Evaluasi kemampuan lahan, analisis neraca air dan erosi tanah untuk arahan penggunaan lahan optimal di sub daerah aliran Sungai Sumani Sumatera Barat

WIDYATMOKO, Dwi Yuli, Prof. Dr. Ir. Putu Sudira, M.Sc

2010 | Tesis | S2 Ilmu Kehutanan

Tujuan penelitian ini untuk: (i) mengetahui kemampuan lahan, (ii) mengkaji kesesuaian penggunaan lahan saat ini terhadap kemampuan lahan; (iii) mengetahui potensi air, (iv) mengkaji dampak penggunaan lahan saat ini terhadap erosi tanah, aliran permukaan, dan indeks kekeringan, dan (v) menentukan arahan penggunaan lahan optimal di sub DAS Sumani. Penelitian ini menggunakan pendekatan satuan lahan sebagai unit analisis dan unit pemetaan. Klasifikasi kemampuan lahan dilakukan dengan metode matching menggunakan software Land Classification and Landuse Planing (LCLP). Potensi air menggunakan metode neraca air Thorntwaite Mather, pendugaan erosi menggunakan metode Universal Soil Loss Equation (USLE), dan arahan penggunaan lahan optimal sub DAS Sumani menggunakan analisis kuantitatif dan kualitatif dengan Arc View GIS dan mempertimbangan kemampuan lahan, kesesuaian penggunaan lahan, erosi tanah, dan potensi air setiap satuan lahan. Hasil penelitian menunjukkan lahan di sub DAS Sumani yang didominasi pertanian lahan kering/ladang dan intensitas curah hujan tinggi terdiri dari kelas kemampuan lahan II,III,IV,V,VI,VII, dan VIII. Kelas kemampuan lahan ini didominasi oleh lahan dengan potensi rendah atau kelompok lahan yang tidak bisa diolah/digarap (kelas kemampuan V,VI,VII, dan VIII) seluas 36.783,80 ha.(81,68%). Kesesuaian penggunaan lahan di sub DAS Sumani seluas 12.045,48 ha (26,75%) telah sesuai, seluas 5.973,24 ha (13,26%) tidak sesuai positif, dan seluas 27.014,37 ha (59,99%) tidak sesuai negatif dengan kemampuannya. Potensi air di sub DAS Sumani sangat besar, yaitu rata-rata 7.532,55 m3/ha/th. Rata-rata laju erosi aktual di sub DAS Sumani diperkirakan sebesar 61,21 ton/ha/th. atau 2.756.443,62 ton/th. dan erosi yang diperbolehkan sebesar 1,36 mm/th atau setara dengan 17,14 ton/ha/th. Neraca air menunjukkan bahwa indeks kekeringan di sub DAS Sumani kurang dari 16,7 % dan termasuk dalam kriteria sedikit kekurangan air. Indeks koofisie n aliran sub DAS Sumani rata-rata 0,28, termasuk dalam kategori baik. Untuk menekan laju erosi dan degradasi lahan, hasil penelitian ini merekomendasikan arahan penggunaan lahan optimal di sub DAS Sumani untuk menambah luasan hutan dan agroforestry, yaitu hutan seluas 9.066,20 ha dan agroforestry seluas 10.491,23 ha. Prediksi erosi aktual setelah arahan penggunaan lahan optimal adalah 20,14 ton/ha/th.

This study aims to (i) determine land capability, (ii) review current landuse suitability with the land capability, (iii) find out the water capacity, (iv) assess the impact of current landuse on soil erosion, surface runoff, and dryness index, and (v) determine the allocation of optimal landuse in Sumani sub-watershed as well. This research applied land units approach as analysis and mapping units. Land capability classification was done using matching techniques, and Land Classification and Land use Planning (LCLP) software. Water capacity was determined using the water balance Thorntwaite Mather method. Prediction of erosion was calculated using the method of Universal Soil Loss Equation (USLE), and the allocation of optimal land use in Sumani sub-watershed was implemented using quantitative and qualitative analys is by Arc View GIS and considering land capability, landuse suitability, soil erosion, and water capacity in each land unit. The results indicated that the land in Sumani sub-watershed was dominated by unirrigated agricultural field and high rainfall intensity consists of land capability classes II, III, IV, V, VI, VII, and VIII. It was dominated by unarable land (classes IV, V, VI, VII, and VIII) an area 36.783,80 ha.(81,68%). In terms of landuse suitability in Sumani sub-watershed, an area of 12.045,48 ha (26.75%) had suitable usage, of 5.973,24 ha (13,26%) had not suitable positive, and of 27.014,37 ha (59,99%) had not suitable negative with land capability. Water capacity in Sumani sub-watershed was enormous, that is, 7.532,55 m3/ha/year in average. Average erosion rate in Sumani sub-watersheds was expected to be 61,21 tons/ha/year, or 2.756.443,62 tons/year, and tolerated erosion was 1,36 mm/year or 17,14 ton/ha/year. The water balance showed that the dryness index in Sumani sub-watershed is less than 16.7% and included in the criteria of somewhat lack of water. Coefficient runoff index of Sumani sub-watershed 0,28, and was included in good categories. To reduce the rate of erosion and land degradation, the study recommended the allocation of optimal landuse in Sumani sub-watershed to increase the area of forests and agroforestry, which are forest area of 9.066.20 ha and of 10.491.23 ha, respectively. Predicted erosion of optimal landuse allocation was 20,14 ton/ha/year.

Kata Kunci : Daerah aliran sungai, Kemampuan lahan, Erosi tanah, Potensi air, Penggunaan lahan optimal