Time Lag Modeling in Modification of Nakayasu Synthetic Unit Hydrograph

Abstract

In the development of the synthetic unit hydrograph method, the empirical formula expresses the relation between the watershed morphometry characteristic and the constituent elements of hydrographs that are peak discharge (Qp), time to peak (tp), time lag (TL), and time base (Tb). This research intends to build a model of time lag in order to modify the Nakayasu SUH modeling. The Nakayasu synthetic unit hydrograph (SUH) model, seen as very popular and widely used by practitioners, needs to be adjusted for the main parameters that form the hydrograph to be able to be directly used based on the watershed morphometry characteristic. Based on the specific characteristic of the Nakayasu SUH formulation, this research will focus on developing the Nakayasu SUH model by the modification with adding the watershed characteristic parameter to the time lag parameter (tL). This research was conducted in 23 watersheds in Indonesia. The previous research in the same watersheds indicates five morphometry parameters with a positive effect on the time lag with the ranking due to the strongest effect as follows: watershed area (A), the river length from outlet to the weight point of the watershed (Lc), H = L.Lc, and the main river length (L). However, the two other parameters negatively affect the time lag with the ranking due to the strongest effect as follows: river network density (D) and river slope (K). Then, the modeling of time lag is carried out based on the research result, and the result of the time lag model is as follows: tL=Lc0.1809H-0.00159A0.003459