Feeding the world: The potential of desalinated water for irrigation

October 26th, 2015

Shmuel Assouline (A.R.O. – Volcani Center, Israel), Yair Israeli (Jordan Valley Banana Experiment Station, Israel), Avner Silber (Northern R&D, Israel)

The world’s population is estimated to exceed 9 billion by 2050, a 30% increase in the current population.1 The associated food requirements are expected to rise by 70 to 100%.2 This increase in food demand, more than twice the predicted change in population, also reflects expected individual income growth and an elevation in the global standard of living.2

Matching the demand for food from a larger and more affluent population is unlikely to be able to be met by an expansion in the area of land used for agriculture, which is estimated to contribute around 20% of the required increase in crop production.2 The main part of the required increase will therefore need to come from “increasing the efficiency of existing means of food production and distribution instead of converting new areas”.3 Since most of the world’s agriculture is rainfed, a key strategy will be increasing the relative contribution of more efficient irrigated agriculture to increase crop yield per unit area. Indeed, transitioning from dryland to irrigated agriculture typically increases crops water use efficiency (WUE) and yields by a factor of 3.4

Concurrent with population growth, the pressure on global freshwater (FW) resources is expected to increase significantly, especially in countries chronically short of water where the population is projected to increase from half a billion to four billion people by 2050.5 These processes will increase the competition over allocations of FW resources between domestic and irrigation needs. Hence the urgent need for developing alternative and unconventional water resources to alleviate pressure on FW and for adopting commensurate water resource management methods.

Irrigation water quality is a crucial factor in the sustainability of irrigated agriculture, especially the issue of salinity which is known to adversely impact the productivity of agricultural crops. It is estimated that the cost of salinity to global agriculture is around $US 12 billion a year, and it is expected to increase as soils are further affected.6

Extensive irrigated agriculture invariably leads to a buildup of salinity in the soil profile and in groundwater or river systems that supply irrigation water. This trend is accelerated by intensive modern agricultural practices involving the application of fertilizers with irrigation water, which may require periodical leaching using significant amounts of water beyond what is needed for irrigation. Because of the low efficiency of natural (rainfall) and artificial (irrigation system) salt leaching, soil salinity often increases in crop root zones especially in semi-arid regions with limited rainfall.

On the other hand, salt leaching increases the salinity of groundwater and surface water resources7 leading to a vicious cycle and a gradual increase in water demand for irrigation. Irrigation-induced salinity buildup is one of the earliest man-made ecological disasters; responsible for the demise of the civilizations of Mesopotamia and the Indus valley.6 It is estimated that 20% to 50% of the world’s irrigated land is salt-affected to some extent.6

A rapidly emerging source of water that could address both the increasing need for high quality water and the risk related to salinity is desalination of saline or sea water. Desalination techniques have improved and costs have been reduced dramatically in the last decades.8 Desalinated water (DW) is now becoming a competitive source of water for irrigation, especially for high value cash crops.

The impacts of DW on plant growth, yield, and water use efficiency were studied experimentally in a banana plantation located in the Jordan Valley, Northern Israel.9 This was done by comparison with the plant performances in field plots and lysimeters (Figure 1) when irrigated with water from the Sea of Galilee (also known as the Kinneret Lake), considered to be the best FW source for irrigation in the region. Banana plants in that region require between 2000 and 2200 mm of irrigation water per growing season with mean commercial yield of 70 tons/ha.

A pilot reverse osmosis desalination plant (Nirosoft, Carmiel, Israel) was installed to desalinate FW pumped from the lake. The experimental plots were drip irrigated with FW directly pumped from the lake and with DW from the desalination plant. After the addition of fertilizers, the electrical conductivity, EC, of the applied water was 1.5 dS m-1 for the FW irrigation and 0.3 dS m-1 for the DW irrigation treatment. A detailed description of the experimental set-up and the irrigation treatments could be found in Silber et al. (2015).9

The impact of using DW for irrigation on fruit productivity is depicted in Figure 2, and is compared to the performances of conventional FW irrigation. The relationship between the mean fresh weight of banana bunches and the annual amount of applied water for irrigation is also depicted in Figure 2 (results are significant at the level of p<0.0001).

