Quantifying the impacts of climate and land use changes on the hydrological response of a monsoonal catchment

The effect of climate change and land use change on runoff generation and flooding has received great attention in many hydrological modelling studies. However, currently many hydrologists are still uncertain how much these two factors contribute to runoff generation, particularly in monsoon catchments. The river Kelantan is in one of the states in Malaysia, which experiences monsoon flooding, was used to investigate these two factors in effecting hydrologic response changes. Therefore, this study tries to provide a framework mainly to i) identify trends in the River Kelantan streamflow and explore the possible causes of that change, including precipitation change and land use changes; ii) disentangle and quantify the precipitation and land use and change effects on hydrological response and potential flooding in the River Kelantan catchment using past and current hydrological events; iii) simulate the future runoff scenarios (i.e. 2020s, 2050s and 2080s) using precipitation and land use changes projections. Historical data on the streamflow of the River Kelantan and precipitation in the Kelantan catchment were investigated for trends using the Mann-Kendall non-parametric method. In summary, a general pattern has been revealed in which streamflow is increasing in all seasons upstream, but is decreasing in the dry season downstream. The pattern in streamflow downstream is fairly well matched by increases in precipitation in the wet season and decreases in precipitation in the dry season. In the upstream area, the increases in streamflow are not matched by universal increases in precipitation, but rather by increases in the wet season only and decreases in the dry season, as for the downstream sub-catchment. The increases in streamflow in the dry season are, thus, more difficult to explain and land use change been performed and has been proven to cause a partial contribution of such observed trend in the upstream area. Subsequently, a study using the lumped HEC-HMS model to disentangle these two factors in causing hydrologic response changes (i.e. peak discharge and runoff volume) was performed. The results demonstrate that for the upstream area precipitation and land use changes led to the greatest increases in peak discharge and runoff volume. In contrast, in the downstream area the results suggest that precipitation trends may have led to significant increases in runoff generation. The simulation of hydrologic response in the future (i.e. 2020s, 2050s and 2080s) showed that climate change (i.e. precipitation change) has positive links with the peak discharge and runoff volume. If precipitation estimated to decrease using PRECIS A1B storyline from the SRES scenario, runoff was predicted to decrease and vice-versa. For the land use change impact, the scenario involved reducing the forested area, increasing the agricultural and built-up land caused runoff estimated to increase from 2020s to 2080s. The combined scenario demonstrated that precipitation change coupled with land use change has a significant impact to changes in peak discharge and runoff volume for the study area compared to climate change and land use change studies alone