Estimating the volumetric water footprint of swiss chard (Beta vulgaris) and carrot (Daucus carota) grown in the Highveld Region South Africa

Abstract

This study estimated the total volumetric water footprint (WF) of two vegetable crops, a root vegetable crop (carrot) and a leafy vegetable crop (Swiss chard) grown in Gauteng Province, South Africa. South Africa is a water scarce country and demands for food have increased due to population growth, and hence more water is required to ensure sufficient production or more efficient production methods. However, this is unlikely with dwindling water resources and the increased production of high value horticultural crops that are dependent on irrigation to ensure sufficient and profitable yields. To date, irrigation water efficiencies are often low, and there is still a lack of information on the long-term sustainability of water for current and future food production. WF accounting can potentially provide better information on the impact of human activity, such as crop production under irrigation, on water resources and to guide a more sustainable management of these resources. The Soil Water Balance (SWB-Sci) model was used together with field trials to estimate crop evapotranspiration (ET) and the volumetric blue water footprint (WFblue) and green water footprint (WFgreen) of carrot (Daucus carota L.) and Swiss chard (Beta vulgaris L.) grown at different planting dates in different locations in Gauteng Province, South Africa. The volumetric grey water footprint (WFgrey) was estimated separately from WFblue and WFgreen because of the difference in methodology. Field trials were established at the UP Hatfield Experimental Farm and Greenway Farms in Tarlton to monitor carrot and Swiss chard (Hatfield only) growth and water use over two seasons (summer, autumn). For Swiss chard, the volumetric WFblue and WFgreen was measured at three harvest interval dates in each of the two seasons (summer and autumn). At the different planting dates, seasonal ET of carrot grown at Tarlton was relatively lower than seasonal ET of carrot grown at Hatfield in the summer growing season and relatively higher than for autumn growing seasons. High crop yields obtained at Tarlton reduced the total volumetric WF of carrot, which was relatively lower than for carrots grown at Hatfield in autumn and summer. There were differences in the ratio of blue/green water use in addition to the volumetric WFblue and WFgreen throughout the different growing seasons. During the summer growing season at Hatfield, the crop water requirements were met by green water even though blue water was used as a supplement. However, in autumn crop water requirements were met only by blue water resources as the autumn season is categorised by cool and dry weather conditions with the absence of rainfall. As a result of different agronomic practices at the two locations, WFgrey in Tarlton was relatively higher than the WFgrey for Hatfield in summer and autumn. On average, the volumetric WF of carrot was less than 200 L kg-1 for all growing seasons, with the highest carrot volumetric WF obtained for the summer growing season at Hatfield (182 L kg-1) , followed by autumn grown carrot crop at Hatfield (179 L kg-1) and then carrots grown for the autumn growing season at Tarlton (155 L kg-1). The difference in planting dates, crop management, weather conditions and environmental characteristics influenced the total water use and volumetric WF of carrot at different planting dates for the two locations. Swiss chard was grown in the two growing seasons with the average yield measured at three harvest intervals because Swiss chard has the ability to re-grow, thus several harvests can be made for one sowing date. Therefore, it was hypothesized that Swiss chard would have a relatively lower WF than other similar crops which cannot be harvested multiple times. For the summer growing season, water use was met by both blue and green water resources, with high crop water requirements observed for the first harvest, followed by the 3rd harvest and then the 2nd harvest. The same trend was observed for the autumn growing season even though crop water requirements were met using blue water resources exclusively. During the summer growing season, the highest Swiss chard yield was observed during the 1st harvest, with the 2nd and 3rd harvest yields. Similar trends were also observed for autumn growing season with the reduction in plant size. Swiss chard consumptive WFs for different harvests were observed to decrease following the first harvest, potentially due to the fact that the crop had already established a root system. The WFgrey of Swiss chard grown in autumn was slightly higher than in summer due to differences in crop yield and nitrogen (N) leaching. For the two growing seasons, autumn-grown Swiss chard had higher total volumetric WF of 222 L kg-1, while the summer Swiss chard had a total volumetric WF of 140 L kg-1. The variation in the summer and autumn production results from different weather conditions where high temperatures in the summer season increased crop water use, driven by the higher atmospheric evaporative demand, while cooler temperatures in autumn led to a longer growing season. Although vegetable production is dependent on irrigation water, green water clearly remains to be an essential component in ensuring food security in Gauteng Province. Thus, effective use of rainwater can help reduce the use of blue water resources and decrease pressure on scarce freshwater resources, especially in semi-arid regions. However, optimal management to ensure high crop water use efficiency and yield may contribute significantly to reducing blue water use in food production. Most importantly, the results illustrate the importance of separating blue and green water use in order to get reliable results on how production influences water availability thus ensuring sustainability. For future research it is important to consider the possibility of growing each crop in a season where it is more efficient, and to focus on the reduction of blue water resources for each of the different study sites.