Jonathan Brookes
The spring season to date has started off like the last couple, not being over wet in late winter but ending up with regular spring rainfall topping up the soil to where field capacity in some locations is exceeded, and drainage events are occurring.
It is also the start of the AgFirst led Sustainable Food and Fibre Futures Fund project “What’s Coming out of Tile Drains”.
We are all aware of how important tile drain systems are to the success of fruit growers, as significantly more trees die in Hawke’s Bay every year from excess water than from dehydration. As an industry we need to understand the behavior of tile systems with regard to flow and nutrient concentrations, which is the focus of this 3 year project. With an improved understanding of the impacts horticultural has on surface water quality we can finetune our practices around efficiency of water and nutrition, to ensure water quality is sustainable in the long term.
The project is only just underway, but we are already seeing some interesting differences in flow from September and October between the different tile drain monitoring sites.
- The first key finding is around the urban myth that this season was dry – a better description would be “not saturated”. At the start of September, many soil moisture readings were at 70% to 100% of field capacity from 10cm to 1m depth. A good winter rain event and a lack of ET kept the soil moisture up.
- Secondly, since then, there has been about 130mm rainfall across a number of events, and about half of farms have captured tile drain flows after some of the larger rain events. Other sites have not yet recorded a flow event, we do not fully understand why, but the tiles have been cleaned. This is surprising.
- Thirdly, not all tile drains are the same, some have not run for the entire program, some have been going flat out for the full 2 months, but we also have examples of 2 cleaned tiles in the same orchard where one is running and the other is not.
- When trying to predict if tiles will run, some seem to be well tied into the rainfall events. Others seem to be strongly connected to the level of the water table, with a greater connection to nearby river levels than local rainfall events. Some just seem to have the excess water just drain through the profile and past the drains, possibly the events have not been significant enough yet to flood the soil and force the tiles to run?
The key messages so far are that tile drains systems are a lot more complicated than many people think and are very location specific. The more we understand them, the more likely we can manage water and nutrition on our properties to find the sweet spot that allows for optimum block performance outcomes as well as minimising our surface water effects, helping to ensure we keep our social license to keep doing what we do well – fruit growing.
