What determines a drought?

After the past few years of drought in Idaho, the wet winter was a welcome relief for many Idahoans. However, according to the U.S. Drought Monitor map, dry conditions still linger in northern Idaho. With 40% of Idaho lands experiencing drought conditions this summer, you may be wondering how drought maps are decided. The USDM map classifies drought conditions under six intensity categories. Normal indicates a lack of drought, while the abnormally dry (DO) category shows areas where drought may be starting or ending. The additional four categories indicate the severity of drought and range from moderate (D1) to exceptional drought (D4). Northern Idaho and surrounding counties in Washington, Oregon, and western Montana are experiencing abnormally dry to severe drought conditions this year.

Twice a month, a team of Idahoans meet to evaluate drought conditions in Idaho. These hydrologists, meteorologists, water district and irrigation canal managers track how environmental conditions like snowpack, precipitation, soil moisture, and streamflow are trending across the state. Every state has a team of experts that evaluate drought-related data at the county and state levels to provide local feedback on changing conditions to the U.S. Drought Monitor authors.

These recommendations are pooled by USDM authors who have the Herculean task to redraw the official national drought map each week after reviewing hundreds of maps, datasets, drought impact reports, and local insights. The USDM map does not directly trigger political actions, but decision-makers use it to determine drought responses, eligibility for drought relief programs and whether to declare drought emergencies.

So why did most of central and southern Idaho see drought conditions end while northern Idaho remains in drought? Conditions in northern Idaho started diverging from the rest of the state as early as November when the first big rain and snow events occurred. These November storms brought more moisture to watersheds south of the Salmon River Basin with that pattern continuing throughout the winter. Our third La Niña winter differed from the previous two with well above normal snowfall in central and southern Idaho and the least amount of snow accumulating farther north. By the end of winter, snowpack peaked at 113 to 160% of normal in watersheds south of the Clearwater River Basin, providing the promise of enough water to overcome the multiple years of drought. In northern Idaho, snowpack peaked at only 88% of normal in the Kootenai-Pend Oreille watershed and reached near normal conditions in the Coeur d’Alene, St. Joe and Clearwater River watersheds. Although May and June precipitation were near normal, by spring a large enough precipitation deficit had grown in northern Idaho to prevent overcoming the preceding years of drier-than-normal conditions.

With the snowpack providing approximately 75% of the surface water flowing in our rivers each year, the health of each winter’s snowpack plays a major role in the Idaho drought equation. How much snow fell is only part of the story of whether Idaho feels water rich or water poor during the summer. Soil moisture, groundwater conditions, and how quickly the snowpack melts influence how much water reaches the rivers, lakes and reservoirs. Dry soils act like a sponge, soaking up the snowmelt. The slower the snowpack melts, the more runoff seeps into the soil and less water runs directly into streams, making streamflow another important drought indicator. This year, warm temperatures in May rapidly melted the snowpack and filled depleted reservoirs and lakes. While a rapid spring runoff was good for replenishing reservoir water storage, streamflow levels are now below normal in North Idaho. Without a lot of snow remaining at high elevations to feed the rivers during the remaining summer months, we will continue to see low water levels in streams. NOAA’s Climate Prediction Center forecasts that drought will continue to persist this summer in North Idaho.

Visit https://droughtmonitor.unl.edu/ to find up-to-date drought information for your area.

Erin Whorton is an NRCS water supply specialist.