UKH

Can you have too many trees ? My wife has just returned from a local environmental think-tank (informal) in which river ecology was debated. There's not enough water in the river ! In fact it is known that 18 different village communes benefit from water drawn off at source - so that's the primary reason, however a vocal participant at the meeting was seriously proposing the need to deplete the forests which wick away 'local lifeblood'.

Is this possible - what do you believe ?

It's not a question of belief!

I guessed from the thread title this was going to be one of yours. I think that says something about what I believe...

Simple answer: remove the trees and rivers wash out quicker, less shade increases ground level temperatures speeding up evaporation. The local source of wisdom was talking bullocks. 

That’ll explain why the Sahara is so full of those pesky wesky trees, sucking up all that moisture n stuff n that n wotnot.

The Amazon is famously dry by the time it reaches the Atlantic after all those trees sucking up the water.

That you just spout drivel.

Get yerself a hobby mate.

Damn thirsty barstards. Maybe cut them in half as a compromise?

“Thank you. Since we decided a few weeks ago to adopt the leaf as legal tender, we have, of course, all become immensely rich. [...]

"But we have also," continued the management consultant, "run into a small inflation problem on account of the high level of leaf availability, which means that, I gather, the current going rate has something like three deciduous forests buying one ship's peanut." [...]

"So in order to obviate this problem," he continued, "and effectively revalue the leaf, we are about to embark on a massive defoliation campaign, and...er, burn down all the forests. I think you'll all agree that's a sensible move under the circumstances.”

On a slightly more scientific level, there are a lot of factors at play here to give any kind of useful answer.

There is some evidence that entirely afforested river catchments have lower water yields than comparable unforested examples but it's not a universal truth and the studies I've seen only indicate relatively minor changes. To blame the trees for sooking up all the water seems a bit nonsense.

I'm new to this debate, for which reason I tried to present it without bias. However I am biased. I have planted more than 500 trees on our 8 acre property and am further encouraging natural regeneration.

What I can see is that density and extent of forests may locally control water flow in a certain river - but to consider this alone without regard for other environmental factors would be complete folly.

My water supply comes from a small spring that runs into a thousand gallon tank about a quarter of a mile above my house. The spring itself is in Coed y Brenin forest and would just about cope during the drier periods and we'd have to be pretty sparing at times. A few years ago, being a commercial forest, the trees around my spring were felled and despite having much, much drier Spring/ Summer months recently, my water supply has never been better. The area has just been re-planted, so I'll keep you posted. 😉

So, has anyone been looking at rainfall figures?

Or water abstraction, either for potable water supplies, or for irrigation?

Or other agricultural activity that might be retaining water?

Trees do take a lot of water. Their root systems might also cause physical retention of water in the ground, preventing it running off the ground into the river.

A simple google search yields useful-looking answers, e.g.

https://www.forestresearch.gov.uk/research/forestry-and-water-resources/how...

"The impact on water supplies can be even greater in the lowlands, where a conifer forest can reduce the annual volume of water recharging a groundwater aquifer by 70% or more compared to grass."

A local, informal environmental think tank ey. Sounds fun!  We call that the pub around here and the only thing discussed is why the mines are shut and how ace it would be for everyone if they reopened them and the place could return to the good old days of everyone drinking from 2pm in the street and fighting. Apparently they even had police on horseback every weekend trotting round. Now they won’t even come round if someone burgles your house. 

this is, frankly, off the scale idiotic bullshit. 

I'll see if I can summarise what's already been said. sorry if this is going a bit primaryschool in tone. 

trees do increase the transpiration component of the water cycle. extremely dense foliage leads to a lot of moisture being sucked up from the ground and entering the atmosphere via the leaf canopy. this is what why rainforests are so wet - and not just "a forest".  in the real world you need extremely dense coverage before you get an actual rain-forest with it's own climate.

moving to rivers; during times when it's not raining the river's baseflow is a function of soil moisture. after it rains and the surface runoff has passed, this can take anything from hours to days depending on the size of the catchment, the river's baseflow depletes (comparatively) extremely slowly. but it does go down, and will eventually run dry. clear enough: the more moisture in the soil the higher the baseflow, and the more drought resistant the river is. 

