Modeling the effect of rainfall intermittency on the variability of solute persistence at the soil surface

Abstract

[1] A number of processes contribute to the decrease in solute concentration in the near surface; these include leaching and degradation. The combined effect of all loss processes is termed dissipation; its converse is termed persistence. In this paper we derive analytical expressions for the statistics of the time for a chemical, which undergoes linear equilibrium sorption and first-order degradation under the influence of a random series of rainfall events, to dissipate to a specified concentration. Rainfall is characterized by a random time between events and a random storm depth, both of which are assumed to be exponentially distributed. Analytical expressions for the mean and variance of the time for dissipation to 50% (DT 50 ) and 10% (DT 90 ) of the applied concentration are derived. The model is in qualitative agreement with empirical observations of pesticide dissipation. The model results indicate that on average these solutes tend to dissipate exponentially with time. Rapid dissipation initially, followed by slower dissipation later, is more likely to occur. This may contribute to observations of apparent multirate dissipation which can be explained without recourse to sorption or biological kinetics. A larger average storm magnitude and/or more frequent storms reduce the mean and increase the coefficient of variation of dissipation times. A normal distribution, parameterized using the derived statistics, approximated the probability distribution of dissipation times reasonably well. On the basis of this we obtain a measure of the potential amount of chemical available for rapid off-site leaching at a given time since application.

A preferential flow leaching index

“…We model here a thin layer near the soil surface containing pesticides with a concentration in the liquid phase given by C [M L −3 ], which fully mixes with water from intermittent rainfall events [ Leonard , 1990; Jarvis et al , 1994; Steenhuis et al , 1994; Zhang et al , 1997, 1999; McGrath et al , 2008b]. The near surface mixing layer is parameterized by a depth z l [L] and constant volumetric water content θ l [L 3 L −3 ].…”

“…In order to begin to assess the role of the timing and magnitude of rainfall events on leaching we assume that the duration of rainfall events is small in comparison to the time between events. This allows us to consider the loss of solute from the source zone to be a piecewise continuous process, with instantaneous leaching losses due to rainfall events and continuous losses due to degradation between storms [ McGrath et al , 2008b]. Using the transformed variable x ( t ) = ln[ C ( t )/ C (0)] the dimensionless concentration remaining just prior to a rainfall event is given by where t b the time till a rainfall event.…”

“…More recently, McGrath et al [2008b] derived statistics of the persistence of chemicals in the source zone, subject to a random rainfall signal. It was assumed there that pesticide dissipation can be described by a piecewise continuous process, with continuous degradation between rainfall events and discrete losses during them.…”

Identifying a rainfall event threshold triggering herbicide leaching by preferential flow

“…Recently the structure of rainfall has begun to be included within a developing theory of preferential flow and pesticide leaching [ Heppell et al , 2002; Malone et al , 2004; Struthers et al , 2007; McGrath et al , 2007, 2008a, 2008b; Nolan et al , 2008]. Heppell et al [2002] used repeated applications of a mobile tracer at the soil surface to identify a combination of rain event duration and rain event intensity as controls on the amount of preferential flow occurring in a heavy clay soil.…”

Modelling pesticides transfer to surface water at the catchment scale: a multi-criteria analysis

“…The pesticide fluxes generated within an agrosystem are governed by the movement of water and the hydrologic and hydraulic characteristics of the catchment. Pesticide persistence in soil and fluxes are also related to rainfall (Ramwell et al 2002), in particular the intensity of the rainfall event (Pot et al 2005;McGrath et al 2008McGrath et al , 2010.…”