KAJIAN EROSI DAN NERACA HARA DALAM RANGKA REHABILITASI HUTAN DAN LAHAN (Kasus Di DAS Olonjonge,Parigi Moutong Sulawesi Tengah)

Ir. Herman Harijanto, MP., Prof. Dr. Ir. H. Djoko Marsono

2014 | Disertasi | S3 Ilmu Kehutanan

Penelitian ini dilaksanakan di DAS Olonjonge denganr tujuan: (i) mengkaji laju erosi tanah yang terjadi pada berbagai satuan lahan yang terdapat di kawasan DAS Olonjonge,(ii) mengkaji Debit air sungai dan Sedimen melayang,(iii) mengkaji neraca hara di kawasan DAS Olonjonge dan(iv) membuat arahan rehabilitasi hutan dan lahan di DAS Olonjonge. Penelitian ini menggunakan pendekatan satuan lahan sebagai unit analisis dan unit pemetaan. Prediksi erosi tanah menggunakan metode USLE, debit air dan sedimen melayang menggunakan analisis korelasi, analisis neraca hara menggunakan analisis deskriptif dan penentuan arahan rehabilitasi hutan dan konservasi tanah menggunakan analis silang (crostab) dengan mempertimbangkan tingkat bahaya erosi,fungsi kawasan dan neraca hara. Hasil penelitian menunjukkan bahwa 1).laju erosi yang terjadi di DAS olonjonge berkisar antara 1,47 - 693,38 ton/ha/thn. DAS Olonjonge didominasi oleh tingkat bahaya erosi sedang (35,07% dari total luas DAS) yang diikuti kelas tingkat bahaya erosi sangat ringan (25,61%),. tingkat bahaya erosi ringan (13,01%), berat (13,09%), dan sangat berat (13,23%). 2). Debit air sungai yang mengalir pada penampang sungai Olonjonge berkisar antara 0,15 m3/detik – 13,70 m3/detik dengan debit rata-rata sebesar 3,70 m3/detik. Selanjutnya sedimen yang terangkut bersama limpasan air sungai berkisar antara antara 0,094 kg/m3 – 4,621kg/m3 dengan rata-rata sedimen yang terangkut sebesar 1,744 kg/m3. Hubungan antara debit air dan debit sedimen melayang menunjukkan korelasi yang sangat kuat (r ≥ 0,8), hal ini mungindikasikan bahwa meningkatnya debit air sungai akan diikuti dengan meningkatnya sedimen melayang (suspended load) yang terangkut bersama aliran sungai 3). Hasil analisis keseimbangan hara pada berbagai tipe lahan di DAS Olonjonge menunjukkan bahwa pada tipe penggunaan lahan tegalan terjadi defisit unsur hara sebesar-11,73 kg/th hara N, -2078,33 kg/thn unsur hara K dan - -845,36 kg/thn hara P. Sedang pada lahan coklat monokultur, terjadi defisit unsur hara sebesar -25,56 hara K dan -5,32 hara P. Dengan demikian perlu dilakukan upaya rehabilitasi dengan penambahan pupuk agar produktivitas lahan tetap terjaga.4). Bentuk-bentuk perlakuan arahan rehabilitasi hutan dan lahan di wilayah DAS Olonjonge adalah kombinasi sipil teknis dan vegetatif. Pada kasawasan penyangga, lindung dan budidaya tanaman tahunan (BTT), dengan erosi ringan sampai sedang, arahan rehabilitasi lahan dan konservasi tanah dapat berupa teras gulud, teras kredit ,SPA, rorak mulsa vertikal, dan strip rumput, . Sedang pada satuan lahan dengan erosi berat sampai sangat berat, arahan rehabilitasi lahan dan konservasi tanah dapat berupa teras guludan, teras kredit, teras individu, SPA, barisan sisa tanaman, rorak mulsa vertikal, pengelolaan tanah minimum, agroforestry. Selanjutnya pada lahan-lahan yang terjadi defisit unsur hara, maka dapat dilakukan pemupukan agar produktivitas lahan tetap terpelihara.