the split between how much of the rainfall contributes to surface-runoff vs infiltration (groundwater) is complex and is influenced by may factors: topography, soil-type, geology, foliage, ground-cover. aside from urbanization (buildings, paving, etc.), foliage is the main component which we can and do drastically affect.

tree cover does two main things; root systems loosen the underlying soils and prevent the soil becoming compacted and dense, and the trees themselves provide shade, wind and rain protection to the ground below. this all helps the rainfall infiltrate into the ground to then reappear as baseflow later once the rain stops.

treeless ground sheds more water off as surface water, quicker, and with more risk of surface erosion - so less to groundwater and less to river baseflow. (the fact that tree cover both simultaneously loosens soils and prevents erosion seems to be too much for some people - but I digress)

the amount of water lost to evapotranspiration is a fraction of the benefit from reduced surface runoff and increased infiltration. I do hydrology, hydraulics, water resource and flood risk fro a living - but I can't off the top of my head give you figures for this. a tree-covered catchment essentially acts like a sponge, releasing water slowly over a longer period of time. this is usually considered a benefit to people who want to abstract downstream during dry weather. the only potential argument for trees reducing water yield would be if you had a system whereby you immediately catch, collect and store all surface water runoff during rainfall. in my experience this is essentially unheard off in anything other than very small scale; e.g. gray-water from roofs.

well that's already far to many words.

not everyone knows everything, and I'v heard some pretty dumb and miss informed opinions at public consultations, but this one takes the whole tray of biscuits and the cake stand. I have my suspicions that your man isn't as stupid as all that and has some other motive to object. there's nothing like making a monumentally stupid statement just to waste everyone's time in addressing it - ahem!

That is right, little-known fact, it's 2 inches wide when it reaches the Atlantic

Whilst you’re on the line…

How much do you think the coastal limestone denes in County Durham tend towards a temperate European rainforest?  The partial enclosure by the steep sides seems to raise the moisture and help similar types of ferns, lichens and so on grow in the understory as you’d see in bits of west coast forest.  They have a very different feel to other local forests.

I don't know. but what you describe sounds quite cool, I'll need to seek these things out next time I'm in that area of country. I love a good micro-climate.

there's a few places (can't remember off the top of my head) that claim to be a temperate equivalent to rainforest. I guess it's like everything else, it's all a sliding scale of grey.

I think, don't quite me, that to be considered actual rainforest the clouds of mist and fog that you see billowing off any forest in the early morning need to be big enough to actually cause local rainfall.

This is already being done.  We have a local nature reserve where some trees are being felled to thin the woodland and restore the wet environment

what lives in the woods can take a lot of water.  There has been a lot of discussion in local papers about the lack of proper analysis of the impact of water consumption at Whinfell, the Center Parcs (ex Oasis) complex (rapidly and ever developing and expanding) that sits a couple of k from here.

words well spent, thanks ! I notice that you didn't even mention decarbonisation, as indeed the focus is on hydrology; altogether the impressive array of services to the environment are multifarious

A couple of questions. In a built environment with a lot of hard cover, a high proportion of rainfall must never infiltrate. Do we observe significantly slower growth of urban trees, and are they thus susceptible to higher disease / failure ?

Do tree root systems ever reach right down to aquifer depths, or are they general well separated ?

Castle Eden Dene is the one to visit, proper karst landscape with the stream sinking, and the remains of it’s time as a Victorian pleasure garden including an excellent footbridge.

Yep, trees are often planted to dry the land (one of our neghbour farmers has done exactly that to dry a boggy field and hopefully dry out a field culvert). A tree pulls 500-2000l of water from the soil and transpires it into the atmosphere every day in the summer.

Home to the mighty Archangel (VS 4c)! I remember going for a walk round the dene in 1990 and thinking it looked like an impressive line. I believe climbing is banned nowadays?

