Simple models for phosphorus loss from manure during rainfall.

the two kinetic equations are the maximum amount of water-extractable phosphorus (WEP) and a characteristic desorption time. Water- nure for total loss per rainfall event. Predicting the timing extractable P can be measured directly but currently the characteristic duringthestormis,however,neededwhentheinfiltration time can only be obtained by fitting experimental data. In addition, rate during the storm varies and part of the released P we evaluated two models usually used to estimate P loss from soil, from the manure enters the soil (Steenhuis et al., 1981). the Elovitch equation and power function, both of which relate P loss Moreover, regression-based models do not give insight to time. The models were tested against previously published data of into the processes and may fail for situations outside P release from different manures under laboratory conditions. All the range for which they were tested. equations fit the data well. Of the two kinetic equations, the second- Thus, our objective was to develop and evaluate relaorder model showed better agreement with the data than the first- tively simple, mechanistically defensible mathematical order model; for example, maximum relative differences between the expressions to predict the release of P from manure in model results and measured data were 2.6 and 4.7%, respectively. rain or other leaching waters. This paper presents four The characteristic times varied between 20 min for dairy manure and analytical models that predict the release of dissolved almost 100 min for poultry manure. The characteristic time did not P from manure with time. We used previously published appear to change with flow rate but decreased with smaller manure laboratorydatabySharpleyandMoyer(2000)totestthe aggregates. The parameters for power-function relationships could models’ ability to predict total dissolved phosphorus not be related to measured manure characteristics. These results pro(TDP) release from a variety of manure types. Total disvide the first step to process-based approximations for predicting solved P is defined as the total P after filtration through P release from manure with time during rainfall shortly after land application, when P losses are the greatest. 0.45-m filter (Haygarth and Sharpley, 2000). Experimentsby Muck(1978) wereemployed tofurther explain the dissolved P loss.