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United States Department of Agriculture

Agricultural Research Service

Research Project: Improved Agricultural Management Practices to Reduce Adverse Impacts on Water Resources in Semi-Arid Regions

Location: Contaminant Fate and Transport Research

2012 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 relates to objective 1 of the parent project, "Assessment and improvement of existing modeling technologies for simulating regional scale hydrogeology, including land surface processes associated with agricultural water management; modeling assessments of the impacts of existing and changing agricultural water and land management practices on water quality for regions of interest". The coupled transport of liquid water, vapor, and energy in the surface soil layer influences a variety of hydrological and ecological processes. 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 were conducted to test various methods to measure soil moisture content, soil heat flux and evaporation in a 15 cm (ID) by 75 cm column fitted with several types of moisture and heat-flux sensors. The sensors included: a dual needle heat pulse sensor, a triple needle heat pulse sensor, a 5TM water and temperature sensor (using capacitance/frequency domain technology), a water matric potential sensor Campbell Science 229-L (heat dissipation method), and soil relative humidity probes HC2-C05. Preliminary results indicate that estimation of the soil energy balance with the heat-pulse probe was affected by the placement of the probes. It appears that the evaporation zone was very near the soil surface; possibly within a few millimeters of the soil surface. Since it is likely that evaporation was occurring within the upper few millimeters of the column, it proved challenging to estimate surface evaporation with a heat-pulse sensor. Future experiments will increase the sampling time to determine if the probes can be utilized to measure evaporation as the front proceeds downward. Another difficulty was obtaining accurate moisture values across a range from very dry to near saturated conditions. In general, none of the tested probes were able to give accurate moisture contents over the full spectrum. Further research is needed to find the optimal method for measuring the full range in water contents. Preliminary tests were conducted in Field 2-B, University of California-Riverside Field Station using the eddy covariance technique to measure energy and water fluxes in field settings. Useful data was obtained and this methodology will be used in future experiments comparing micro-meteorological evaporation measurements with soil-based measurement obtained using water and heat-flux sensors.

4. Accomplishments

Last Modified: 2/23/2016
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