I know nothing about this topic. but I'd hope 'Forest Research' (which is the research group of the Forestry Commission; they're still at Alice Holt) ought to be a reliable source. Did you have a look at their page I linked, because it seems to give a different impression to the one you have given? I'm struggling to reconcile the two views.

https://www.forestresearch.gov.uk/about-us/

Sounds like the think tank needs one of those trucks to bring their suction tube and empty it. 

It's like humans are in competition with trees. Centuries ago we had the luxury of gay abandon in regard for them, but are now obliged to recognise a deeper dependence.

There are signs of the coming tension over water availability, with hints of nimby-ism. Trees may be  deemed a nuisance in our back yard but the amazon rainforest should certainly not be depleted any further.

Or is it just a question of scale? Anyone who keeps a garden knows about balancing competition between trees and their spreading roots - and the needs of the vegetable patch. The trigger seems to be 'forest' depletion or management.

If the rewilding campaign gathers momentum we must surely see forests spreading in the UK, where rain and water supply don't seem to be such an issue. Perhaps the conflict over water rights will first get tense in southern europe where changing weather patterns will oblige adaptation and innovation. 

We aren't competing with trees for water, trees are locking in carbon saving us from ever higher temperatures and likely less certain rainfall. 

Water shortages are due to our misuse of it, think how many litres of drinking water go down the toilet, on the lawn, over the car... per household every year. This has to come from somewhere. We are the problem.

I'm going to look into some of these in a bit more detail, I vaguely remember reading about some of the mid-wales stuff years ago.

my expertise in predominantly flood risk and water resource for energy, so not directly landuse and hydrology, but it's been playing a bigger part in recent years. 

from an irrigation and groundwater recharge point of view, annual volume of streamflow isn't very helpful; the same volume than could pass thorough in a couple of days of thunderstorm with extreme drought for the rest of the time. it doesn't give any indication of flow duration, or change in Q95 (the level at which you're typically no longer allowed to abstract from a river)

I'm not skeptical of the FR's work, but these research projects seem to throw up some quite vague conclusions. certainly for me in a professional capacity we still can't reliably quantify or predict change in catchment hydrology from landuse. it appears FR are mostly looking at the impact commercial plantations. yes, these will have a water demand. the point still remains that you cant assume that taking this demand away will unequivocally result in more water for your use downstream.

personal opinion here: (rant warning - please ignore!)

my previous post was just base on my understanding of general principles. land use planning and hydrology is a "now" topic in the UK, and natural flood management (NFM) is the next big thing. unfortunately there seems to be a tendency to talk big and do fuc all. to me is seems to be highly, but subtly, politicised. there's all manner of funding-streams for projects to re-wild this catchment, re-meander that stream, all with a 10yr landowner agreement and with zero (and I mean zero) data collection, before, during or after. this concept has been on the radar for decades, but it's still impossible to work with as there is very little to go on in regard to empirical research from which you can derive action equals effect. if I was being cynical I might think that some people don't want to know how their stewardship of uplands, mooreland and peatland affects downstream riverflow.

that's a UK perspective. on the continent there's going to be a bit of a reckoning in the next decade, particularly spain and italy, where growing water-intense crops (lettuce, tomato, etc.) isn't going to be possible at current levels of production due to drought.

groundwater recharge is going to become a serious issue in the future. I don't know what type of forest, and in what context, the OP is looking at. but, if cutting down trees was the answer we'd already be there.

re-reading, I think there's a wee bit of misunderstanding in what I'm saying.

the forestry research stuff talks about the influence of forestry on "water yield" - from what I can see their use of yield applies as it would apply to a hydro-reservoir, they're looking at the total flow that goes downstream, irrespective of it's runoff characteristics.

this is not directly useful if you're looking at abstractions, e.g. irrigation, where you can pump-out or take off water from a river, up to your allowed or paid-for usage, whenever you want provided you're not in low-flow conditions (in the uk I think the "hands-off" flow condition is typically when the river is at or below Q95) [i.e. you can expect to be in this low-flow state for about 5% of the year]

if you're abstracting without a reservoir, what you don't want is long periods of low flow interspersed with short high flow events, you can't use this water in the time it's available. this change in runoff characteristics isn't tackled afak in the FR stuff, and IoH reports shed little light on it also. that is, it isn't researched enough for us to usefully use in the real world. I may be wrong, I might be behind in my CPD.

