The UK Green Film Festival returns for a fifth edition between 3-10 May. Sponsored by Friends of the Earth, this is a truly nationwide event, with over 20 cinemas across the country - including the Ultimate Picture Palace - screening some or all of the seven films on a range of environmental issues in the programme. In addition to the documentaries (five of which are UK premieres), there will also be a series of talks and Q&As with film-makers and eco-thinkers and campaigners. So there will be plenty to discuss before the award of the Palme Verte and the UKGFF Audience Award on the closing night.

The precious resource of water comes under scrutiny in two features at UKGFF 2015. In H2Omx, José Cohen and Lorenzo Hagerman consider the state of the water supply and sewage systems serving Mexico City, which is suffering from the fact that it was constructed by the conquering Spanish on top of a series of ancient lakes. With 22 million people requiring fresh water, the crisis is growing exponentially more serious and this cogently argued and splendidly illustrated wake-up call not only highlights the roots of the problem, but also proffers a few strategies for managing water in the Valley of Mexico that could contribute towards a sustainable solution.

It comes as something of a surprise to discover that such a megalopolis still employs a waste and drainage network that was established in colonial times, but the sheer size of the population has put enormous strain on the imported supplies, while subsidence has started to impact upon the artificial outlets and exposed the city to the risk of flooding from the surrounding mountains. Moreover, as the outlying regions become more resentful of having their water earmarked for the capital, an increasing number of neighbourhoods have no access to running water and have to be supplied by tankers whose owners are anything but philanthropists.

Even more alarming, however, is the condition of the drainage canal that runs through Mexico City into Hidalgo state. This putrefying waterway is so badly maintained that effluence seeps into the ground, while clouds of detergent foam often bounce across neighbouring agricultural land. As there is no alternative irrigation channel, crops are polluted by the toxic waters, as are fish stocks and farmers jokingly consider it an act of glorious revenge to sell produce back to city dwellers who have no idea of its origin.

Cohen and Hagerman (who inherited the three-year project when the original director baled) make fine use of arresting aerial images of sites like the Cutzamala supply line to reinforce their message. The statistics contained in the graphics and animated segments produced by Diego Huacujai and Humberto Zamorale are also potently presented. But they avoid hectoring and also discuss the efforts of the water harvesting initiative Isla Urbana without seeming preachily smug. Civil engineer David Vargas and industrial designer Enrique Lomnitz are shown teaching plumbers how to instal systems. But they also listen to the locals to learn about specific problems and the film concludes that such grass roots co-operation can do much to alleviate the problems until the government can finally be persuaded to intervene and regulate the distribution of fresh water and renovate the sewers.

A little of Ariel Guzik's electronica score goes a long way, but this is a sensible and persuasive picture that finds an intriguing companion in French video artist Antoine Boutet's Sud Eau Nord Déplacer, which assesses the excavation and operation of the world's biggest water transfer system, the Nan Shui Bei Diao, which was devised by Chairman Mao Zedong to bring supplies from southern China to the populous, but arid regions of the north. However, the project was not achieved without a considerable toll being taken on the environment. Yet, while Boutet excels at capturing the scars on the landscape, he lacks the curiosity and compassion to examine the human cost with any intimacy or depth.

In many ways, this resembles Jennifer Baichwal and Edward Burtynsky's Watermark (2013), in that Boutet is as much concerned with the aesthetic value of his striking images as he is with their eco ramifications. Indeed, the opening third of the film feels like a gallery installation, as Boutet dwells on the spectacular nature of the vistas rather than informing the viewer of their significance. However, he is no stranger to this subject, as he focused on the Three Gorges Dam in Zone of Initial Dilution (2006).

It's clear from the quote taken from Bruce Lee's Longstreet tele-series (1971-72) that Boutet wishes the audience to empty its mind and be as free to flow as water. But, while the Nan Shui Bei Diao has forced rivers to change direction in order to turn deserts into pastures and woodlands, Boutet leaves onlookers to draw their own conclusions, as he flits between images of monolithic concrete structures, parched riverbeds and Communist propaganda materials proclaiming the efficacy and ingenuity of the entire project.

On occasions, Boutet ventures into reportage territory, as he eludes his minders to meet with farmers who are angry that they have been evicted from fertile land and compensated with rickety shacks and dead-end plots. The work party planting trees in the wilderness seems like a sign of progress, as does the shot of the dissident teacher who had spent almost two decades behind bars, but is now happy to swim in the lake serving a ginormous dam. But Boutet's camera refuses to be fooled and it is pretty clear that all claims about taking China forward 80 years in one leap. It's just a shame that this compelling topic hasn't been approached with a little more sociological trenchancy.

