EVALUATION OF RAINFALL-SURFACE RUNOFF MODEL FOR SIMULATING THE IMPACT OF CLIMATE CHANGE ON THE CITARUM WATERSHED'S BASEFLOW

• Main Author: Diaz Primadita, Brigita
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/71281
• Institution: Institut Teknologi Bandung

Several studies have shown that the Citarum Watershed is affected by climate change. This is proved by decrease in baseflow volume per total flow and a decrease in the dependable flow (Q80) over the past three decades. Decreasing baseflow will have a very vital impact, considering that the Citarum Watershed supplying Java-Bali electricity, irrigation, raw water, and industrial needs in West Java and DKI Jakarta. Currently, a mutually agreed climate change projection is available, namely the General Circulation Model (GCM), but unfortunately most of the widely used hydrological models do not really suit the characteristics of the Indonesian watersheds. In addition, studies related to baseflow modelling are still rarely carried out compared to flood modelling. In Indonesia, the main executor in formulation and implementation of water resources policies is the Ministry of Public Works and Public Housing. Therefore, it is necessary to evaluate the general and popular rain-flow model used within the Ministry by aiming to simulate climate change impact on baseflow. This study also examines spatial and temporal changes in rainfall distribution in the Citarum Watershed and projections of baseflow in the upper and middle parts of the Citarum Watershed during 2006-2045 using climate change scenarios. The study evaluates six popular runoff-rainfall lumped models (Mock, NRECA, HBV96, NAM, Sacramento, Empiric) to simulate the impact of climate change on baseflow in the Citarum Watershed, with seven CORDEX GCM AR-5 RCP 8.5 rainfall projection models (CNRM CM5, CNRM RCA, CNRM v2 RegCM, CSIRO MK3,6, EC EARTH, GFDL ESM, and IPSL). Separation of the baseflow from the total stream of observations is obtained with the Recursive Digital Filter Lyne and Holick one filter. The rainfall projection models are bias corrected using the Quantile Mapping method per quarter based on the seasonal characteristic of the study location. Then Standardized Precipitation Index and Standardized Runoff Index methods are used for analysing the changing conditions of rainfall and discharge use. Projections of the impact of climate change on the distribution of rain in the Citarum Watershed show that the distribution of rain in 2006-2015 tends to be normal, then tends to be wetter evenly by 55% in 2016-2025, to be wetter twice as much in 2026-2035, especially in the middle and downstream part, and much drier up to three times in 2036-2045 in almost the entire Citarum Watershed. The topography of the watershed tends to be contoured, especially in the upper and middle parts, causing weather phenomena at the study site to be heavily influenced by local effects, this is what causes the response to the effects of climate change in each sub-watershed to be different, as seen in two studied sub-watersheds. In 2016-2025, the Nanjung and Cisokan Sub-Watersheds are projected to be in a wet-prone status, but in 2026-2035 the Nanjung Sub- Watershed is in a dry-prone status and the Cisokan Sub- Watershed is iv prone to being wet, while in 2036-2045 the Nanjung Sub-Watershed is very prone to dryness and the Cisokan Sub- Watershed fluctuates with conditions prone to dryness and wetness. The Mock model is recommended to simulate baseflow by showing the most stable results, namely a correlation of 58% in the Cisokan Sub-Watershed and 51% in the Nanjung Sub-Watershed and the RMSE is the smallest compared to the other five models. The Mock model is also relatively easy to work on with fewer parameters and assumptions than other models, is open access, and is already included in the BPSDM Training Module so it is relatively familiar within the Ministry of Public Works and Housing of Indonesia. In the Nanjung Sub-Watershed, in 2016-2025 it is projected that the average baseflow will increase, especially during the transitional season with the frequency of baseflow above normal increasing 1.4 times. In 2026-2035 the average base flow is projected to decrease, especially in March April May (MAM) and the frequency of below normal baseflow will double. Whereas in 2036-2045: the average base flow is projected to drop drastically throughout the year with the frequency of base flow below normal increasing up to 2.3 times. In the Cisokan Sub-Watershed, in 2016-2025 it is projected that the average baseflow will increase with the frequency of baseflow above normal increasing 10 times. In 2026-2035 the average baseflow will increase, especially in December January February (DJF) and September October November (SON), with the frequency of baseflow above normal increasing up to five times. For 2036-2045 baseflow is projected to be more volatile, where the average base flow will decrease in MAM and June July August (JJA) but will increase in DJF and SON. This research shows that the relatively easy, accessible, and familiar, Mock Model can be efficiently used in projecting the impact of climate change on baseflow. It is hoped that with this research, more climate change projections, especially for baseflow analysis, can be carried out with higher spatial resolution, at least sub-watershed scale, considering that the meteorological and climatological responses to increasing Greenhouse Gases in one area can vary.