Proposing an appropriate land use based on hydrological analysis and in accordance with state regulations :: A case study of Samin Watershed Central Java Indonesia

CAHYOLESTARI, Diah Pratiwi, Prof. Dr. H.A. Sudibyakto, M.S

2010 | Tesis | S2 Geo-Information for Spatial Planning and Risk M

Samin Watershed has suffered from flood at the end of December 2007. The land cover change in Samin Watershed is mainly caused by both economic and population growth factors. Farmers are likely to plant the commodity, such as sugarcane plant, that has higher economically value than other plants without taking consideration on soil conservation. The second is population growth which needs more room for settlement and farming. The main objective of this research is to develop a land use assessment model for the Samin Watershed to determine the optimum land use types for overland flow (floods) reduction in accordance with state regulations. In this study, calculation on actual direct surface runoff employs Curve Number Method of US Soil Conservation Service. Data prepared to estimate Curve Numbers are: land cover, soil texture and infiltration rate, daily rainfall data of year 2006, sub-watershed delineation, information on land treatment and/ or practices, and hydrological condition of each land treatment. Sample points for infiltration measurements and soil texture analysis are based on land mapping unit, which has the same land cover and soil type. Runoff depth is calculated for each sub watershed after Curve Numbers are estimated. Estimated runoff using CN Method over Samin Watershed is 1,364.55 milimeters. Method for calculating peak discharge of the watershed uses a synthetic unit hydrographs, used by Soil Conservation Service. The unit hydrograph is determined by factors, i.e. lag time, time to peak, peak discharge, and base time. Watershed characteristics needed are watershed area, maximum length of travel, and watershed‟s slope. Discharge calculation is performed for each sub-watershed, since the calculation of runoff depth is done on sub-watershed. The value of watershed‟s discharge comes from the sum of all sub-watersheds‟ discharge. Discharge calibration with observational data is conducted within several sub-watersheds, since there is only one ungauged discharge station located in the midde of Samin Watershed. Estimated peak discharge in the area is 1,619.68 m³/s, Compare to observational discharge data 2006, estimated peak discharge is 935.18 m³/s, which 125% bigger than observational discharge data. Four land use models are proposed in accordance with state regulations. Models are developed manually, by using different land treatment. Model-1 reduces surface runoff 0.35% compare to runoff 2006. Model-2 and Model-3 decrease runoff up to 0.26% and 0.03% respectively than runoff 2006, while Model-4 reduces surface runoff until 0.41%. Among the four models, Model-4 is the most appropriate land use to reduce surface runoff. Converting upper slope‟s agricultural into woodlands has little significance for decreasing runoff over Upper Samin Sub-watershed. In this area, application of Model-1, Model-2 and Model-3 increases surface runoff, especially in agricultural and sugarcane coverage.

Kata Kunci : surface runoff, hydrological analysis, state regulations, land use model