Progress doesn't have to be destructive, but it will seemingly always be contentious, as Phie Ambo reveals in Good Things Await, which reports on the biodynamic methods employed by Danish octogenarian Niels Stokholm and the efforts of the European Union to close down the Thorshøjgaard farm that he and wife Rita Hansen have been working for four decades. In spending a year on the premises, Ambo seeks to retain a vérité detachment. But passive observers have no place on a busy farm and, at one point, she finds herself putting down her camera to help Niels and Rita with the breach birth of a bull calf.

The theory of biodynamic agriculture was posited by the German philosopher Rudolf Steiner in 1924 and its holistic insistence on the interdependence of soil fertility, plant growth and animal husbandry takes into account spiritual, mystical and celestial influences, as well as hard terrestrial fact. However, Ambo provides little contextual background, as she arrives at Thorshøjgaard, where Niels and Rita are fighting to bolster stocks of the increasingly rare Danish Red dairy cattle. These are not the only creatures on the farm, as sheep, pigs, hens, butterflies and bees all play their part in Stokholm's eco-ethical methodology.

Even the family cats and dogs do their bit, as do the students from the local high school, who spend a week on the farm at busy times of the year, lending a hand with the heavier chores and learning about the patented `growth drop' brand of fertiliser that is made from manure-filled cow horns that are buried in the ground during a growing season and then mixed with boiling water. The kids seem to appreciate the care and consideration that goes into the raising of crops and the rearing of livestock. Moreover, some of Denmark's finest eating establishments, including NOMA and Restaurant Julian, are sufficiently impressed to place regular large orders for the organic produce.

But, unsurprisingly, the European Commission's Directorate-General for Agriculture and Rural Development is not so readily swayed and inspectors are frequently dispatched to Thorshøjgaard in a bid to find sufficient evidence to close it down. As the rules on animal welfare are so particular and strict, Stokholm is denied an organic certificate, even though the Danish Reds are fed on hay during the winter months and allowed to graze on grass and herbs during the summer. Moreover, the breeding methods employed are entirely natural and the calves remain with their mothers for a minimum of six months.

Dismissing biodynamics as quackery, the EU inspectors impose fines and issue writs that could result in Stokholm losing his licence to keep cattle. As times are already hard because of the recession, he is in no position to make the recommended renovations to his deteriorating out-buildings. But a period of inclement weather and the opening of a court case take Niels and Rita to the brink. But that is when Restaurant Julian chef Jesper Møller makes a timely and inspired intervention that is fully backed by his NOMA rivals, Trevor Moran and René Redzepi.

Ambo is clearly in Stokholm's corner, just as Peter Gerdehag championed Swedish sisters Britt Georgsson and Inger Gusafsson in Women With Cows (2011) and Andy Heathcote backed Sussex maverick Steve Hook in The Moo Man (2013). But this is one of those films that is likely to have viewers shouting at the screen at the intransigence and idiocy of jobsworthy bureaucrats who know how to push a pen, but have no idea whatsoever about the mysteries of the land. They can quote regulations about animal nutrition and crop rotation at the drop of a hat, but they cannot get their heads around ideas that are not covered in the rulebook.

Lyrically photographed by Ambo and Maggie Olkuska and paced by editor Theis Schmidt to match the measured routines of the farm, this eloquent, if mud-spattered charmer is wonderfully complemented by Rasmus Winther's sound and the score composed by Icelander Johann Johannsson and performed with spine-tingling brilliance by Paul Hillier's choir, Theatre of Voices.

The focus shifts from biodynamics to biodiversity in Sandy McLeod's Seeds of Time, which profiles seed bank pioneer Cary Fowler, the director of the Global Crop Diversity Trust in Rome who came to the renowned music video director's attention when she read a John Seabrook article in the New Yorker in 2007. Born in Tennessee, Fowler has survived cancer and seen two marriages break-up because of his determination to reinvent the planet's food system, which has been knocked out of kilter by the growing demands of industrial farming. Travelling the world to lecture and collect new seeds, Fowler strives to avert an agricultural catastrophe by conserving species that produce insufficient yields to satisfy the big corporations, yet which might just be crucial in staving off famine in the event that a new form of blight wipes out a profitable variety of a staple crop like wheat, corn or soya beans.

Fowler is not the first to gather seeds. Soviet botanist Nikolai Vavilov dedicated his life to improving cereal crops before he was arrested on an expedition to the Ukraine in 1940 for criticising the theories of Stalinist acolyte Trofim Lysenko. He was sentenced to death, but succumbed to starvation in prison in 1943. However, his loyal staff at the Leningrad seed bank protected his life's work during the 28-month Nazi siege by hiding in the basement of the Virilov Institute with a cross-selection of the 250,000 seeds, roots and fruits in what was then the largest store of its kind in the world.

