Pop Quiz: What do you need to interpret a streamflow forecast?
A) Knowledge of the basin B) Understanding of climate conditions C) Appreciation for probabilities D) All of the above
Answer: D. All of the above
There’s a lot of data and expertise that goes into predicting Colorado water conditions. One powerful tool used in predicting drought scenarios is the USDA’s Natural Resources Conservation Service’s (NRCS) streamflow forecast. The NRCS provides monthly streamflow forecasts from January until June, or until most of the mountain snow has run off.
Water providers often look to these forecasts to help them determine how drought-like a year will be. These forecasts may influence water decisions like municipal drought responses, how much water to keep in a reservoir, or how much water to lease from other sources.
So, how do you access these forecasts and what else should you consider to predict streamflow and drought conditions?
How to Access NRCS Streamflow Forecasts
The forecasts are accessible to anyone who knows the right place to look. They are published in the USDA’s Basin Data Reports. When visiting the page, select “Colorado” as the state and “Forecast” as the report type.
You can also view maps summarizing basin-wide forecasts for Colorado. You can access current or historical forecasts of both the reports and maps from these dashboards.
Understanding NRCS Streamflow Forecasts
There are several items to consider when interpreting streamflow forecasts. First, realize that the streamflow forecasts for specific gages often represent “unimpaired” streamflows. These are the streamflows that would exist absent diversions, transmountain inflows, and reservoir releases. This is an important distinction to realize when comparing current year forecasts to historical gaged streamflows.
It’s typical to focus on the 50% Chance of Exceedance Forecast, which NRCS estimates has a 50% probability of being exceeded by actual streamflows that year. However, if you’re looking for the lowest streamflow that the current year is likely to produce, the 90% Chance of Exceedance Forecast can provide a useful, and statistically more likely, reference.
And while it’s tempting to make drought predictions as soon as data is available, late winter and spring storms, can strongly influence forecasted streamflows. This is a major reason why early-winter streamflow forecasts can differ significantly from later-spring forecasts.
Additional Considerations for Predicting Drought Scenarios
The NRCS Streamflow Forecasts are a powerful tool, but they aren’t the only data point helpful for understanding future hydrologic conditions. When making water supply decisions based on streamflow forecasts, we recommend reviewing a wider array of indicators, including:
The Standardized Precipitation Index. In the “Select Product” dropdown, choose “Standardized Precipitation Index (SPI)" and select your desired timeframe and region.
The Surface Water Supply Index calculated by the Colorado Division of Water Resources. Check the “Surface Water Supply Index” layer to view.
The Evaporative Demand Drought Index (EDDI) is an experimental drought monitoring and early warning guidance tool that shows promise. EDDI can be mapped over timescales from one week to one year in the EDDI Map Archive tab.
Pairing NRCS’s Streamflow Forecasts with historic knowledge of the basin and climate conditions can provide water providers with critical information they can use to predict drought scenarios and make water resource decisions. If you need assistance with water resource planning or water-related analysis, reach out to a Martin and Wood consultant today!