Yhulandari, Afifa (2026) Optimasi Alokasi Bendungan Semantok di Kabupaten Nganjuk Berdasarkan Variabilitas Inflow. Masters thesis, Institut Teknologii Sepuluh Nopember.
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Abstract
Operasi Bendungan Semantok masih menggunakan pola operasi tetap sehingga belum mampu mengakomodasi variabilitas inflow yang memengaruhi ketersediaan air irigasi pada setiap periode operasi. Kondisi tersebut berpotensi menimbulkan surplus maupun defisit air yang berdampak pada efisiensi pemanfaatan tampungan dan keandalan pelayanan irigasi. Penelitian ini bertujuan mengembangkan adaptive rule curve sebagai dasar operasi Bendungan Semantok melalui integrasi prakiraan curah hujan, prediksi inflow, evaluasi neraca air, dan penyesuaian kalender tanam sehingga keputusan pelepasan air dapat disesuaikan dengan kondisi hidrologi aktual. Penelitian menggunakan data inflow Bendungan Semantok periode 2001–2021, data curah hujan dan klimatologi periode 2015–2025, serta data kebutuhan air irigasi. Variabilitas inflow dianalisis menggunakan Kurva Durasi Aliran (Flow Duration Curve) untuk memperoleh empat skenario debit andalan, yaitu Q35, Q50, Q65, dan Q80. Selanjutnya dilakukan simulasi tujuh alternatif pergeseran kalender tanam melalui analisis neraca air. Setiap alternatif dievaluasi berdasarkan indikator Reliability, Resilience, dan Vulnerability (RRV), luas layanan irigasi, intensitas tanam, serta keuntungan usahatani. Alternatif dengan kinerja terbaik digunakan sebagai dasar penyusunan adaptive rule curve dan pedoman operasi waduk. Hasil penelitian menunjukkan bahwa Alternatif 5 dengan awal tanam Desember Dasarian III merupakan alternatif yang memberikan keseimbangan kinerja terbaik. Pada kondisi Q80, alternatif ini meningkatkan Reliability dari 66,67% menjadi 69,44% (meningkat 2,77%), menurunkan defisit dari 58,52 juta m³ menjadi 8,29 juta m³ (berkurang 85,84%), serta menurunkan Vulnerability dari 4,88 juta m³ menjadi 0,75 juta m³ (berkurang 84,63%). Luas layanan irigasi meningkat dari 4.525,79 ha menjadi 4.575,39 ha, sedangkan intensitas tanam meningkat dari 238% menjadi 241%. Keuntungan usahatani mengalami penurunan sebesar 1,69% dibandingkan kondisi eksisting, namun masih berada pada tingkat yang dapat diterima karena diikuti peningkatan keandalan operasi waduk dan efisiensi pemanfaatan tampungan. Pada kondisi Q35, Q50, dan Q65, sistem mampu mempertahankan Reliability sebesar 100% tanpa mengalami defisit. Kontribusi penelitian ini adalah pengembangan kerangka operasi waduk adaptif yang mengintegrasikan prakiraan curah hujan, prediksi inflow berbasis hubungan hujan–inflow, klasifikasi kondisi hidrologi menggunakan Flow Duration Curve, evaluasi neraca air, penyesuaian kalender tanam, adaptive rule curve, serta indikator Reliability, Resilience, dan Vulnerability (RRV) ke dalam satu prosedur operasional. Kerangka tersebut menghasilkan pedoman operasi waduk yang dapat digunakan untuk menentukan target elevasi, pelepasan air, dan pengelolaan layanan irigasi berdasarkan kondisi hidrologi aktual sehingga mendukung operasi Bendungan Semantok yang lebih adaptif, andal, dan efisien dibandingkan pendekatan operasi menggunakan satu rule curve tetap.
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Reservoir operation at Semantok Reservoir has traditionally relied on a fixed operating rule that is unable to adequately accommodate inflow variability, resulting in fluctuations in water availability throughout the irrigation season. This limitation increases the risk of water surpluses during wet periods and water deficits during dry periods, thereby reducing reservoir utilization efficiency and irrigation service reliability. This study aims to develop an adaptive rule curve for Semantok Reservoir by integrating rainfall forecasting, inflow prediction, reservoir water balance evaluation, and cropping calendar adjustment to support more responsive reservoir operation under varying hydrological conditions. Historical inflow data from 2001–2021, rainfall and climatological records from 2015–2025, and irrigation water demand data were analyzed. Inflow variability was characterized using the Flow Duration Curve (FDC) to define four dependable flow scenarios (Q35, Q50, Q65, and Q80). Seven alternative cropping calendars were subsequently evaluated through reservoir water balance simulations. Each alternative was assessed using the Reliability, Resilience, and Vulnerability (RRV) indices together with irrigated area, cropping intensity, and farm profit. The best-performing alternative was then adopted as the basis for developing an adaptive rule curve and an operational guideline for reservoir management. The results identified Alternative 5, with planting initiated during the third ten-day period of December, as the optimal operating strategy by achieving the best balance between hydrological reliability, irrigation performance, and reservoir operational efficiency. Under the Q80 scenario, Reliability increased from 66.67% to 69.44% (an increase of 2.77%), while the irrigation water deficit decreased from 58.52 million m³ to 8.29 million m³ (a reduction of 85.84%) and Vulnerability decreased from 4.88 million m³ to 0.75 million m³ (a reduction of 84.63%). The irrigated area increased from 4,525.79 ha to 4,575.39 ha, while cropping intensity improved from 238% to 241%. Although farm profit decreased slightly by 1.69% compared with the existing operation, this reduction remained acceptable because it was accompanied by significant improvements in operational reliability, reservoir storage utilization efficiency, and irrigation service performance. Furthermore, the system maintained 100% Reliability without water deficits under the Q35, Q50, and Q65 scenarios. The primary contribution of this research is the development of an adaptive reservoir operation framework that integrates rainfall forecasting, rainfall–inflow prediction, hydrological classification using the Flow Duration Curve, reservoir water balance evaluation, cropping calendar adjustment, an adaptive rule curve, and Reliability–Resilience–Vulnerability (RRV) assessment into a single operational procedure. The proposed framework provides practical operational guidelines for determining target reservoir water levels, water releases, and irrigation allocation based on prevailing hydrological conditions, thereby enabling reservoir operation to become more adaptive, reliable, and efficient than conventional operation based on a single fixed rule curve.
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