Modelling nitrogen leaching from overlapping urine patches

Urine depositions have been shown to be the main source of N leaching from grazing systems and thus it is important to consider them in simulation models. The inclusion of urine patches considerably increases the complexity of the model and this can be further aggravated if the overlaps of urine patches are also considered. Overlapping urine patches are potentially important sources of N loss because the N load in these areas can be very high. In this work, we investigate a methodology to simplify the process of accounting for overlapping urine patches. We tested a two-stage approach, where, on one hand, the urine of two consecutive depositions could be aggregated and deposited at the time of the second deposition. This was called the Delayed Representation (DR) and would be useful when the delay (T"d) between overlaps is short. On the other hand, if T"d is sufficiently large, the depositions would become functionally independent and the urine patches could be considered separately. We called this the Independent Representation (IR). We tested this methodology by comparing simulations where the overlapping urine patches were considered explicitly and using the DR or IR in several combination of climates, soils and management options, chosen to span the likely range in New Zealand, using the Agricultural Production Systems Simulator (APSIM) model. The results from the simulations indicated that when T"d 180 days, the IR was found to be acceptable in all the climates, soils and management options simulated. This left a generic window, 20 < T"d < 180 days, where explicit simulations would be required. In some conditions, that window was considerably shorter. In a dry climate and shallow soil, the window was found to be 75 < T"d < 180 days, while for the simulations with a wetter climate and deep soils, that window was about 70 < T"d < 90 days, except for depositions in the middle of winter. These limits should apply only to long-term simulations where the user is interested in the average behaviour of the system, as considerable year-to-year variability was observed. We suggest that these windows can be used to guide the development of simulation models that include representations of urine patches under many conditions but where the soil and/or climate conditions vary markedly from those used here the analysis should be repeated.