Such was their dedication that nine members of the round-the-clock team perished from hunger rather than consume any of the samples in their care. Fortunately, similar heroism is not expected of the staff at the Svalbard store some 810 miles from the North Pole in Norway. This sub-zero Eden is as costly as it is ambitious. But McLeod and Fowler also travel to Peru to reveal how a cabal of indigenous farmers has come together to save the 1500+ varieties of potato native to their country. As these have a ceremonial significance, as well as a nutritional value, the project to create a potato park is deemed doubly important. However, the experts are also willing to venture into the mountains to pass on tips that will help isolated communities fight against the ravages of climate change.

The fact that diversity among American vegetable crops is down 7% from the 1910s may not sound daunting. But the very real danger that disease could wipe out existing cash crops should disconcert us all, especially as so much of what is grown is fed to animals in the human food chain. Svalbard and the International Potato Centre in Lima are remarkable facilities and Fowler has achieved a great deal in his career to date. But preserving genetic seed diversity is not enough in itself and McLeod makes her point with eloquent simplicity.

Conservation of a different kind takes John Fiege to rural East Texas in Above All Else, which chronicles the efforts of David Daniel and his neighbours to prevent TransCanada's Keystone XL pipeline from crossing their pristine land during its passage from the Alberta tar sands to the refineries on the Gulf Coast. However, while Fiege succeeds in capturing the drama of the stand-off between the amateur activists and the hired guns of the faceless conglomerate, he fails to provide enough background information to give the heroic rearguard its much-needed rationale. Consequently, those familiar with documentaries like Peter Mettler's Petropolis (2010) or Tom Radford and Niobe Thompson's Tipping Point (2011) will have a better understanding of what is at stake on both a human and an environmental level.

David Daniel spent around 30 years working as a gymnast and a stuntman before he moved his family to a plot in the East Texas forest. He worked for three years building his dream home and had only just completed the project when he intercepted surveyors on his property. Six weeks later, Daniel received notification from TransCanada asking for permission to roam. He soon found out that his neighbours had experienced similar treatment and further deceptions followed as the company tried to convince the locals that deals had already been concluded in order to coerce them into selling up for a bargain price.

In fact, the Obama administration had yet to reach a decision about allowing Keystone XL over the 49th Parallel. So, when TransCanada started sending private security guards to secure their patch, Daniel got in touch with the volunteers at Tar Sands Blockade and began to fight back. As in all good eco conflicts, however, they adopted the tactics of civil disobedience and passive resistance. People began padlocking themselves to pipe trucks and barricading roads. Even the kids were taught how to stand up to bullies and how to escape potentially dangerous situations.

But the protest reached its zenith in the winter of 2012 when a small encampment of treehouses was erected in the forest canopy in precisely the places TransCanada had earmarked for the initial clearance. While it sounds noble and romantic, this was a perilous enterprise and many sacrifices had to be made in order to keep the site occupied and guarded. Daniel had his own share of domestic turmoil, while some of his older neighbours faced the real prospect of losing their homes.

Fiege cannily crosscuts the sit-in with clips from corporate videos boasting how sensitively TransCanada deal with landowners on their pipe route. But, while the ironic juxtaposition is drolly trenchant, this actuality is every bit as polemical as the promos it mocks. Naturally, audiences will side with David against the Canuck Goliath, especially when he is used to acts of high-wire derring-do and prepared to get himself arrested outside the White House to get his point across. But, by opting not to lay out the case against tar sand excavation and transportation in worthwhile detail, this winds up feeling like a calculatingly emotive exercise in Capracornery that too often lays itself open to accusations of tree-hugging nimbyism.

Kris Kaczor handles another underdog story with a little more finesse in Divide in Concord, which stands shoulder to should with 84 year-old Jean Hill and her adjutant Jill Appel, as they try to persuade the local council to pass a law banning single-serving, non-reusable plastic water bottles in the town of Concord, Massachusetts. As Lexington and Concord were the scenes of the first battles in the American Revolutionary War of 1776 and native son Henry David Thoreau returned home to write his rustic masterpiece, Walden (1854), this is a highly symbolic place to launch an eco campaign. But, while it is easy enough to admire Hill for standing up for what she believes at her advanced age, her tactics leave a lot to be desired and Kaczor struggles to convert her tetchy tenacity into anything approximating winning empathy.

