Design and Construction of a Hillslope-Scale Rainfall-Runoff Simulator in Phoenix, Arizona

As one of the fastest growing cities in the Sonoran Desert, the Phoenix metropolitan area is positioned to lead the way for interdisciplinary investigations of weather extremes, and the associated challenges related to the urbanization of a dryland region. In particular, rainfall-induced erosion in this region is responsible for significant infrastructure and property damage, and potential harm to humans in the urban setting. The cost of rebuilding infrastructure and the potential for reducing future flood-related catastrophes prioritizes the need for physically-based research focused on the relationships between rainfall and erosion in the region. A hillslope-scale rainfall simulator (Norton Ladder Type) and soil test bed experimental apparatus is currently under construction at the Center for Bio-mediated and Bio-inspired Soils Field Lab (CBBG-SFL) at Arizona State University’s Polytechnic Campus in Mesa, Arizona. The rainfall simulator will produce synthetic storms across a wide range of intensities that cover a minimum effective area of 40 feet long by 8 feet wide. The water used in rainfall simulations is filtered and treated by a reverse osmosis system. A hydraulically operated tipper is utilized to vary the slope gradient of the soil test bed from horizontal to approximately 1.5:1 (33.7°). Soil in the testbed area (1 foot depth) may also be varied to meet specific research criteria or the requirements from industry partnerships. A sediment collection system is designed to capture, measure, and store all runoff and baseflow throughout the test duration from the soil test bed. The diversity of rainfall events from summer storms (July-September) to winter storms (November to March) offers a unique opportunity to characterize natural precipitation across this region from direct measurements of the drop size distribution (DSD) and calculated kinetic energy (KE) and to quantify their relation to erosion. Site-specific meteorological data is collected including wind speed and direction, air temperature and relative humidity, solar radiation, soil moisture and temperature to a depth of 30 cm, and three precipitation sensors including two tipping bucket rain gauges and one laser-type disdrometer to determine appropriate testing periods in conformance to the American Society for Testing and Materials standard test method D6459-15. Analyses of the experimental results will be focused on quantifying the hillslope-scale variability in soil moisture, sediment transport, and effectiveness of engineered solutions to erosion hazards in the region. The design and development of an experimental rainfall-runoff simulator set in Phoenix presents challenges that are unique to erosion research. Simulated rainfall is subject to high air temperatures and winds which will disturb and modify the DSD across all intensities. Variability in the DSD coupled with eolian deposition may contribute to variability in runoff sediment concentrations from the test bed area. Collection and consideration of the unique properties of natural storm events in this region is tantamount to producing synthetic storms in an experimental facility that aims to accurately represent rainfall-runoff processes in a dryland region. Initial calibration and conformance testing will be done to quantify the variables of greatest concern. Subsequent conformance simulations are done to facilitate meaningful comparative analyses to simulations carried out by other facilities as well as ensure repeatability and confidence in produced results.