Runoff Generation Mechanism in the Appalachian Plateau Region of Alabama - A Field Investigation

Abstract

Alabama is one of the largest poultry producing state in the country. However, excessive land application of litter to pastures in Alabama has resulted in buildup of phosphorus (P) in soils of major poultry producing counties located in the Appalachian Plateau region in North Alabama. Alabama has developed P-index to alleviate water quality impacts of P loss from soils. Alabama's P-index is based on agronomic soil P thresholds and treats an entire field as the surface runoff (and P) contributing area. This approach often leads to over application of litter to areas with high probability of P loss via surface runoff and an improper and incomplete assessment of fields for litter application. Since the primary mechanism of P transport is surface runoff, it is important to identify hydrologically active areas (HAAs) and the surface runoff generation mechanisms within a field. Through an intensively-instrumented field study we are identifying HAAs. Specific objective of this study is to delineate spatial and temporal distribution of HAAs and identify surface runoff generation mechanism (infiltration excess vs. saturation excess) using distributed sensors. Results from two events showed that the surface runoff generation mechanism is mostly infiltration excess. During intense storm (e.g., Event 1) surface runoff was observed from fairly large areas within the field. For these events, the surface runoff generation areas originated at the same location and expanded and contracted depending on the rainfall and other hydrologic characteristics. Certain hydrologic characteristics (e.g., rainfall intensity, antecedent soil moisture conditions, saturated hydraulic conductivity and presence of sandstone layer) seem to play dominant role in surface runoff generation in this region. Thus, this study demonstrates that spatial and temporal distribution of HAAs can be characterized. If spatial and temporal distribution of HAAs can be predicted by a few key variables, it will have a huge impact on the management of P from land-applied poultry litter. Hence, this study will greatly contribute to science-based decision making and will increase the acceptability of Federal and State regulations that are based on sound science.