For both water qualities, the mean weight of the banana bunches increases with irrigation amounts. However, irrigation with DW increases weight by 25% on average. This figure illustrates the significant potential of DW irrigation on water savings and on yield increase. The highest bunch fresh weight for FW irrigation is achieved with 2500 mm of water. The same yield could be obtained with only 1100 mm/year of water using DW irrigation, corresponding to a 56% water saving. Similarly, maintaining the same amount of 2500 mm/year of water for irrigation, the bunch fresh weight increases from 24.7 kg for FW to 32.3 kg using DW – a 30% increase in yield with DW irrigation.

The lysimeters in the experimental set-up enabled monitoring of the main components of the water budget. DW irrigation resulted in an increase of the plant water uptake, as expected from the observed higher root density and the less negative osmotic potential in the soil water solution. Lysimeter data allows for evaluating the impact of DW irrigation on plant water use efficiency (WUE= ratio between the total dry weight of the harvested fruits over one year and the total amount of water uptake by the plant during that year). The results for 2012 corresponding to the different irrigation treatments are shown in Figure 3. As set out in the Figure, DW irrigation improved the plant WUE by 40% for the recommended irrigation amount of 2000 mm per year.

The experimental results establish the tremendous potential of DW use for irrigation on plant yield and water use efficiency. DW application opens new possibilities for significant water savings, meaningfully reducing the estimated amount of water required to meet expected food demand by cutting sharply the non-productive water use for leaching salt accumulation out of the root zone. Finally, the observed improvement in fruit weight indicate that higher income could be expected, thus contributing to the economic feasibility of the shift towards a more expensive water source.

The results presented here were obtained on one crop, requiring further validation on a wider range of crops. Nevertheless, if these results reflect a general trend, irrigation with DW could provide an important contribution to the problems of global land and water scarcity on the one hand, and to the challenge of meeting the expected increase in food demand, on the other hand.

References :

Roberts, Nine billion? Science 333, 540-543 (2011). Tilman, C. Balzer, J. Hill, B. L. Befort, Global food demand and the sustainable intensification of agriculture. PNAS 108, 20260-20264 (2011). C. J. Godfray, J. R. Beddington, I. R. Crute, L. Haddad, D. Lawrence, J. F. Muir, J. Pretty, S. Robinson, S. M. Thomas, C. Toulmin, Food Security: The Challenge of Feeding 9 Billion People, Science 327, 812-818 (2010). A. Howell, Enhancing water use efficiency in irrigated agriculture. Agronomy Journal 93, 281-289 (2001). Oki Taikan, Shinjiro Kanae, Global Hydrological Cycles and World Water Resources, Science 313, 1068-1072, doi: 10.1126/science.1128845 (2006). Ghassemi, A.J. Jakeman, H.A. Nix, Salinization of Land and Water Resources (Univ. of New South Wales Press Ltd., Canberra, 1995). Russo, A. Laufer, A. Silber and S. Assouline, Water uptake, active root volume and solute leaching under drip irrigation: a numerical study. Water Resour. Res. 45, W12413, doi:10.1029/2009WR008015 (2009). Elimelech, W.A. Phillip, The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 333, 712-717. doi 10.1126/science.1200488 (2011). Silber, A., Y. Israeli, I. Elingold, M. Levi, I. Levkovitch, D. Russo and Assouline, Irrigation with desalinated water: a step toward increasing water saving and crop yields. Water Resour. Res. 51, 450-464, doi:10.1002/2014WR016398 (2015).

Shmuel Assouline is an ARO research scientist (the Israeli Agricultural Research Organization) at Bet Dagan, Israel. His fields of research include soil physics, irrigation, and surface hydrology. Yair Israeli is a research scientist (emeritus) affiliated to Jordan Valley Banana Experiment Station, Tsemach, Israel. Avner Silber is an ARO research scientist (emeritus) affiliated to the Northern R&D, Rosh Pina, Israel. His fields of research include soil chemistry, irrigation and plant nutrition. This article is based on an original research paper entitled ‘Irrigation with desalinated water: a step toward increasing water saving and crop yields.‘ published in Water Resources Research in 2015 by Silber, A., Y. Israeli, I. Elingold, M. Levi, I. Levkovitch, D. Russo and S. Assouline.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.