Snow to Streamflow: A Story of Decline [Part 2]

In snowy regions like the Henry’s Fork, the amount of water held in the snowpack on April 1st can help predict summer streamflow. But over the past two decades, the relationship between the water stored in spring snowpack and summer streamflow has changed, making summer streamflow less predictable. New research by HFF and Idaho State University scientists show how we can improve our summer streamflow predictions by better understanding when snowmelt and rain enter the ground relative to the forest growing season.

Key Takeaways:

Why is it important to be able to accurately predict water supply?

Part 1 discussed how warmer temperatures and changing forest dynamics are contributing to water supply decline in the Henry’s Fork. At the Henry’s Fork Foundation, if we want to reduce drawdown of Island Park Reservoir and increase winter flows to grow more trout, we need to be able to accurately predict water supply. Understanding watershed processes are key for those predictions—but these processes are changing and impacting the quality of HFF’s predictions.

How do watershed processes impact water supply predictions?

Since the severe drought of the early 2000s, less water from spring snowpack is turning into streamflow, leading traditional approaches to over-predict summer streamflow. This poses the risk of us going into the summer thinking we have more water than we actually do. Predicting this change presents a new challenge, since most approaches do not account for changes in forest cover or evapotranspiration (ET; how much water evaporates or is used by plants).

Precipitation becomes streamflow more effectively when incoming snowmelt and rainwater exceeds demand (potential evapotranspiration or PET). Potential evapotranspiration, or PET, simply has to do with how much water would evaporate if there was no limit to how much water was available. In the Henry’s Fork, high snowmelt and springtime rain lead to peak water inputs in the late spring. Demand (PET) has historically been lower during the early spring months. As a result, streamflow is higher during spring months when supply is high, and demand is low. We call this the ‘recharge period,’ because snowmelt and rain are recharging or adding more water to the system than is being taken out by ET. Changes to the recharge period will affect how much water becomes streamflow.

Changes in the recharge period can be due to:

What has HFF’s research shown?

To better understand the impact of a changing recharge period, HFF research presented a new unit of measurement, which we call the Vegetation-Water Alignment Index (VWA). The VWA calculates how water supply aligns with the growing season. The VWA helps us identify:

Created by Sarah Newcomb

We found that streamflow is lower after years when more water entered the ground during the forest growing season (smaller recharge period) instead of before the growing season. Since the VWA provides us with information about future streamflow, it can be used to help us improve our streamflow predictions from year-to-year.

One reason predicting streamflow is challenging is due to the limited information known on April 1st, when most summer predictions are made. When applied to the major tributaries of the Henry’s Fork, the Vegetation-Water Alignment metric improved summer streamflow predictions in each subwatershed. Specifically, we found that water input and growing season alignment is particularly important in the low-elevation forests surrounding Island Park.

Check out the peer-reviewed scientific publication in Hydrologic Processes in June 2024 at https://onlinelibrary.wiley.com/doi/abs/10.1002/hyp.15211. If you do not have access via a subscription or library, please contact Rob Van Kirk (rob@henrysfork.org) to request a copy.