Climate-Driven Shifts in the Geographic Dependency of the NRCS Initial Abstraction Ratio

The appropriate selection of the initial abstraction ratio in the Natural Resources Conservation Service (NRCS) curve number method has been debated within the hydrologic engineering community. Building on recent work that introduced the concept of hydrologic conservatism zones to guide the choice of the initial abstraction ratio based on rainfall frequency and geographic location, this paper investigates how projected future climate conditions may shift these zones. Using pilot data from National Oceanographic and Atmospheric Administration (NOAA) Atlas 15 for the State of Montana, future precipitation frequency estimates were analyzed under global thermal index (GTI) scenarios representing global temperature increases ranging from 1.5°C to 5.0°C, focusing on a 24-h storm duration. The results demonstrate how rising temperatures, and corresponding increases in extreme precipitation, are likely to shift the relative proportions of the landscape that produce higher estimates of runoff from an initial abstraction ratio of 0.05 to a ratio of 0.2, particularly in the southwestern and mountainous western United States. The findings presented here show a statewide trend toward an initial abstraction ratio of 0.2 yielding the more conservative runoff estimate with both return period length and GTI scenario. These findings are, to the best of our knowledge, the first of their kind to make use of NOAA Atlas 15 pilot data and foreshadow broader implications across the United States. This work also calculates the return period lengths relative to current values of projected future precipitation depths as a function of return period and GTI.