MODELLING THE SALINITY BUILDUP AND UNDERSTANDING TREE STRESS IN A CHANGING CLIMATE AT A PECAN ORCHARD IN TORNILLO, TX: IS DRYLAND IRRIGATION SUSTAINABLE AT AMERICAN SOUTHWEST? (Invited Presentation)

Along the Rio Grande valley in western Texas, soils are developed on floodplain sediments and have been irrigated to grow crops like pecan, cotton, and alfalfa. Soils become saline and sodic after just 100 years of cultivation. In a pecan orchard near Tornillo, TX, our studies have shown that soil salinity has exceeded the pecan trees’ tolerance levels, stunting their growths, especially in soils with finer texture. To assess if irrigated agriculture can be sustained in the region, it is critical to better quantify and model the rate of salt accumulation in future scenarios. To do so, we have carried out extensive field investigation on chemistry of soil, drilled core samples and groundwaters, water dynamics, and tree physiology, as well as reactive transport modeling, and focused on how this managed critical zone will respond to climate variability, and a shift of irrigation water source. Our results show that the salt accumulation is observed as deep as 10 m, and its rate is not constant but projected to increase with time, especially when precipitation of secondary calcite clogs the pores, reduces infiltration and leads to more evaporative water losses. Indeed, a petrocalcic layer (caliche) can potentially be developed at soil surface after just another 100 years, leaving little to no pores for water infiltration or salt leaching. This could occur in even shorter period of time (30-50 years) if the deep groundwater is used for irrigation during droughts, since it is characterized by much higher total dissolved solids and loads more salts than the Rio Grande river. The American southwest is projected to become dryer and hotter in the summer, potentially losing more water by evaporation and consequently demanding more irrigation, but also stressing the trees via the heat waves and changes in osmotic water pressure. This study helps to assess the soil quality deterioration by irrigation at aridland soils, and more importantly, better predict how this will challenge the future agriculture due to climate variability and intense management practices. A better understanding of the interconnected processes of water, trees, soils and salts in irrigated dryland agriculture allows us to invest in projects that would increase the resilience of this anthropogenically impacted Critical Zone to seek sustainable solutions.