Estimating Urban Stormwater Generation Under Future Land Cover Scenarios

Abstract

Over 80% of the U.S. population currently lives in urban areas, and this is projected to rise to over 90% by 2050. As population growth fuels urban expansion, stormwater runoff is expected to intensify, especially in arid regions, yet the hydrologic consequences of changing land cover remain underexamined. This study quantifies present and future stormwater runoff in urban areas across Arizona’s six Active Management Areas (AMAs) using Geographic Information Systems (GIS), land cover data from the National Land Cover Database (NLCD) and Integrated Climate Land Use Scenarios (ICLUS), and the Soil Conservation Service Curve Number (SCS-CN) method with event based design storms from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14. Runoff was estimated for 1-hour and 12-hour durations under 1-, 10-, and 100-year return intervals, comparing 2021 baseline land cover conditions to low (SSP2) and high (SSP5) land cover projections through 2100. Results show the watersheds of Pinnacle Peak–Paradise Valley and North Branch Santa Cruz Wash are projected to experience the most substantial increases in urban development, 93 km² and 89 km², respectively, placing them in the 99th percentile of urban growth across all decades and scenarios. These changes are also reflected in projected runoff volume, where the mean runoff across all AMAs increases from approximately 30 ac-ft in the 1-hr, 1-yr storms to over 1,300 ac-ft in the 100-yr storms, and from 185 ac-ft to more than 2,300 ac-ft for 12-hr storms. The highest projected runoff occurs in the Santa Cruz and Tucson AMAS, exceeding 4,500 ac-ft and 2,500 ac-ft, respectively, under the 12-hr, 100-yr storm. Watersheds such as Pinnacle Peak-Paradise Valley (~4,800 ac-ft), City of Phoenix-Salt River (~4,600 ac-ft), and Painted Rock Reservoir-Gila River (~4,500 ac-ft) consistently produced the greatest watershed-scale runoff through 2100. These findings underscore the importance of proactive, spatially targeted stormwater management strategies that account for both the magnitude and variability of future urban expansion.