Modeling Halophytic Plants to Improve Agricultural Production and Water Quality in Arid and Semi-Arid Regions
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A major problem for irrigated agricultural production in arid and semi-arid environments is salinization of land. Irrigated land accounts for about one third of the world's food, but nearly one fifth of irrigated lands are salt affected and suffer from reduced yield due to soil salinization. Many farmers worldwide currently leach their lands to remove salt, but this practice can create further problems such as polluting nearby water sources with salt, fertilizer, and pesticides. Most common cultivated crops are known as glycophytes and suffer from reduced yield when subjected to salt stress. However, about 1% of the world's flora are known as halophytes, or plants that are capable of completing their life-cycle in higher saline soil or water environments. Halophytes are not commonly cultivated, but may be useful for human consumption, biofuel, or animal consumption. As a first step to assessing the potential of halophytic plants for salinity management, the Agricultural Policy/Environmental Extender (APEX) model was updated with a module to simulate plant-water-soil salinity dynamics using electrical conductance. The halophytes Atriplex nitens, Climacoptera lanata, and Salicornia europeae were parameterized in the APEX model's plant database. Plant, soil, and water data from field sites in the Central Kyzylkum and Khorezm regions of Uzbekistan were used to set up APEX models for two field sites. Measured data collected from the two field sites in 2013 were used to assess model performance. Although APEX ran with the salinity module and produced output, analysis of the output indicated that further work is needed to produce a model that will be useful for assessing salinity management with halophytes. A sensitivity analysis was performed on 47 parameters in APEX, and 14 were found to be sensitive for biomass, crop height, and soil electrical conductance (EC) output, including soil parameters such as soil albedo, sand content, and silt content. After running 500 simulations with random combinations of sensitive parameters, best fit results between observed and modeled values for crop biomass, crop height, and soil EC had deviations of as much as 42.5 tonnes/ha biomass, 200 cm of crop height and 23 mS/cm of EC, respectively. Suggestions for model improvements include enabling the modeling of individual salt ions because plants may experience toxic effects of different ions. Additionally, halophytes and conventional crops will die or fail to germinate under threshold salinity levels, but this relationship was not demonstrated with the model. Some halophytes have an increased yield under moderate soil salt levels, and salt can percolate into deeper soil layers, but these phenomena were also not simulated by the model. Future iterations of this project will benefit from more field data. Daily weather from the modeled time period should be used instead of generated weather based on monthly statistics. Plant and soil data should be taken at more frequent intervals, preferably at least once a month.