Flood frequency analysis and a regional peak streamflow correction factor model for Kashkadarya Region, Uzbekistan

Floods are the most common natural hazard in the Republic of Uzbekistan and cause loss of life for humans and livestock, damage infrastructure, destroy or impair habitats, and disrupt the economy. To inform infrastructure design and water management scenarios, statistical flood frequency analyses are performed. Ideally, statistical flood frequency analyses are based on instantaneous annual peak streamflows because peak streamflows are causative to maximum flood inundation surfaces. When instantaneous annual peak streamflows are not available, such as the historical hydrologic data portfolio in Uzbekistan, the largest daily mean streamflow becomes the surrogate for the annual peak. These 1-day annual maxima are usually an underestimation of the true peak streamflow for the year, particularly in a region where flood hydrograph durations are short and flashy. This problem in hydrologic risk analysis is exemplified in the Kashkadarya Region of Uzbekistan where long-term (50+ years) daily mean streamflow data exist, but digitized streamflow data is limited to 1991 to present at ten streamgages. Given that instantaneous peaks are not available for the Uzbek streamgages, a correction factor was calculated based on 3,466 station-years of daily mean streamflow and peak streamflows at 185 streamgages in, New Mexico, USA. New Mexico was chosen because it is a comparatively data-rich region with somewhat analogous topography and precipitation to Kashkadarya, Uzbekistan. The analysis showed that on average, instantaneous annual peaks were 38% higher than annual daily maxima. A regional statistical model was made using basin characteristics as explanatory variables to estimate an adjustment factor to increase flood streamflows based on the annual daily maxima. The modeled adjustment factor was then applied to annual exceedance probability streamflows from a flood frequency analysis performed at the ten streamgages in the Kashkadarya Region. The frequency analysis was performed using generalized extreme value probability distribution on daily streamflows from 1992 to 2020.