By Mohamad Ali Mashuri, M.Sc. – (Thesis M.Sc. Unesco-IHE, Delft, Netherland, 2014), Supervised by Prof. Charlotte de Fraiture,PhD, M.Sc, Prof. Robiyanto H. Susanto, PhD, M.Agr.Sc. and F.X. Suryadi, PhD, M.Sc.
Brebes District which has the second largest of its area in Central Java Province has a lot of potential from existing resources, especially for the agriculture and aquaculture sector. By the geographical conditions, Brebes District has an area range from coastal to mountainous areas. From the development of the two sectors above, Brebes District well known as “Shallot City” because of more than 30% of national shallot production is produced from Brebes District, and since 2009, Brebes District is also set to be “Seaweed City”.
Gangsa Sub River Basin is one of the 11 Sub River Basin in Brebes District has an area of 8,000 ha and an average height of 0-5 m+MSL. With the existing natural conditions, especially in the northern area where more than 90% of it land use as fish pond area suffered from flood/inundation problem regularly as result of the high tide from Java Sea and discharge from Sigeleng Main Drainage. In addition to these problems, in this northern area also has wetland/agriculture area where people cultivated it for shallot or rice as their activity. Although they are included in technical irrigation system, they have only 150 for the cropping intensity which is very low. Therefore, some improvements are necessary to deal with those problems. This research was carried out by DUFLOW and AQUACROP modelling. It aims to analyze the existing drainage and flood protection of study area, to discuss the alternative scenarios for flood protection, and to evaluate the yield respond of agricultural commodities through existing irrigation conditions.
The methodology of this research consists of preparation, field work and data acquisition, data analysis and modelling, results analysis, and conclusions and recommendations. Preparation phase mainly consists of literature study by reviewing journals, reports, books, etc. related to the topic. Field work and data acquisition phase includes compiling the spatial data and hydrological data from several offices related to the study, and also collecting primary data through field measurement such as river discharge and water level fluctuation (tide).
Prior to DUFLOW modelling development, model calibration was done and resulted in water level fluctuation that close to the data gained from field measurement, meaning that the DUFLOW modelling is appropriate for further scenarios development. In this study, three specific locations was selected for more discussion in describing hydraulic performance of the water as effecting on flood/inundation problem on those areas, namely first area (transition area of agriculture and aquaculture area), second area (settlement area) and third area (fishpond area located near estuary). Meanwhile, from the existing cropping pattern, climate data and the amount of water delivered from Krasak Secondary Canal, AQUACROP modelling is used to estimate the productivity of agriculture commodities.
In the first area, which is a transition area between the agricultural area with fishpond area, have objectives to maintain the area from flooding or inundation problem, and on the other hand is the improvement of water management to be achieved (increasing cropping intensity). The proposed measures to reach the objectives are by constructing new diversion canal into Gangsa River, install the gate at the end of new diversion canal in order to maintain water level at irrigation canal during dry season, and rehabilitation and improvement of irrigation system.
In the residential area which is the second object, it takes several scenarios to anticipate flood in the area since it is evident that the main factor of inundation that is derived from high tide and discharge of the main drainage during high rainfall. The combinations of these scenarios are among others, to conduct normalization, heightening the dyke, installing the gates at each secondary canal and constructing retention pond.
In the third area (downstream area) which is a pond area, by DUFLOW modelling used to calculate the water circulation into the pond through a secondary canal. From the existing condition (without rainfall) obtained the result that the circulated water by high and low tide reach 3,580 m3 per day or less than 10% of the standard level. Several scenarios applied are installing the gate at the end of the secondary canal in anticipation of the discharge from Sigeleng Main Drainage during high rainfall, and constructing the control structures at tertiary/field level to ensure circulation of the waste water.
AQUACROP modelling gives some evidences that during the first, second, and off season, there is a problem related with the yield reduction due to water stress, either because of excess or lack of water. Meanwhile, at the second planting season, farmers can get the optimum yield for their crop. However, an inadequate drainage system in agriculture area will influence on fishpond area below. Thus, it needs an integrated approach towards irrigation management system and to determine some preferred alternatives of flood mitigation measures, besides technical evaluation, holistic economic and social analysis are necessary to conduct for policy making.
Keywords: Rural drainage, flood modelling, DUFLOW modelling, flood protection, AQUACROP modelling, integrated water management system