Storage tank (3.5 m3-180 m3) built partially or entirely underground using mortar-, clay- or plastic-lining. Captures and stores surface runoff or rooftop rainwater. Shape: cylindrical or square. Applying cover, inlet/outlet pipes, screen, first-flush or sediment tank, and pump reduce contamination/evaporation. Advantage is that under ground tanks need less strength than above ground (less costly). Roofs need however more attention. In areas with severe winters (such as Mongolia) underground tanks also give frost protection.
USD7 (per m3) with plastic-lining, USD10 (0.2 m3) ferro-cement jar (Bolivia). Ferro-cement tank ~ USD1,900 (60 m3). Cost distribution: 60% material; 30% transport, labor and 10% replacements. Affordable, short lifespan alternative (~2 years): plastic-lining at USD1.6 per m3 (Uganda).
Rainwater tanks for household- to community-level. Community-funding limits scale (costs). Donor-supported (NGO/government) large-scale projects potentially successful above 50 tanks. Equal participation of genders/class in committee and OM group advised. Local resources and labor to save costs and ensure lifelong caretaking.
Rainwater use implies no direct negative impact on environment. Surface runoff may contain chemical and physical contamination. Water storage increases climate change resilience (floods, droughts) – greater the scale, the more resilient. Tank should be placed above the water table.
Material lifespan: plastic 25 years; ferrocement 25-40 years; wood/earth/clay 2 (low grade), 20 (polyethylene/PVC) and 40 (polypropylene) years. Water testing/treatment especially important with runoff. Tanks require biannual cleaning. Repair may be required for deterioration from rodents or for broken linings.
User effort: rainwater harvesting requires first rainfall for flushing/cleaning system. Livestock should be kept away from tanks. Water treatment is necessary, else cause pathogen illness. Tanks are accessible for all. Governing with all genders and economic levels is important. Water temperature is more stable and not as hot as with above ground tanks.
No relevant remarks for Below ground tanks.