|Weatherington-Rice, J - BENNETT&WILLIAMS CONSULT.|
Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 22, 2004
Publication Date: April 22, 2004
Citation: Malone, R.W., Shipitalo, M.J., Ma, L., Ahuja, L.R., Rojas, K.W., Logsdon, S.D., Weatherington-Rice, J. 2004. Role of macropores in pesticide transport to groundwater. International Workshop on Applications, Enhancements, and Collaborations of ARS RZWQM and GPFARM Models. p. 39-40. Technical Abstract: Agricultural system models are useful for a variety of reasons. Models are tools which policy-makers can use to evaluate decisions and which scientists can use to investigate water quality problems. A model can gauge how well we understand a system or process - if we can't model it, we don't understand it. In structured soil, pesticide transport to shallow groundwater and subsurface drains is usually dominated by macropore flow. Therefore, the macropore flow process must be understood to accurately predict pesticide transport to shallow groundwater and subsurface drains. Two discoveries were made concerning the interaction of macropore flow and pesticide transport by investigating pesticide transport through 30 x 30 x 30 cm undisturbed blocks brought into the lab, applying rainfall shortly after pesticide application, and applying the Root Zone Water Quality Model (RZWQM) to this data. The first discovery was that greater "effective" macroporosity results in less chemical transport through macropores when runoff is zero. Part of this discovery included confirmation that water and chemical movement through macropores is dominated by a fraction of total macropores (effective macroporosity). If runoff is not produced, greater effective macroporosity results in more soil volume for pesticide sorption and lower RZWQM simulated pesticide concentration in percolate. This is counterintuitive because it seems like more macroporosity would result in higher pesticide transport. Decreasing the number of percolate producing macropores (nmacro) by -50% resulted in about +200% increase in RZWQM simulated alachlor transport through macropores. The second discovery was that greater tillage intensity has little to no affect on the number of effective macropores, but increased tillage intensity may decrease the macropore radius and affect the soil hydraulic properties such as increase soil matrix saturated hydraulic conductivity. At present, RZWQM allows input of a static effective macroporosity. Research suggests that effective macroporosity increases with increasing soil water content. Also, RZWQM simulates that macropores are destroyed during tillage and slowly reform during soil reconsolidation. As a result, RZWQM may incorrectly simulate pesticide transport after tillage. Future research should address development of changing effective macroporosity with changing soil water content and RZWQM should be modified to adjust macroporosity with tillage as current research suggests.