Hohonu Tide Predictions

Understanding tides is essential for a range of use cases, from coastal management and environmental studies to navigation and recreation. This document provides an overview of the intricacies of tides, the methods Hohonu uses to predict them, and how users can access this valuable data.

---

Hohonu tide predictions, used alongside local observations, enable more robust coastal management.

The Difference Between Total Water Level and Tides

Hohonu sensors measure the total water level at a specific location. However, at sites where water level is tidally influenced, this total level is a combination of two key components:

1. Astronomical Tides: The predictable, regular rise and fall of the water caused by the gravitational pull of the moon and sun.

2. Non-Tidal Residual: The remaining component driven by weather, wind, river discharge, seasonal variability, and storm events (among other factors).

To understand flood risk and coastal conditions, it's essential to separate these two components. Without an accurate, local astronomical, it is difficult to isolate the non-tidal residual which tells us exactly how much higher (or lower) the water is than "expected".

How Hohonu Predicts the Tide

Hohonu models and predict the local tides at all tidally-influenced stations in its network using a well-established method called harmonic analysis. The method deconstructs the overall tidal signal into many simpler, individual waves called tidal constituents (or harmonics). Each tidal constituent represents a repeating pattern that comes from the motions of the Moon, the Sun, and Earth, and how those motions are expressed in the rise and fall of the tide at a particular location. For each constituent, an amplitude and phase are determined, allowing for an infinite reconstruction of the tidal signal into both the past and future.

There are hundreds of these constituents, but Hohonu uses approximately 30 of the most important including M2 (lunar semidiurnal), S2 (solar semidiurnal), K1, and O1 (lunar and solar diurnal, respectively). Constituents like SA (Solar Annual) and SSA (Solar semi-annual) typically represent seasonal changes in water levels caused by things like the seasonal heating and cooling of ocean water. We have omitted these to ensure the tide predictions are only from “astronomical” forcing. However, this may result in larger non-tidal residuals in seasonally influenced water bodies.

Harmonic analysis is conducted for all coastal stations with at least 35d of data. Analyses include up to two years of data, and are repeated monthly for months 1 - 12 of a station's deployment, and yearly thereafter. While it may appear that short data records would negatively influence tidal prediction accuracy, in testing we found most of a station's tidal signal can be explained after 1 month of data was collected, and that the signal's improvement tapers after approximately 6 months. If a harmonic analysis does not yield significant tidal constituents, the station is likely not tidally-influenced and thus no tide predictions will be generated for that location.

Non-tidal residual (i.e. total water level minus tides) is clearly visible at this station, driven both by seasonal increases in overall water level due to heightened temperature and riverine discharge, and by acute storm surge events.

Accessing Hohonu's Tide Prediction Data

Hohonu provides users with easy access to tidal data through the Hohonu web dashboard and via our developer API.

1. On the Web Dashboard

When you view a station's data on the dashboard.hohonu.io, you will see a number of time series datasets:

• Observed: This is the total water level measurement coming directly from a Hohonu sensor.

• Hohonu Tide Predictions: This is the tidal signal calculated for the location using the harmonic analysis method described above. Tide predictions can be rendered in any unit or datum that are also available for observed data for the station.

In the case that Hohonu Tide Predictions are not available for a station, predictions from a nearby NOAA station may be displayed as a blue line. Note that these NOAA predictions often come from "virtual" gauges located far away from the Hohonu station, leading to inaccurate prediction of tide magnitudes and timing.

2. Using the Hohonu API

For users with granted access, tidal data streams are available through our API. This allows you to integrate Hohonu tide data directly into your own tools, models, or applications. To retrieve data for a given sensor and time range, see our complete API Reference Documentation.