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Research Project: Understanding Ecological, Hydrological, and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Quantification of curve number for arid watersheds

Author
item DUAN, J. - University Of Arizona
item ARJMANDI, ALI - University Of Arizona
item CANFIELD, E. - Pima County Flood Control District
item DEMARIA, E. - Pima County Flood Control District
item Goodrich, David
item QI, K. - University Of Arizona

Submitted to: Journal Hydrologic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/9/2024
Publication Date: 5/13/2025
Citation: Duan, J., Arjmandi, A., Canfield, E., Demaria, E., Goodrich, D.C., Qi, K. 2025. Quantification of curve number for arid watersheds. Journal Hydrologic Engineering. 30(1). Article 04024059. https://doi.org/10.1061/JHYEFF.HEENG-6267.
DOI: https://doi.org/10.1061/JHYEFF.HEENG-6267

Interpretive Summary: Heavy rainfall routinely causes runoff in arid regions. Computer models are used to estimate how much runoff is produced by these heavy rains. One method used to estimate the amount of rainfall that is infiltrated into the soils of a watershed is the Curve Number (CN). The physical interpretation of CN is still challenging in arid watersheds. This paper analyzed the historical rainfall and runoff event data from 1975-2021 for two arid watersheds, the Santa Rita and the Lucky Hills experimental watersheds near Tucson, Arizona. The results showed CN varies event by event depending on precipitation, landuse, types and density of vegetation, as well as soil properties. The event-based curve numbers increase with the initial amount of rainfall that is abstracted (rain falling on plants and in small depressions). Assuming the watershed storage is the total infiltrated water for each rainfall event, the CN values can be estimated using the more physically-based Green-Ampt infiltration model. This study applied the Green-Ampt infiltration model to estimate the CN values for all the historical events and these estimated with the back-calculated CN values using observed rainfall and runoff. The (G-A) calculated CN values are relatively close to the back calculated CNs from observations for roughly 70% of the events. This indicated that CN can be calculated using the soil properties and rainfall duration, which sheds light for quantifying the CN value for ungaged watersheds in remote areas.

Technical Abstract: Although the curve number (CN) method has been widely used in rainfall and runoff models, the physical interpretation of CN is still challenging, especially for arid watersheds. This paper analyzed the historical rainfall and runoff event data from 1975-2021 for two arid watersheds, the Santa Rita and the Lucky Hills experimental watersheds in Tucson, Arizona. The results showed CN varies event by event depending on precipitation, landuse, types and density of vegetation, as well as soil properties. The event-based curve numbers increase with the initial abstraction. If assuming the storage as the cumulative infiltrated water in each rainfall event, the CN values can be estimated using an infiltration model. This study applied the Green-Ampt infiltration model to estimate the CN values for all the historical events and compared with the back-calculated CN values using observed rainfall and runoff. We found 72% of the calculated CN values falls in the 90% confidence interval, and 96% are within the 80% confidence interval. This indicated that CN can be calculated using the soil properties and rainfall duration, which sheds light for quantifying the CN value for ungaged watersheds in remote areas.