Application Of Soil And Water Conservation Technology In Water Resource Management Of Kaligarang Watershed, Central Java

Abstract

Kaligarang watershed had a significant role as a major supplier of drinking water for Semarang city. Land use of Kaligarang watershed wich was dominantly used for dry land and mixed crops dry land farming (59.71%), and only a small part for forest (11.92%), without adequate soil and water conservation techniques had significanly decreased the watershed hydrological functions. This conditions were indicated by the increasing of direct runoff, fluctuations of rivers discharge (floods in rainy and drought in dry season) and in turn decreasing of water availability. On the other hand, the increasing of population and economic development in Semarang city had caused a significant increase of needed potable water. The decreasing of water availability and the increasing of water demand had significanly increased water deficit, especially in the dry season. The application of adequate soil and water conservation technologies were badly needed to solve the problem sustainably. Sustainable water resource management was considered achievable when the participation of commonly increased through practicing adequate soil and water conservation technique and/or paying the cost of the technologies. The aims of this study were (a) to predict the effect of land use on hydrological function of Kaligarang watershed; (b) to predict the demand and availability of water for Semarang city, as well as water stakeholders WTP for Kaligarang watershed management; (c) to analyze the effect of soil and water conservation practices on hydrological characteristics of Kaligarang watershed; and (d) to formulate alternatives recommendations of soil and water conservation technologies far in the development of water resources of Kaligarang watershed. The prediction of land use effect on hydrological condition were carried out using linier regression analysis, and prediction of willingness to pay for Kaligarang watershed management were conducted using WTP method. The erosion prediction value of each development scenario predicted using USLE method (Weischmeier and Smith, 1978) and the volume of daily surface runoff were predicted using SCS method (US-SCS 1973 in Arsyad 2006). Calculation of surface runoff and erosion predictions were carried out using Soil and Water Assessment Tool (SWAT) hydrological model. The development plan of sustainable water resources of Kaligarang watershed was arranged in four scenarios. The best scenario of water resource managements were determined based on ecological, social, and economical considerations. The ecological indicators were base on predicted erosion value, river regime coefficient and direct surface runoff coefficient. Sosial indicator were based on community acceptability and economical indicator were determined based on the percentage of water user respondents WTP and the amount WTP for improvement cost. In addition to ecological, social and economical considerations, the alternative development scenario determined based on the most rational implementation time. Sustainable water resource management plan formulated by applying soil and water conservation practices on dry land farming, mixed dry land farming and resettlement. The result showed that the land use change of Kaligarang watershed (2000 – 2013) resulted in the increasing direct runoff coefficient, maximum daily discharge and the decreasing of the daily minimum discharge, as well as the increasing of the river discharge fluctuation. The total willingness to pay value for the rehabilitation of Kaligarang watershed was Rp 6.09 billion/year, which were derived from domestic activities (56.7%), industrial activities (33.5%), commercial activities (5.7%) and public fasilities (4.1%). Soil and water conservation practice could reduce the river regime coefficient value from 20.2 (the existing condition or 1st scenario) to 17.4 (2nd scenario), 16.2 (3rd scenario) and 16.5 (4th scenario), lower the direct surface runoff coefficient from the 1st scenario (40.5%) to 35.0% (2nd scenario), 33.8% (3rd scenario) and 33.4% (4th scenario). Application of soil and water conservation techniques had been able to reduce erosion prediction to less than tolerable soil loss (62.4 tons/ha/year), which was from 324.25 tons/ha/year of 1st scenario to 31.53 tons/ha/year of 2nd scenario, 32.69 tons/ha/year of 3rd scenario, 31.67 tons/ha/year of 4th scenario, increase the baseflow from from 218.7 mm (1st scenario) to 325 mm (2nd scenario), 445.0 mm (3rd scenario) and 557.4 mm (4th scenario). Monthly water yields showed the increasing during dry season and decline during wet season, which can use an indication of the increasing of water resource availability. The best scenario of technology soil and water conservation to improve water availability DAS Kaligarang was 4th scenario (planting strip croping combined with retention pond on <8% slope, gulud terracing combined with contour planting and mulching crop residues 6 tons/ha/year on 8 – 25% slope, application of agrosilvopastural or agrosilvicultural agrotechnology models on > 25% slope in PLK and PLKC land use and construction of retention ponds in settlements on <15% slope). Scenario-4 was able to hold the erosion prediction down to 31.67 tons/ha/year (Etol = 62.4 tons/ha/year), the river regime coefficient down to 16.5, the direct surface runoff coefficient down to 33.4%, increase the baseflow up to 557.4 mm/year and land improvement time was 23 years.