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Will it ever stop - or start - raining?


I have this memory from years ago when traveling in the UK over the summer of overhearing a small boy complaining loudly to his parents “When will it stop raining?” I sympathized wholeheartedly – it was wet and grey and not really the summer holiday we had imagined. Some would say, unkindly, what would you expect from a UK summer?

A country road, seen through a rain covered windscreen

For farmers questions about rain are more important to business than to the small boy and his holiday. If it’s raining, how much it’s rained, when it’s next going to rain are valuable pieces of information. What will be done based on if it rained or not and when will it next rain can be very different scenarios. Being confident about the answer to those basic questions – has it rained and when will it rain – is important.

At Regen, we are always striving to get better at answering these questions for our customers. There certainly has been more than one occasion when a farmer had told me that the forecasts we use are terrible! So we decided to do a bit of investigation ourselves – how terrible is terrible? This is what we found when we checked out how close two different weather forecasts were to what actually happened.

A chart showing that MetService rainfall forecasts are slightly more accurate than Yr

For three months, at three different locations, we recorded the rain forecast from MetService (the service Regen uses) and YR (a service we know of lots of farmers look at) and compared this to the rain we recorded at those sites. Turns out – not surprisingly – neither got it to spot on. It also turned out that the amounts off they were varied – it wasn’t that one service consistently forecast rain that didn’t come or vice versa. They both had times when they forecast rain and it didn’t arrive or missed forecasting rain that did arrive. I want to stress – this isn’t a science level comparison, so we aren’t drawing any conclusions, but it is enough to show that, at these particular locations, MetService was out a bit more often for small amounts of rain but YR was more out, more often for large amounts of rain.

The good news was that when the forecast from each service was for more than 5mm of rain, two-thirds of the time the actual rain that fell was within 5mm of the forecast rain amount. Given it is a forecast and New Zealand’s landscape is incredibly variable across small distances, I think either forecast service gives a reasonable level of confidence about what is likely to happen rain-wise.

So what does this mean? Both MetService and YR use some of the same international weather models to generate their forecasts, but they also have their own processes they lay over this. We also know from talking with lots of farmers that in some locations they feel one is better than the other. And, generally, farmers will look at a range of forecasts and mash up the predictions to get a sense of what is more likely to happen at their location.

The takeaway for me is that it is worthwhile to actively look at forecasts and use this in decision-making. That’s the approach we take in our services, aiming to support our farmers make the best decision for the day by getting the MetService forecast (for Regen this is the choice as they also provide an ET forecast as well as rain) for their location every day and putting that into the “should I irrigate or not” decision. We can be confident it will be right more than its wrong.

Bridgit Hawkins

CEO/Executive Director