Analysis of water balance and aridity index in catchment and irrigation area of Jatiluhur Reservoir

Abstract

One of the functions of Jatiluhur Reservoir is to supply irrigation water for 264.000 ha of agricultural land in Pantura, West Java. Most of the farmlands in these area had water deficit during dry season. The purpose of this study were to analyze the water balance and aridity index in catchment and irrigation area of Jatiluhur reservoir, to calculate crop water requirements, irrigation water requirements and efficiency of irrigation channels on farms in the region Pantura West Java, and also to identify and mapping the areas that should receive high priority irrigation water supply. Water balance analysis using Thornthwaite- Mather method was done to obtain water resource potential representation of upstream Citarum watersheed, that hystorically had higher rainfall than its downstream. Irrigation water requirement was calculated by Penman – Monteith method of water balance analysis, using CropWat for Windows version 8.0 software. The result of rainfall analysis showed that dry season in Pantura was occured from April until November (eight months), and wet season was occured from December until March (four months). The result of water balance analysis showed that soils in this region experienced water deficit during April to December. Analysis of aridity index showed that the coastal region Pantura experienced severe drought from June to October, with August as the peak. Water balance of Citarum river and Jatiluhur reservoir indicated that irrigation water demand for agricultural land in Pantura could not fulfilled only by Citarum River, so it need subvention from local resource. Moreover, calculation using CropWat for Windows showed that water requirement for all agricultural land in Pantura was 3.797,7 million m3/year, while data from PJT II showed that the average of irrigation water supply for this area was 5.519,6 million m3/year. It indicated that efficiency of the irrigation Jatiluhur channels was about 68,8%.

Salah satu fungsi Waduk Jatiluhur sebagai pengendali aliran Sungai Citarum adalah memberikan pasokan air irigasi bagi lahan pertanian seluas 264.000 ha yang merupakan daerah sentra produksi beras nasional di wilayah Pantura Jawa Barat. Saat musim kemarau, sebagian besar lahan pertanian di wilayah ini sering mengalami kekeringan. Tujuan penelitian ini adalah untuk : (1) menganalisis neraca air dan indeks kekeringan di DTA Waduk Jatiluhur dan daerah irigasinya di wilayah Pantura Jawa Barat; (2) menghitung kebutuhan air tanaman, kebutuhan air irigasi dan efisiensi penyaluran air irigasi pada lahan pertanian di wilayah Pantura Jawa Barat; serta (3) mengidentifikasi dan memetakan daerah yang perlu mendapat prioritas pasokan air irigasi. Penelitian dilakukan di Wilayah Kerja Perum Jasa Tirta II (PJT-II) seluas 1.364.072 ha, dengan fokus pada Daerah Aliran Sungai (DAS) Citarum Hulu, yang merupakan Daerah Tangkapan Air (DTA) Waduk Jatiluhur, dan pada lahan pertanian di Daerah Pantura Jawa Barat yang mendapat pasokan air irigasi dari Waduk Jatiluhur. Analisis neraca air dengan metode Thornthwaite-Mather dilakukan untuk mendapatkan gambaran potensi sumberdaya air di DAS Citarum bagian hulu yang secara historis memiliki curah hujan yang lebih tinggi dibandingkan daerah hilir. Analisis neraca air waduk bertujuan untuk mengetahui potensi simpanan air di Waduk Jatiluhur yang dapat dimanfaatkan guna berbagai kebutuhan di daerah hilir, khususnya kebutuhan air irigasi. Kebutuhan air di daerah irigasi dihitung melalui analisis neraca air yang diperoleh dengan menggunakan metode Penman – Monteith pada program CropWat for Windows versi 8.0.