Application of hydrological models to determine of rainfall and run off harvesting system of Singkarak Catchment

Abstract

The biophysics characteristic of Singkarak Catchment including topography, slope, drainage densities, land use and annual rainfall ware analyzed using the Geographycal Information System (GIS) model. The design, dimension and location of rainfall and run off harvesting system were analyzed and determined using MAPDAS model combined with GIS model. The MAPDAS model was also used to develop the model for flood risk prevention and drought impact mitigation. The impact of established rainfall and run off harvesting system were analyzed and developed using MAPDAS with Stella Program. Biophysical analysis showed that the topography of Singkarak Catchment was hilly to mountainous with 30-100% slope; the soils ware dominated by Andosols, Inseptisols and Ultisols, forest cover was only 25%, annual rainfall was 2 800 mm; average run off coefficient was 22%, and it reached up to 45% during wet months; and drainage densities was ranged from 2.64 m ha-1 to 3.8 m ha-1. To prevent flood in Paninggahan watershed it required to harvest about 0.816 mm of rainfall which equivalent to 48 086 m3 of run off. The result of run off and geomorphological instantaneous unit hydrograph analysis showed that about 47- 94 small reservoirs (Embung) which covered about 0.8-1.6 ha areas need to be established in Paninggahan watershed. Which the same purpose Malakotan watershed required to harvest about 2.7 mm of rainfall which equivalent to 189 622 m3 of run off. The resalts of run off and geomorphological instantaneous unit hydrograph analysis showed that about 63-126 small reservoirs (Embung) which covered about 3.2-6.3 ha areas in Malakotan watershed. Impact of rainfall and run off harvesting analysis showed that effective run off models to prevent flood and drought risk was a combination of MAPDAS model and GIS. The location of reservoirs construction, reforestation and afforestation activities should be covered out in the zone within 8-11 km and 13-17 km from the outlet at Paninggahan and Malakotan watersheds respectively. Impact of the established rainfall harvesting system was the stability of the lake water level, with the minimum and maximum water level were 360 and 363 meters asl. Respectively will not disrupt the continuity of water supply for irrigation purpose of Ombilin river and Singkarak hydroelectric power plant.

Faktor biofisik DTA Singkarak dengan topografi yang berbukit sampai bergunung dengan lereng curam, aktifitas pertanian yang intensif, serta curah hujan yang tinggi, mengakibatkan terjadinya debit puncak yang ekstrim, yang disertai tingkat sedimentasi yang tinggi. Untuk itu perlu upaya menurunkan aliran permukaan serta tingkat sedimentasi dengan perbaikan agroteknologi serta sistim panen hujan dan aliran permukaan di DTA Singkarak Oleh sebab itu tujuan penelitian adalah untuk 1) mengkaji karakteristik biofisik, 2) menentukan desain sistem panen hujan dan aliran permukaan berdasarkan aplikasi model hidrologi, 3) mengembangkan model aliran permukaan DAS untuk menahan resiko banjir dan kekeringan, serta 4) mengkaji dampak implementasi sistem panen hujan dan aliran permukaan pada DTA Singkarak. Model hidrologi yang digunakan adalah model debit sesaat model aliran permukaan daerah aliran sungai (MAPDAS) berbasis hidrograf satuan sesaat geomorfologi. Karakteristik biofisik adalah imput dari model MAPDAS, yang dapat ditentukan dengan sistem informasi geografi (SIG) dalam data raster. Analisa sistem panen hujan dan aliran permukaan terdiri atas penentuan ambang batas debit puncak di bendung sebagai volume panen hujan dan aliran permukaan yang juga dilakukan dengan model hidrologi MAPDAS. Analisa lokasi sebaran sistem panen hujan dan aliran permukaan dapat menurunkan debit puncak secara efektif dan signifikan yang memakai konsep hidrologi satuan sesaat geomorfologi (GIUH). Analisa model dinamik dengan program Stella merupakan model yang dapat mengubah parameter sebagai input yang digunakan untuk melihat dampak implementasi sistem panen hujan dan aliran permukaan terhadap tinggi muka air Danau Singkarak.

Sinukaban, Naik