Simulasi limpasan permukaan dan kehilangan tanah pada berbagai umur kebun kopi: Studi kasus di Sumberjaya, Lampung Barat

Conversion of forest to agriculture on sloping land raises concern over the degradation of watershed functions. On sloping land, a considerable amount of rainfall can be lost as surface run off, reducing water availability to crops and carrying soil particles leading to soil erosion. Simulation with the WaNuLCAS (Water, Nutrient and Light Capture in Agroforestry Systems) model was used to explore changes of surface run off and soil loss as a result of changes in soil structure in response to land use changes in coffee gardens in Sumberjaya, Lampung. Runoff and soil loss are, according to the model, specifically influenced by 4 main parameters related to rain intensity (Rain_IntensMean – RIM), surface infiltration (S_SurfInfiltInit – SSI), decay rate of macropores per day (S_KstrucDecay – SKD) and ease with which soil is carried by surface run off (E_EntrainmentCoefBarePlot – ECB). By adjusting values for these four parameters and using defaults as well as measured inputs (e.g. rainfall), the simulation shows that the surface run off and soil loss for all land uses qualitatively agreed with run off and soil loss measured in the field. Rain_IntensMean (RIM) 50 and S_SurfInfiltInit (SSI) 1000 mm day-1 are the values representing rainfall and soil conditions in Sumberjaya, Lampung. The pattern of increase and subsequent decrease of runoff with increasing age of the coffee can be accounted for through a simple representation of the dynamics of soil structure. AS_KStrucDecay (SKD) of 0.0005 day-1 reproduced the pattern for surface run off, while soil lossprediction was best for an E_EntrainmentCoefBarePlot (ECB) of 0.002 m2 kg-1 soil mm-1 m2. Model simulation withWaNuLCAS after this ‘calibration’ can be used to explore the changes of surface run off and soil loss as a result ofland use changes and rainfall distribution.

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