A method to measure hydrologic restoration attributed to low impact development by computing similarity between runoff frequency spectra

Abstract

Low impact development is to restore natural hydrology in the urban environment. Thus, hydrologic characterization and hydrologic restoration measurement are crucial to low impact development research. The present study uses runoff frequency spectrum to present years of daily runoff events for characterizing flow regime, and develops a method to quantify similarity between runoff frequency spectra for measuring hydrologic restoration. The method was applied to case study of rainwater harvesting of a roof across five climate patterns with annual precipitation from 465.1 to 1807.9 mm/y. 26–30 years of daily runoff data were obtained by continuous mass balance simulation at daily time step that considered stochastic end-use water demand/usage of tanks. For comparison, similarity of total runoff amounts (St) and similarity of runoff frequency spectra (Sd) between undeveloped and developed scenarios were computed to measure hydrologic restoration. Maximum St = 1 was found to be always achievable and only attained by a single rainwater tank size. Maximum Sd = 0.34–0.78, which could be achieved by a size range larger than that with St = 1. Compared with natural condition, total cumulative frequencies of rainwater tank with maximum Sd were declined by 59.2–84.3%. The proposed method solves the problem of quantitative comparison of runoff frequency spectra with different frequencies and lengths, which has advantages of: 1) quantifying the extent of hydrologic restoration at daily temporal resolution, 2) finding a trade-off between reduction of roof runoff intensity and maintenance of natural runoff frequency, and 3) supporting decision-making on strategies to restore natural flow regime in the urban environment as much as possible.