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Salt Management for Sustainable Degraded Water Land Application under Changing Climatic Conditions Runbin Duan* and Clifford B. Fedler Department of Civil and Environmental Engineering, Texas Tech University, Lubbock, Texas 79409, United States agricultural production land is losing sustainability due to salinization. The percentages of the soils with salinity problem on irrigated lands in “different countries are 27% for India, 28% for Pakistan, 13% for Israel, 20% for Australia, 15% for China, 50% for Iraq, and 30% for Egypt”.1 One common practice to solve the problem is using additional water to flush the salt out of the crop root zone. Currently, under changing climatic conditions, the trend is for surface waters to dramatically increase in salinity due to higher evaporations rates coinciding with increased average daily temperatures. In addition, groundwater appears to be increasing salinity due to increased deep percolation of high saline irrigation water. Global warming leads to higher water demand for crop production. More salt mass is brought into the soil profile through more irrigation water with higher salinity that accelerates soil salinization. As a result, salt management requires more additional water to wash salt out of or away from the crop root zone.
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SOME PROBLEMS IN CURRENT SALT MANAGEMENT Currently, the salt mass balance approach in common use often causes markedly erroneous results in the management of the salts because of the invalid assumption. This assumption is that there is no, or only minor, salt precipitation or salt dissolution occurring in the irrigated soil profile. Salt mass balance is widely regarded as a traditional tool to evaluate the effects of salt management and is identified as one of main components for designing a sustainable degraded water land application system.2 A long-term, worldwide accepted concept managing salt on irrigated lands is that salt mass out from the crop root zone should be equal to or higher than salt mass in so that soil salinization can be effectively reduced or even completely avoided. However, farm managers or farmers do not fully understand the mass balance of salts where it consists of more than mass in from irrigation and mass out via leaching. Salt precipitation can lead to an apparent unnecessary overuse of irrigation water, which is originally used for flushing salts below the root zone. Conversely, soil salt dissolution can cause more unexpected salt to be washed from the root zone. As a result, the salt management objective is wrongly regarded as completed. The current salt management inaccurately estimates the required leached water used to flush salt downward. There are two important terms in salt management: leaching fraction (LF) and leaching requirement (LR). LF is the ratio of the
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MORE LAND APPLICATION OF DEGRADED WATER IS REQUIRED BY CHANGING CLIMATIC CONDITIONS More frequent and severe drought and global warming are increasingly exacerbating the water crisis and threatening the global food security in arid and semiarid regions. Approximately 90% of the developed water has been consumed by irrigated agriculture that supplies 40% of the grain and fiber production. Therefore, more degraded waters must be used as irrigation water to reduce water pressure under changing climatic conditions. These waters mainly include treated wastewater from municipalities and industry, drainage water collected from irrigated agriculture, and waste streams from concentrated animal feeding operations. Possibly, hydraulic fracturing wastewater will be reused as irrigation water in the future. However, salt management is crucial for sustainability of land application systems.
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SALT MANAGEMENT IS BEING CHALLENGED BY CHANGING CLIMATIC CONDITIONS Changing climatic conditions are pushing salt management toward the unprecedented critical point such that it will determine the success of future irrigated agriculture and food security, especially in the arid and semiarid regions of the world. Land application using degraded water without appropriate management has caused high soil salinity for a number of years. Around half of the global irrigated land requires tremendous effort to deal with soil salinity. More than 45 million hectares of © 2013 American Chemical Society
Received: Revised: Accepted: Published: 10113
August 14, 2013 August 23, 2013 August 23, 2013 September 5, 2013 dx.doi.org/10.1021/es403619m | Environ. Sci. Technol. 2013, 47, 10113−10114
Environmental Science & Technology
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(2) Duan, R.; Fedler, C. B.; Sheppard, C. D. Field study of salt balance of a land application system. Water, Air, Soil Pollut. 2011, 215, 43−54. (3) Corwin, D. L.; Rhoades, J. D.; Šimunek, J. Leaching requirement for soil salinity control: Steady-state versus transient models. Agric. Water Manage. 2007, 90, 165−180. (4) Schnoor, J. Salt: The final frontier. Environ. Sci. Technol. 2013, 47 (5), 2152−2152. (5) Rozema, J.; Flowers, T. Crops for a salinized world. Science 2008, 322, 1478−1480.
leached water to the applied water. LR is the ratio of the required leached water to the applied water. Theoretically, the LF should be higher than the LR so that the soil salinity can be kept in an appropriate range for avoiding unexpected crop yield loss. Currently, the widespread method used to determine the LR is based on the steady-state, crop-specific leaching requirement model. With the aid of modern high-speed computers, the LR determined by traditional methods is found to have been overestimated compared with the newly proposed transient LR models.3 Overestimation of the LR definitely causes overuse of water and increases the burden of local water supplies. However, these transient LR models are still not proven to be applicable and effective in salt management in large fields under varying field conditions. Furthermore, current salt management still does not fully consider and even highlight current changing climatic conditions in regular management practices, either sustainability of agricultural production and food security in environmental, economic, and social dimensions. For examples, current salt management does not consider methods to handle the salinity increase in surface water and groundwater brought about by global warming in arid and semiarid areas. The targeted salinity level used for estimating the LR does not account for the salinity impact to local ecosystems. As we know, small changes of salinity may result in changes of species composition of local ecosystems.4 The LR calculation solely focuses on the target yield without consideration of profit optimum in irrigated agriculture. Current salt management mainly highlights the targeted salinity level in the crop root zone, but ignorance of salinity increases in downstream water resources due to salt wash off or salt percolation may significantly damage production of downstream staple crops and trigger social instability.
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IMPROVING SALT MANAGEMENT IN THE FUTURE A holistic survey should be conducted to investigate soil salinization, salinity levels in various water bodies, and salinity impacts to local ecosystems, economy and social communities in the degraded water irrigated agricultural regions. The nearreal-time salinity monitoring technology and systems are in emergent and great need and should be developed and established for sustainable salt management. Under changing climatic conditions, future salt management calls on innovative technology, strategy and policy so that food security can be ensured without compromising environmental, economical, and social sustainability. Halophytes with high water-use efficiency and drought tolerance, sufficient market demand and biomass production potential should be pursued and encouraged5 as part of the crop rotation systems so that salt can be effectively removed from the sites and the salt impacted lands can be sufficiently remediated.
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AUTHOR INFORMATION
Corresponding Author
*(R.D.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Stockle, C. O. Environmental impact of irrigation: A review. http://www.swwrc.wsu.edu/newsletter/fall2001/IrrImpact2.pdf (accessed July 18, 2013). 10114
dx.doi.org/10.1021/es403619m | Environ. Sci. Technol. 2013, 47, 10113−10114
