Location: Contaminant Fate and Transport Research2013 Annual Report
1a. Objectives (from AD-416):
(a) Determine the impact of soil characteristics (hydraulic conductivity- and pressure-moisture relationships), heat and water vapor transport and energy/mass exchanges (e.g., solar radiation, water) on soil water and chemical status (i.e., salinity, nutrients, trace elements) of the soil. (b) Test, and develop when necessary, sensors to measure soil water content covering the range from saturation to very dry conditions. (c) Test an improved non-isothermal, two-phase water flow model for simulating water movement and the accumulation and leaching of salts and trace elements in irrigated soils that was developed by Unit scientists in FY2010. (d) Relate this information to agronomic production and the protection of water resources. Ensure that this information can be incorporated into regional scale assessments of ground water quantity and quality that will be a focus of future research.
1b. Approach (from AD-416):
(a) Conduct laboratory and field experiments to measure the status of soil water in semi arid regions over wetting and/or drying cycles. These experiments will elucidate the interactions between irrigation, water and water vapor movement in soil, and evaporation from soil during irrigation and drainage cycles and as affected by soil heating and solar energy inputs. Several methodologies for measuring soil water content will be tested and compared to gravimetric sampling (e.g. capacitance, TDR, psychometric). Obtain direct measurement of evaporation and sensible heat flux using the eddy covariance method. The information will be used to develop a measurement database, for model testing, and to obtain modeling uncertainty estimates for use in future regional scale salinity and nutrient assessments. (b) Test a newly develop non-isothermal water and solute transport model to elucidate the accuracy of simulated soil water content and evaporation. Compare simulation to field-scale soil water content and evaporation measurements. This research will include measurement of various soil physical properties, evapo-transpiration, heat transfer, energy balances and soil-atmospheric exchanges to characterize the moisture status of soils. Models will be tested and improved models will be developed, where necessary, to enable prediction of soil moisture status from saturated to very dry conditions.
3. Progress Report:
This research is related to objective 2.2 of the parent project, "Development of new technologies for improved modeling of coupled overland and subsurface flow and transport". The coupled transport of liquid water, vapor, and energy in the surface soil layer influences a variety of hydrological and ecological process. Theoretical models of evaporation processes, based mainly on the work of Philip and de Vries, are often in disagreement with experimental data collected on moderately dry soils. A major limitation to improving theory is inadequate measurement methods and technologies. In these studies, data from the column provided detailed measurements of evaporation, soil moisture, and soil temperature during soil drying. Laboratory studies continued to test methods of measuring soil water content in soil columns. Several moisture and heat-flux sensors are being evaluated, including: dual needle heat pulse sensors, triple needle heat pulse sensors, TM water & temperature sensor which uses capacitance/frequency domain technology, Campbell Science 229-L (heat dissipation method) water matric potential sensors, and soil relative humidity probes, HC2-C05. To date, obtaining reliable and accurate soil water content measurements from a soil column has been elusive. In general, none of the tested probes were able to give accurate moisture contents over the full spectrum of water content. Further research is needed to find the optimal method for measuring the full range in water contents. Work will continue on this topic, since accurate water content measurements are needed to fully test non-isothermal heat and water transport theory, the primary goal of this research project.