Developing Hydrogeophysical Tools to Study Hydro-bio-geo-chemical Processes in Dryland Agricultural Fields at the Nexus of Food, Water and Soils

Abstract:

Critical zone processes in drylands are typically considered inactive due to their low precipitation and vegetation cover. However, with increasing demand for food, more drylands are converted into agricultural systems. Once driven by irrigation water and growing crops, drylands become hydro-bio-geo-chemically dynamic. We have begun several pilot studies near El Paso, TX, along the Rio Grande Valley to understand the changes in the critical zone processes of these transformed drylands. Our study area is hot and dry, and is filled with fluvial sediments with layers of contrasting particle sizes and permeability. Typically, 1 m of water is applied every year in pecan orchards by flood irrigation, mainly from Rio Grande with some addition from local groundwater.

Combining hydrological, geochemical and hydrogeophysical tools (e.g., resistivity, GPR) to study these managed agricultural systems has great implications for soil sustainability and water resources management. Irrigation alters surface and subsurface hydrologic regimes and impacts the regional water balance and quality of the adjacent aquifers. Indeed, pathways and residence time of water within the heterogeneous soil substrates are critical for identifying efficient and effective irrigation strategies, assessing the water connection among Rio Grande, groundwater and agricultural return flows, and examining the variations in the groundwater level by pumping and recharge during intensive irrigation. Furthermore, solute transport is controlled by water flow. High evapotranspiration rates in these systems lead to water loss and salt accumulation, especially in fine-textured soils, significantly degrading soil quality. In addition, soils are typically amended with fertilizers and pesticides, and irrigation moves nutrients and toxic trace metals from soils to other water reservoirs along its flow paths, impacting drinking water quality.

In summary, dryland agricultural fields represent an important Critical Zone end-member that are highly modified by human activities. They are biologically and chemically active, driven by alternating wet and dry soil conditions, but challenging to study at regional scales due to layered geological structure and heterogeneity in their hydrological properties.