Kaji Ulang Neraca Air Waduk Sempor Kabupaten Kebumen berbasis Model Soil Water Assessment Tool (SWAT)

Abstract

Waduk Sempor di Kabupaten Kebumen, Jawa Tengah, memiliki peran penting sebagai sumber air irigasi, penyedia air bersih, dan pengendali banjir. Namun, perubahan tata guna lahan di daerah tangkapan air (DTA), khususnya konversi lahan hijau menjadi permukiman, telah menyebabkan peningkatan limpasan permukaan dan sedimentasi, yang berdampak pada penurunan kapasitas tampungan dan ketersediaan air. Penelitian ini bertujuan untuk mengevaluasi kinerja model Soil and Water Assessment Tool (SWAT) dalam memodelkan debit masuk (inflow) ke Waduk Sempor serta menganalisis neraca air tahunan sebagai dasar pengelolaan sumber daya air yang berkelanjutan. Model SWAT dikalibrasi dan divalidasi menggunakan data debit observasi dari BBWS Serayu Opak. Hasil validasi menunjukkan kinerja model yang sangat baik dengan nilai NSE sebesar 0,83 dan R² sebesar 0,87. Hasil simulasi operasional waduk menunjukkan Waduk Sempor berada dalam kondisi penuh selama 36 hari, cukup selama 311 hari, dan defisit selama 18 hari, dengan tingkat keberhasilan pemenuhan kebutuhan air mencapai 95%. Total inflow tahunan sebesar 184,02 juta m³ dan outflow sebesar 186,45 juta m³ menghasilkan defisit 2,43 juta m³. Hal ini menunjukkan bahwa pengelolaan waduk perlu dievaluasi agar distribusi air bisa lebih menyesuaikan dengan perubahan kondisi musim.

The Sempor Reservoir in Kebumen Regency, Central Java, plays an important role as a source of irrigation water, a provider of clean water, and a flood control measure. However, changes in land use in the catchment area, particularly the conversion of green areas into residential areas, have led to increased surface runoff and sedimentation, which have impacted the reservoir's storage capacity and water availability. This study aims to evaluate the performance of the Soil and Water Assessment Tool (SWAT) model in modeling inflow to Sempor Reservoir and analyzing the annual water balance as a basis for sustainable water resource management. The SWAT model was calibrated and validated using observed flow data from the Serayu Opak Water Resources Management Agency (BBWS). Validation results showed excellent model performance with an NSE value of 0.83 and an R² value of 0.87. Operational reservoir simulation results indicated that Sempor Reservoir was at full capacity for 36 days, sufficient for 311 days, and in deficit for 18 days, with a water demand fulfillment rate of 95%. The annual total inflow of 184,02 million m³ and outflow of 186,45 million m³ resulted in a deficit of 2,43 million m³. This indicates that reservoir management needs to be evaluated to better align water distribution with seasonal condition changes.