OPTIMASI PEMANFAATAN SUMBERDAYA AIR WADUK PANDANDURI DI LOMBOK TIMUR NUSA TENGGARA BARAT

Abdul Aziz, Dr. Ir. Rachmad Jayadi, M.Eng

2012 | Tesis | S2 Mag.Pengl.Sumberdaya Air

Suplai air dari Jaringan Interkoneksi Lombok Selatan belum mampu memenuhi kebutuhan air sebagian daerah irigasi di Lombok Selatan secara optimal, termasuk Daerah Irigasi (DI) Pandanduri dan Swangi. Untuk mengatasi masalah ini pemerintah tengah membangun Waduk Pandanduri, yang disiapkan menjadi regulator pengelolaan sumberdaya air di daerah Pandanduri dan sekitarnya. Rencana tersebut memerlukan informasi akurat tentang kemampuan suplai air dari waduk sesuai dengan pola budidaya tanam dan karakteristik ketersediaan air. Informasi dari penelitian sebelumnya, bahwa terdapat potensi pemanfaatan aliran kembali (return flow) yang terjadi di lahan dalam sistem irigasi tersebut. Untuk itu perlu dilakukan studi lebih detail untuk mengetahui kemampuan optimum Waduk Pandanduri dalam pemanfaatan air sesuai potensi yang dimilikinya. Pada penelitian ini dilakukan kajian optimasi pemanfaatan air Waduk Pandanduri untuk keperluan suplesi air irigasi pada Sistem Jaringan Irigasi Pandanduri Swangi dengan memperhitungkan potensi return flow sehingga diperoleh pola pengaturan pelepasan air waduk yang optimal. Kemampuan optimum Waduk Pandanduri dalam pemanfaatan air dilihat dengan melakukan analisa optimasi alokasi air menggunakan metode Program Linier, yang dikombinasikan dengan teknik simulasi operasi pengaturan pelepasan air (release) waduk. Kinerja alokasi air diukur dari pencapaian intensitas tanam per tahun dengan mempertimbangkan nilai faktor k yaitu, rasio antara release nyata dan target release berdasarkan kebutuhan air dan reliabilitas operasi waduk. Simulasi operasi waduk dilakukan selama 12 tahun (1999 - 2010). Inflow waduk dalam perhitungan menggunakan debit rerata tengah bulanan yang tercatat pada AWLR Suradadi. Besarnya outflow diperhitungkan berdasarkan target release yang terdiri dari kebutuhan air untuk penggelontoran dan kebutuhan air irigasi baik DI Pandanduri-Swangi juga Sistem Irigasi Gambir dengan beberapa alternatif pola tata tanam dan awal periode tanam. Besarnya release air waduk memperhitungkan nilai return flow dari sistem irigasi DI Pandanduri. Nilai return flow diperoleh dengan pendekatan model Tangki. Hasil Penelitian menunjukkan bahwa dengan pendekatan model Tangki dalam menentukan nilai return flow, diperoleh peningkatan pemanfaatan air Waduk Pandanduri. Penelitian sebelumnya, luas lahan yang dapat diari berdasarkan intensitas tanam hanya 258,4% setahun dari total areal 4.477 ha dengan keandalan operasi waduk 82,4% dan nilai faktor k sebesar 0,854. Hasil optimasi pada penelitian ini menunjukkan peningkatan intensitas tanam menjadi 269,5% dan keandalan operasi waduk sebesar 83,7% untuk nilai faktor k sama yaitu 0,854. Pemanfaatan air waduk optimal diperoleh pada penanaman tiga golongan, pola tanam padi-padi/palawija-palawija awal tanam bulan Oktober dengan intensitas tanam sebesar 237,20% dan keandalan operasi waduk sebesar 90,0% serta nilai faktor k sebesar 0,920.

Water supply from South Lombok Interconnection Irrigation Network has not able to meet water demand of irrigation areas in South Lombok optimally, including Pandanduri and Swangi irrigation areas. To overcome this problem the goverment has been building a construction of Pandanduri reservoir, as the regulator of water resource management in Pandanduri and surrounding areas. The plan needs accurate information about water supply capacity of the reservoir according to the cultivation pattern of planting and characteristics of water availability. From previous studies, there was potential of return flow that occurs on land within the irrigation system. Therefore, it needs more detailed study to determine the optimum capacity of Pandanduri reservoir in water utilization by considering all its potentiality. This research is designed to optimize water utilization of Pandanduri Reservoir to supply water irrigation of Pandanduri-Swangi Irrigation System by taking into account the potential of return flow in order to obtain the pattern of optimum reservoir water release. In order to obtain the optimum water resource utilization of Pandanduri reservoir, an optimization model based on Linear Programming method was formulated and then a water balance of Standard Operating Rule reservoir simulation model was applied in combination with the optimization model. Performance of water allocation is measured by the achievement of annual cropping intensity by considering the ratio of actual release and target release (k factor) and the reliability of reservoir operations. The amount of outflow was calculated based on target release that consisted of water requirements for flushing and irrigation water needs of both Pandanduri-Swangi and Gambir Irrigation System with some alternative cropping patterns and beginning of planting periods. Return flow from drainage water of Pandanduri irrigation area was taken into account to determine the amount of reservoir water release. Modification of Sugawara Tank model was applied in order to determine the amount of return flow. The result shows that the application of the Tank model to determine the amount of return flow can increase the water utilization of Pandanduri reservoir. Previous studies showed that the available water in Pandanduri reservoir was able to meet the irrigation water demand with maximum annual cropping intensity 258,4% for 4.477 ha of command area. The output of simulation model concludes that supply capacity under constraint of k factor was 0,854 and the reliability of reservoir operation was 82,40%. Optimization results in this study showed an increase in cropping intensity to 269.5% and the reliability of reservoir operations is 83,7% for the same k factor is 0,854. The optimum water utilization of Pandanduri reservoir based on annual cropping intensity is 237,20%, it can be obtained by planting three classes of rotation water supply and cropping pattern paddy-paddy/palawija-palawija that the cultivation is begin in early October. The output of simulation model concludes that supply capacity under constraint of k factor is 0,920 and the reliability of reservoir operation is 90,0%.

Kata Kunci : Return flow, model Tangki optimasi Program Linier