Modeling a system where values do not change is a generally simple task. But when dealing with systems like rivers, the flow in those rivers is constantly changing due to water being added from high water tables or tributary streams, or water is being lost from the stream due to diversions or loss to low water table aquifers. The flow in rivers is constantly changing both in time and location. To model river systems at LWS we develop tools known as point-flow models.

Point-flow models estimate water flow in rivers between known measurement points through time. These points where flow is estimated are generally at diversion structures since the models are most often used to evaluate the ability to divert or exchange water at these points.

The general concept of the model is quite simple:

• We begin with known data from as many points as possible from the study reach of the river on a set time interval, which is usually daily data. Let’s say we have 4 flow gages and 10 diversion structures along our study reach of river. Starting with the flow on a particular day at the upstream flow gage, we can work out way downriver and reduce the flow by the daily diversions from our diversion structures.

• When we reach the next flow gage downstream we would only expect our estimate to match the downstream flow gage if the river is not gaining or losing any water; since that rarely happens, our estimate will likely be higher or lower than the actual measured flow.

• If our estimate is higher than the actual value, that means the river is losing water as it flows.  And if our estimate is lower than the actual value that means the river is gaining water. So the difference flow between our estimate and the flow tells us how much water was lost or gained and, when divided by the distance between the gages, we know the approximate loss or gain is in the river per mile. This estimate of daily gain or loss in each section of the river between gages allows us to approximate the flow in the river at any location.

That is the basic framework of a point-flow model. The fun starts when we begin adding hypothetical water structures to test their impact on the river system. Point-flow models can be used to estimate exchange capacities, water availability for new diversions, or impacts to downstream water rights from new diversions. If the structure we are testing is a diversion for a reservoir, a point-flow model pairs nicely with a reservoir operations model. For more information about reservoir operations models, check out our previous blog on the topic.

Additional diversions from the structure we wish to test are added to the point-flow model to see the impact to the river and other water rights. The really complicated part of a point-flow model comes in this step. Water rights are not simple, and so modelling the impacts to a complicated legal web of water rights from a new diversion must be very carefully done to make the model results as accurate as possible to make the results valid. These complicating factors include things like Compact seasons, reservoir fill seasons, the priority of downstream water rights, conditional water rights, in-channel flow minimums, and many more. Each point-flow model comes with its own set of unique challenges, but if there is enough data to generate a point-flow model, they can be invaluable tools for evaluating future water rights.

LWS is very experienced in the development of point-flow models; from simple streams with a new diversion to highly complicated rivers modeling multiple interlinked diversions and storage structures. If you think you have a problem that may be helped by a point-flow model, give us a call. And if you have a need for a water resources firm for well drilling, groundwater modeling, water rights cases, or more, please reach out to any of the people below and we will be happy to put together a plan for your needs. We help with projects big and small.

Bruce Lytle, President of LWS: bruce@lytlewater.com

Chris Fehn, Senior Project Engineer: chris@lytlewater.com

Anna Elgqvist, Senior Engineer: anna@lytlewater.com