Vietnam Notebook: Effects of Geography and Climate

Mekong River Delta, Cần Thơ, Vietnam.

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The basic geography and climate of Vietnam

Indochina is dominated by three main geographic features:

(1) The first is the Annamite mountain chain that starts in the highlands of China’s Yunnan province and traverses the spine of Indochina until it meets the east coast in central South Vietnam. The crests of the Annamite mountains follow the border between Laos and Vietnam before crossing over into Vietnam just south of the point where the borders of Laos, Cambodia, and Vietnam meet and ending some fifty miles east northeast of Saigon.

(2) The second feature is the Red River. Along with its principal tributaries, the Black River to the south and the Clear River to the north, the Red River rises in the highlands of Yunan and flows in a southeastern direction to the Gulf of Tonkin, forming the broad delta in northern Vietnam that was the cradle of Vietnamese culture and civilization. We know from the archaeological record, based on skeletal measurements, that the people who lived there as far back as the third millennium B.C. were the ancestors of the people who live there today. That conclusion is also supported by the linguistic record; ancient place names are preserved in the Red River Delta that aren’t used anywhere else in Vietnam. We also know that almost from the beginning, the people of the Red River Delta were growing paddy rice, that is they were damming up the land and creating paddies in which to grow their rice.

(3) The third important geographic features is the Mekong River. It rises in the mountains of Yunnan not far from the Red River’s point of origin, flows southward through the north-easternmost of the Annamite mountains, then through the Mekong plain. It marks the border between Laos and Thailand for most of its length, and then forms the Mekong Delta in southeastern Cambodia and southernmost Vietnam. Like the Red River Delta, the Mekong Delta is a major rice growing region, but has several advantages to its northern counterpart; First, since it is further south, the climate is hotter and that results in significantly larger yields per acre; Second, unlike the Red River, the Mekong floods gradually and relatively predictably. That predictability allows for a special kind of rice, floating rice, that grows fast enough to stay ahead of the rising water.*

There is another important difference between the Red and Mekong deltas, and that is that the Vietnamese didn’t reach the Mekong Delta until comparatively recently. The original populace of the Mekong Delta, at least in historical times, was Khmer, that is Cambodian, and there is still a significant Cambodian minority in Vietnam’s Mekong Delta provinces. The Vietnamese didn’t reach the Mekong Delta until about the time of the American Revolution and didn’t finish conquering it from the Cambodians until the 1850s, shortly before the French arrived.

To recap, the dominant physical geographic features of Indochina are the Annamite mountains, the Red River and the Mekong River. Next let’s take a look at the climate …

There are two monsoons: the southwest monsoon, running from about mid-October to late March; and the northeast monsoon that runs the remainder of the year. The most important of the two for our purposes is the southwest monsoon. It brings warm, moist air off the Indian Ocean and South China Sea and deposits it as rain. Vietnam’s elevation increases from south to north. As air rises it cools, at a rate of two and a half degrees per thousand feet, and as it cools its capacity to hold moisture diminishes. Since the warm air coming off the ocean is saturated to begin with, it turns to rain, providing the water needed to grow rice year after year.

The rain comes down so hard, particularly up against the west side of the Annamite mountains that the ground turns to soup and nothing can move. Those downpours are in full force by May in central Laos and the rains move northward as the season progresses. By July or August Indochina is saturated, with one exception; the coastal strip of central South Vietnam on the east side of the Annamite range, from about 80 miles north of the Ben Hai River to an area just northeast of Saigon, is sheltered by the mountains and depends on the northeast monsoon for its rain.

The northeast monsoon brings dry weather to most of Indochina. It is based on the clockwise circulation around the Siberian high, the high pressure system over the eastern part of Russia. It generally runs from mid-September to late December. By late October or early November things start to dry out and by December most of the peninsula is dry.

There is, however, a partial exception. The easternmost air coming off the Siberian high passes over the Gulf of Tonkin before reaching Indochina and picks up a fair amount of moisture en route. It brings enough moisture to irrigate the coastal strip of central Vietnam  and in the Red River Delta farmers are able to double crop, that is they can grow two full crops of rice per year.

Mid-October to mid-May is the traditional winter-spring campaign season around which the communists based their military planning.  If you look carefully at the weather cycle, you’ll find that the period between the beginning of January and mid-May, roughly 1 January through 15 May, is the only time of the year when the weather is suitable for offensive operations throughout all of Indochina. If you look at Vietnamese history, almost every decisive battle or campaign all the way back to the Chinese conquest in 111 B.C., were launched during that period.

Wet rice cultivation is the basic reality of Vietnamese culture. Given a suitable climate, you can grow more food calories per acre with wet rice cultivation than with any competing grain and you can do it year after year in the same paddy without a loss of fertility. You’ve got a ready source of fertilizer: human waste, supplemented by manure from the water buffalo you use to plow and whatever pigs, chickens, and dogs you may have. And I should add that the Vietnamese were growing rice in paddies long before the Chinese; in fact the Chinese seem to have learned wet rice cultivation from the ancestors of the Vietnamese fairly late in the game, perhaps as late as the time of Christ.

