Trees can be bad for the environment

Our hydro-geologist friend has been looking into our situation and, as well as climate change, he has also pointed the finger at inappropriate land management.

In our case, that means the plantations of sitka spruce planted by previous owners.

This is an edited version of his views:

I’d say that the effective catchment for yours and your neighbours boreholes is somewhere around 8000-12,000 sq metres.  This would provide a maximum yield of around 3100 to 4800 litres per day (766,500 to 1,200,000 litres per year) assuming your boreholes are both 100% efficient and capture all groundwater infiltrating into the mini catchment.

In reality you can only expect to ever capture 70% of this, ie 2,100 to 3,300 litres per day or typically 2 to 3 times a typical UK households average consumption.

However,this is not the whole story as there are other users taking groundwater from the system, not least the conifers that you have on your land!

The catchment receives in terms of rainfall between 5,200,000 to 7,800,000 litres per year (14,246 to 21,370 litres averaged per day over the year).

When we start to look at what volumes are actually retained for use either from surface water runoff or soaking into groundwater the figures start to reduce rapidly.

I have done a quick nett balance of what enters your mini catchment and what is removed by the prevailing conditions, and land management can have a huge effect.

Conifer trees can evapotranspire huge amounts of water, typically up to 60% of rainfall landing on them, especially in the first 20-40yrs of growth (they peak at around 20 years and then declines thereafter).

Conifers are worst type of tree for this as they typically intercept and evapotranspire 1.8 times more than deciduous trees.

In your case, if up to 50% of your mini catchment has been planted with conifers (then they are probably capturing and returning to the atmosphere around 3,900 to 6,000 litres per day on average.

This volume of water removed by the trees is made up of the following likely components:

• Removed from ground by 4000-6000 sq.metres conifer trees 1,800 to 2,700 litres per day (averaged over year)
• Rainfall intercepted on leaves and evaporated by 4000-6000 sq. metres conifer trees 2,100 litres to 3,200 litres per day (averaged over year)

Obviously this will peak in summer when your need is also greater and reduce in winter as in line with their growth.

However, when we look at what is going into the ground and what is available nett of water removed by trees and your boreholes pumping efficiency it starts to all get very marginal!

• Total recharging into groundwater: 3,100 to 4,800 litres per day
• Less loss from conifers: 1,800 to 2,700 litres per day
• Nett available for abstraction assuming 100% efficiency: 1,300 to 2,100 litres per day
• Assume 70% efficiency for removal by boreholes 910 to 1,400 litres per day or 1 to 1.4 average UK households.

Given how marginal this is, you can then begin to see how the catchment has been mismanaged by planting the conifers and this combined with climate change, no matter how subtle, can have such a devastating effect.

In summary then, I estimate that the daily averages for your mini catchment water balance is as follows:

• Incident Rainfall Landing 14,246 to 21,370 litres per day
• Rainfall lost by interception by conifers 2,100 to 3,200 litres per day
• Groundwater removed by conifers and lost back to atmosphere 1,800 to 2,700
• Rainwater Interception from roofs guestimate 400 litres per day
• Evapotranspiration from grass areas (say 40% of catchment) 2200 to 3280 litres per day
• Nett available from above 7,740 to 11,690 litres per day.

However, with exception of balance left in groundwater (following removal of proportion by conifers) of 1,300 to 2,100 litres per day, I expect that the amount running off and lost to the river Urie catchment is 6,440 to 9,590 litres per day averaged over the year.

With climate change this will increase and the nett volume percolating down into groundwater increase.

The other nett effect of climate change is that the conifers grow quicker and have a longer growing season due to mild winters.

As a result they will also start to remove more from the groundwater leaving even less for you from an ever dwindling resource.

If you were to start to remove conifers and replace with deciduous trees (suggest you do this little by little from centre out using conifers as wind break around edges and to generate favourable micro climates) you could improve volume of water entering groundwater system by on average 90 to 140 litres per year for every square metre of conifer removed and replaced by deciduous trees.

It should give people food for thought when they start extrapolating that out over Scotland and consider how many conifer plantations there are.

Not only are they taking up vast amounts of water, but climate change is making them grow faster and take up even more water.

But given the commercial interests involved, does anyone think the Government will take action about this? Thought not.