Integrating temporal inequality into conservation planning to improve practice design and efficacy

Authors: Williams, Mark; King, Kevin; Penn, Chad

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2018
Publication Date: 6/11/2018
Citation: Williams, M.R., King, K.W., Penn, C.J. 2018. Integrating temporal inequality into conservation planning to improve practice design and efficacy. Journal of the American Water Resources Association. 54(5):1029-1054. https://doi.org/10.1111/1752-1688.12662.
DOI: https://doi.org/10.1111/1752-1688.12662

Interpretive Summary: It is widely known that the majority of nitrogen (N) and phosphorus (P) losses occur during storm events, but there are currently few approaches that utilize this valuable information in conservation planning. In this study, we develop a framework to include these 'hot moments' in the conservation planning process by analyzing data from 40 tile-drained agricultural fields in Ohio. Results showed that 80% of nutrient losses occurred between 7-12 days per year in surface runoff and between 32-58 days per year in tile drainage. Using data from the monitored fields, we were able to develop design flows for structural conservation practices such as woodchip bioreactors, saturated buffers, and P removal structures. The results of the study highlight that the combination of in-field (e.g., nutrient and water management), edge-of-field (e.g., structural conservation practices), and in-stream practices will achieve water quality targets in tile-drained watersheds.

Technical Abstract: In contrast to spatial inequality, there are currently no methods for leveraging information on temporal inequality to improve conservation efficacy. The objective of this study was to use Lorenz curves to quantify temporal inequality in surface runoff and tile drainage, identify controls on nutrient loading in these flow paths, and develop design flows for structural conservation practices. Surface runoff (n=94 site years) and tile drainage (n=90 site years) were monitored on 40 fields in Ohio, USA. Results showed that, on average, 80% of nitrate-N, SRP, and total P loads occurred between 7-12 days per year in surface runoff and between 32-58 days per year in tile drainage. Similar temporal inequality between discharge and load provided evidence that loading was transport-limited and highlighted the critical role hydrologic connectivity plays in nutrient delivery from tile-drained fields. Design flow criterion for sizing structural practices based on load reduction goals was developed by combining Lorenz curves and flow duration curves. Comparing temporal inequality between fields and the Maumee River, the largest tributary to the western Lake Erie Basin, revealed challenges associated with achieving watershed load reduction goals with field scale conservation. In-field (i.e., improved nutrient and water management), edge-of-field (i.e., structural practices), and in-stream practices will all be required to meet nutrient reduction goals from tile-drained watersheds.