In this the third critical rainy season of the full-fledged war in Rhodesia the white army brass is privately confiding its surprise and relief.

Once again its black nationalist guerrilla adversaries have made less headway than was feared during the November-through-March rainy season, which gives them the tactical advantage of cover, food and water.

The army's satisfaction is of a most limited nature and is confined to the knowledge that given its manpower problems - and financial limitations - the tactical situation could well be very much worse.

This is not a war the Rhodesian army says it can win in the field. Gen. Peter Walls, the overall military commander, has said in public that the conflict can only be brought to an end through political means.

The war, all but taken for granted now, has meant convoy traffic in some parts of the country and an end to driving after dark outside the main towns.

Security guards in shops and offices have become a permanent part of Salisbury life since August when guerrillas set off two bombs in the capital. One killed 11 persons and wounded more than 70 in a local Woolworths store.

Almost every area in the country has been the target of guerrilla hit-and-run attacks and the quickening of the war's tempo is obvious in the official casualty figures.

In 1977 alone almost as many guerrillas and government troops were killed - 1.759 and 197, respectively - as during the four previously years when the toll was 1,932 and 214. Last year 1.055 black and 56 white civilians were killed, compared to 285 and 79 in the previous four years.

Despite serious internal problems involving tribally based rivalries, the guerrillas gradually have loosened government control over the large areas close to Rhodesia's borders.

By all accounts, the Rhodesian army still controls cities, roads, all major economically important areas and can go anywhere it wants if it is willing to risk paying the price.

Government statistics show, however, that by the end of 1977, guerrilla pressure had forced the closure of 338 black primary schools - teaching 85,221 students - and 15 secondary schools teaching nearly 4,000 others. The greatest number of these schools were located in areas close to the Mozambique border in the east where guerrillas nominally loyal to Robert Mugabe's Zimbabwe African Nationalist Union operate.

Another large cluster lay in the northwest and southwest, where Zambia-based Joshua Nkomo's Zimbabwe African Peoples' Union is the undisputed guerilla organization.

Further evidence of the erosion of civil administration has been the guerrillas' ability to close down 17 mission hospitals and six others run by the government in rural areas.

Last month, a leaflet dropped over the Maranke and Makoni regions near the Mozambique border ordered a noon to dusk curfew as punishment for its inhabitants' alleged willingness to "feed, shelter and assist communist terrorists."

All schools and stores were order closed. No vehicles, including bicycles and buses, were permitted inside the large area and children and dogs were not allowed out any time on pain of being shot.

In other areas in eastern Rhodesia, the guerrillas have attacked the burned down the so-called protected villages, which are the local version of the strategic hamlets of the Vietnam conflict.

Designed to prevent the guerrillas from obtaining food and shelter from the local population, the protective villages now house half a million blacks.

Were it not for their cost, it is widely believed that more would be built.

Other evidence of guerrilla effectiveness is to be seen in various border areas where few able-bodied men are to be found. Presumably they have been recruited by the guerrillas and taken abroad for training.

The guerrillas are also credited with having infiltrated African townships around Salisbury and other major cities.

Recently two guerrillas walked into a Bulawayo factory, set off explosives and escaped to the city's sprawling black townships.

Elsewhere, especially around the eastern highlands area near the Mozambique border, the tea and coffee plantations have been able to continue working at moderate efficiency amid suggestions that their white owners have come to an accommodation with the guerrillas.

Just how many guerrillas are operating inside Rhodesia is a matter of secrecy and conjecture even within the Rhodesian establishment. Before the rainy season began, Mugabe's forces were estimated at roughly 3,300 men and Nkomo's at 300.

Traditionally, the rainy season favors guerrilla infiltrations from Mozambique and Zambia, and some sources estimate that anywhere from 3,000 to 12,000 men have entered the country since the end of the year.

All but 300 are thought to have entered from Mozambique - apparently forced across the border by Mozambiques President Samora Machel, according to Rhodesian military sources.

He is said to have acted after Rhodesian forces staged two devastatingly effective raids on guerrilla camps inside Mozambique last November.

Said to lack proper weapons and logistics support, the newly arrived guerrillas should provide easy pickings for the government forces.

Somewhat ominously, for the Rhodesian army, however, the Rhodesian leaflet campaign offering "safe return" to guerrillas who surrender - in fact simply guaranteeing their lives will be spared - has yet to produce any known dividiends.

In the meantime, Rhodesia's badly stretched career army and para-military police have recruited increasing numbers of blacks.

Published statistics claim four-fifths of the security forces are now black and the first black career army officers are already serving with units.

An indication of the army's popularity - and possibly also of widespread black unemployment - was provided recently when 7,000 black svolunteered for the 400 openings available each month in the rapidly expanding Rhodesian African Rifles.

It is part of Smith's gamble that the present internal constitutional talks will produce a black government that can rely on the increasingly black security forces to cope with the guerrillas.

Even if the present internal talks are broght to a successful conclusion, no one here thinks the guerrillas will lay down their arms simply because the goal of black majority rule has finally been achieved.

If anything, it is feared that Nkomo would the commit his better trained, armed and led guerrilas to the conflict, which he has refused to do so far in apparent hope of somehow finding a negotiated solution that would meet his demands.