Hydrology Monitoring
Hydrology is the study of the movement of water. Volume of water, timing with respect to rainfall, and duration of high flows are important aspects of stream hydrology. Applied hydrology includes predicting rates and amounts of runoff as in flood forecasting, engineering storm water conveyance systems, pollutant loading estimations, and many other water management tasks. Most applications of hydrology are dependent upon monitoring of the streams, rivers, and ditches that are the subject of management. Hydrology monitoring in streams most often is typically composed of two tasks: water level monitoring and flow estimation.
Water level monitoring is often done with automated equipment. While single measurements can be useful, frequent and regularly timed measurements of water level are needed for an in-depth understanding of stream hydrology. These data a typically are presented as a graph called a hydrograph. A hydrograph shows water levels over time. Other useful data might also be presented on a hydrograph too, such as daily rainfall totals. Hydrographs for different streams may look very different, even under the same climate conditions. For instance, a stream with a highly urbanized watershed will typically have a "flashy" hydrograph, rising quickly and intensely in response to rain and then receding just as quickly. A stream with a less urbanized watershed would likely respond more gradually and have lower maximum water levels.
Many applications of hydrology data require knowing not only how high the water got, but also the volume of water. For instance, installing the properly sized storm water structures requires this information. Less intuitively, analysis of water quality problems can require flow data. Water quality tests reveal the concentration of pollutants; only when paired with flow data can the amount of pollutants be calculated.
Flow volumes are most often estimated by entering monitored water levels into rating curve equations. Rating curves are the mathematical relationship between flow and stage. Determining this equation requires manually measuring both flow and stage simultaneously from the stream. Water level is measured from a fixed point. The water's velocity (often if feet per second), is measured with a current meter. Velocity is measured at regular intervals across the stream. At each interval the depth is also noted. Velocity is measured at a depth equivalent to 6/10 of the stream depth at each interval. Then, the flow (usually in cubic feet per second) in each interval is obtained by multiplying the depth, width of the interval, and water velocity. Then, the flow in each interval is summed to yield the total stream flow. This process is repeated (usually at least 15 times) under a variety of water levels. Then, the all paired water level and flow readings are graphed on an x-y axis. The equation of the line that best fits these points is the rating curve. The rating curve can be used to calculate flow when only water level is known.
To download or view stream hydrology data for Anoka County Streams, use the links in the "Rivers and Streams Directory."