Thanks. I'm glad I'm not the only one struggling with this...

I'm not really sure the research will ever get there in terms.of empirical evidence - there are too many site specific factors. The cycle of UK forestry means that impacts vary over the forest lifespan. I've read some stuff which talks about higher interception losses post forest harvesting due to the large brash piles on the ground! With most forests harvested in blocks, effects at the catchment scale will be really difficult to identify.

The Coalburn work is interesting and (I think) one of the longest running studies.

fair point, there's definitely a lot of variables. we could do with a lot more catchment research across the board rather than just forestry. I think I'm just a bit miffed at the inability to engineer things, i.e. prove a benefit. we have local authorities (scotland) come to us and ask us to engineer them out of flood risk. then they ask, what about catchment management / natural flood management? and we say - yea, but we can't quantify any benefit for you. even though it's a good idea, you'll need evidence

coming back to the OP's question; every plant has a water demand, so, get rid of all plants and you'll have yourself the maximum amount of water for your exclusive use. I'm being a tad facetious, but what sort of a landscape would we have if we did take this approach - probably something resembling much of scotland

There are two studies reported in Scientific American where a number of identical sites were either planted or left, an up to 50% reduction in base flow for one study after planting and 18-22% for the other study, the difference being the density of planting and the age of the trees. 

The way trees grow clearly reduces the amount of run-off to feed rivers and also reduces the amount going into deeper ground water to feed springs so obviously river levels will be reduced.

The river Job rises on the Flanks of Pic du Cagire 1912m, captured at 800m approx. The forest is indigenous beech with spruce, mixing into other varieties with lowering altitude. All managed by the french forestry commission (ONF) who cut swathes out up to about 1300m, normally leaving a few mature trees to re-seed.

The thinktank meeting discusses renewable energy - a point of contention for us - we have a 2m dam which could be useful for a micro-hydro project but the EU wants to flatten it by order of continuité ecologique . If we resist then a v expensive fish pass must be built, and our cut even at 10% share  would still blow a hole in the bank account.

Your opinion about presence and frequency of dams w r t flood mitigation ??

Is our use of water itself acting as a temporary "reservoir"?

Most of our domestic use must go into drains and eventually be recycled (hopefully after treatment). That water would otherwise have flowed towards the sea in the first instance.

My old house had spring water until commercial softwood upthehill dried it up, when we moved in it was useable, 20 years on the trees were huge and the springs dry for 3/4 seasons. I can see where the idea comes from but I suspect the reality is complicated by the trees, density and terrain.

Jk 

Or remained in the deep aquifers which aren't refilling as quick as we pump water out. 

Zero human influence water sits in the forest and marshes (farmed fields don't exist), a large volume of water slowly percolates down filling aquifers, some hits impermeable layers forming springs..streams..rivers and back to the ocean.

We've cut a whole series of natural processes out, dams in the uplands, drainage channels to fill them, drainage everywhere we live, we have fast tracked the water to either supply use with clean water and the remainder going straight to the nearest big river and sea. 

As was mentioned above soon there will be real problems in southern europe, north africa and near east, where deep old aquifers are being pumped for watering our crops and many could literally have taken decades to fill, when they run out food production will switch to another country and the local population left without drinking water. It's not as bad in the uk, but we are very wasteful with water. 

The moves to reduce the N° of dams on European rivers, or to at least allow the passage of migratory species (particularly but not only fish,) is primarily a biodiversity measure - there are few European rivers remaining where spp can migrate from mouth to source and that's had an impact on wildlife*. But of course dams also have an impact on river flow & the interactions can be very complex, especially once land use & climate change are taken into account.

