Tamera water retention landscape to restore the water cycle and reduce vulnerability to droughts

Progressive desertification is a major problem in southern EU countries, especially the Iberian Peninsula. Alentejo is arid (summer maximum temperatures above 30C, average precipitation about 600mm/m2/year) with periodic droughts; decades of erosion have stripped the humus topsoil needed to absorb rainwater, recharge aquifers, buffer floods and maintain water quality. Portugal's National Adaptation Plan projects temperature rises of 2-3C (RCP4.5) to 5C (RCP8.5) by 2100, greatest in summer and inland areas, with seasonal precipitation reductions of 10-50% under RCP8.5, worsening desertification and biodiversity-loss risk across southern Portugal.

Create a water retention landscape (WRL) to counteract erosion, desertification and drought, enabling Tamera to become self-sufficient in water and food and reduce its vulnerability to climate change, while demonstrating a replicable model for other Mediterranean areas prone to desertification.

Water Retention Landscapes restore the full water cycle by retaining rainfall on-site (no runoff, only spring outflow), combining retention lakes/ponds, reforestation with mixed ground cover, holistic grazing management, keyline design, terracing, swales and runoff infiltration. Construction uses a compacted clay/fine-material sealing layer tied into the subsoil, meandering (non-reinforced) banks aligned with the prevailing wind for oxygenation, and varied depth zones supporting aquatic-habitat diversity. 'Lake 1' (6,400 m2 capacity) was built in 2007 and filled by its second winter, with a new perennial spring emerging within the first year; a further, roughly three-times-larger retention area followed in 2011. Between 2006 and 2015, 29 lakes and retention spaces were built, expanding water-body area from 0.62 ha to about 8.32 ha; after 2015 effort shifted to swales, ditch planting, mulching and dam maintenance. The highest retention space, at the top of the valley, gravity-feeds lower ponds without pumping, and riverside forest and fruit trees (chestnut, alder, ash, elder, plus olive, cork oak and other natives further out) were planted alongside wildlife corridors.

A cost-benefit analysis using Net Present Value (2015-2050, 3% discount rate) accounted for construction, licensing, fees and taxes as costs, excluding unquantifiable costs such as construction-phase pollution/wellbeing effects. Quantified benefits: transitional woodland grew from 9.34 ha to 19.50 ha (2006-2014), giving an estimated 9.4%/year increase in carbon storage; estimated net tourism/water-event income (2014-2050) of 810,000 EUR; land/property-value benefits estimated at 150,000-400,000 EUR. Many benefits (agricultural productivity gains, ecosystem service value) could not be quantified. The resulting NPV was negative (-261,551 EUR), meaning quantified discounted benefits did not outweigh construction costs, though the analysis notes this is sensitive to discounting assumptions and omits major unquantified benefits. Since 2011, Tamera's community has sourced all drinking water from wells fed by the WRL, and the network also supports neighbouring farms and firefighters during drought or fire.

Financial investment for WRL construction is estimated on the order of half a million euros, a major implementation obstacle addressed at Tamera through private fundraising and donor communication; the complex legal/regulatory framework was another obstacle. Success factors were the designers' technical and local-climate knowledge and Tamera's capacity to mobilise the community for a multi-functional, long-term investment.