Climate impacts on water supply and demand zones in the South Platte

For almost a half century, the Northern Colorado Conservancy District (NCCD) has diverted runoff from the western slope of the Front Range to agricultural and municipal water users on the eastern plains of Colorado. Among other factors, the annual demand for such water is driven by the regional rainfall anomalies over eastern Colorado during the growing season. Although the NCCD reservoirs are large enough to buffer against shortfalls of individual years, the amount of vulnerability of this regional trade between west slope water supplies and east slope water demands against the vagaries of climate is unknown.

Therefore, we analyzed the precipitation co-variability on seasonal time scales over the area managed by the NCCD. Long-term climate records in mountain valleys and eastern plains location are compared against two decades of automated SNOTEL records in the higher elevations of the Front Range. Seasonally varying associations between west slope and east slope precipitation anomalies are diagnosed and related to runoff data. A sizable fraction of this variability is related to ENSO, with a seasonally dependent sign of this association. Nevertheless, the ENSO-precipitation links present the opportunity to assess the potential predictability of water supplies and demands in the NCCD domain.

Another major concern for water management in the NCCD is the occurrence of drought on multi-annual time scales, for which existing infrastructure would be unable to provide the necessary supply buffer. We evaluate the historical risk of long-term drought on both sides of the Continental Divide, and for this purpose are exploiting paleo-climatic reconstructions of annual streamflow in the South Platte Basin that extend back into the 18th century.