Judul: Variabilitas Ruang Sifat Hidrolika Tanah di Lereng Berhutan Sub DAS Cipeureu Hutan Pendidikan Gunung Walat, Sukabumi
Abstrak: Theoretically, physical soil properties of forested hill slope varies vertically as well as horizontally, from crest to footslope, due to soil erosion process. Variation in soil proporties cause the soil hydraulic properties are different. Klute (1986) mentioned that soil hydraulic properties that determine water movement in the soil, both saturated and unsaturated are soil hydraulic conductivity and soil water retention function. This soil hydraulic properties is vary spatially (Hendrayanto 1999, Kosugi 1996, Mallants et al., 1993) and spatial variation of soil hydraulic properties affect the water flow behavior in the soil. Spatial variation of soil hydraulic properties could be analyzed by using semivariogram approach. The purpose of this research are to know the spatial variability of soil hydraulic properties and their impact on water storage and one dimensional vertically water flow in the soil of a forested hillslope. Such information can be used as basic data as input for the simulation of land management in Gunung Walat Educational Forest (GWEF). Soil samples were taken from six points, in a forested slope, from crest to footslope. At each point, nine soil samples from difference depths were taken. Soil water retention curves (SWRC) were measured from those samples. SWRC measurements were conducted in Kyoto University, Japan (Kosugi et al., 2009). Soil hydraulic properties were analized by using Lognormal (LN) model parameters of soil water retention and soil hydraulic conductivity functions Kosugi (1996), and spatial variability of soil hydrualic properties were analyzed using variance and semivariogram analyses. The effect of soil hydraulic properties on soil water flow were solved by using one dimensional vertically water flow based on Richard equation (1931) and numerical solution was done by using Hydrus 1D 4.14 software (Simunek et al., 2008). The results show that in general, the hydraulic properties of soil on a forested hillslope in Sub Watershed Cipeureu GWEF vary spatially (vertical and horizontal direction). Analyses of variances and semivariogram showed that two parameters of soil hydraulic properties of LN model, those are soil effective porosity (􀟠􀝏 􀵆 􀟠􀝎) and standard deviation of pore radius distribution (􀟪) are vary spatially, but not for the saturated soil hydraulic conductivity (Ks) and the median of pore radius distribution (􀟮􀝉). The variation of soil hydraulic properties are random or not sequences from the crest to the footslope or vice versa. The variability of soil hydraulic properties give an affect on the variation of water flow in forested hill slope. Generally, flux in the vertical direction at the crest to footslope have the same trends, the top layer of soil water flux responded more quickly than the middle and bottom layers. However, the response rate of each slope position of the water flow are different. The sequence of the response of water flux at each position of the slope is: the lower slope (plot 4) > crest (plot 1) > lower slope (plot 5) > upper slope (plot 2) > mid-slope (plot 3) > footslope (plot 6).
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