<![CDATA[ABSTRAK Pengelolaan air di Bali sering direduksi menjadi objek wisata estetis atau sebatas mitos keagamaan belaka, padahal eksistensi Pura Ulun Danu menyimpan logika sains tradisional sebagai instrumen tata ruang dan rekayasa lanskap hidrologis. Penelitian ini bertujuan untuk mengidentifikasi dan menganalisis relasi indeksikal (kausalitas fisik) antara kondisi ekologis lanskap air Danau Beratan dan Danau Buyan terhadap wujud arsitektur serta orientasi tata letak Pura Ulun Danu. Pendekatan yang digunakan adalah kualitatif induktif dengan desain studi kasus interpretatif Sharan B. Merriam (2009). Data dikumpulkan melalui observasi kawasan, wawancara mendalam semi-terstruktur, dan studi dokumen, yang dianalisis menggunakan kerangka semiotika Merrell. Berdasarkan hasil analisis, ditemukan empat mekanisme relasi indeksikal yang membuktikan keterhubungan pura dan danau urutan ontologis kausal yaitu danau mendahului pura, ko-presensi fisik bangunan di tepian langsung danau, ketergantungan kausal hidrologis dari hulu ke hilir, serta perlindungan ekologis aktif kawasan danau melalui sakralisasi ruang kosmik. Kesimpulannya, Pura Ulun Danu bukan sekadar artefak religius, melainkan sebuah tanda indeksikal (indeks) spasial yang eksistensinya secara mutlak didikte oleh keberadaan objeknya, yaitu danau. Arsitektur pura beroperasi sebagai instrumen rekayasa lanskap air yang rasional untuk menjaga keberlanjutan ekosistem. Kata kunci: Pura Ulun Danu, Semiotika Arsitektur, Relasi Indeksikal, Rekayasa Lanskap, Lanskap Air. ABSTRACT Water management in Bali is often reduced to aesthetic tourist attractions or mere religious myths, whereas the existence of Pura Ulun Danu holds traditional scientific logic as an instrument of spatial and hydrological landscape engineering. This study aims to identify and analyze the indexical relationship (physical causality) between the ecological conditions of the Lake Beratan and Lake Buyan waterscapes and the architectural manifestation and layout orientation of Pura Ulun Danu. The approach used is an inductive qualitative method utilizing Sharan B. Merriam's (2009) interpretive case study design. Data were collected through spatial observation, in-depth semi-structured interviews, and document study, analyzed using Merrell's semiotic framework. The study found four mechanisms of indexical relationship proving the connection between the temple and the lake: the causal ontological sequence where the lake predates the temple, physical co-presence of the building directly on the water's edge, hydrological causal dependence from upstream to downstream, and active ecological protection of the lake area through cosmic spatial sacralization. In conclusion, Pura Ulun Danu is not merely a religious artifact but a spatial indexical sign whose existence is absolutely dictated by the presence of its Objek, the lake. The temple's architecture operates as a rational waterscape engineering instrument to maintain ecosystem sustainability. Keywords: Pura Ulun Danu, Architectural Semiotics, Indexical Relation, Engineered Landscape, Waterscape.]]>
