Hutan mempunyai fungsi ekonomi, konservasi dan perlindungan system keseimbangan air pada Daerah Aliran Sungai (DAS). DAS Cemoro-Modang merupakan areal hutan tanaman jati milik PT. Perhutani wilayah I Jawa Tengah. Pengelolaan hutan oleh PT. Perhutani saat ini belum mempunyai standar lingkungan dari aspek tata air. Penelitian ini bertujuan untuk mengetahui pengaruh empirik penebangan hutan pada DAS Cemoro-Modang terhadap debit puncak dan volume banjir. Diharapkan dengan penelitian ini dapat menentukan jatah tebangan yang dapat dilakukan oleh PT. Perhutani pada DAS Cemoro-Modang tanpa menimbulkan banjir yang nyata pada bagian hilir DAS. Kalibrasi parameter DAS dan simulasi hidrograf banjir dilakukan dengan model hidrologi HEC-HMS. Pengaruh penebangan hutan dialihragamkan menjadi nilai curve number (CN). Simulasi hujan-aliran dilakukan pada lima skenario perubahan CN dan tujuh skenario perubahan kedalaman hujan. Hasil simulasi debit puncak dan volume banjir dilakukan analisis varian dan uji lanjut beda nyata terkecil pada tingkat kepercayaan 95%. Nilai CN awal sub DAS Modang dan sub DAS Cemoro masing-masing sebesar 78,559 dan 82,403. Penebangan hutan pada sub DAS Modang seluas 292,518 ha dan sub DAS Cemoro seluas 448,826 ha menyebabkan peningkatan CN pada kedua sub DAS tersebut masing-masing sebesar 9,812% dan 4,223%. Peningkatan nilai CN pada sub DAS Modang tidak menimbulkan perbedaan debit puncak dan volume banjir yang nyata pada sungai Modang. Peningkatan nilai CN sub DAS Cemoro berpengaruh nyata terhadap debit puncak, sedangkan volume banjir sungai Cemoro tidak menunjukkan perbedaan yang nyata. Hujan berpengaruh nyata terhadap hidrograf banjir pada kedua sungai tersebut. Pengurangan kedalaman hujan hingga 30% tidak berpengaruh nyata terhadap debit puncak. Penambahan kedalaman hujan hingga 20% berpengaruh nyata terhadap debit puncak banjir, tetapi volume banjir akan berbeda nyata jika terjadi peningkatan kedalaman hujan sebesar 30%.

The functions of the forest are economic, conservation and protection of the water balance system on the watershed. The Cemoro-Modang watersheds are area of cultivated teak forest owned and managed by PT. Perhutani. The management of PT. Perhutani has no hydrologic standard to prevent the degradation of the water balance system. The objective of this research is to understand the empirical impacts of forest in the Cemoro-Modang catchments. In particular how the forest cutting influences the peak flow and flood volume. This research is also to predict the numbers of forest cutting on the Cemoro-Modang watersheds without happening significant flood. The calibration and simulation of flood hydrographs are calculated by using the HEC-HMS hydrology model. The curve number (CN) is applied to value forest cutting. The result of rainfall runoff simulation is transferred into five CN and seven rainfall intensity scenarios. The analysis variant and least significant difference test method are then used for testing both peak flow and flood volume that have resulted by simulation. The spatial analysis shows that the existing curve number values both Modang and Cemoro catchments are 78,558 and 82,403 respectively. The numbers of hectares of forest cutting at Modang and Cemoro watersheds are 292,518 and 448,826 correspondingly. These have increased the CN value of both Modang and Cemoro watersheds to 9,812% and 4,223%. Increasing of CN does not significantly affect the peak flow and flood volume of watershed Modang and flood volume of watershed Cemoro. The peak flow of watershed Cemoro showed a significant differential after the increase of CN to 4,223%. Rainfall intensity has significant effect on both peak flow and flood volume of the Cemoro-Modang watersheds. There is no effect of decreasing rainfall intensity until 30% for peak flow. Increasing it until 20%, shows significant effect to peak flow, but flood volume will be change as it increases until 30%.

Kata Kunci : Sungai,Debit Volume,Penebangan Hutan,forest cutting, flood peak flow, flood volume, curve number