Transit loss in a river reach is generally defined as the decrease in flow rate between two points of the reach. The losses occur due to various factors including infiltration and seepage into groundwater, evaporation, transpiration by riparian vegetation, unmeasured diversions, and other hydrological processes. Thus, transit loss is a complex physical process (or combination of processes) which is not consistently defined across all relevant contexts and applications. 

Common applications for estimating transit loss include: 

1. Determining appropriate reservoir release volumes to meet downstream obligations

2. Estimating impacts of loss reduction strategies (e.g., phreatophyte removal)

3. Projecting changes in transit losses under climate change scenarios

4. Quantifying losses in conserved water programs between source and destination 

The last application is applicable for the UCRAF RiverWare models and required a methodology for quantifying water from a conserved water program lost during transit. This question is increasingly asked of water managers in the western United States as the need for conservation programs and associated policy development is constantly growing. While important and necessary for this application, transit loss is difficult to precisely quantify and administer in practice. The approach developed for the UCRAF models quantifies the incremental increase in transit losses attributable to conserved water as it moves through a river system rather than attempting to directly measure the physical absolute losses.

Under baseline conditions, system water in a river reach is subject to established hydrological patterns of gains and losses. When conserved water is introduced “on top” of system water in a river reach, an incremental increase in losses (or an incremental decrease in gains) occurs. A fundamental challenge with quantifying these incremental changes as transit losses is that the magnitude of associated water volumes is often impossible to measure directly. Conserved water volumes are typically less than gage uncertainty which is estimated to be ±8% for well-rated USGS gages. The proposed methodology reframes the quantification approach from direct measurement to incremental impact assessment by asking the question: 

“In a specific river reach, how much less System Water makes it downstream in the presence of Conserved Water?” 

Intrinsic to this question is an acknowledgement that the amount of system water reaching a downstream destination is reduced when conserved water is added to the system regardless of whether a reach is in a gaining or losing hydrologic condition. This allows for an accounting exchange from conserved water to system water which prevents negative impacts to system water due to the presence of conserved water.

The method of estimating incremental transit loss uses channel geometry to develop a relationship between flow and wetted perimeter for a given river reach then quantifies incremental loss proportional to the change in this relationship given the presence of conserved water flowing along with system water. Therefore, the proposed method requires physical characteristics of the river reach through which conserved water flows. 

We leverage existing methods of estimating bathymetry from digital elevation models and streamflow data (Follum et al., 2013, 2017, 2020, 2023; Gutenson et al., 2025) to estimate physical characteristics of river reaches. These methods were originally developed for flood forecast systems and military mobility and can be applied to any river in the United States.

The following data is required (either input or developed) for each river reach in the system to implement the proposed method: amount of conserved water at a designated upstream location to begin charging transit loss (e.g., the program source), reach length, loss rate versus flow (percent loss per distance), and average wetted perimeter versus flow. Implementation in RiverWare assumes a single cross section at each gage and that channel geometry is constant between gages.  

TL = BL × WPI

Where:

TL = Total Loss (flow)

BL = Base Loss (flow)

WPI = Wetted Perimeter Increase (ratio)

Example:

TL = 50 cfs × 1.025 = 51.25 cfs

IL = TL - BL

Where:

IL = Incremental Loss (flow)

TL = Total Loss (flow)

BL = Base Loss (flow)

Example:

IL = 51.25 cfs - 50 cfs = 1.25 cfs

CW(Out) = CW(In) - IL SW(Out) = TW - CW(Out)

Where:

CW(Out) = Conserved Water at outflow (flow)

CW(In) = Conserved Water at inflow (flow)

IL = Incremental Loss (flow)

SW(Out) = System Water at outflow (flow)

TW = Total Water at outflow (measured at gage) (flow)

The SW(Out) and CW(Out) amounts then become the inflow to the next downstream river reach.

Example:

CW(Out) = 100 cfs - 1.25 cfs = 98.75 cfs

Outflow Gage = 2,500 cfs

CW(Out) = 98.75 cfs

SW(Out) = 2,500 cfs - 98.75 cfs = 2,401.25 cfs

The incremental transit loss estimation method described here provides a practical approach for quantifying transit losses specific to conserved water programs focusing on the incremental impact of conserved water on transit losses in a river reach. This method is effective in accounting for conserved water regardless of gage readings or how small the conserved water flow is relative to system and total flow and is easily implemented in a RiverWare. By accounting for conserved water’s incremental impact, the approach ensures that the remaining water is unaffected by the presence of conserved water.

Future work can improve upon the method through further study and fine-tuning of physical parameters. For example, the method would be improved with more accurate estimates of gradations in channel geometry instead of assuming a constant channel cross-section over a river reach. While there is room for improvement, the baseline method offers a technically sound and administratively feasible approach to transit loss calculation and accounting.