Hydrological Assessment

The hydrologic monitoring program works to verify delivery of the Montana Water Trust’s contracted water, and also to further understand the hydrology of these various watersheds. The techniques and tools administered are site specific. The following provides a discussion of the equipment and methodologies currently implemented:

Discharge measurements:
The Montana Water Trust takes discharge measurements throughout the irrigation season using a Marsh-McBirney flow meter to measure the amount of water flowing down the project stream or diversion ditch. We establish strategic cross-sections for all water transactions to measure the delivery of the agreed upon water. Discharge measurements follow USGS protocol for open channel flow measurements.

R2Cross Measurements:
The Montana Water Trust applies the R2Cross instream flow incremental methodology to initially assess the minimum stream flow needs of Project streams. The Colorado Water Conservation Board developed the R2cross methodology in 1996 after recognizing a need for an instream flow methodology that expressed the following: 1) cost-effective data acquisition, 2) easy-to-learn and implement procedures, and 3) simple data interpretation (Colorado Water Conservation Board 2001).

The R2Cross instream flow methodology requires on-the-ground measurements from a single stream transect and links these variables to the natural environment being protected. The method is based on two biological assumptions: 1) in coldwater ecosystems fish are the species most sensitive to minimum instream flows and 2) riffles are a critical component to healthy aquatic ecosystems and most sensitive variation in discharge.

The hydrologic data (slope, depth, distance from stake, velocity) collected during a single field visit is entered into the R2Cross Microsoft Excel database. The majority of the data plots and tables generated by the R2Cross program are designed to allow the user to cross-check hydrologic information, and to further understand of the dynamics the system.

Stream flow requirements for habitat protection in riffles are determined from flows that meet criteria for three hydrologic parameters: mean depth, percent of bankfull wetted perimeter, and average water velocity. During the summer, all three hydrologic requirements must be met. During winter flows only two of the three requirements must be met. These requirements are made to account for seasonal variability that occurs in most unregulated systems.

In addition to the hydrologic physical parameters measured, biological observations are an optional component of the R2Cross methodology. A fish sample and an aquatic invertebrate sample are suggested for each stream reach. This biologic information is separate from the R2cross hydrologic modeling but can be considered when making instream flow recommendations. The biologic data is not necessary for the model to make instream flow recommendations, and is primarily a parameter to provide better understanding of the aquatic ecosystem.

Staff Gage Installation:
For long- term flow transactions, the Montana Water Trust installs manually read staff gages on project streams. Stage discharge rating curves are developed for each gage location using a minimum of three discharge measurements. Stage-discharge relation (rating curve) is obtained from stage readings from the staff gage plotted against the measurements of discharge using a current meter over a range of varying gage heights at the gauged site.

Parshall Flume Installation:
For certain transactions, flow is most effectively managed by installing a flume in the associated diversion. A Parshall flume has a specially shaped open channel flow section, which may be installed in a ditch, canal, or lateral to measure the flow rate. Parshall flumes are the most widely used types of flumes for fixed flow monitoring installations. They have wide flow ranges, are resistance to submersion, and are simple to calibrate.