Maui Rainfall and Water Resources

Ola i ka wai – water is life. This is not just a nice phrase but it is critically important for us here in Hawai’i – if rain were to suddenly stop it would be an end for our ability to live here. So it behooves us to understand our water, and that of course all of our water originates from rain.

Anecdotally, there are lots of observations that rain has slowed down on the North Shore (the windward) side of Maui. The goal of this post is to try to put some actual numbers to that.

This first data set is from the NWS (National Weather Service), captured in Kahului. It spans more than 100 years and is probably the most comprehensive set of rain data, although the period 1925 – 1954 is missing. The data set is referred to as Kahului Airport, but the airport didn’t come into existence until 1954, so the data from that point on is OGG (the airport). Unfortunately, the Kahului set is the only official rain data that goes back more than 50 years. Kahului is also 15-20 miles from the north slopes of Haleakala, which receives most of East Maui rainfall. But nevertheless, while the absolute numbers from Kahului are not important, the time history we will assume is similar to the variability in East Maui.

The rain fall is obviously quite variable, ranging from less than 10″ per year to over 30″. To assess trends I broke the data into 20 year buckets and took the average annual rainfall at Kahului within each 20-year period. The results are this:

1905-1924: 16.1″
1955-1974: 15.7″
1975-1994: 16.5″
1995-2014: 12.6″
2015-2025: 13.3″

There is about a 20% decrease in rain in the period 1995 until now, relative to the period before 1995.

It is interesting to compare the rainfall data from Kahului to what the level of the freshwater lens was within one monitoring well. This well was drilled in 2002 for the specific purpose of monitoring the Haiku aquifer. It is called the Hogback well and is located between Haiku Town and Maliko Gulch. The well report is located here.

The Hogback well also shows a downward trend from roughly 2005, although 2005 is definitely greater than what the level was in 2002. The “spike” at the Hogback well in around 2005 seems to line up with the “spike” in rainfall at Kahului, in about 2005. If so, this would imply that the effect of rainfall has little lag until it percolates down to the freshwater lens. This is illustrated with the side-side comparison of rainfall (left side) and well level (right side).

The following data set comes from an individual who lives up Awalau Rd in Haiku. That area is known for getting a lot of rain, probably due to elevation where marine clouds rise sharply. This data set runs from about 1978 to 2017. It does not show a clear downward trend at around 1990-2000 as the Kahului set does.

Future posts here will further explore the relationship between rain and Haiku groundwater, as well as rain variability. Stay tuned!