Rainfall in the Lake Brunner catchment is frequent and often heavy. Annual rainfall is around 4-5 meters per year. Therefore many of the farm dairy effluent management recommendations that are suitable for much of New Zealand are not practical in the Lake Brunner catchment.
Normally, around the country, regional guidelines for the application of FDE to land only allow irrigation if the soil water deficit (SWD) is over 10 mm, but given the practical difficulties presented by the wet climate, regional guidelines for the Lake Brunner catchment allow for FDE to be applied to land when the SWD is over 0.5 mm. Note that a deficit means water the soil doesn't have, and indicates the soils capacity to soak up water. So a larger deficit is good for land where FDE is applied.
When the SWD is zero (ie no deficit), soil is at 'field capacity' and all the soil 'micropores' are full. Water is held in the micropores and is removed via evapotranspiration. With the addition of more water the larger 'macropores' begin to fill, but these pores operate under gravity and as they fill drainage begins, normally down through the soil at first. If enough water is applied to fill the macropores, the soil will be saturated and surface run-off is likely. However, on free draining soils, complete saturation is unlikely and drainage of water in excess of field capacity will be continuing as water is applied. An example would be filing a bucket with a hole in it. Where the bucket never fills to the point of overflow because the rate of filling or the total volume of water added is not sufficient to overcome the water loss from the hole.
By allowing a maximum irrigation depth of 10 mm per day when the soil water deficit is >0.5 mm, we are relying on the macropores to take up some water. We are relying on sufficient surface contact between invading water and soil without causing saturation conditions or bypass flow. Modelling shows that SWD's over 10 aren't common in this catchment.
Analysis by Ag Research showed that different local soil types had negligible effect on determining whether irrigation is allowable or not. The same applied to rainfall: when different rainfall sites within the catchment were compared there was little difference. So one weather station at Inchbonnie provides the rainfall data for this model. The soil type used is 'Mosquito', which was a suitable intermediate from a selection of predominant soils found in the catchment.
Some conditions are:
- Irrigation is not allowed when SWD <0.5 mm. Ideally irrigation will be done when SWD is >10 mm but SWD's of 10 mm may not be common.
- The application depth on a given day should not exceed 10 mm.
- The model assumes FDE is applied to a different paddock each day.
- The instantaneous application rate should be less than the soil saturated hydraulic conductivity. As a rule of thumb a maximum application rate of 5 mm per hour is practical (for no more than 2 hours = 10 mm/day).