SUMMARY
VERA AGUSTINA. Sensitivity Evaluation of parameters Used in ANSWERS erosion prediction model. Supervised by Suria Darma Tarigan and Nailc Sinukaban. Many erosion predictions on watershed scale have been conducted by scientists. The result of predicton will be accurate if the necessary data used in are also representative. One of the erosion prediction model is Areal Nonpoint Source Watershed Environtment Response Simulation (ANSWERS). The ANSWERS model is one that uses distributed parameters to simulate correlations between rainfall and sediment. The input data of model are rainfall, soil, land use and land surface condition, characteristic of channel and individual element unit. Those input data consist of parameters which show the characteristics of each element of the watershed. The output goined from the model is runoff and erosion. The changing of values within the parameters of input model effects output in varied levels. To know the amount of changing levels in parameters values with different methods towards the output, it is required that output sensitivity be evaluated towards the changing of model input. The purpose of the research is to evaluate the sensitivity of model output towards any value changing of parameters of model input of erosion prediction. The research began on September 1999 until Mei 2000. The soil example was taken from research site is used for determining bulk density, total porosity, field capacity and antecedent soil moisture. The meansure of permeability was conducted on the site using permeameter. Sensitivity analysis was done in several scenarios of simulation which are: (1) the simulation of value changing of model parameters, (2) The simulation of the chailging of rainfall intensity and (3) the simulation of the changing of watershed size on runoff and erosion. Sensitivity analysis was conducted for 14 model input parameters which are: total porosity (TP), field capacity (FP), the steady state infiltration rate (FC), the different between the maximum and steady state infiltration rates (A), infiltration exponent (P), infiltration control zone depth (DF), antecedent soil moisture (ASM), soil erodibility (K), the potential interception volume (PIT), the actual percentage of cover (PER), the roughness coefficient, the maxinlum roughness height (HU). Manning's n (N) and relative erosiveness (C). Composition of the simulation is: (1) parameters increase of 1% (Pl), 5% (P5), 10% (P10) and decreasing it 1% (Ml), 5% (M5), 10% (M10); (2) rainfall intensity change simulation which consist of 75 mm/hour (Cl), 100 m d h o u r (C2) and 125 mdhour; ( 5 ) the watershed size change simulation which consist of 1.725 Ha (Ll) and 10.081.25 Ha (L2). Sensitivity simulation result shows that runoff and erosion are decreasing along the increasing of TP, FC, A, DF, PIT, RC, HU, K, C and decreasing of FP, P, ASM. PER. According to the intensity value of many kinds of parameters value change level, the 5% change levels is considered representative to determine runoff
and erosion sensitvity order. At the rainfall intensity C1 (75 d o u r ) parameters order based on the runoff sensitivity is RC>ASM>FC>P>N>TP>FP>PIP HU>DF>A> PER>K dan C. Parameters order based on the runoff sensitivity with the increasing rainfall intensity to C2 (100mm/hour) is RC>ASM>FC>N>HU>P>TP>DF>A>PI?,FP> PER>K,C and C3 ( 1 2 5 d o u r ) RC>N>ASM>FC>P>TP>A>HU>DF>PI?,FP >PER> K, C. Parameters order based on the erosion sensitivity with the increasing rainfall intensity to C2 (100 mmlhour) is RC>FC>ASM>P>TP, A, DF, K, PIT, N, C>FP>PER and C3 (125 mmhour) RC>FC>K>ASM>C>P>HU >N> FP, A>TP, DF, PIPPER. Parameters order based on the runoff sensitivity with the increasing of the watershed size, with the watershed size L1 (1.725 Ha) and rainfall intensity C1 (75 m d l o u r ) is RC>ASM>FC>P>N>TP>FP>PIPHU>DF>A>PER>K dan C, rainfall intensity C2 (100 mmlhour) RC>ASM>FON>HU>P>TP>DF>A>PIPFP>PER >K, C and C3 (125mm/hour) RC>N>ASM>FC>P>TP, A>HU>DF> PI?,FP>PER> K, C; at watershed size L2 (10.081,25 Ha) with rainfall intensity C1 (75 rnmhour) is RC>ASM>FC>P>HU>TP, FP>F, PIT>A>N>K, PER, C; with rainfall intensity C2 (100 mmhour) RC>FC>ASM>P>HU>FP>TP,A,N>DF,PIT,PER>K,C; and rainfall intensity C3 (125mrnljam) adalah RC>FC>ASM>N>P>FP,A>TP>DF>PIT>HU> PER>K, C. Parameters order based on erosion sensitivity at watershed size L1 with rainfall intensity C2 (100 mmhour) is RC>FC>ASM> P, HU>TP, A, DF, K, PIT, N>FP>PER and rainfall intensity C3 (125 mm/hour) RC>FC>K>ASM>C>P>HU >N>P,A>TP, DF, PIT PER; mean while at watershed size L2 with rainfall intensity C2 only RC which is sensitive and rainfall intensity C3 is RC>K, N, C. Soil example taking method shows variaty of bulk density, total porosity and field capacity. The result of permeability meansurement using permeameter tend to be variatif. Antecedent soil moisture which was determined from disturb soil example was less various at the same soil.
