
Southern Oscillation Cycle (ENSO) is known to have major climate impacts around the world.
Figure 4: 1000mb Vector Wind for high (left) and low (right) flow years.
Nevertheless, if I look at the sea surface temperatures for high vs. low flow years, they are
remarkably similar, and do not indicate La Nina conditions as far as I can tell. This may mean
there is some lag time between the state’s precipitation and the sea surface temperature that my
analysis is not picking up on. A lag time is likely because the Pacific basin system is so large.
Figure 5: Surface Skin Temperature (SST) for high (left) and low (right) flow years.
IV.
Summary of results and discussion
While I did see some correlation between stream flow and climate conditions I did not find
any conclusive evidence that I would recommend a watershed manager monitor in order to
predict stream flow. The hydro-climate linkages for this watershed are more complex than I
anticipated and I would need to do a more detailed analysis to generate more decisive results.