As a main part of hydrologic losses, infiltration plays an important role in runoff estimation, which can further serve as design criteria and flood forecast information. Among various infiltration models, the initial abstraction and constant loss method (IA&CL) is widely applied based on a concept that any watershed can be assumed to store an absolute depth of rainfall at the beginning of the rainfall as initial abstraction (IA) and then reduce the rainfall rate at a constant loss (CL) rate. Due to the simplicity and the lack of physical equivalent properties, conceptual methods like IA&CL are often subject to issues in parameterization/calibration. Therefore, the UTA team is tasked by the USACE–Fort Worth District to better evaluate these initial and constant loss rates that have been observed to occur during rare or extreme precipitation events in Texas.

The UTA research team was particularly tasked to evaluate and correlate the initial and constant losses with soil types, development of storage in the basins, and rare frequency peak discharges (i.e. 2%, 1%, and 0.2% annual chance exceedance) for a list of watersheds that the USACE-Fort Worth District has already developed models for. This loss rate study would help USACE gain a better knowledge in understanding how hydrologic responses react to initial and constant losses during extreme events by completing the following tasks:

A storm catalog with at least 50 observed storms in Texas that have precipitation depths greater than 10 inches in 24 hours.

  • A sensitivity analysis of hydrologic loss rate parameters used in the current hydrologic models for the selected watersheds.
  • Calibration of loss parameters using extreme storms from the storm catalog.
  • Evaluation of antecedent soil moisture and watershed storage for the selected events.
  • Incorporation of soil types, basin storage and rare frequency peak flow into initial and constant losses parameters based on available information.