RINGKASAN
VERA AGUSTINA. Evaluasi Sensitivitas Parameter yang Digunakan dalam Model Prediksi Erosi ANSWERS. Dibawah bimbingan Suria Darma Tarigan dan Naik Sinukaban. Berbagai prediksi erosi pada skala DAS telah banyak dilakukan para peneliti. Hasil prediksi tersebut akan akurat apabila data yang digunakan representatif. Salah satu model prediksi erosi adalah model Areal Nonpoint Source Watershed Environtment Response Simulation (ANSWERS). Model ANSWERS mempakan model dengan parameter terdistribusi yang mensimulasikan hubungan-hubungan hujan limpasan dan hasil sedimen. Data masukan model adalah data curah hujan, data tanah, data penggunaan lahan dan kondisi permukaan lahan, data karakteristik saluran serta data satuan individu elemen. Data masukan tersebut terdiri dari parameter-parameter yang menunjukkan karakteristik setiap elemen dari DAS. Keluaran dari model adalah aliran permukaan dan erosi. Perubahan nilai parameterparameter masukan model mempengaruhi keluaran dengan berbagai taraf yang berbeda. Untuk mengetahui seberapa besar taraf perubahan nilai parameter-parameter dengan metoda berbeda terhadap keluaran, maka perlu dievaluasi sensitivitas keluaran terhadap perubahan masukan model. Tujuan penelitian adalah evaluasi sensitivitas keluaran model terhadap perubahan nilai parameter-parameter masukan model prediksi erosi. Penelitian dimulai pada bulan September 1999 sampai Mei 2000. Pengambilan contoh tanah pada lokasi penelitian digunakan untuk penetapan bobot isi, porositas total, kapasitas lapang dan persentase kejenuhan tanah. Pengukuran permeabilitas dilakukan pada lokasi penelitian dengan menggunakan alat permeameter. Analisis sensitivitas dilakukan dalam beberapa tahap skenario sin~ulasi sebagai berikut: (1) simulasi perubahan nilai parameter-parameter model, (2) simulasi perubahan intensitas hujan dan (3) simulasi perubahan ukuran DAS terhadap aliran permulcaan dan erosi. Simulasi dilakukan pada 14 parameter masukan model yaitu: parameter porositas total (TP), kapasitas lapang (FP), laju infiltrasi konstan (FC), selisih antara laju infiltrasi maksimum dan lconstan (A). koefisien eksponen persamaan infiltrasi (P), kedalaman zone kontrol (DF), erodibilitas tanah (K), \;olcme intersepsi potensial (PIT), persentase penutupan lahan (PER). koefisien kekasaran permukaan (RC), tinggi kekasaran maksimum (HU). koefisien kekasaran Manning's (N)dan faktor tanaman dan pengelolaannya (C). Susunan tahapan simulasi adalah sebagai berikut: ( I ) parameter-parameter diubah dengan menaikkan 1% (PI), 5% (P5), 10% (P10) dan diturunkan 1% ( 1 ) 5% ( 5 10% (Ml0); (2) simulasi perubahan intensitas hujan terdiri dari: 75 mmljam (CI), 100 n~m/jam(C2) dan 125 m d j a m ('23); (3) simulasi peubahan ukuran DAS terdiri dari: 1725 Ha (LI) dan 10.081,25 Ha (L2).