Moreover, the way in which the food staple is grown affects the culture and its values. Growing wheat in dry land tilled with an ox or horse-drawn plow shaped the mentality of the European peasant. Since the number of calories per acre per year is relatively modest, at least by wet rice cultivation standards, Europe historically had relatively low population densities by Asian standards. And just as dry land grain cultivation shaped us, the realities of wet rice cultivation shaped the Vietnamese. Since you can raise a crop from the same paddy, year after year, generation after generation, you develop an overriding attachment to your ancestral village. This cultural phenomenon cannot be understated. What’s more, each villager had to cooperate to keep the paddy dams in good repair; they had to cooperate to keep the flood control dams in place in order to keep the Red River under control, no mean feat because the Red River and its tributaries flow down steep and narrow valleys, and when the rains come they rise quickly.

The population density in the rice growing regions of the Red River Delta can run between 2,500 and 4,000 people per square mile. In the south it’s around 150 to 250 per square mile of cultivatable land. The reason for the disparity is historical: Vietnamese were relative late comers to the lower Mekong Delta and rice growing became commercialized before population densities could rise to traditional levels. The population density of the Red River Delta, and the Than Hoa region to its immediate south, was determined by how much rice could be grown and that in turn was limited by the relative coolness of the climate. As a result, the Red River Delta has never enjoyed a significant rice surplus and bad growing conditions can quickly lead to famine. Modern data indicates that the peasants of the Red River Delta work significantly harder than those in the lower Mekong Delta to produce the same amount of rice. That has been true from the beginning and has had cultural consequences for the region.

The annual weather cycle has some interesting aspects. Crachin is a period of light rain accompanied by low stratus clouds and poor visibility which frequently occurs in the area of the China Sea between January and April. In late March the rice farmers burn off their stubble. That produces large amounts of smoke that, when combined with the rains of crachin, produces big chunks of ash falling out of the sky.

In addition to the wet rice farmers burning off their stubble, many hill tribes in the mountains of Laos and Vietnam annually practice slash and burn agricultural techniques. That produces only a fraction of the per acre yield, but requires very little labor. It also depletes the fertility of the land very rapidly.

So, as it stands, these same cycles prevail over and over, driving not only agriculture but also war…

The monsoon cycle is important militarily for a number of reasons. First, the only time during the annual cycle when the weather is suitable for military campaigning throughout all of Indochina is the period between 1 January and 15 May. Unsurprisingly, many of the important battles in the region’s history were fought during that time. Second, most of Indochina is suitable for campaigning between mid-October and mid-March. Nearly every decisive battle or campaign in the history of Indochina was fought or at least started during that winter/spring campaigning season, including the major battles and campaigns of antiquity and the middle ages. A remarkably high percentage of them were fought or started during the brief period between the beginning of January and mid-March when the weather over all of Indochina is suitable for campaigning.

What is karst? Limestone rock. The formations commonly seen in Southeast Asia featuring vertical structures of limestone rock, whole mountains, and they can go straight up—absolutely vertical—several thousand feet, straight up and straight back down, range after range. It’s very spectacular and very rough. It’s part of the culture, too. There are areas of Laos, North Vietnam, and China where the mountains really do look like that. Rather spectacular, very beautiful… and it’s difficult to move military forces through them!

History and human geography of Vietnam:

When you look at the map of Vietnam, you see that the entire coastal strip is marked with river deltas, smaller than the Red and Mekong deltas, but economically significant. What do you find in them? In the north, you find people who live in same villages where their ancestors lived five thousand years ago. As you move southward these ancestral ties extend for progressively shorter periods as you go. Starting in pre-historic times, the Vietnamese expanded from north to south, from delta to delta, displacing the original inhabitants. They grew rice. It’s back-breaking work and produces an enormous amount of food per acre, but not a super-abundance of calories in terms of the amount work expended per person. In addition, you have to maintain the dikes. Every step is hard work. But it does support a dense population.

One more cultural reality of the human geography in Vietnam worth noting is the interplay between lowland Vietnamese and the hill tribes. Remember that slash and burn agriculture practiced by the hill tribes produced much less food per acre than rice paddies, but it produced much more food per person hour expended. It doesn’t support high population densities, and that is why the lowland, wet rice growing cultures rule Indochina. But there is a cultural backlash: the lowland paddy farmers know that the hill tribes don’t work as hard as they do, so they look down on them as soft and lazy. There are very hard feelings and prejudices on both sides, right down to today. Cultural memory is at work here: the hill tribes were forced up into the hills in antiquity by the wet rice growing Vietnamese conquerors who seized the best land. When the French finally came in, they took advantage of the enmity, and cultivated the hill tribes: the hill tribes of northern Indochina, the Hmong, Man and T’ai, sided with the French against the Vietminh; later, the Hmong, Nung and some of the more primitive southern tribes sided with the U.S. They were not rewarded for their efforts in the end.

* Part of the reason is the big lake in the middle of Cambodia, the Tonle Sap. During the dry monsoon the water flows out of the Tonle Sap into the Mekong and from there out to the South China Sea. During the wet monsoon, the flood waters coming down the Mekong from China and the Mekong Plain back up into the Tonle Sap, flowing the other way. That absorbs a lot of the flood water and releases it gradually.

You might also check the maps on

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