Hydrologic modeling of runoff from a livestock manure windrow composting site with a fly ash pad surface and vegetative filter strip buffers

Abstract

<p>Windrow composting of livestock manure materials provides a strategy for converting organic wastes into a recyclable soil fertility product that is less hazardous to the environment. Although outdoor windrow composting can produce runoff that is detrimental to surface water quality, vegetative filter strip (VFS) buffers were reported to significantly reduce runoff and contaminants from a windrow composting research site. To estimate the efficacy of VFS buffers and other best management practices on runoff from future windrow composting facilities, a computer hydrologic model may provide a valuable tool for predicting runoff losses from these proposed sites. This research evaluated a windrow composting/vegetative filter strip buffer (WCVFS) hydrologic model for estimating runoff volume losses from a livestock manure-based windrow composting site with a fly ash composting pad surface and VFS buffers. Runoff and physical attribute data from six rainfall events during 2002 to 2004 at a central Iowa windrow composting research site were used in the WCVFS model evaluation. Three rainfall events were designated as “wet” composting period events (2002 and 2003 seasons), and three were designated as “dry” composting period events (2004 season). Runoff data were comprised of average observed runoff volumes from three compost windrow area:VFS buffer area ratio treatments that included 1:1, 1:0.5 (large and small VFS buffer areas, respectively), and a 1:0 (no buffer) control. The WCVFS model performance was good to very good for the 2003 wet composting period model validation rainfall event with no significant differences among 1:1, 1:0.5, and 1:0 ratio treatments for simulated versus observed runoff volumes. In contrast, WCVFS model performance was unsatisfactory for the 2004 dry composting period validation event with significantly higher simulated runoff volume from the 1:0.5 ratio treatment versus observed runoff volumes. There were no significant differences for the 1:1 and 1:0 treatments. The WCVFS model effectively estimated 1:1, 1:0.5, and 1:0 treatment runoff volumes from the earlier wet composting period and 1:1 and 1:0 treatment runoff volumes from the later dry composting period rainfall events. However, the soils data-derived VFS buffer runoff and infiltration functions in the WCVFS model flow routing component may not have sufficiently accounted for some short-term hydrologic changes in VFS buffer soil and fly ash pad surfaces. This could have resulted in overestimation of dry composting period simulated runoff volume from the smaller 1:0.5 ratio VFS buffer area treatment. Consequently, the use of other alternatives to soils data-derived VFS buffer runoff and infiltration functions should be evaluated in future WCVFS model simulation trials to potentially improve runoff volume prediction accuracy.</p>