Salvadorean guerrillas have altered their military tactics in response to increased bombing missions by El Salvador's Air Force and expanded United States reconnaissance flights here, guerrilla commanders and Western observers say.
Rebels in the provinces of Chalatenango, San Vicente, and Cabanas say their tactical changes include:
* Disbanding of large guerrilla columns in favor of mobile units of four to seven fighters who spread out over several hundred yards.
* Deployment of rebel forces close to the line of fire during combat to make air strikes against guerrilla positions hazardous for regular Army troops.
* Massing of large numbers of guerrilla fighters only at night, and only for one hour, before breaking again into small units.
* Suspension of the use of field radios for some military operations.
The Salvadorean Air Force, composed of six US A-37 fighter-bombers, five Israeli Fouga planes, and 19 Hughes 500 helicopters, in March increased the number of its air strikes to 30 a day from its previous level of 10, according to US and Salvadorean military officials here. This increase in bombing will be augmented by the purchase of five A-37s and 10 helicopters if El Salvador receives an additional $117 million in US military aid requested by the Reagan administration.
The Salvadorean Air Force now sends three to five planes at a time on bombing runs, compared with one or two planes previously. Salvadorean pilots also are winging in closer to their targets. In the past the planes reportedly unloaded bombs several hundred feet above targets; now US advisers are urging Salvadorean pilots to move in closer to improve the accuracy of their strikes.
Increased reliance on air power in the civil war here coincided with American and Salvadorean disclosure in April that US aircraft flying out of Honduras are providing intelligence reports to Salvador pilots and field commanders.
US reconnaissance flights out of US Southern Command headquarters in Panama have been a factor in the war for some time. The new factor, however, is that American-piloted flights by OV1 Mohawk observation planes and C130 Hercules aircraft out of Honduras's Palmerola airfield are sending immediate intelligence data to the Salvadorean military. The planes also intercept and monitor rebel field communications.
The ''Iron'' fragmentation bombs in the Salvador Air Force arsenal now include antipersonnel bombs that explode a yard above the ground. Above-ground explosions allow hunks of metal shrapnel to be hurled in a wider, deadlier circumference than bombs already in use which explode on contact. The Air Force has 250-, 500-, and 750-pound US-made ''Iron'' bombs.
The increase in air assaults and plans to further augment Air Force capacity stand in contrast to the Army's lack of mobility and ground strength, say rebel fighters.
''The Army is increasingly using the Air Force to respond to guerrilla strikes,'' says one rebel standing on a road outside the town of Tejutepeque in Cabanas Province. ''They realize that their troops no longer have the capacity to break the rebel forces.''
As this guerrilla speaks, small units of rebels enter the town to buy plastic bags of sweet bread. There appear to be dozens of fighters in the area who could mass into a large force in a matter of minutes, but who now deploy themselves in seemingly autonomous cells.
''The guerrillas can be spotted by the reconnaissance flights,'' says one US military observer, ''but it's just not worth dropping 500-pound bombs on groups of four or five people.''
Rebels contend that new aerial infrared photography taken by US nighttime reconnaissance flights means the rebels can gather in large numbers only briefly each night.
''We hold our large meetings at night for one hour,'' says a rebel patrol leader in the guerrilla-held town of Comalapa in Chalatenango Province, ''so that by the time the infrared photography reaches the Air Force we have disbanded again into small groups. But this tactic is not new; we have been doing this for over a year.''
''When we deploy ourselves during combat,'' says a rebel unit leader called Matilde in Santa Clara in San Vicente Province, ''we are up next to the belts of the soldiers. Any aerial attack endangers the enemy, and if they bomb close to us the soldiers often run away. When the bombs kill the soldiers themselves, it has a very demoralizing effect'' on their fellow fighters.
''We are aware that the US can monitor our radio conversations,'' Matilde says, ''so we find ways to communicate without our radios.''
The Human Rights Office of the Roman Catholic Archdiocese in San Salvador contends that the increased air strikes have primarily endangered the lives of civilians, who live in fixed locations and cluster together in small population centers. The rebels say that the air strikes are targeted at civilian population centers in an effort to break the rebels' infrastructure. The Air Force denies this.
Rebel radio Venceremos recently condemned an Air Force strike which it said resulted in the deaths of some 30 to 40 civilians in the villages around Tejutepeque.
There is some evidence that the air strikes can be effective against the rebel army, especially if guerrillas are casual about their new deployment tactics.
Civilians in the town of Jutiapa in Cabanas Province contend that the Air Force attacked and killed many in a large column of guerrillas that moved out of Tejutepeque after the town was captured by rebels on March 25.
''Most of them were killed,'' says one Jutiapa resident, ''and the remaining guerrillas entered the town with many wounded. The guerrillas denied that many had died up there,'' he says pointing to a steep volcanic rise on the outskirts of the town, ''but the civilians who live near there came back with stories of 90 or 100 bodies. You didn't hear about this on the guerrilla radio.''
This air strike was mentioned by Col. Joseph Stringham, the former head of the US military group here, during his June 4 farewell conference.
''The bombing and close air support, I'm convinced, have been very effective, '' the stocky Vietnam veteran said. ''I can cite numerous examples. The attack on Tejutepeque the first election round caught them trying to get out of there in the daylight and tore them up. Without actually going down and counting, they caught them in the middle of the afternoon. The Hughes 500s were up, the A-37s were up, everybody in the world was up. Nobody's counting bodies on this thing, thank God, but I can tell you for sure that I am aware that it hurt the subversives.''