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An Instantaneous Unit Hydrograph for Estimating Runoff from Windrow Composting Pads

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  21st Century Watershed Technology: Improving Water Quality and the Environment Conference Proceedings, May 27-June 1, 2012, Bari, Italy  12-12227.(doi:10.13031/2013.41454)
Authors:   Ernest W Tollner, Owen J Duncan, Herbert Ssegane
Keywords:   Runoff Models, Gravel Aggregate, Windrow Composting Pads, Tank Model, Curve Number

A MatLab-Simulink compartmental dynamic model was developed for a windrow composting pad. The physically based MatLab-Simulink approach offered the benefit of effectively evaluating the effects of different pad configurations on runoff rates and amounts. Between December 23, 2010 and June 11, 2011, we recorded measurements of rainfall and containment pond storage and pond pumping on 10-minute time intervals. Infiltration rate and percolation through the crusher-run media were measured using double ring infiltrometers. We compared the containment pond stage predictions from the MatLab-Simulink compartmental models to the observed pond stage and found R2values greater than 0.9, Nash-Sutcliffe statistics greater than 0.9 and Root-mean-square-errors (RMSE) less than 1% of the full pond volume on 16 storms occurring from Dec 23, 2010 to June 21, 2011). A continuous simulation over the calibration period (December 23, 2010 to January 30, 2011) revealed good agreement of MatLab-Simulink and observed pond volume. The MatLab-Simulink compartment model appeared to capture the physical runoff process occurring on the pad, except for the impact of the compost material itself. With the MatLab Control Systems Toolbox, one could linearize the MatLab-Simulink model at several levels of compartmental storage. A transfer function for each linearized model made possible the determination of the impulse response at each linearized condition. The instantaneous unit hydrograph is the impulse response at the appropriate linearization. The analysis yielded a typical unit hydrograph reminiscent of the tanks-in-series analyses. From these analyses, we proposed an instantaneous unit hydrograph for similarly constructed windrow compost pads, which is also usable in other situations.

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