Hydroclimate zonation for optimized agricultural landuse using SWAT model: case of Barito Hulu watershed in Central Kalimantan

Abstract

Landcover changes, particularly due to deforestation, may affect the hydrological response of watershed. Deforestation of natural forest represents a major part of forest land degradation that affected the watershed functions, such as loss of land productivity due to increase soil erosion, runoff, evapotranspiration, and decrease of water storage capacity. The main objective of this study was to determine landuse zones for agricultural uses based on water resource aspects. The specific objectives of this study were: (1) to determine water yield and evapotranspiration for landcover conditions of 1990 and 2003; (2) to determine the rate of erosion for 1990 and 2003 conditions; and (3) to optimize land use zonation for agricultural uses for Barito Hulu watershed. The hydrological response for each landcover was computed using SWAT model. The result of the study showed that decreasing 8% of forest area from 1990 condition would increase water yield, evapotranspiration, and erosion ofabout 9%, 6%, and 13 ton/Ha/year, respectively, while water storage decrease of about 15%. The response for 1990 landcover condition with input rainfall of 3.117 mm/year generated water yield of approximately 44%, evapotranspiration 37%, water storage 19%, and erosion rate 20,5 ton/ha/year, while for the 2003 landcover condition with rainfall 2.613 mm/year generated water yield 53%, evapotranspiration 43%, water storage of 4%, and erosion of about 33.2 ton/Ha/year. The study found that the landuse zones composition are as follows: forest cover of 76%, mixed garden 0,5%, community rubber area 1%, settlement area of 10%, dry-land agriculture of 12%, and oil palm garden of 0,4%. Those landuse and landcover compositions provide water yield of approximately 56%, evapotranspiration of 40%, water storage of 4%, and erosion of about 27,5 ton/ha/year, less than the tolerable soil loss.

Perubahan tutupan lahan khususnya deforestasi dapat mempengaruhi respons hidrologi suatu Daerah Aliran Sungai (DAS). Deforestasi adalah salah satu faktor yang menyebabkan degradasi lahan DAS, di mana secara nyata menurunkan produktivitas lahan akibat adanya peningkatan erosi tanah, limpasan permukaan, dan menurunnya simpanan air. Tujuan utama dari penelitian ini adalah untuk menentukan zona tutupan dan penggunaan lahan dalam rangka pengembangan pertanian berdasarkan pasokan sumber daya air DAS Barito Hulu. Penelitian ini juga mempunyai beberapa tujuan khusus, yaitu: (1) menentukan hasil air dan evapotranspirasi pada tutupan lahan tahun 1990 dan 2003; (2) menentukan besarnya erosi pada tutupan lahan tahun 1990 dan 2003; (3) optimasi penggunaan lahan pertanian untuk menentukan zonasi penggunaan lahan DAS Barito Hulu. Analisis respons hidrologi terhadap masing-masing tutupan lahan dilakukan menggunakan model SWAT. Pengembangan model optimasi dilakukan dengan solver (Microsoft office Excel). Optimasi komposisi tutupan dan penggunaan lahan dilakukan dalam rangka menghitung hasil air (water yield) maksimum, aliran permukaan (runoff), evapotranspirasi, dan erosi tanah yang minimum. Validasi dilakukan menggunakan koefisien determinasi (R2) dan Nash-Sutcliffe Index (NSI).