Precipitation
How does precipitation impact water quality?
Precipitation is a key factor in understanding a stream's chemical, physical, and biological characteristics. Increased flows from rainfall may temporarily lower the concentration of pollutants and move contaminants through the system. At the same time, water flows through our communities, picking up contaminants like pesticides, fertilizers, and sediments. This polluted runoff then enters storm drains which drain directly into local streams.
Heavy rainfall can also stir up the stream-bed, releasing metals and nutrients from the soil, and in extreme cases lead to erosion of the stream banks, adding yet more sediment and nutrients to the stream.
Over time, sediment settling on the stream bed will cover gravel substrates that are crucial for organisms like fish, insects and invertebrates. This can reduce habitat availability for spawning fish (such as trout) and macroinvertebrates, which rely on clean, coarse substrate for shelter and reproduction.
Analysis
The following section illustrates precipitation daily and monthly levels within the KICP monitoring areas, based on three NOAA weather stations (one in each key watershed).
- Boulder Creek Station - GHCND:USC00050848, Boulder, CO US
- St. Vrain Creek Station - GHCND:US1COBO0352, Longmont 1.8 ENE, CO US
- Coal Creek Station - GHCND:US1COBO0557, Lafayette 0.5 ENE, CO US
Precipitation Events are listed in tables below each of the daily graphs. These represent periods of measurable precipitation. For this report, a Precipitation Event has been defined as a period of consecutive days that experienced more than 0.1 inches of rain each day and totaled at least 1 inch over the event, or a single day with greater than 0.5 inches of rain.
2023 Monthly Precipitation Totals within Boulder, St. Vrain, and Coal Creek Basins
Rainfall in most months was typical of historical averages, and lower than average in March, October, and November. However, in May and June, the basin saw nearly double what is expected for this area. Roughly 10 inches of rain fell in total, over those two months, leading to peak flows in May and June that measured 3 to 10 times what is normal.
1991 through 2020 Historical Monthly Average Precipitation for the Boulder Creek Station
See individual years from 1893 to current: NOAA Physical Sciences Laboratory
2023 Precipitation Graphs
Boulder Creek Station
The months of May and June saw multiple high precipitation days, including 3 precipitation events within just those two months of over 2 inches of total rain. Another late season storm can be seen affecting the Boulder Creek Station in mid-September (see the Precipitation Events table below the graph).
2023 Precipitation Events
*A precipitation event is defined as a period of consecutive days that experienced more than 0.1 inches of rain each day and totaled at least 1 inch over the event, or a single day with greater than 0.5 inches of rain.
St. Vrain Creek Station
Higher precipitation in the St. Vrain was concentrated more in May than in June, with a notably high event during the second week of May. Later summer and fall months were typical.
2023 Precipitation Events
*A precipitation event is defined as a period of consecutive days that experienced more than 0.1 inches of rain each day and totaled at least 1 inch over the event, or a single day with greater than 0.5 inches of rain.
Coal Creek Station
The same storm seen on the St. Vrain in the second week of May also affected Coal Creek, to an even greater degree, with more than 4 inches of rain falling in the basin in that one week alone. The rest of the year followed normal precipitation patterns for Coal Creek.
2023 Precipitation Events
*A precipitation event is defined as a period of consecutive days that experienced more than 0.1 inches of rain each day and totaled at least 1 inch over the event, or a single day with greater than 0.5 inches of rain.