Placing a value on Kenya's largest forest
They fall as mere raindrops but quickly transform into cogs in a billion-dollar machine crucial to the future of a nation's economy.
That's the startling conclusion of new research into the economic value of the preserving Kenya's Mau Forest, the country's largest.
In the jargon of environmental science, the forest's ability to generate rain and to store water is "an ecosystem service" worth huge sums to activities downstream.
The forest stretches over hills between the Rift Valley and Lake Victoria and is the source of no fewer than 12 rivers flowing through the heart of Kenya.
Prized as a "natural water tower", the forest has also been the target for aggressive clearance and timber logging in recent decades and its size has been cut by at least 40%.
Research by the Kenya Forest Research Institute (KEFRI) and the UN Environment Programme (UNEP) estimates the economic benefit of the forest to be more than $1.3bn per year.
The aim of the work is to bring a financial focus to the cause of conservation.
UNEP's executive secretary Achim Steiner told me: "Having the hard numbers for the value of nature changes the way people think about it.
"If we destroy the forest, we compromise nature's ability to supply water, and if we lose the water supply we'll have to spend lot of money finding alternatives."
The study comes as officials and ministers gather at Nagoya in Japan for a major conference on the UN Convention on Biodiversity.
With targets for conserving the natural world repeatedly missed, the hope is that introducing an economic argument will help to halt the losses.
Key industries that depend on water from the Mau Forest are already aware of its critical importance.
Bordering the trees are some of Kenya's largest tea plantations - tea is one of the country's key exports and the research calculates that it benefits from the forest to the tune of $163m a year.
Standing in a field of tea bushes owned by the food giant Unilever, Florence Mitei, a company official, describes the forest as essential.
"The forest gives us rain. Without the trees we don't get rainfall, therefore we do not get our tea.
"During the dry spells, the plants dry up and we cannot support the livelihoods of our employees."
Further downstream, Kenya's state power company Kengen operates a Japanese-funded hydro-electric power station on the Sondu River - half of the country's power is driven by water.
Some 90% of the Sondu's flow comes directly from the Mau Forest and the station itself generates as much as 6% of Kenya's total supply.
A massive system of barriers channels part of the river's flow towards a steep pipe that leads to a turbine hall - but output fell drastically during a drought last year.
The study calculates that the Mau Forest's value to the electricity sector is $131.6m.
Manager Alfried Abiero says that, "long-term you get worried about the future of the forest because for the sustainability of this project, the Mau has to be there."
I ask him what would happen if the forest were to be cleared.
"God forbid," he says. "We'd get reduced precipitation and reduced flows and it would affect the viability of the whole system."
Further from the forest are six lakes that receive the Mau's water, among them Lake Nakuru, renowned for its population of brilliant pink flamingos.
This blaze of lurid colour helps to make tourism one of Kenya's biggest earners, and the study reckons this industry receives $65 million in benefits from the forest.
Further services provided by the forest include an estimated $89million in storing carbon, $98million in controlling soil erosion and $21million in support for fisheries.
For Jacob Mwanduka, of the campaign group Friends of the Mau Forest Watershed (FOMAWA), the forest is "the leading ecosystem in this country, supporting a third of country's population.
"Without water, there is no life. And without forests, there is no water.
"It's as simple as that. It's painful that we are losing our forest, so we need to act now."
Aware of these pressures, the Kenyan Government is committed to saving the Mau Forest and has plans to confront the main cause of its destruction: the presence of 20,000 families inside it. Some have been resettled already.
But this raises highly sensitive questions - how much should people be compensated? What about illegal settlers? And where should they be moved to?
Beside one track through the forest, Margaret Kwamboka and her husband Kennedy are planting pea seeds on a patch of cleared land. The charred stumps of felled trees still stand around them.
Margaret says she understands the value of the trees but cannot afford to leave.
"There's nothing we can do. I am afraid because I don't have anything. How will I feed my children?"
The fate of the forest raises a difficult dilemma: the balance between the immediate needs of impoverished people in a developing country and the long-term ability of the natural world to support for key elements of the economy.
Usually, struggles of this kind end with nature losing.
That is why there's such a clamour for governments to commit themselves to tougher targets for conservation at the talks in Nagoya.
The raindrops trickling from the leaves of the Mau Forest - and their worth in hard currency - are a reminder of what is at stake.