Modelling the influence of irrigated terraces on the hydrological response of a small basin

A geomorphologic-unit-hydrograph based model (H2U) was used to simulate the discharge of the Kali Garang river (Central Java, Indonesia). Even if the peak discharge and concentration time fitted well the observed data, the base time was systematically underestimated. Assuming that the terraced areas delayed part of the streamflow, we first developed a hydraulic model, based on the Cascaded Reservoirs Law, to simulate the outflow of a system of n terraces. This model was calibrated with observed data collected in a three-terrace-system monitoring. Afterwards, the digitized hydrographic network was divided into 2 components: terraced and non-terraced areas. Within the latter, the H2U model simulated directly the transfer to the outlet and it was assumed that the given peak discharge was used to calibrate the global Runoff Coefficient (kr) of the catchment. Concerning the terraced areas, the delayed outflow computed by the hydraulic model was then routed to the H2U model. The total discharge was given by the sum of the convolutions. The method was applied on the Banyumanik sub-basin (75 km2). The results fit well the observed discharge, even the descending limb. But some more assumptions are needed concerning the way farmers are managing the water within the terraces or the way to assess the average values of the parameters for the entire basin.