3.3 Parameter Estimation
The observed event-based runoff in the XDG watersheds were divided into two groups by plots: the datasets of cropland and one grassland plot were used for calibration and the remaining datasets of another grassland plot were used for validation. All the parameters used in the test methods were optimized using the Marquardt (1963) algorithm for solving constrained least squares problems. In order to examine the applicability of the models, λ=0.2 assigned in its original development is adopted for the original SCS-CN, Huang et al.(2006), Huang et al.(2007) and the proposed method (Method 1), and the slope parameters (a1 and a2) obtained from Huang et al.(2006) were also used for Huang et al. (2006) and the proposed method (Method 1). Moreover, the effect of initial abstraction and slope on the proposed method were tested with the optimized λ (Method 2) and both optimized λ and slope parameters (Method 3).