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Copy of Copy of Copy of Alerts, Analysis, and Automation: Hohonu's Water Level Thresholds
- Kevin Mukai
- Sep 25
- 6 min read
This document provides an overview of Hohonu’s water level thresholding system, including information on how thresholds for a station are set and utilized.
In recent months, Hohonu has:
Established an automated system that utilizes historical data to set default advisory level thresholds at sites across Hohonu’s diverse sensor network
Built support for low water thresholds and alerts
Updated how thresholds are displayed in Hohonu’s Live Map and are used for sending text alerts to community members
What are Water Level Thresholds and Why Are They Useful?
A water level threshold is a pre-defined water level height that, when crossed, distinguishes an event of extreme water level and may subsequently trigger an alert. Visually, it is fixed elevation that provides immediate context for real-time water level or trends.
One primary purpose of thresholds is to automate monitoring and provide early warnings for conditions that may require attention and action. Instead of constantly watching a graph, you can rely on threshold alerts to notify you when water levels reach a significant point, such as one that could lead to minor flooding or create hazardous low-water conditions for navigation.
Another purpose is for planning and analysis. With thresholds, you can ask questions like “will my property flood in two weeks?” or “how many times did water level rise above the bulkhead in the past 5 years?"
Hohonu uses two different levels to help you distinguish between the severity of an event:
Advisory Level: This is a "heads-up" notification. An advisory threshold can be used to indicate that water levels are higher or lower than normal or that conditions are favorable for a potential event. It serves as an early warning to pay closer attention to the conditions.
Warning Level: This is a higher-level alert that indicates a more serious event is imminent or already occurring. A warning threshold can signify the point where water levels are of such concern that action should be taken. For example, a warning could mean that flooding is expected and may pose a significant threat.
With thresholds, you can ask questions like, "will my property flood in two weeks?" or "how many times did water level rise above the bulkhead in the past 5 years?"
Establishing Water Level Thresholds
The primary component of this system is to automatically set meaningful default advisory thresholds for nearly every station in Hohonu’s network, providing immediate context to your water level data. Should you wish to use specific, custom thresholds, we make it easy to replace the default thresholds with advisory and warning levels of your choice. Some characteristics of this approach to thresholding system:
Generalization: The system works across most of our sensor and deployment types, determining default thresholds in minimal time after a sensor is deployed.
Data-Driven: A station’s thresholds are based on best practices from NWS and NO-COOPS, and rigorous modeling and statistical approach using the sensor’s data.
Flexibility: Users retain full control and can easily override any default with their own custom thresholds.
How Hohonu Sets Default Thresholds
The method for calculating default thresholds depends on the station's Deployment Type. The system automatically classifies each station to ensure the correct calculations are applied. Presently, only Advisory level thresholds are calculated by default.
Tidal Stations
For tidally-influenced deployments, the system calculates a High Water Advisory threshold after collecting at least 35 days of data.
The primary method involves modeling a station's data against the nearest official NOAA tidal gage using a Generalized Least Squares regression. If a good fit between the gages’ data is found, the NOAA NWS or NOS Minor Flood threshold is transferred to the Hohonu gage using the regression fit
If a reliable model cannot be established, a statistical method is used based purely on your station's collected data. Here, the 99th percentile daily high water value is used to indicate the line of extreme water levels in the historical data
These thresholds are automatically recalculated monthly for the first year of a station’s deployment and annually thereafter to adapt to new data and seasonal changes.
Over Land / Flood Stations
These stations are automatically assigned a constant Flood Advisory threshold set at a Flood Depth of 0.25 ft. This provides an immediate, standard reference for when water begins to accumulate on normally dry land, and the value is equivalent to that used by the National Weather Service’s National Water Prediction Service.
Water Depth Stations
These stations receive a constant Low Water Advisory threshold set at a Water Depth of 3 feet. This is useful for monitoring minimum water levels in channels, reservoirs, or other bodies of water where depth is critical.
Stage, Water Quality, and Others
Currently, no default thresholds are automatically calculated for our Stage and Water Quality station types. However, users can still set custom High or Low Value thresholds manually.
Hohonu uses two different levels to help you distinguish between the severity of an event
Incorporating Your Custom Thresholds
While our default thresholds provide a great starting point, you always have the option to set your own. When you supply a custom threshold, it will override the default for that specific type and level (e.g., you can override the "High Value Advisory" default but keep a "Low Value Advisory" default if one exists).
To set a custom threshold, simply send us the desired advisory and/or warning levels to kokua@hohonu.io. Feel free to include a contextual label for that threshold. No need to convert between datums and units - our system will do that automatically.
Putting Thresholds to Work
The threshold system is seamlessly integrated into Hohonu’s dashboard and back-end, providing at-a-glance insights, automated alerts, and powerful tools for analysis and planning.
View Thresholds on the Hohonu Public Dashboard
You can visualize thresholds directly on the water level plots as clear, horizontal lines, giving you an immediate reference for the current conditions at your station. Navigate to past or future dates to visualize when water levels may have or may potentially exceed the threshold.
Hovering over the ‘i’ icon will provide you the threshold level and value. In the station's Options dropdown menu, a 'Thresholds' section allows you to toggle the visibility of "High Water" and "Low Water" lines. By default, threshold lines are turned ON for tidal stations and OFF for land stations to provide the most relevant view.
Receive Advisory and Warning Alerts
Our system uses these thresholds to automatically notify you of significant events.
Your custom thresholds always take precedence. If you set one, it will be used for alerts instead of the system-generated default, ensuring you are notified about the levels that matter most to your operations.
You will receive alerts for both High Value events (when water rises above a threshold) and Low Value events (when water drops below a threshold).
To sign up for alerts, navigate to your station’s page and click on the alert (bell) icon in the upper right corner:
In the popover, input your phone number, and select the desired thresholds you want to receive alerts for:
After you click ‘Sign Up’, you should receive a welcome text message. Follow the instructions to complete signing up for the alerts. You may opt out of alerts at any time.
Analyze the Past - Plan for the Future
Thresholds are powerful tools for contextualizing historical data and preparing for future conditions.
Understand Past Events: Look back at your historical data to see how frequently and for how long water levels have crossed your defined thresholds. This can help you identify patterns and understand the impact of past storms or droughts. Hohonu offers a service to perform Water Level Threshold Analysis & Hindcasting for its customers.
Prepare for Upcoming Events: Combine thresholds with our forecast data to see when water levels are predicted to cross into Advisory or Warning levels. This is especially useful for anticipating events like King Tides or storm surge, allowing you to plan and take proactive measures.
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