Rainwater harvesting with solar and gravity powered irrigation for high tunnels

Abstract

High tunnels use clear plastic film over a large metal frame to trap solar radiation as heat in order to extend the crop growing season and reduce environmental stress. High tunnels differ from high tech greenhouses in that they use the natural soil as the growing media and use natural ventilation without fans or heaters to control the growing environment. High tunnels can provide some of the benefits of greenhouses at a much lower cost. However, the protective cover cuts-off rain water to the crop and significantly modifies the crop-water use environment. In order to reduce reliance on external sources of water, The University of Tennessee–Biosystems Engineering and Plant Science Departments implemented three types of rainwater harvesting (RWH) for high tunnels that did not require an external source of power for irrigation: gravity flow, solar battery-powered pumping, and solar transfer pumping. RWH by gravity-flow stored water captured off the high tunnels at a high enough elevation to deliver water for irrigation at very low pressure while solar battery-powered pumping delivered pressurized water directly to the irrigation system. Solar transfer pumping moved harvested rainwater to a higher elevation tank that used gravity flow to irrigate at intermediate pressures. These RWH systems were designed to store 64 mm of rainfall from the high tunnel cover (6400 L per 100 m2 of footprint area) and were able to provide 75% to 100% of the spring crop and 90% to 100% of the fall crop irrigation based on 16 experiments over six years. The RWH systems were ranked in order of increasing cost, maintenance, and complexity as follows: 1) gravity flow, 2) solar transfer pumping, and 3) solar battery-powered pumping. However, all RWH systems had high initial cost when compared to well and municipal water supplies, $12,750 to $15,950 for two 9.2- × 29-m high tunnels. Significantly lower cost rain gutters and water storage were identified but not yet tested for RWH in high tunnels.