Critical loads. Which mode of phosphorus transfer (PT) and which pathway? (see Figure 1). Also a need to consider the focus - farm, field and river are inter-dependent and cannot be considered separately. Farm balances affect field balances and thus loads to rivers. Where and when is P applied – incidental transfers may present a short-term risk, whereas dissolution transfers may be long term (thus risk when)? When are the surface waters must vulnerable? If ultimately we want to determine and control critical loads to rivers we need a comprehensive understanding of hydrological mechanisms and pathways – this will provide the basis for assessing hydrological risk.

Hydrological risk. How much water will go through the system and where? The role of hydrology is central and fundamental in the phosphorus transfer (PT) regime (hydrology is the ‘energy and carrier’ of P – Figure 1), but remains difficult to simplify. We need to agree and define the spatial and temporal controls on P transfer via the various hydrological pathways.

Temporal considerations are rainfall intensity, duration and the interval between rainfall events. Risk will vary temporally. These influence the discharge (and hence P load) to receiving waters. Some research has shown that two ‘populations’ of data can be identified in terms of hydrology and water flow  . Perhaps there is an opportunity for us to define threshold levels of hydrological activity? For understanding mechanisms, hydrological activity may be nominally classified at two levels. Level 1 activity occurs during light or little rainfall for a high proportion of time; in contrast Level 2 activity occurs less frequently, but is more energetic and has a large capacity for P transfer over a small time (storm) period. Concepts such as soil water status and effective rainfall are important here.

Spatial considerations are tremendously variable and there is definitely an opportunity for us to define scales and pathways. Perhaps spatial variation in the hydrological pathways can be classified at two scales: (i) soil profile, (ii) slope/field, which subsequently can help understanding of P transfers into the wider catchment, where problems occur. Risk will vary with pathways.

Terminology/definitions. The range of potential hydrological pathways present scope for confusion, as terminology varies – an attempt at defining pathways is given in Table 1. Often process terms, such as leaching, or generic terms, such as drainage, are confused with hydrological pathways per se. Runoff is also an ambiguous term consideration – is it a process, a pathway or a water sample? These terms need careful consideration.

1. Identify pathway and identify catchment/watershed area.
2. Weir/tipping bucket system will be necessary for determining flows.
3. Identify threshold flows/rainfall intensity to separate level 1 and level 2. You may wish to identify extra ‘threshold’ levels.
4. Automatic water samplers will be necessary for level 2 or more, and flows/rainfall will need to be logged digitally and connected to the water sampler as a potential trigger.
5. Level 1 sampling will be appropriately sampled on a ‘policing’ basis – eg. daily, at all times when soils exceed field capacity. Automatic samplers may not be necessary for this.
6. Level 2 sampling will need to be flow proportional and will require some automated mechanism for increasing sample frequency in relation to flow – the aim being to capture the rise and fall of the storm hydrograph.
7. Calculate export coefficients using flow weighted means.

 Table 2. Terminology commonly associated with hydrochemical transfer pathways, nominally classified by discipline, time and spatial scale.