Pergerakan air tanah tiga dimensi di tanah berhutan dan agroforestry.

Abstract

Water movement in soil is an important phenomenon to be studied, especially in relation to the implementation of proper forest management on water cycles and nutrient cycling in soil on plant growth and flood control. The soil water movement, especially in the unsaturated soil, is driven by the non-equilibrium system which causes water flow from high potential to low potential, therefore, the water saturation level always change. Richard (1931) developed the model of water movement in unsaturated soil based on Darcy's law, where the unsaturated water movement is influenced by the unsaturated hydraulic conductivity as a function of water retention. Therefore, the knowledge or data of K() and () functions are needed. One of models of K() and () functions is lognormal (LN) model developed by the Kosugi (1994). The research objective is to determine three-dimensional soil water movement in forested and agroforestry catchment area by applying computational techniques of threedimensional unsaturated flow introduced by Kosugi (2000). Computation uses three-dimensional (3D) unsaturated water flow model of Richard's equation modified by Šimůnek et al. (2006), and LN models of soil water retention curve (()) and hydraulic conductivity (K(). The soil hydraulic properties of forest soil of Rokko Mountain (HTNr), forest soil of Gunung Walat Educational Forest, HPGW (HTNgw), and agroforestry soil of HPGW (AF) are used as inputs. The three-dimensional computation technique shows the good result of computation, which is indicated by the changing of pressure head with time and rainfall. Based on the characteristic of (), the ability to retain water of both layer of HTNgw soil is highest in comparison with AF and HTNr. Based on the characteristics of K(), the ability to drain water of HTNgw soil in the layer 10 cm is highest in comparison with HTNr and AF, while in a layer 50 cm the ability to drain water of HTNr soil is highest in comparison with AF and HTNgw. Generally based on the characteristics of soil hydraulic flow in layer 10 cm and 50 cm of soil, when no rain, the three-dimensional drainage proccess in HTNr is fastest in comparison with HTNgw and AF, while when there is a rain, three-dimensionally drainage process in HTNgw and AF is faster than HTNr

Pergerakan air dalam tanah merupakan fenomena penting untuk dipelajari, terutama dalam kaitannya dengan pelaksanaan pengelolaan hutan yang tepat terhadap siklus air dan siklus hara di dalam tanah terhadap pertumbuhan tanaman dan pengendalian banjir. Pergerakan air tanah, terutama di tanah tidak jenuh, terjadi karena terdapat sistem yang tidak setimbang (nonequilibirium system) yang menyebabkan aliran air dari potensial tinggi ke potensial rendah. Richard (1931) mengembangkan model untuk menduga aliran air dalam tanah tidak jenuh berdasarkan pada hukum Darcy, dimana aliran air dalam tanah tidak jenuh dipengaruhi oleh kondusktivitas hidrolika tanah tidak jenuh sebagai fungsi dari retensi air tanah. Dengan demikian diperlukan pengetahuan atau data tentang fungsi hubungan K() dan (). Salah satu model fungsi () dan K() adalah model lognormal (LN) yang dikembangkan oleh Kosugi (1994). Tujuan penelitian adalah mengetahui pergerakan air dalam tanah secara tiga dimensi di suatu daerah tangkapan air berhutan dan agroforestry menggunakan teknik komputasi aliran tidak jenuh tiga dimensi yang diperkenalkan oleh Kosugi (2000). Model aliran air 3D tidak jenuh menggunakan persamaan Richard yang telah dimodifikasi oleh Šimůnek et al. (2006), dan model retensi air tanah dan konduktivitas hidrolika tidak jenuh menggunakan model LN kurva retensi air tanah (()) dan model LN konduktivitas hidrolik (K()). Sifat hidrolika tanah yang digunakan sebagai masukan model adalah sifat tanah berhutan di Pegunungan Rokko-Jepang (HTNr), tanah berhutan di Hutan Pendidikan Gunung Walat (HTNgw), dan tanah agroforestry di Hutan Pendidikan Gunung Walat (AF).