Hill became aware of the problem caused by plastic bottles when her grandson told her about the plastic contributing to the 100 million tons of waste trapped in the Pacific Trash Vortex by the currents of the North Pacific Gyre. She quickly discovered that 1500 bottles of water were consumed every second in the USA and that only a fifth of them were recycled. So, in 2010, she enlisted the help of Appel and some like-minded green warriors and went door to door drumming up support for a bill in the municipal assembly.

Many are swayed by the mere mention of the International Bottled Water Association. But Hill runs into difficulty when local Republican Adriana Cohen points out that it makes no sense outlawing one of the healthiest beverages on sale in Concord when sugary drinks that have been proven to be detrimental to teeth and the waistline are sold in similar containers. Cohen proclaims that people have a democratic right to choose the products they purchase and, perhaps unsurprisingly, shopkeeper Jim Crosby concurs.

Yet when they challenge Hill to debate the issues on the radio, she relies on taciturn obstinacy rather than cogent argument to undermine their case. Similarly, a dismaying number of her supporters resort to cheap political insults instead of countering their opponents and quite what the artist who steals bottles from recycling bins to produce a sculpture that supposedly exposes their lack of commitment to saving the environment is anyone's guess.

Although he devotes time to the naysayers, Kaczor makes little attempt to hide his sympathies with Hill's crusade. Thus, he shares her disappointment when the bill is rejected on its first two readings. However, she only lost by seven votes the second time and she returned to secure a 39-vote majority on 5 September 2012. Subsequently, Concord's example has been followed by San Francisco and Harvard University. But, while Hill may have carried the day, her methods are as flawed as her reasoning and this failure to win the audience over to a little old lady rabble-rousing from the best of intentions makes this a very unsettling film to watch.

Finally, Fredrik Gertten switches focus in his latest documentary, as the Swede who forged his reputation with Bananas!* (2009) and Big Boys Gone Bananas!* (2012) takes a look at the merits of pedal and horse power in Bikes vs Cars. Having been born in Malmö, Gertten has always cycled and believes that a combination of the multi-billion dollar multi-national oil, automotive and construction industries has ensured that national and civic leaders are powerless to introduce urban planning initiatives that would reduce the number of cars on our roads. It is projected that, by 2020, there will be twice the one billion cars currently on the worlds roads and Gertten travels across Europe and the Americas to examine how the eco lobby is striving to fight back.

Among the pioneers he encounters is Aline Cavalcante, who was so appalled by the statistic that a cyclist is killed every four days in the Brazilian city of São Paulo that she launched Ciclofaxia, which ensures that, each Sunday, one lane of Paulista Avenue is open to cycles only. Urban planner Raquel Rolnik and film-maker Fabio Mendonza endorse her approach. But car dealer Nicolas Habib knows there are big bucks to be made from supplying the aspirational residents of a thriving megalopolis and he is bent on expanding the five dealerships his father Sergio opened after concluding a deal with the state-owned Chinese firm, JAC Motors.

Since disgraced Toronto mayor Rob Ford and deputy Denzil Minnan-Wong also thought that the motor vehicle has had a bad press. Indeed, Ford was so convinced there is a war being waged against cars that he spent $300,000 in 2012 to remove the bike lanes from Jarvis Street. However, he hadn't counted on the civil disobedience of the anonymous members of the Urban Repair Squad, who sneak out at night and use spray cans and stencils to replace the street markings.

By contrast, Joel Ewanick has seen the light after spending much of his career as a marketing director for such car companies as Porsche, General Motors

and Hyundai. In 2013, he founded FirstElement Fuel with the aim of dotting the Californian landscape with hydrogen filling stations that would encourage consumers to switch to fuel cell vehicles. Elsewhere in the same state, Los Angelino Don Ward has founded the underground cycling club Wolfpack Hustle, while urban explorer Dan Koeppel has encouraged his fellow citizens to rely on Shanks' Pony by organising the annual two-day walking event known as The Big Parade.

Among the most emulated strategies is the Ciclovia opened in the Colombian capital, Bogotá by liveability specialist Gil Peñalosa. But such schemes are still few and far between, even though Copenhagen cabby and occasional tour guide Ivan Naurholm believes such a dedicated route would do much to improve even a relatively enlightened city like the Danish capital, where 40% already make their daily commute under their own steam.

As with any advocatorial documentary, it is easy to establish heroes and villains for the audience to latch on to. Gertten is rather guilty of playing to the crowd here. But his motives are honourable and few will disagree that something has to be done about the gridlock choking major cities and causing road rage and accidents, as well as pollution. Quite whether this `on yer bike' crusade will have the desired effect is extremely doubtful. But Gertten is very strong on the historical context and the section on how the US car industry sold the dream of the freeway, out-of-town shopping and leisure motoring is potent and sobering.