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Characterizing Mass And Energy Transport at Different Scales
O. O. Wendroth
Department of Plant and Soil Sciences
Wide regions of Kentucky are covered by karst landscapes. Due to the soil genesis, soils are well-drained and exhibit a short connection to the groundwater. We need to quantify, how fast fertilizer nutrients and pesticides applied during agricultural management are transported downward with incoming rainfall, in order to avoid groundwater pollution. The purpose of this long-term-research project is to quantify surface and subsurface fluxes of water, chemicals, and solutes to better understand the water dynamics in these landscapes and to contribute to strategies for managing soils in a way that maintains water quality while minimizing environmental risks. Moreover, since sinkholes and their associated hillslopes and catenas can differ in size and geometry, elevation, and soil profile features, rules for generalizing transport processes for different scales need to be derived.
2009 Project Description
In this report, outcomes are based on two studies, i.e.,
1) Field-scale bromide transport as a function of rainfall amount, intensity and application time delay, and
2) Using remote sensing and crop observations for identifying nitrogen deficiency and predicting crop yield of winter wheat .
1) Rainfall events following application of fertilizers and agrochemicals to the land surface can cause rapid and deep leaching of solutes to deep soil layers and pollute ground water. The rate and transport phenomenon of leaching strongly depend on the amount and intensity of a rainfall. Moreover, the time lag after a chemical application when the rainfall occurs can strongly influence the transport rate of a solute. The objective of this study was to quantify bromide transport at the field scale for different rainfall amount, intensity, application time delay and initial soil water content, while amount, intensity and time delay were varying at four levels each to derive strategies for avoiding rapid leaching of solutes. The experiment was carried out on a Maury silt loam soil at the Spindletop Research farm of the Agricultural Experiment Station of the University of Kentucky, Lexington, KY. The field site was divided into 32 plots, each being 2 m long and 4 m wide. These plots were located next to each other along a 64-m transect. One half of the field (32 plots) was pre-irrigated with water to cause a slightly larger initial soil water content than in the other half.
2) Although farmers usually manage their wheat fields uniformly, e.g., treat their fields with homogeneous amounts of mineral nitrogen fertilizer, grain yield strongly varies. This observation implies that nitrogen fertilizer is not applied in an optimum way, i.e., in some zones within a field it has been over-applied, in others under-applied, causing environmental or economic damages. Active and passive optical sensors may reflect zones with deficit or surplus nitrogen crop status. However, it is unknown to what extent crop indices, such as NDVI (Normalized Difference Vegetation Index) or RIRR (Red-Infrared Ratio) measured early during the growing season are related to the final yield. The objective of this study is to identify whether remote sensing devices used on-the-go across farmers' fields can identify zones of crop nitrogen status, and to find out whether a crop growth model applied to site specific soil information are able to capture final crop yield pattern for predicting crop yield and improving management decision support.
Field-scale bromide transport as a function of rainfall amount, intensity and application time delay
1) A KBr tracer was applied in October 2008 in a spatial design and time schedule that allowed subsequent rainfall to be applied in groups of four plots with a sprinkler irrigation system. This type of design allows studying transport processes and analyze observations with spatial and frequency domain approaches, such as geostatistics and spectral analysis. Center of mass was calculated for the bromide concentration depth profile of each core. Results were analyzed with respect to spatial covariance, i.e., semi- and crosssemivariance, power spectrum, cross- and quad spectrum and coherency.
Results are focused here to the center of bromide mass distribution along the profile and across the transect. Increasing the amount of irrigation water generally increased the leaching of bromide as expected. However, for the same rainfall amount, leaching was the more effective the lower the rainfall intensity. In other words, a high intensity rainfall may not cause as much leaching as a slow rainfall that would last longer. In our study, the time delay effect on bromide leaching depth was not strongly pronounced, i.e., regardless of whether the rainfall set in 1 hour, 4 hours, 1 day, or four days, the leaching occurred to a similar extent. The effect of initial profile soil water content, rainfall amount and intensity is manifested in the spatial covariance behavior of the center of bromide mass. Cospectral analysis reflects common variance components at the same wave lengths (1/frequency) between spatial rainfall variation and bromide center of mass.
2) Using remote sensing and crop observations for identifying nitrogen deficiency and predicting crop yield of winter wheat In an on-farm study, mineral nitrogen fertilizer was applied in a sinusoidal spatial distribution pattern at rates between 0 and 160 kg/ha. The nitrogen fertilizer response differed across the farmer's field due to underlying differences in soil texture and therefore soil fertility. Through crop indices, remote sensor results coincided with yield differences based on nitrogen response and spatial soil differences.
At the same time, a crop growth simulation model was applied with site-specific soil input and management information. The model was able to capture the spatial pattern in the yields caused by different nitrogen application rates, however, the model did not reflect the impact of soil differences across the field. Remote sensing devices were more sensitive to changing soil and management conditions and therefore provide a promising tool for farmers to apply nitrogen fertilizer in a site-specific and more sustainable way.
Wendroth, O., G. Schwab, D. Egli, and L. Murdock. 2009. Soil Mineral Nitrogen and Crop Biomass Dynamics of Winter Wheat in Space and Time in a Farmers Field. Annual Research Report of the University of Kentucky Wheat Science Group.
Wendroth, O., G. Schwab, L. Murdock, and K.C. Kersebaum. 2009. Field sampling for agricultural model input and parameterization. Abstract, Enhancing and Facilitating Use of Agricultural System Models in Field Research, Annual Meeting, ASA-CSSA-SSSA, Nov. 1-5, 2009, Pittsburgh, Pennsylvania.
Wendroth, O., R.J. Walton, and S. Nambuthiri. 2009. Spatio-temporal soil water processes at the field scale. Abstract, Application of Soil Physics to Resolving Environmental Problems: Honoring the Impact of M.Th. van Genuchten, Annual Meeting, ASA-CSSA-SSSA, Nov. 1-5, 2009, Pittsburgh, Pennsylvania.