About the Hydrologic Monitoring Program
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Recording stream gages operate electronically. The techniques used follow those described in manuals by the United States Geological Survey and other standard references. Stream stage or water level is measured with an appropriate device and recorded by a data logger at a programmed time step, usually every 15 minutes. Discharge or streamflow is determined by direct measurement of water depth and velocity at a cross-section. On small streams, standard control devices such as weirs or flumes may be used. The gages are maintained and the data loggers uploaded at least ten times a year. Intermediate site visits are made to obtain high flow discharge measurements during rain storms and check the condition of the equipment after extreme weather events. Data processing is by micro-computer programs written by WLRD staff members. Stage-discharge rating curves are constructed from the series of discharge measurements at each site and a rating table made from points taken from the curve. Continuous discharge records are produced by applying the rating table to the corrected continuous stage record. Many stations have unstable stage-discharge relationships. Multiple ratings are developed at these sites and shifting-control methods are used to apply the rating table to the stage record. For a station recording every 15 minutes, 96 values are produced for each day. Mean daily discharge is the arithmetic mean of the set of values taken from midnight to midnight each day. Equipment failure may produce a period of no or poor stage record. In many cases we have estimated the mean daily discharge from observations, discharge measurements, rainfall records, and nearby gage records.
Recording rain gages use tipping bucket gages to measure rainfall and data loggers to store the information. The gages are uploaded, maintained and the calibration checked about ten times a year. The data loggers total record the rainfall every 15 minutes in 0.01 inch increments. Data accuracy is checked by comparing nearby gage records for anomalies. Where equipment failure produces a gap in the record, rainfall is estimated from nearby recording gages and volunteer observations. Streamflow and rainfall data are stored in electronic database files and can be reported in a variety of formats.
Water Quality Sampling
Stormwater grab samples are usually obtained by sending out teams of two workers to predetermined sampling sites once sufficient rainfall has fallen to generate runoff. Samples are collected in bottles supplied by test laboratories. The sampling teams a re trained in field procedures by WLRD staff water quality specialists at the beginning of the wet season. Sample analysis is performed by certified laboratories. Flow proportional composite samples are collected for some projects. To collect these samples, automatic samplers are signaled by data loggers at predetermined flow quantity intervals. The series of samples are collected into a single bottle until the runoff event is over. The paper Automatic Stormwater Sampling Made Easy (1990) by Thrush and Grant describes many of the procedures commonly used in operating these sampling programs. The storm composite sample is subsampled to test for various parameters at the laboratory.
Staff and Crest Stage Gages
Periodic visual readings of water level are made at some lake, detention pond, wetland, groundwater well, and stream sites. Staff plates are installed or tape down points established to provide a benchmark to compare the water level to. Some benchmarks are surveyed to a point of known elevation. Stream sites may have a stage-discharge rating established so that the gage height reading can be converted to discharge. Crest-stage recorders are installed at some sites. These record the highest water level reached since the last reading. These consist of a closed end tube with holes to allow water in, a rod inside, and an indicator substance such as ground cork that floats with the rising water level and remains on the rod at the highest level reached. These sites are checked every 4 - 6 weeks. The data are stored in the database.
The gage number is a short identifier assigned to a monitoring site for ease of use. Stream gage numbers usually consist of the sub-basin number and a single letter from A to T. Rain gages are similarly named but the letters range from U to Z.
Gage Name, Descriptive Location and Address
Gage names are assigned to describe the general location and purpose of a monitoring site. The descriptive location elaborates on the gage name to further describe the location with nearby landmarks. Many of the sites have actual addresses or in some cases we have estimated a street address as a further aid to the site location.
Latitude and Longitude
Sites have NAD 27 latitude and longitude coordinates obtained by a Global Positioning Satellite receiver. These data are available on request.
Date Installed, Date Removed
The month and year monitoring began is given for each site. If a gage has been removed, the month and year of removal is given.
Accuracy of Data
Streamflow data are prepared by applying a stage-discharge rating to the recorded stage. We use techniques similar to the U.S.G.S. and the accuracy of our records is affected by similar factors. The Water Resources Data books describe these factors succinctly: The accuracy of streamflow records depends primarily on: (1) The stability of the stage-discharge relation or, if the control is unstable, the frequency of discharge measurements, and (2) the accuracy of measurements of stage, measurements of discharge, and interpretation of records. Many of the streams we gage have hydraulic controls that change seasonally, or change as the result of high flows, or are affected by human activities. For this reason, although many periods have higher accuracy, in general accuracy should be considered to be within 15% of true, except for estimated periods, which are less accurate. Equipment failure or freezing weather causes occasional gaps in the stage record. For many of these periods, we have estimated the mean daily discharge from site visit observations, rainfall records and comparing nearby and similar streams. These estimates are made to give continuity to the record and enable the calculation of monthly and annual mean discharges. For details on the accuracy of a specific gage or time per iod, contact WLRD with a request for gage data. Rainfall records represent a point sampling of rainfall amounts. Topography and local conditions can cause nearby sites to accumulate different amounts of rain over a period. The accuracy of our records is assured by regular maintenance and calibration o f the equipment. Where equipment failure creates a gap in the record, the missing period is estimated with data from nearby rain gages or volunteer observers.
Totals and Statistics
Rainfall data tables list monthly rainfall totals, the maximum of the daily totals for each month, and an annual rainfall total for the water year. The annual hyetograph graphs the daily totals over time to give a visual look at the rainfall distribution throughout the year.