* see eg https://www.eea.europa.eu/data-and-maps/figures/free-flowing-rivers-in-euro...

 and also reduces the amount going into deeper ground water to feed springs so obviously river levels will be reduced.

it's the 2nd bit I'm not convinced of. there are a fair few sources that'll say GW recharge is improved, so much so that I had it as accepted theory.

in the UK the Pontbren farms "project" showed that the roots of trees and hedges helped water get into the ground. this project wasn't set up to study hydrology specifically, essentially it was a farm project with a bit of research on the side. unfortunately I'm not aware that this type of project being repeated in the UK with a view to properly study hydrology - which is a great shame.

I'd be concerned that deforestation, though you may well see higher river flows, would in the long term not be sustainable. there's a lot more going on, for instance the tree canopy surface-cooling effect contributing to rainfall formation.

The root structure loosens and opens the soil allowing it to store more water in that area, the tree does this to store a supply for transpiration in dry periods. This water would normally not get to the deep groundwater anyway but since it will be evaporated by the tree that is moot. There must be a net shortfall in the water available to build rivers as a large quantity of the rainfall will dissapear into the atmosphere (to fall elsewhere but that doesn't help that tree or river).

ah, you're on the dry side of the Pyrenees - not surprised you've got challenges.

dams can be really useful for flood mitigation, a typical flood storage reservoir would be kept empty with a permanently open culvert through the dam, this acts as a throttle to limit the downstream peak flow. Trouble is, it get a decent effect they need to be quite big and impound a serious volume of water.

2m high is about as small as a dam gets while still being called a dam (slight exaggeration), unless you've got a very big lake behind it it's unlikely to achieve a worthwhile effect on riverflow. the other limiting factor is the reduction in peak flow only last a limited distance downstream before nature (runoff from the catchment downstream of the dam) catches up again and the river is more or less back up to peak flow again. (very roughly)

a fish pass can be relatively simple and "relatively" cheap to install (although I wouldn't want to pay for it myself out of my own pocket), although a 2m drop is not that big, it's still high enough that would make a fish pass bigger than you would think it's going to be. They do need to be properly designed w.r.t hydraulics, expected riverflow and fish size / species.

if you're looking into (or arguing about) water abstraction as well as flooding - I'll stand by this as a universal concept: you want to do as much as you can to slow down the speed at which water sheds off the land; e.g. contour-planting, creating temporary pools and ponds etc. slowing down runoff will do two things, reduce peak flow and increase infiltration to ground which helps the river survive better into dry weather.

(sorry of you already know this): for your hydropower project, the height of the dam is potentially unimportant - the generating head is the difference between the water level behind the dam and the elevation of the generator in the powerhouse. ideally you have a small dam, no more than an off-take weir, high up up a steep hill above your powerhouse. you could generate power from a 2m low-head arrangement, bit it's unlikely to be much to write home about.

In Scotland, their very last monarch passed a decree authorising the chopping of trees down for this exact reason, so blame Forrest Wickingter. 

How long is a piece of string? How much flooding do you get, how much does this river/stream go up at max.  How downstream are you by they way as if I look at Cazounas it's very small.  Put the right dam at the right place and you can do whatever you want with consequences.

Given that it's got going to be fed by snowmelt, you want a soil that's going to 'store' that water and stop it all ending up in the river in one short time period.  I doubt it's too many trees that are reducing flow

No flooding issues in the last 2 decades, but it's helpful to understand all the pros and cons. Ours is not the only dam between Izaut & next village - Cabanac. Erosion of river banks during flood volumes is perhaps a bigger issue, also helped by buffering heavy runoff. Just read interesting testimony from a french hydrologist who proposes that surface floods used to be an essential part of buffering and aquifer restocking - but containment walls now hurry the water along it's course.

There's a lot of homes and villages drawing off that source already. The pressure to flatten dams has come from the EU directive and further driven by the fishing federation. However, we have multiple octagenarian witnesses that fish levels in the river were healthy 40-50 yrs ago, despite dams blocking upstream migration. Further we can't quantify how many such fish came from re-stocking operations. 

There are many elements to the whole picture for which evidence is anecdotal; but finally IMHO the real problem is people unwilling to 'cut according to the cloth'.