2011 Annual Report 1a.Objectives (from AD-416) Objective 1. Develop improved understanding and quantitative descriptions of the effects of degraded irrigation waters on root water uptake and soil hydraulic properties. Objective 2. Develop improved knowledge and predictive capabilities of unsaturated flow and transport processes affecting retention, transformation, and transport of pesticides, pathogens, pharmaceuticals and other contaminants. 1b.Approach (from AD-416) Measurements of compensatory uptake are needed to formulate improved models of root water uptake. We will conduct greenhouse studies aimed at observing compensatory uptake in cotton for different stress conditions. Cotton is deep rooted, has high salt tolerance, and is an agronomically important crop in San Joaquin Valley, CA. In the past cotton has been observed to exhibit some compensatory behaviors, e.g. utilizing shallow groundwater to varying degrees depending upon root zone conditions. Current guidelines for degraded water use are based on the effects of sodium and salinity on the soil saturated conductivity, and do not consider the impact on unsaturated hydraulic properties. Experimental data for the unsaturated properties is sparse and contradictory. To aid in the formulation of improved guidelines for irrigating with degraded waters, we propose experiments aimed at elucidating the effects of saline-sodic irrigation water on unsaturated soil hydraulic properties. The HYDRUS software will be expanded to include features that enhance its suitability as an advanced simulation model and decision support tool for testing and implementing site-specific soil, water and crop management practices. Among the features to be added are modules to account for the transport of pesticides, pathogenic microorganisms, colloids, pharmaceuticals, and related constituents in unsaturated soils. Planned modifications include improved representations of straining processes, colloid aggregation, colloid transport in unsaturated soil, size exclusion in structured soils, colloid-facilitated transport, and chemical reaction, sorption, and degradation processes. 3.Progress Report The objectives of project 5310-61000-014-00D are to improve mathematical models of critical biogeochemical processes in irrigated agricultural soils and to use computer simulation to investigate alternative water and soil management practices and their effects on agricultural productivity and environmental quality. FY2011 was the final planned full year of the project. The project initiated in 2007 with 2.6 Scientists and 3.0 technical support staff. Following the retirements of 4 key personnel (2 scientists and 2 support staff), the project has been operating with 1.3 scientists and 1.0 technical support staff. Consequently, it has been necessary to prioritize milestones over the last couple of years. In FY2011, progress was made in modeling several key biogeochemical processes, supporting national program efforts to develop decision support systems for managing degraded irrigation waters, including tools for determining the fate and transport of salts, trace elements, and pesticides. Progress includes: 1. The permeability of a porous material (such as soil or rock) is a measure of the rate at which water can flow through the material and it is a critical parameter in a number of important hydrological processes such as infiltration, flooding, erosion, evaporation, groundwater flow, and the movement of pollutants belowground. Scientists have long sought to understand how different arrangements of pores in rock or soil lead to different permeabilities. In FY2011, we developed a new theoretical pore-scale model explaining the relationship between the permeability and electrical conductivity of porous materials and demonstrated that it was more accurate than earlier models. The findings will assist researchers seeking to measure or predict permeability and related important hydrological parameters and processes. (Obj. 2C) 2. Pesticide volatilization from agricultural soils is one of the main pathways in which pesticides are dispersed in the environment. Past investigations have shown that the volatilization rate results from non-isothermal transport process in the soil and depends critically on the water content of the soil surface. In FY2011, progress was made in modeling the effects of soil moisture status on pesticide volatilization. The new knowledge may contribute to management practices which will better protect agricultural workers and surrounding communities from potentially toxic emissions. (Obj. 2B) 3. Improved mathematical models for simulating the accumulation and leaching of salts and trace elements in irrigated soils are needed to maintain productivity with degraded irrigation waters. In FY2011, extensive salt accumulation and leaching data collected over four years were analyzed and compared with model predictions. The analysis will contribute to improved predictive models and decision support tools which will allow for greater utilization of marginal quality waters. (Obj. 1A) 4. A GUI was developed for the CHAIN-2D simulation model. (Obj. 2B) 5. Due to retirements and changing priorities, HYDRUS development has been transferred to our colleagues at the university and in industry (www.pc-progress.com). (Obj. 2A) 4.Accomplishments 1. Pore-scale physics explains soil and rock properties. The permeability of soil and rock is a critical parameter affecting a number of important hydrological processes such as infiltration, flooding, erosion, evaporation, groundwater flow, and the movement of pollutants below ground. Scientists have long sought to understand how different arrangements of pores in rock or soil lead to different permeabilities. In FY2011, ARS researchers at Riverside, California used techniques from theoretical physics to develop a new model explaining permeability in terms of pore-structure and demonstrated that the model was more accurate than previous models. The findings will assist researchers seeking to measure or predict permeability and related important hydrological parameters and processes. Review Publications Skaggs, T.H., Trout, T.J., Rothfuss, Y. 2010. Drip irrigation water distribution patterns: Effects of emitter rate, pulsing, and antecedent water. Soil Science Society of America Journal. 74(6):1886-1896. Perez Guerrero, J.S., Skaggs, T.H., Van Genuchten, M.T. 2010. Analytical solution for multi-species contaminant transport in finite media with time-varying boundary conditions. Transport in Porous Media. 85(1):171-188. Siyal, A.A., Skaggs, T.H., Van Genuchten, M. 2010. Reclamation of saline soils by partial ponding: Simulations for different soils. Vadose Zone Journal. 9(2):486-495. Corwin, D.L., Lesch, S.M., Segal, E., Shouse, P.J., Skaggs, T.H., Bradford, S.A. 2010. Comparison of model- and design-based sampling strategies for characterizing spatial variablity with ECa-directed soil sampling. Journal of Environmental & Engineering Geophysics. 15(3):147-162. Shouse, P.J., Goldberg, S.R., Skaggs, T.H., Soppe, R.O., Ayars, J.E. 2010. Changes in spatial and temporal variability of SAR affected by shallow groundwater management of an irrigated field, California. Agricultural Water Management. 97(5):673-680.