Research Accomplishment Reports 2007

Characterizing Mass And Energy Transport at Different Scales

O.O. Wendroth
Department of Plant and Soil Sciences

 

Project Description

Under certain conditions, surface-applied agrochemicals, i.e., fertilizers and pesticides can move rapidly downward through the unsaturated zone and cause negative effects on groundwater quality. Little is known on the impact and the coincidence of soil structural conditions, initial soil moisture, and upper boundary conditions subsequent to chemical application, especially amount and intensity of rainfall (Warnemuende et al., 2007) on leaching depth of surface-applied chemicals.

Plot experiments will be performed with continuously changing surface boundary conditions to study the impact of different rainfall scenarios on leaching depth and vertical solute distribution. An important question is: How many soil samples are necessary and how big should a sample be to obtain the local solute concentration? Bronswijk et al. (1995) determined solute depth profiles from 1-m-soil cores, and derived field-average solute transport behavior. Wendroth et al. (1998) observed field-average bromide transport using the same method but observations of solute concentration were spatially not structured.

Spatial representativity of solute concentrations can generally not easily be determined due to pronounced heterogeneity (Tseng and Jury, 1994). The objective of this study was to apply an anionic tracer (KBr) to a field with changing initial soil moisture conditions and to measure solute concentration at a high spatial resolution of 0.25 m sampling distances using two augers of different size spatially alternatingly. Spatial analysis should show whether solute concentration was spatially distributed randomly or with structure, and whether solute concentrations at different depths were crosscorrelated with each other.

Results show the different initial moisture conditions distinctively. In the 0-10 cm sampling depth, water contents fluctuate regularly from 6 to 11 m. These alternatingly higher and lower values do not appear in any other zone, nor at any other depth. Spatial distributions of anion concentration at the upper three and the subsequent three sampling depths yielded the following results: Anion concentration data in the 0 - 10 cm depth show no distinctive pattern and exhibit a large variance. The variance does not exhibit any spatial structure as obvious from the autocorrelation function. On the other hand, anion concentration values observed at depths between 10 and 40 cm show a distinct pattern and exhibit spatial structure.

Concentration measurements are spatially related (95 %) over a distance of four lags, corresponding to 1 m. Below 50 cm depth, the concentration values become relatively small, and do not exhibit a distinct spatial pattern, nor spatial autocorrelation structure. Probably, this result manifests that the applied tracer did not reach soil depths below 50 cm. Observations in each of the subsequent depths between 10 and 40 cm depth are significantly spatially crosscorrelated over distances of up to 1 m. Ellsworth and Boast (1996) derived shorter ranges of spatial dependence than in this study but in general, their findings of solute concentration being spatially correlated at distances below 1 m corresponds to our findings.

Impact

Objective 1: To develop an improved understanding of the fundamental soil physical properties and processes governing mass and energy transport, and the biogeochemical interactions these mediate The objective of the project is to study the impact of soil structural properties, initial profile soil moisture status and different rainfall scenarios on flow and transport of surface applied solutes. This goal requires appropriate sampling strategies for determining solute distribution in the soil profile in order to address the impact of initial and boundary conditions on solute transport. The determination of resident concentration and its spatial representativity is one of the main experimental conditions to be fulfilled for the subsequent experiment.

Objective 2: To develop and evaluate instrumentation and methods of analysis for characterizing mass and energy transport in soils at different scales In this pilot study, a sampling scheme was tested and validated that it is eligible to determine a measure of the locally representative solute concentration. This will be important for distinguishing between inherent random variability and treatment induced solute transport. Soil samples for solute concentration will be taken every 0.25 m in horizontal direction. This result is essential for the ongoing experiment this spring when rainfall intensity and amount as well as timing between chemical application and rainfall are evaluated with respect to their impact on leaching depth of surface applied solutes.

Objective 3: To develop and evaluate scale-appropriate methodologies for the management of soil and water resources Measurements of solute concentration taken over sampling distances larger than 1 m support an estimation of the average field leaching behavior. In order to obtain a site-specific estimate of leaching behavior, resident concentration values need to be obtained at spatial distances below 1 m. The results will show what state variables taken at the land surface will be important in the future to estimate leaching behavior in order to avoid unfavorable soil and weather conditions. Further impact: PI (Ole Wendroth) is now chair of the regional project W1188, and is member of a chore group writing the new regional project.

Publications

Loescher, H.W., J. Jacobs, O. Wendroth, D.A. Robinson, G.S. Poulos, K. McGuire, P. Reed, B.P. Mohanty, J.B. Shanley, W. Krajewski. 2007. Enhancing Water Cycle Measurements for Future Hydrologic Research, Bulletin of American Meteorological Society (BAMS) 88:669-676. DOI:10.1175/BAMS-88-5-669.

Wendroth, O., and N. Wypler. 2007. Unsaturated hydraulic properties: Laboratory evaporation. In: M.R. Carter and E.G. Gregorich (Eds.) Soil sampling and methods of analysis. Canadian Society of Soil Science, 2nd ed., CRC Press, Boca Raton, FL, pp. 1089-1106.

Lebron, I., M.D. Madsen, D.G. Chandler, D.A. Robinson, O. Wendroth, and J. Belnap. 2007. Ecohydrological controls on soil moisture and hydraulic conductivity within Pinyon-Juniper Woodland. Water Resour. Res. 43, W08422, doi:10.1029/2006WR005398.

Kersebaum, K.C., H.I. Reuter, K. Lorenz, and O. Wendroth. 2007. Model-based nitrogen fertilization considering agro-meteorological data. In: Bruulsema (ed.) Proc. Symposium "Integrating weather variability into nitrogen recommendations", Soil Science Society of America, Indianapolis, IN, Publ.: International Plant Nutrition Institute.

Flynn, E.S., C.T. Dougherty, and O. Wendroth. 2007. Assessment of Grassland Condition with the Normalized Difference Vegetation Index. Agron. J. 100:114-121.

Wendroth, O. and D.A. Robinson. 2007. Scaling Processes in Watersheds. Encyclopedia of Water Science 2. Taylor and Francis. (accepted, in press).