This study was conducted in the area of Olonjonge watershed in order to: (i) examine the erosion rate in several area units existing in the area of Olonjonge watershed; (ii) discuss the discharge of river water and the suspended sediment; (iii) investigate nutrient balance in the area of Olonjonge watershed; and (4) determine some directions land and forest rehabilitation in Olonjonge watershed. This study used approaches of area units as analysis and mapping units.The prediction of land erosion had used the method of USLE, discharge and suspended sediment had used correlation, analysis of nutrient balance had used descriptive analysis, and the determination of directions of forest and land rehabilitation had used crosstab analysis considering the level of erosion danger, areal function, and nutrient balance. The study results showed that:1)The erosion rate that occurred in the area of Olonjonge watershed ranged between 1.47-693.38 ton/ha/year. The Olonjonge watershed was dominated by the medium level of erosion danger (35.07% of total wide of watershed), followed by the level of light erosion (13.01%), weight (13.09%), and heavy weight (13.23%). 2) The flowing discharge in the longitudinal section of Olonjonge river ranged between 0.15 - 13.70 m3/second with the average of discharge 3.70 m3/second. Then, the transported sediment together with the water track ranged between 0.094 - 4.621 kg/m3 with the average of transported sediment of 1.744 kg/m3. The discharge had strong correlation with the suspended load (r ≥ 0.8), it showed that the higher discharge would be followed by the higher suspended load that transported by the watershed. 3) The analysis results of nutrient balance in several land types in Olonjonge watershed showed that in the land using type of field, there were some deficits of hara element of N of -11.73 kg/year, hara element of K of -2078.33 kg/year, and hara element of P of -845.36 kg/year. Whereas in the land using type of chocolate monoculture, the deficit of hara elements of K and P were -25.562 kg/year and -5.32 kg/year, respectively. Therefore, it needed to perform some rehabilitation efforts with several additions of fertilizer in order to maintain or increase the rate of land productivity. 4) Mode of treatment of direction of land and forest rehabilitation in the area of Olonjonge watershed was combination of technical and vegetative civil. In the buffer zone, protection, and cultivation of annual plants with the light and medium level of erosion, the direction of and land rehabilitation could be performed by conducting gulud terrace, credit terrace, SPA, rorak mulsavertical, and grass strip, . Whereas in the unit of land with weight and heavy weight level of erosion, the direction of forest rehabilitation and land conservation could be performed by gulud terrace, credit terrace, individual terrace, SPA, residual plants line, rorak mulsa vertical, minimum land processing, and agroforestry. Furthermore, it could be performed some fertilizations in order to maintain the land productivity in the fields that experienced deficit of hara elements.This study was conducted in the area of Olonjonge watershed in order to: (i) examine the erosion rate in several area units existing in the area of Olonjonge watershed; (ii) discuss the discharge of river water and the suspended sediment; (iii) investigate nutrient balance in the area of Olonjonge watershed; and (4) determine some directions land and forest rehabilitation in Olonjonge watershed. This study used approaches of area units as analysis and mapping units.The prediction of land erosion had used the method of USLE, discharge and suspended sediment had used correlation, analysis of nutrient balance had used descriptive analysis, and the determination of directions of forest and land rehabilitation had used crosstab analysis considering the level of erosion danger, areal function, and nutrient balance. The study results showed that:1)The erosion rate that occurred in the area of Olonjonge watershed ranged between 1.47-693.38 ton/ha/year. The Olonjonge watershed was dominated by the medium level of erosion danger (35.07% of total wide of watershed), followed by the level of light erosion (13.01%), weight (13.09%), and heavy weight (13.23%). 2) The flowing discharge in the longitudinal section of Olonjonge river ranged between 0.15 - 13.70 m3/second with the average of discharge 3.70 m3/second. Then, the transported sediment together with the water track ranged between 0.094 - 4.621 kg/m3 with the average of transported sediment of 1.744 kg/m3. The discharge had strong correlation with the suspended load (r ≥ 0.8), it showed that the higher discharge would be followed by the higher suspended load that transported by the watershed. 3) The analysis results of nutrient balance in several land types in Olonjonge watershed showed that in the land using type of field, there were some deficits of hara element of N of -11.73 kg/year, hara element of K of -2078.33 kg/year, and hara element of P of -845.36 kg/year. Whereas in the land using type of chocolate monoculture, the deficit of hara elements of K and P were -25.562 kg/year and -5.32 kg/year, respectively. Therefore, it needed to perform some rehabilitation efforts with several additions of fertilizer in order to maintain or increase the rate of land productivity. 4) Mode of treatment of direction of land and forest rehabilitation in the area of Olonjonge watershed was combination of technical and vegetative civil. In the buffer zone, protection, and cultivation of annual plants with the light and medium level of erosion, the direction of and land rehabilitation could be performed by conducting gulud terrace, credit terrace, SPA, rorak mulsavertical, and grass strip, . Whereas in the unit of land with weight and heavy weight level of erosion, the direction of forest rehabilitation and land conservation could be performed by gulud terrace, credit terrace, individual terrace, SPA, residual plants line, rorak mulsa vertical, minimum land processing, and agroforestry. Furthermore, it could be performed some fertilizations in order to maintain the land productivity in the fields that experienced deficit of hara elements.

Kata Kunci : erosi, neraca hara, DAS