Crop water and irrigation requirements of Jatropha curcas L. in semi-arid conditions of Botswana: applying the CROPWAT model

Abstract

Globally, second-generation biofuels have been promoted as an alternative fuel to fossil fuels. This shift in energy sources was envisaged to reduce dependence on fossil fuels, cut down greenhouse gas (GHG) emissions, and ultimately to relieve national economies from the ever-escalating prices of fossil fuels. Jatropha curcas L. (Jatropha) appeared to have great potential as a feedstock for biodiesel production. It attracted interest from governments and private investors in Asia and Africa, including Botswana. Despite the global promotion of Jatropha biodiesel production, there is surprisingly limited information about its water use during the cultivation stage. The aim of this study was to determine the crop water, irrigation requirements and the effects of irrigation scheduling on Jatropha cultivation in Botswana. Climate and soil data from 2014–2016 were obtained from the weather station at the Department of Agricultural Research Station, Sebele, Botswana. Secondary data sources were consulted to obtain data such as crop coefficients, crop duration length, rooting depth, and crop height. The CROPWAT model was then used to estimate the rate of reference evapotranspiration (ETo), evapotranspiration of Jatropha (ETc), irrigation water requirements (IWR) and yield response to irrigation scheduling in Botswana. The results showed that the annual ETo from 2014 to 2016 at the station was 1456 mm. The lowest monthly ETo (50.10 mm) was observed in June and the highest (182.59 mm) was observed in January. The results also indicated that the total Jatropha evapotranspiration rate (ETc) from 2014–2016 was estimated to be 955.4 mm/ growing season and the irrigation requirement was estimated to be 665.4 mm/ growing season with an irrigation efficiency of 70%, indicating that supplementary irrigation is needed. Furthermore, high yield reductions are expected in the late developmental stage if limited water is supplied. The late developmental stage requires more water than other stages. Given the low and highly variable rainfall in Botswana, satisfactory yields for biodiesel production will only be realized if irrigation is done. It is therefore recommended that shifting planting dates to coincide with rainfall should be considered and also allocating water to the most critical crop developmental stages which would help to maximise yield.