Design and development of a borehole pumping and water level monitoring control system in Botswana

Abstract

Botswana experiences recurrent droughts and has limited surface water resources with the average annual rainfall varying from a minimum of less than 250 mm in the Southwest region and a maximum of about 650 mm in the Northeast area, which is below the world’s average. Day time temperatures are very high, with a maximum of 43.9oC, resulting in high average annual evaporation rates of up to 2000 mm. Globally, the agricultural sector is the main water user, with irrigation subsector accounting for most of it. Botswana, however, has a slightly different situation where the livestock subsector accounts for about 75% of agricultural water use. Most livestock farming takes place in rural areas, where water is normally pumped from boreholes and stored in storage tanks with no control automation. The same pumping-storage set up is also used to provide most rural areas with potable water for domestic use. Manual operation of these systems results in lots of water spillage from storage tanks and waste of energy involved in pumping. Such a scenario motivated the need to conduct a study where automatic water pumping and control system could be developed and be used to ensure that both water and energy are not only conserved but also used sustainably. In this regard, a study was undertaken to design, develop and test an automatic water level monitoring prototype with the ultimate aim of saving water and energy. The research further sought to determine the amount of water and energy savings that can be achieved through the use of such a system and also determine its economic viability. A prototype, typifying a Botswana rural farm, was developed following the engineering design process. Consequently, an arduino-controlled prototype consisting of a control unit, storage tanks and electrical energy supplies was assembled and tested. Magnetic float sensors were placed in tanks to measure the level of water and send the information to the arduino controller, which then made necessary decisions by sending an appropriate control signal to the pump through a relay switch. Tests carried out showed that the control prototype prevented overflow of water from the storage tank during a pumping process. Extrapolation on water and energy savings to the national level revealed that the automatic control system could potentially save 6,825,000 cubic metres of water and 3,627 MWh of electrical energy in a year. This translated to BWP 66,356,400 or USD 6,635,640 of annual monetary savings. When powered through solar energy, the system has been found to have a short payback period of about four years. The system is recommended for more extensive studies and tests in real farms in order to ascertain more accurately its merits and be ready for adoption by farmers in the country. Further, since the country has abundant solar energy with the annual average sunshine hours varying between 8.2 to 9.7 hours/day, the system would provide huge opportunities for sustainable use of both water and energy. This would then go a long way in improving the reliability of water supply, reducing greenhouse gas emissions from diesel-powered water pumping engines and consequently lower the overall farm operational cost.