August 20, 2010

What a wild ride this growing season has been. Excessive moisture dominated the state during the first two months. This trend continued for much of western Nebraska during the past four to six weeks, while much of south central and southeast Nebraska experienced less than 50% of normal moisture. Coupled with temperatures consistently in the 90s, with an occasional 100 F plus reading, dryland crop stress widely materialized during the past two weeks.

The recent dry spell across south central and southeast Nebraska has led the U.S. Drought Monitor authors to upgrade the region to D0 (abnormally dry) based on data through 7 a.m. Tuesday, August 17. (Precipitation that fell later on Tuesday will be considered in the next assessment.) This categorization was further strengthened by the fact that the Standardized Precipitation Index placed the average return period of this recent event as something experienced every three to five years.

Precipitation Forecast

If short range weather models prove true, this upgrade to an abnormally dry conditions will be short-lived. Crops are rapidly progressing toward maturity in the impacted areas and water demands will drop off rapidly during the next three weeks. The corn crop is three to four weeks ahead of last year’s crop across southern Nebraska and two weeks ahead of last year across extreme northern Nebraska. Decent moisture chances exist for Aug. 20, Aug. 24, and during the Aug. 27-Aug. 31 period, with a potential for heavy rainfall Aug. 28-29.

Reservoirs at or Near Capacity

The continued moisture across western Nebraska has been a pleasant surprise. Reservoirs on the Platte watershed had completely filled by the end of May with the Wyoming lakes well into their flood pools. Low irrigation demand has kept the reservoirs unusually high for this time of year and streamflow rates continue to average triple their normal rates.

As of Aug. 18, Lake McConaughy was at 1.568 million acre-feet or 90.0% of capacity. Upstream, Glendo Reservoir was at 83.7% of capacity, Pathfinder Reservoir was at 84.5%, and Seminoe Reservoir was at 91.1% of capacity. In other words, it would only take 507,000 acre-feet of water to fill all four reservoirs. Federal regulations require that these reservoirs remain below 90% of full pool from October through March to minimize the impact of wave action on the dam facing.

Implications for Streamflows and Lowland Flooding

Because irrigation demand has been lacking, regulators are facing a real quandary as we approach the beginning of the snowpack season. Regulators don=t want to release too much water early and throw water away if snowfall is poor. However, another above normal snowpack this winter will have them scrambling to make room for the inflows. Even in a poor snow season, at least 600,000 acre-feet of water will be captured by the reservoirs during the spring runoff.

What does all of this mean? Streamflow rates are likely to remain above normal for at least the next 30 days and any water entering Lake McConaughy will be equalized through releases. Depending on how the Bureau of Reclamation approaches this upcoming winter, streamflow rates downstream of Glendo may continue to run over 2,000 cubic feet per second (300% of normal). If rainfall is above normal, these flows could be significantly higher.

Even with normal snowfall this winter, all four reservoirs will likely reach full pool and high streamflows will continue into next May. There is a high probability that lowland flooding will return to areas west of Lake McConaughy this spring. Flooding downstream of Lake McConaughy will depend on the extent and intensity of rainfall events coupled with water releases.

Al Dutcher
State Climatologist, Lincoln