It is noon in Mali and a film of heat coats the West African landscape. The earth is not yet desert, but it becomes increasingly leached and ashen the farther north one travels; miles of red soil give way to sand.

Villages are occasionally visible from the road, their huts, granaries, and spired mosques all built of dun-colored adobe. In front of the compounds, there are invariably women pounding millet, rhythmically hurling long wooden pestles over their heads, then twirling them down into mortars of grain.

Under a powder-blue sky, the scalding sub-Saharan expanse stretches away, the flat land broken only by scattered bushes, baobab trees, and - not far from the Niger River town of Mopti - a single road sign.

It advertises ''Operation Mils Mopti,'' an agricultural project that is credited with doubling the production of millet and sorghum in the Niger River delta. The project, funded by the United States Agency for International Development (AID), underwrites well-digging, provides training for extension agents in agrononmy, and supplies credit to Malian farmers for purchases of fertilizers and donkey-drawn plows.

The sign brings back a conversation held a week earlier, nearly 6,000 miles northwest of Mali. ''Consider some facts,'' said the Washington-based AID official, leaning back in his chair and glancing at the gray row of State Department offices out his window. ''The eight nations of the Sahel region - Senegal, Mali, Mauritania, Upper Volta, Chad, Niger, Gambia, and the Cape Verde Islands - have a total of 30 million people, only a quarter of whom have access to clean drinking water. The average intake of calories is below - 20 percent below - the minimum recommended by the United Nations.

''The infant mortality rate is over 15 percent. Life expectancy is 38 years, and a host of diseases ruin lives well before that age. The literacy rate is 10 percent. Annual per capita income is under $200.''

He paused and leaned forward. ''I could go on. The needs of the Sahel are enormous.''

The Sahel - the long swath of semiarid land separating tropical Africa from the Sahara Desert - has for the past five years been the stage for a major US development effort, one born in response to the drought and famine of 1968-1973 during which tens of thousands of Sahelians died. The vehicle for the US economic assistance is the Sahel Development Program (SDP), honored as a separate line item in the foreign aid budget and allocated $95 million in fiscal 1981. (The appropriation for fiscal 1982 is still hanging fire.) While representing less than 2 percent of the US foreign aid budget, the SDP embodies a shift in the type of foreign aid sent to the Sahel - one away from traditional short-term relief shipments to long-term development projects, regionwide in scope. (Indeed, by addressing the needs of an entire geographical region rather than simply single countries, the SDP has served as a model for AID projects elsewhere in Africa and the Caribbean.)

The flow of US aid dollars has been to the countryside, where 90 percent of the Sahel's population live as farmers and herders. ''We want to increase agricultural production by changing the technology of the farmer, his instruments, his fertilizers and pesticides, the way he uses his resources,'' explains an AID official in Mali's capital of Bamako. Smiling, conscious of the immodest scope of his proposal, he adds, ''It took 40 or 50 years to change agricultural technology in the US. It's what we want to do here in 20.''

Like Operation Mils Mopti, other SDP projects tend to focus on basic needs: health clinics, farm-to-market roads, reforestation, livestock management, the construction of a rainwater dam in Mauritania, the installation of solar-powered pumps to lift water and grind grain in Upper Volta.

''We've had our share of failure,'' concedes an AID worker in Mali. ''The common thread of success has been those projects that are small-scale, that offer a return to each local participant. You'd be surprised,'' he adds ruefully , ''how long it took us to figure that out.''

The SDP dovetails with the efforts of more than 20 other nations and development banks. Together with the Sahelian states themselves, the donors make up the ''Club du Sahel,'' an informal organization designed to plan and coordinate long-term development projects across the Sahel. In terms of financial contribution, the US has generally ranked fourth or fifth among the club's donors, while France has consistently dispatched the most aid. With the exception of the Gambia (formerly British) and the Cape Verde Islands (formerly Portuguese), the Sahelian nations were French colonies until 1960. France has maintained close contact with them ear-marking $200 million annually for their development.

