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Appropriate routine maintenance of stormwater best management practices (BMPs) after implementation and long term over time is often cited as being critical to ensure they are functioning as intended.  Our experience shows that monitoring and assessing green infrastructure function is also equally important and that monitoring and maintenance should go hand-in-hand.  The source of a BMP not functioning as intended can stem from issues related to a design, a construction process or limited or inappropriate maintenance practices over time.   It is often difficult to understand the functioning condition of a BMP when the system is dry.  Collecting data or information during storm conditions when runoff is occurring provides a much clearer picture of BMP function.  

We are monitoring for water quantity performance, not water quality.  With these methods we are assuming that EPA performance curves are accurate and that if water is being diverted off the road, into the system and infiltrating properly, the soil and plants are working as intended to remove pollutants. Our assessment and monitoring methods are intended to be simple and easy to perform in order to encourage green infrastructure owners and operators to do this work themselves.   The monitoring method chosen can be based on the budget, staff, resources and expertise you have available. 


In order to understand if the intended amount or amount of stormwater is entering a BMP, the first water quantity parameter that needs to be measured is rainfall or precipitation.  The amount and intensity of precipitation received can be quite variable based on location.  As an example, five inches of rain may be recorded at an existing NOAA or USGS precipitation gauge, but 3 miles away in the drainage area of a BMP being monitored, only 0.25 inches of rain may actually fall.  Therefore, if you are expecting to see the equivalence of a 5 inch rain event at your site, your expectations would be incorrect.  That is why it is important to collect or use existing precipitation data that is measured as close as possible to the BMP that will be monitored.  

There are four main methods and types of precipitation data that should be considered when decided to monitor precipitation.

Primary data are measurements that you monitor and collect yourself.

Existing data are gathered from another reliable source that has already collected the data.  

Continuous data are collected, often automated and logged, at short time intervals. 

Daily Discrete data are collected once per day.  

Examples of precipitation data collection types



Primary Daily Discrete - Standard rain gauge that gets manually measured on a daily basis by the user. 


Cost: - $5-20

Advantages: Inexpensive, minimal technical skill required, can install anywhere

Disadvantages: Daily data only,  no rainfall intensity data, needs to be manually read daily


Primary Continuous - Logging tipping bucket rain gauges collect and automatically record rainfall data in 0.01 inch increments.  

Cost: $400-600

Advantages: Automated,  Can install anywhere, rainfall intensity can be computed

Disadvantages: More expensive than other methods, some technical skill required

Existing Continuous - Precipitation data is collected by many federal and state agencies and made available to the public.  The USGS and NOAA are two reputable existing precipitation data sources.


Cost: Free

Advantages: Rainfall intensity can be computed

Disadvantages: Not a good spatial distribution

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Existing Daily Discrete - Daily precipitation data is collected by volunteers through the Community Collaborative Rain, Hail and Snow Network. NOAA provides quality assurance on this existing data.

Cost: Free

Advantages:  Free, Better spatial distribution that federal and state precipitation sites. 

Disadvantages:  Daily data only, no rainfall intensity data

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