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This paper examines the accuracy of high-resolution nested mesoscale model simulations of surface climate. The nesting capabilities of the atmospheric fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) were used to create high-resolution, 5-yr climate simulations (from 1 October 1994 through 30 September 1999), starting with a coarse nest of 20 km for the western United States. During this 5-yr period, two finer-resolution nests (5 and 1.7 km) were run over the Yampa River basin in northwestern Colorado. Raw and bias-corrected daily precipitation and maximum and minimum temperature time series from the three MM5 nests were used as input to the U.S. Geological Survey’s distributed hydrologic model [the Precipitation Runoff Modeling System (PRMS)] and were compared with PRMS results using measured climate station data. The distributed capabilities of PRMS were provided by partitioning the Yampa River basin into hydrologic response units (HRUs). In addition to the classic polygon method of HRU definition, HRUs for PRMS were defined based on the three MM5 nests. This resulted in 16 datasets being tested using PRMS. The input datasets were derived using measured station data and raw and bias-corrected MM5 20-, 5-, and 1.7-km output distributed to 1) polygon HRUs and 2) 20-, 5-, and 1.7-km-gridded HRUs, respectively. Each dataset was calibrated independently, using a multiobjective, stepwise automated procedure. Final results showed a general increase in the accuracy of simulated runoff with an increase in HRU resolution. In all steps of the calibration procedure, the station-based simulations of runoff showed higher accuracy than the MM5-based simulations, although the accuracy of MM5 simulations was close to station data for the high-resolution nests. Further work is warranted in identifying the causes of the biases in MM5 local climate simulations and developing methods to remove them. [ABSTRACT FROM AUTHOR]

The 2013 federal Colorado River Basin Water Supply and Demand Study projected the water imbalance between future supply and demand to increase. The Colorado water supply (WS) exemplifies a pronounced quasi-decadal oscillation (QDO) of 10-20 years throughout its historical record; however, this QDO feature is unaccounted for in the climate models used to project the future WS. Adjacent to the Colorado River, the large watershed of the Great Salt Lake (GSL) in Utah records the hydrologicQDOsignal in its water surface, leading previous studies to explore the cause of decadal fluctuations in the lake elevation and assess predictability. This study reports a remarkable coherence between the Colorado WS and the GSL elevation at the 10-20-yr time scale. Analysis of precipitation and terrestrial water storage anomalies suggests a crossbasin connection in the climate and hydrometeorological variations of the Colorado WS and the GSL. The 160-yr-long and well-kept GSL elevation record makes it an effective indicator for the Colorado WS. [ABSTRACT FROM AUTHOR]

Streamfiow in the upper Colorado River in the western USA is always snowmelt dominated, whereas the lower river's perennial streamflows are snowmelt dominated only 50% of the time. The magnitude and timing of peak flows is important for water resources management. In the upper basin the annual maximum daily discharge usually occurs in May or June, and in the lower basin this peak is observed to occur in any month except May or June. The timing of one-half of the specific runoff is used as a second measure of the variability in timing and magnitude of streamflows. For the upper basin, nine watersheds are used to illustrate streamflow trends, with the Yampa River used as a sample sub-basin. For the lower basin, five watersheds are used, of which Salt River is used as sample sub-basin. The differences in monthly flow variation over 20-year time periods (1920-1939, 1940-1959, 1960-1979, and 1980-1999) are substantial for the Salt River but not for the Yampa River. Three model types were used to estimate streamflow characteristics. An autocorrelation model was used to generate winter specific runoffs, which were more reasonable for the Yampa River than the Salt River. A regression between snow water equivalent (SWE) and winter specific runoff showed a good correlation for the two sub-basins. A weaker relationship exists between SWE and non-winter flows for the sample lower basin watershed. Streamfiow was simulated relatively well using the Precipitation Runoff Modeling System hydrological model. [ABSTRACT FROM AUTHOR]

Postfire runoff and erosion are a concern, and more data are needed on the effects of wildfire at the watershed-scale, especially in the Colorado Front Range. The goal of this study was to characterize and compare the streamflow and suspended sediment yield response of two watersheds (Bobcat Gulch and Jug Gulch) after the 2000 Bobcat fire. Bobcat Gulch had several erosion control treatments applied after the fire, including aerial seeding, contour log felling, mulching, and straw wattles. Jug Gulch was partially seeded. Study objectives were to: (1) measure precipitation, streamflow, and sediment yields; (2) assess the effect of rainfall intensity on peak discharges, storm runoff, and sediment yields; (3) evaluate short-term hydrologic recovery. Two months after the fire, a storm with a maximum 30 min rainfall intensity I30 of 42 mm h-1 generated a peak discharge of 3900 l s-1 km-2 in Bobcat Gulch. The same storm produced less than 5 l s-1 km-2 in Jug Gulch, due to less rainfall and the low watershed response. In the second summer, storms with, I30 of 23 mm h-1 and 32 mm h-1 generated peak discharges of 1100 l s-1 km-2 and 1700 l s-1 km-2 in the treated and untreated watersheds respectively. Maximum water yield efficiencies were 10% and 17% respectively, but 18 of the 23 storms returned ≤2% of the rainfall as runoff, effectively obscuring interpretation of the erosion control treatments. I30 explained 86% of the variability in peak discharges, 74% of the variability in storm runoff, and >80% of the variability in sediment yields. Maximum single-storm sediment yields in the second summer were 370 kg ha-1 in the treated watershed and 950 kg ha-1 in the untreated watershed. [ABSTRACT FROM AUTHOR]