San Salvador — The arms flow to leftist guerrillas in El Salvador appears to have increased dramatically in recent months.
According to United States Ambassador Deane R. Hinton, the flow is now ''at an all-time high.''
Included are arms and ammunition, radio and other communication gear, and medical supplies.
The arms comes in over rugged mountain trails, sometimes on mule-back, from Honduras; by air drops in the remote and rugged guerrilla-infested northern reaches of the country; and aboard small boats and even canoes plying the waters of the Gulf of Fonseca and the smooth coastal waters of the Pacific.
''It is as if you had 40 or 50 Ho Chi Minh trails,'' comments one observer close to the story.
That reference to the Ho Chi Minh trail that supplied communist guerrillas in South Vietnam during the 1960s with their arms, ammunition, and equipment is not mere hyperbole, say these sources. They say it is an accurate comparison to the many routes used by communist guerrillas more than a decade ago in Vietnam.
The amount of arms and equipment flowing into El Salvador can at best be only a guess. But based on data from observations by Salvadorans on the ground and from sophisticated monitoring equipment, the flow is more than enough to supply the approximately 5,000 guerrillas operating in virtually every part of this Massachusetts-size nation.
Observers here, echoing the Reagan administration's statements on the subject , say that Nicaragua serves as the staging area for much of the flow. But these sources are more concerned with the flow itself and its routes, than with the actual origin of the equipment. Some of the flow is stopped by Salvadoran Army and security forces. But it is thought that the vast majority is slipping through.
Nicaraguan officials, it should be noted, deny they are supplying arms to Salvadoran guerrillas.
Among materiel captured entering the country and picked up after encounters with the guerrillas is a good deal of quite sophisticated radio and other communications equipment.
There are also vast quantities of arms and ammunition. The arms include small sidearms, rifles, machine guns, antiaircraft guns, rocket-propelled grenades, M- 79 grenade launchers, and antitank weapons. The latter, together with the rocket-propelled grenades, have a capability of knocking out tanks with their tremendous penetrating power.
One of these grenade launchers was used last year against the US Embassy here. It knocked a hole in the embassy wall 10 feet wide, blew up a room, and sent the projectile through an interior wall as well.
Some of this weaponry is old US equipment left behind in Vietnam, but sources say most is materiel that originates in both East and West Europe. Some is also new equipment from the US.
The flow apparently reached a peak just before the election. The equipment now in the country, available for guerrilla use, would equip a guerrilla force many times larger than the present guerrilla force of 5,000 men and women.
Over the past year there has been a significant shift in supply routes. Most of the material originally came in by land through Honduras, but now sea routes are more popular.
The main entry point apparently is the marshy lands of the Bay of Jiquilisco, the Bay of Jaltepeque, and the delta of the Lempa River. Trawlers and other mother ships in the Pacific unload the supplies into small boats that take the equipment in the dark of the night to drop points on the beaches and in the swampy marshes and lagoons of the region. The area could be compared to the bayous and sluices of the Mississippi River delta in Louisiana.
In the past, the seaborne supplies came across the Gulf of Fonseca, but as Honduran naval forces began cooperating with El Salvador's small navy, this route became less effective.
The land routes converge on three areas of northern El Salvador: the hump of Morazan Province north of Perquin; the largely uninhabited, scrub-brush ravine and gully area of northeasten Chalatenango Province; and the pine forests of western Chalatenango Province, where El Salvador, Guatemala, and Honduras converge.
These areas are largely a no man's land -- a region that was contested by El Salvador and Honduras in their 1969 war, an area in which troops of the two countries are prohibited by cease-fire arrangements from operating.
The air drops are most common in eastern El Salvador -- particularly in guerrilla-infested Morazan Province. Occasionally an airplane will land on a highway to disgorge its contraband.
There are other methods of getting the equipment in - some are hidden on trucks that ply the Pan American Highway. And some may come through normal channels, with payoffs given to customs officials at border points. This method is thought to be used only occasionally.