The eight Sahelian states have also banded together for the purpose of attracting foreign aid to ''famine-proof'' the Sahel. The unified front, called the Permanent Interstate Committee for Drought Control in the Sahel, proved to be a persuasive technique, at least when it came to approaching the US for assistance. One AID official recalls, ''You can bet there was more sympathy on Capitol Hill for the Sahel as a whole than for any one individual state.''

The coordination between donors and the eight sub-Saharan nations has produced - at least for the time being - what seems to be a well-functioning multinational effort. But as one AID official in Bamako points out, ''We're not sure how long the regional approach will appeal to the Sahelian states themselves. The concept is noble. There is a commonality of problems. But there are compelling cultural and political differences, as well as enormous physical distances - the Sahel's nearly the size of the continental US - that can render similarities academic.''

The goal of AID and other donors is to midwife the region into food self-sufficiency and self-sustaining and selt-sustaining economic growth by the end of the century - twin objectives that the Club du Sahel estimates will require aid projects totaling $20 billion to $30 billion over the next two decades.

One former Peace Corps volunteer who has worked in Malian agriculture questions how much of the developmental burden foreign aid can be expected to shoulder. ''Economic aid has to be kept in perspective,'' he cautions, noting that it is only one factor, often competing unsuccessfully against an army of countervailing forces. He offers Mali's own government as one example.

By refusing to pay the nation's farmers fair prices for crops, the bureaucracy can keep city food prices low and urban consumers (who have staged food riots in other West African states) mollified, but the result can be food shortages - farmers growing no more than their families need or else shuttling excess produce into black markets.

''I get furious with people who say Mali can't be self-sufficient,'' says the volunteer. ''Half the food grown where I worked was taken across the border into Upper Volta for better prices.''

He and other development workers cite a litany of impediments blocking agricultural self-sufficiency: changing dietary patterns that result in a nation's populace growing increasingly dependent on imported crops; governments encouring farmers to grow ''cash'' crops for export (and foreign exchange revenue) rather than ''food'' crops for village consumption; and emergency food shipments from abroad - ''It's a no-win situation,'' confesses one relief worker in Upper Volta. ''If the emergency food is not sent, people will starve; and if it is sent, the food can flood the local market and undermine any recovery of local farm prices.''

And then of course, in this land of little rain, there is the weather. Recent studies show that the 1968-1973 weather cycle was not unique; drought has regularly desiccated the Sahel. Climatologists worry now, however, that a a new, long-term pattern of substandard and erratic rainfall - related to overgrazing, desertification and deforestation - may have begun. ''The best foreign aid the Sahel can get,'' goes one common saying, ''is rain.''

Relief workers point out that while the Sahel (though not each state separately) can possibly achieve food self-sufficiency by 2000, the coming years will almost certainly bring unpredictable harvests, and the effectiveness of foreign aid may well be challenged. Says one worker: ''Aid has made a difference , but to see it you have to look where the projects are operating. Farmers are eating better. Life has improved. But the successes are found in the single villages as opposed to the aggregate.''

While the US Congress has often stated its support for the SDP, there is some question whether it will continue unabated. Rep. Stephen Solarz (D) of New York pointed out the peculiar vulnerability of the SDP during hearings for the 1980 SDP budget. Solarz, himself a strong backer of the program, noted the absence in the Sahel of the ''overriding strategic, political or military consideration'' that justified US aid to the Middle East, and added that ''. . . most people in Brooklyn, and I suspect in New Jersey, think, if they think of Ouagadougou (Upper Volta's capital) at all, think of it as some kind of candy bar but hardly as the center for these (developmental) activities.''

But in the view of one AID official who has worked in several Sahelian nations, the region's modest strategic significance should be viewed as an asset. It is the one corner of the world, he asserts, where America operates the popular conception of a foreign aid program - assistance based on the recipient's needs rather than US self-interest: ''We need one program like that, '' he says quietly. ''For our morality. The Sahel Development Program is the last humanitarian economic assistance program run by AID.''

But the SDP's future is unclear. ''We're holding our breath to see what this new Reagan administration will do,'' said one official with a private relief organization associated with AID projects in early February. ''It looks like even developmental aid may be tied to only those countries offering major political or military value to the US. If that happens, the Sahel may well be left out.''

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