Evaluation of the effectiveness of telaga-based rainwater harvesting in arid regions

Abstract

This study evaluates the effectiveness of telaga-based rainwater harvesting (RWH) systems in Tepus and Semanu Sub-districts of Gunungkidul, Yogyakarta, Indonesia, in supporting water availability throughout the dry season. The methods included field surveys of 40 telaga/embung¹ (morphometric/depth measurements, inlet– catchment conditions, and operational status), descriptive statistical analysis, and estimation of water losses using the FAO-56 Penman–Monteith method (ET_o), which was converted into open-water evaporation (E_o). Results show an average designed storage volume of 14,870 m³, while actual effective volume is only approx. 35% due to an average sedimentation rate of 48.25% (50% of telaga fall into the ≥51% sedimentation category). The theoretical rainwater- harvesting effectiveness is 20.85%, whereas actual effectiveness is 6.09% (actual range 0–33.8%; 37.5% of telaga have lost their function). At peak dry-season conditions, E_o ranges from approx. 5–7 mm per day (approx. 31–43 m³ per day for an average surface area of 6,138 m²). Accumulated evaporation over approximately 90 days is equivalent to approx. 2.76–3.87∙10³ m³, or approx. 53–74% of the initial effective storage (approx. 5.20∙103 m³). Operationally, telaga retain their functional capacity for only ±2–3 months after the rainy season, rendering their supply insufficient to meet demand over the entire dry period.