BEIT HOMOTAIM, Occupied Samaria - Despite the population time bomb fused by record-high Arab birthrates, the chairman of one of Israel's most controversial settlements in Occupied Samaria told us that "we can never give the Palestinian Arabs a chance to build a country here."

That sentiment by the mother of two who runs this small Jewish settlement exactly comports with Prime Minister Menachem Begin's view of Israel's "manifest destiny." Yet both Begin and Mrs. Yael Ben Yacoov must contend with the population time bomb - what one of Israel's top politicians calls "the forbidden subject" of Israeli politics.

Boiled down to its statistical essence, this is the problem: Current birthrates in the occupied West Bank and Gaza will double the Arab population in about 20 years. In Israel itself, the present Arab population of almost 600,000 will double even sooner under present birth and death trends. The Jewish population of Israel - now 3 million - is growing at half that rate. Israeli Jews will take 39 years to double, even when adding the current rate of Jewish immigration from other countries.

Thus, in Israel proper, the non-Jewish population will soar to 24 percent of the total from its current 16 percent in the next generation; the Arab population in the occupied territories of Gaza and the West Bank will double to well over 2 million. That raises a specter for Begin's Israel: It will face extreme political hazards as pressure of statehood escalates in the occupied territories and as Israeli Arabs, with full Israeli citizenship, agitate internally in behalf of their stateless Palestinian brothers.

It is this forbidding prospect that explains Begin's insistence on the right to unlimited Jewish settlements in the West Bank. According to some anti-Begin politicians in Jerusalem, the prime minister truly believes that unlimited Jewish settlement - a key part of his limited-autonomy plan for the West Bank - will cause eventual economic atrophy among West Bank Arabs, leading to their mass emigration.

As unexplained census now being taken by the Israeli military government of the occupied territories points to new economic harrassment of Palestinians. West Bank mayors for the first time must turn over the names of all absentee Palestinians who hold Israeli identity cards. Those cards permit them to return from jobs in other Arab countries to vacation with their families on the West Bank.

Although the Israeli military government is saying nothing, King Hussein of Jordan has protested to the United Nations. Jordan worries that the census will lead to harsher procedures against the return to those West Bankers, numbering between 50,000 and 100,000.

With economic development of the West Bank bleak, thousands of West Bankers now work for Israelis, commuting bank and forth into Israel. Restrictions on the better-educated "shadow population" of Palestinians working abroad could cut an economic lifeline, adding new pressures for massive Arab emigration.

If true, such pressures would match Israeli treatment of Arab farmers and farmland on the West Bank; thousands of acres have been seized under ambiguous occupation laws and handed over first to the Israeli army for "security," then to new Jewish settlements.

Partly because of the chaotic system of land ownership under the old Ottoman Empire, Arab farmers in many cases cannot prove their ownership of farms that go back generations. The Israeli have permitted some of the disputed farms to remain in Arab hands - but under a draconian proviso that forbids any addition or improvement to houses. When an Arab family in Marjnarja in the Jordan valley added to its two-room house after the birth of its 10th child, Israeli bulldozers destroyed the addition. West Bank experts say that case was not unusual.

Worse, new Israeli settlements almost always mean construction of deep wells with high-pressure pumping systems that dry up springs used by the Arabs. The Arab village of Ein el-Bayda, for example, lost all its natural water to the Jewish settlement of Mehola. It is now supplied by a pipeline under Israeli control, with no guarantees.

THis economic pressure explains the exodus of thousands of Arab farmers, as it also explains the exodus of tens of thousands of educated Palestinians.

Nevertheless, the population bomb's fuse is burning and realists doubt that Begin's "manifest destiny" can stop it - no matter how bleak the political future can be made to look, how many settlements he plants or how much economic pressure he applies.